a VNC server for real X displays
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README.md

Build Status

This is x11vnc with its development continued by LibVNC and the GitHub community. While 0.9.13 was the last release by the original author Karl Runge, 0.9.14 was the first community-based release here on GitHub.

This repo represents the x11vnc codebase that once resided in the LibVNCServer repo. It was split off LibVNCServer and moved here with https://github.com/LibVNC/libvncserver/commit/498d222976975f53dea885cfe43ef0f805abd412.

Copyright (C) 2002-2010 Karl J. Runge runge@karlrunge.com All rights reserved.

x11vnc README file Date: Mon Dec 27 20:58:57 EST 2010

The following information is taken from these URLs:

http://www.karlrunge.com/x11vnc/index.html
http://www.karlrunge.com/x11vnc/x11vnc_opts.html
...

they contain the most up to date info.

http://www.karlrunge.com/x11vnc/index.html:

x11vnc: a VNC server for real X displays

x11vnc allows one to view remotely and interact with real X displays (i.e. a display corresponding to a physical monitor, keyboard, and mouse) with any VNC viewer. In this way it plays the role for Unix/X11 that WinVNC plays for Windows.

It has built-in SSL/TLS encryption and 2048 bit RSA authentication, including VeNCrypt support; UNIX account and password login support; server-side scaling; single port HTTPS/HTTP+VNC; Zeroconf service advertising; and TightVNC and UltraVNC file-transfer. It has also been extended to work with non-X devices: natively on Mac OS X Aqua/Quartz, webcams and TV tuner capture devices, and embedded Linux systems such as Qtopia Core. Full IPv6 support is provided. More features are described here.

It also provides an encrypted Terminal Services mode (-create, -svc, or -xdmsvc options) based on Unix usernames and Unix passwords where the user does not need to memorize his VNC display/port number. Normally a virtual X session (Xvfb) is created for each user, but it also works with X sessions on physical hardware. See the tsvnc terminal services mode of the SSVNC viewer for one way to take advantage of this mode.

I wrote x11vnc back in 2002 because x0rfbserver was basically impossible to build on Solaris and had poor performance. The primary x0rfbserver build problems centered around esoteric C++ toolkits. x11vnc is written in plain C and needs only standard libraries and so should work on nearly all Unixes, even very old ones. I also created enhancements to improve the interactive response, added many features, and etc.

This page including the FAQ contains much information [*]; solutions to many problems; and interesting applications, but nevertheless please feel free to contact me if you have problems or questions (and if I save you time or expense by giving you some of my time, please consider a PayPal Donation.) Do check the FAQ and this page first; I realize the pages are massive, but you can often use your browser's find-in-page search action using a keyword to find the answer to your problem or question.

SSVNC: An x11vnc side-project provides an Enhanced TightVNC Viewer package (SSVNC) for Unix, Windows, and Mac OS X with automatic SSL and/or SSH tunnelling support, SSL Certificate creation, Saved connection profiles, Zeroconf, VeNCrypt, and built-in Proxy support. Added features for the TightVNC Unix viewer: NewFBSize, ZRLE encoding, Viewer-side Scaling, cursor alphablending, low color modes, and enhanced popup menu; UltraVNC extensions support for: File Transfer, Text Chat, Single Window, Server Input, and 1/n Scaling extensions, and UltraVNC DSM encryption. The SSVNC bundle could be placed on, say, a USB memory stick for SSL/SSH VNC viewing from nearly any networked computer.

Announcements:

Important: If you created any permanent SSL certificates (e.g. via "x11vnc -ssl SAVE ...") on a Debian or Ubuntu system from Sept. 2006 through May 2008, then those keys are likely extremely weak and can be easily cracked. The certificate files should be deleted and recreated on a non-Debian system or an updated one. See http://www.debian.org/security/2008/dsa-1571 for details. The same applies to SSH keys (not used by x11vnc directly, but many people use SSH tunnels for VNC access.)

FAQ moved: The huge FAQ has finally been moved to its own page. If you are trying to follow someone's link to an FAQ once on this page it is now a broken link. Try inserting the string "faq.html", e.g.: from: http://www.karlrunge.com/x11vnc/#faq-singleclick to: http://www.karlrunge.com/x11vnc/faq.html#faq-singleclick

Apologies for the inconvenience, unfortunately it is not possible to automatically redirect to the new page since the '#' anchor is not sent to the webserver.

Background:

VNC (Virtual Network Computing) is a very useful network graphics protocol (applications running on one computer but displaying their windows on another) in the spirit of X, however, unlike X, the viewing-end is very simple and maintains no state. It is a remote framebuffer (RFB) protocol.

Some VNC links:

For Unix, the traditional VNC implementation includes a "virtual" X11 server Xvnc (usually launched via the vncserver command) that is not associated with a physical display, but provides a "fake" one X11 clients (xterm, firefox, etc.) can attach to. A remote user then connects to Xvnc via the VNC client vncviewer from anywhere on the network to view and interact with the whole virtual X11 desktop.

The VNC protocol is in most cases better suited for remote connections with low bandwidth and high latency than is the X11 protocol because it involves far fewer "roundtrips" (an exception is the cached pixmap data on the viewing-end provided by X.) Also, with no state maintained the viewing-end can crash, be rebooted, or relocated and the applications and desktop continue running. Not so with X11.

So the standard Xvnc/vncserver program is very useful, I use it for things like:

  • Desktop conferencing with other users (e.g. code reviews.)
  • Long running apps/tasks I want to be able to view from many places (e.g. from home and work.)
  • Motif, GNOME, and similar applications that would yield very poor performance over a high latency link.

However, sometimes one wants to connect to a real X11 display (i.e. one attached to a physical monitor, keyboard, and mouse: a Workstation or a SunRay session) from far away. Maybe you want to close down an application cleanly rather than using kill, or want to work a bit in an already running application, or would like to help a distant colleague solve a problem with their desktop, or would just like to work out on the deck for a while. This is where x11vnc is useful.

How to use x11vnc:

In this basic example let's assume the remote machine with the X display you wish to view is "far-away.east:0" and the workstation you are presently working at is "sitting-here.west".

Step 0. Download x11vnc (see below) and have it available to run on far-away.east (on some linux distros it is as easy as "apt-get install x11vnc", "emerge x11vnc", etc.) Similarly, have a VNC viewer (e.g. vncviewer) ready to run on sitting-here.west. We recommend TightVNC Viewers (see also our SSVNC viewer.)

Step 1. By some means log in to far-away.east and get a command shell running there. You can use ssh, or even rlogin, telnet, or any other method to do this. We do this because the x11vnc process needs to be run on the same machine the X server process is running on (otherwise things would be extremely slow.)

Step 2. In that far-away.east shell (with command prompt "far-away>" in this example) run x11vnc directed at the far-away.east X session display:

far-away> x11vnc -display :0

You could have also set the environment variable DISPLAY=:0 instead of using "-display :0". This step attaches x11vnc to the far-away.east:0 X display (i.e. no viewer clients yet.)

Common Gotcha: To get X11 permissions right, you may also need to set the XAUTHORITY environment variable (or use the -auth option) to point to the correct MIT-MAGIC-COOKIE file (e.g. /home/joe/.Xauthority.) If x11vnc does not have the authority to connect to the display it exits immediately. More on how to fix this below.

If you suspect an X11 permissions problem do this simple test: while sitting at the physical X display open a terminal window (gnome-terminal, xterm, etc.) You should be able to run x11vnc successfully in that terminal without any need for command line options. If that works OK then you know X11 permissions are the only thing preventing it from working when you try to start x11vnc via a remote shell. Then fix this with the tips below.

Note as of Feb/2007 you can also try the -find option instead of "-display ..." and see if that finds your display and Xauthority. Note as of Dec/2009 the -findauth and "-auth guess" options may be helpful as well. (End of Common Gotcha)

When x11vnc starts up there will then be much chatter printed out (use "-q" to quiet it), until it finally says something like:

.
.
13/05/2004 14:59:54 Autoprobing selected port 5900
13/05/2004 14:59:54 screen setup finished.
13/05/2004 14:59:54
13/05/2004 14:59:54 The VNC desktop is far-away:0
PORT=5900

which means all is OK, and we are ready for the final step.

Step 3. At the place where you are sitting (sitting-here.west in this example) you now want to run a VNC viewer program. There are VNC viewers for Unix, Windows, MacOS, Java-enabled web browsers, and even for PDA's like the Palm Pilot and Cell Phones! You can use any of them to connect to x11vnc (see the above VNC links under "Background:" on how to obtain a viewer for your platform or see this FAQ. For Solaris, vncviewer is available in the Companion CD package SFWvnc.)

In this example we'll use the Unix vncviewer program on sitting-here by typing the following command in a second terminal window:

sitting-here> vncviewer far-away.east:0

That should pop up a viewer window on sitting-here.west showing and allowing interaction with the far-away.east:0 X11 desktop. Pretty nifty! When finished, exit the viewer: the remote x11vnc process will shutdown automatically (or you can use the -forever option to have it wait for additional viewer connections.)

Common Gotcha: Nowadays there will likely be a host-level firewall on the x11vnc side that is blocking remote access to the VNC port (e.g. 5900.) You will either have to open up that port (or a range of ports) in your firewall administration tool, or try the SSH tunnelling method below (even still the firewall must allow in the SSH port, 22.)

Shortcut: Of course if you left x11vnc running on far-away.east:0 in a terminal window with the -forever option or as a service, you'd only have to do Step 3 as you moved around. Be sure to use a VNC Password or other measures if you do that.

Super Shortcut: Here is a potentially very easy way to get all of it working.

  • Have x11vnc (0.9.3 or later) available to run on the remote host (i.e. in $PATH.)
  • Download and unpack a SSVNC bundle (1.0.19 or later, e.g. ssvnc_no_windows-1.0.28.tar.gz) on the Viewer-side machine.
  • Start the SSVNC Terminal Services mode GUI: ./ssvnc/bin/tsvnc
  • Enter your remote username@hostname (e.g. fred@far-away.east) in the "VNC Terminal Server" entry.
  • Click "Connect".

That will do an SSH to username@hostname and start up x11vnc and then connect a VNC Viewer through the SSH encrypted tunnel.

There are a number of things assumed here, first that you are able to SSH into the remote host; i.e. that you have a Unix account there and the SSH server is running. On Unix and MacOS X it is assumed that the ssh client command is available on the local machine (on Windows a plink binary is included in the SSVNC bundle.) Finally, it is assumed that you are already logged into an X session on the remote machine, e.g. your workstation (otherwise, a virtual X server, e.g. Xvfb, will be started for you.)

In some cases the remote SSH server will not run commands with the same $PATH that you normally have in your shell there. In this case click on Options -> Advanced -> X11VNC Options, and type in the location of the x11vnc binary under "Full Path". (End of Super Shortcut)

Desktop Sharing: The above more or less assumed nobody was sitting at the workstation display "far-away.east:0". This is often the case: a user wants to access her workstation remotely. Another usage pattern has the user sitting at "far-away.east:0" and invites one or more other people to view and interact with his desktop. Perhaps the user gives a demo or presentation this way (using the telephone for vocal communication.) A "Remote Help Desk" mode would be similar: a technician connects remotely to the user's desktop to interactively solve a problem the user is having.

For these cases it should be obvious how it is done. The above steps will work, but more easily the user sitting at far-away.east:0 simply starts up x11vnc from a terminal window, after which the guests would start their VNC viewers. For this usage mode the "-connect host1,host2" option may be of use to automatically connect to the vncviewers in "-listen" mode on the list of hosts.

Tunnelling x11vnc via SSH:

The above example had no security or privacy at all. When logging into remote machines (certainly when going over the internet) it is best to use ssh, or use a VPN (for a VPN, Virtual Private Network, the above example should be pretty safe.)

For x11vnc one can tunnel the VNC protocol through an encrypted ssh channel. It would look something like running the following commands:

sitting-here> ssh -t -L 5900:localhost:5900 far-away.east 'x11vnc -localhost
-display :0'

(you will likely have to provide passwords/passphrases to login from sitting-here into your far-away.east Unix account; we assume you have a login account on far-away.east and it is running the SSH server)

And then in another terminal window on sitting-here run the command:

sitting-here> vncviewer -encodings "copyrect tight zrle hextile" localhost:0

Note: The -encodings option is very important: vncviewer will often default to "raw" encoding if it thinks the connection is to the local machine, and so vncviewer gets tricked this way by the ssh redirection. "raw" encoding will be extremely slow over a networked link, so you need to force the issue with -encodings "copyrect tight ...". Nowadays, not all viewers use the -encodings option, try "-PreferredEncoding=ZRLE" (although the newer viewers seem to autodetect well when to use raw or not.)

Note that "x11vnc -localhost ..." limits incoming vncviewer connections to only those from the same machine. This is very natural for ssh tunnelling (the redirection appears to come from the same machine.) Use of a VNC password is also strongly recommended.

Note also the -t we used above (force allocate pseudoterminal), it actually seems to improve interactive typing response via VNC!

You may want to add the -C option to ssh to enable compression. The VNC compression is not perfect, and so this may help a bit. However, over a fast LAN you probably don't want to enable SSH compression because it can slow things down. Try both and see which is faster.

If your username is different on the remote machine use something like: "fred@far-away.east" in the above ssh command line.

Some VNC viewers will do the ssh tunnelling for you automatically, the TightVNC Unix vncviewer does this when the "-via far-away.east" option is supplied to it (this requires x11vnc to be already running on far-away.east or having it started by inetd(8).) See the 3rd script example below for more info.

SSVNC: You may also want to look at the Enhanced TightVNC Viewer (ssvnc) bundles because they contain scripts and GUIs to automatically set up SSH tunnels (e.g. the GUI, "ssvnc", does it automatically and so does this command: "ssvnc_cmd -ssh user@far-away.east:0") and can even start up x11vnc as well.

The Terminal Services mode of SSVNC is perhaps the easiest way to use x11vnc. You just need to have x11vnc available in $PATH on the remote side (and can SSH to the host), and then on the viewer-side you type something like:

tsvnc fred@far-away.east

everything else is done automatically for you. Normally this will start a virtual Terminal Services X session (RAM-only), but if you already have a real X session up on the physical hardware it will find that one for you.

Gateways: If the machine you SSH into is not the same machine with the X display you wish to view (e.g. your company provides incoming SSH access to a gateway machine), then you need to change the above to, e.g.: "-L 5900:OtherHost:5900":

sitting-here> ssh -t -L 5900:OtherHost:5900 gateway.east

Where gateway.east is the internet hostname (or IP) of the gateway machine (SSH server.) 'OtherHost' might be, e.g., freds-pc or 192.168.2.33 (it is OK for these to be private hostnames or private IP addresses, the host in -L is relative to the remote server side.)

Once logged in, you'll need to do a second login (ssh, rsh, etc.) to the workstation machine 'OtherHost' and then start up x11vnc on it (if it isn't already running.) (The "-connect gateway:59xx" option may be another alternative here with the viewer already in -listen mode.) For an automatic way to use a gateway and have all the network traffic encrypted (including inside the firewall) see Chaining SSH's.

These gateway access modes also can be done automatically for you via the "Proxy/Gateway" setting in SSVNC (including the Chaining SSH's case, "Double Proxy".)

Firewalls/Routers:

A lot of people have inexpensive devices for home or office that act as a Firewall and Router to the machines inside on a private LAN. One can usually configure the Firewall/Router from inside the LAN via a web browser.

Often having a Firewall/Router sitting between the vncviewer and x11vnc will make it impossible for the viewer to connect to x11vnc.

One thing that can be done is to redirect a port on the Firewall/Router to, say, the SSH port (22) on an inside machine (how to do this depends on your particular Firewall/Router, often the router config URL is http://192.168.100.1 See www.portforward.com for more info.) This way you reach these computers from anywhere on the Internet and use x11vnc to view X sessions running on them.

Suppose you configured the Firewall/Router to redirect these ports to two internal machines:

Port 12300 -> 192.168.1.3, Port 22 (SSH)
Port 12301 -> 192.168.1.4, Port 22 (SSH)

 (where 192.168.1.3 is "jills-pc" and 192.168.1.4 is "freds-pc".) Then
 the ssh's would look something like:
sitting-here> ssh -t -p 12300 -L 5900:localhost:5900 jill@far-away.east 'x11v
nc -localhost -display :0'
sitting-here> ssh -t -p 12301 -L 5900:localhost:5900 fred@far-away.east 'x11v
nc -localhost -display :0'

Where far-away.east means the hostname (or IP) that the Router/Firewall is using (for home setups this is usually the IP gotten from your ISP via DHCP, the site http://www.whatismyip.com/ is a convenient way to determine what it is.)

It is a good idea to add some obscurity to accessing your system via SSH by using some high random port (e.g. 12300 in the above example.) If you can't remember it, or are otherwise not worried about port scanners detecting the presence of your SSH server and there is just one internal PC involved you could map 22:

Port 22 -> 192.168.1.3, Port 22 (SSH)

Again, this SSH gateway access can be done automatically for you via the "Proxy/Gateway" setting in SSVNC. And under the "Remote SSH Command" setting you can enter the x11vnc -localhost -display :0.

Host-Level-Firewalls: even with the hardware Firewall/Router problem solved via a port redirection, most PC systems have their own Host level "firewalls" enabled to protect users from themselves. I.e. the system itself blocks all incoming connections. So you will need to see what is needed to configure it to allow in the port (e.g. 22) that you desire. E.g. Yast, Firestarter, iptables(1), etc..

VNC Ports and Firewalls: The above discussion was for configuring the Firewall/Router to let in port 22 (SSH), but the same thing can be done for the default VNC port 5900:

Port 5900 -> 192.168.1.3, Port 5900 (VNC)
Port 5901 -> 192.168.1.4, Port 5900 (VNC)

(where 192.168.1.3 is "jills-pc" and 192.168.1.4 is "freds-pc".) This could be used for normal, unencrypted connections and also for SSL encrypted ones.

The VNC displays to enter in the VNC viewer would be, say, "far-away.east:0" to reach jills-pc and "far-away.east:1" to reach freds-pc. We assume above that x11vnc is using port 5900 (and any Host-Level-firewalls on jills-pc has been configured to let that port in.) Use the "-rfbport" option to tell which port x11vnc should listen on.

For a home system one likely does not have a hostname and would have to use the IP address, say, "24.56.78.93:0". E.g.:

vncviewer 24.56.78.93:0

You may want to choose a more obscure port on the router side, e.g. 5944, to avoid a lot of port scans finding your VNC server. For 5944 you would tell the viewer to use:

vncviewer 24.56.78.93:44

The IP address would need to be communicated to the person running the VNC Viewer. The site http://www.whatismyip.com/ can help here.

Scripts to automate ssh tunneling: As discussed below, there may be some problems with port 5900 being available. If that happens, the above port and display numbers may change a bit (e.g. -> 5901 and :1). However, if you "know" port 5900 will be free on the local and remote machines, you can easily automate the above two steps by using the x11vnc option -bg (forks into background after connection to the display is set up) or using the -f option of ssh. Some example scripts are shown below. Feel free to try the ssh -C to enable its compression and see if that speeds things up noticeably.

#1. A simple example script, assuming no problems with port 5900 being taken on the local or remote sides, looks like:

#!/bin/sh
# usage: x11vnc_ssh <host>:<xdisplay>
#  e.g.: x11vnc_ssh snoopy.peanuts.com:0
#  (user@host:N also works)

host=`echo $1 | awk -F: '{print $1}'`
disp=`echo $1 | awk -F: '{print $2}'`
if [ "x$disp" = "x" ]; then disp=0; fi

cmd="x11vnc -display :$disp -localhost -rfbauth .vnc/passwd"
enc="copyrect tight zrle hextile zlib corre rre raw"

ssh -f -t -L 5900:localhost:5900 $host "$cmd"

for i in 1 2 3
do
      sleep 2
      if vncviewer -encodings "$enc" :0; then break; fi
done

See also rx11vnc.pl below.

#2. Another method is to start the VNC viewer in listen mode "vncviewer -listen" and have x11vnc initiate a reverse connection using the -connect option:

#!/bin/sh
# usage: x11vnc_ssh <host>:<xdisplay>
#  e.g.: x11vnc_ssh snoopy.peanuts.com:0
#  (user@host:N also works)

host=`echo $1 | awk -F: '{print $1}'`
disp=`echo $1 | awk -F: '{print $2}'`
if [ "x$disp" = "x" ]; then disp=0; fi

cmd="x11vnc -display :$disp -localhost -connect localhost"   # <== note new opt
ion
enc="copyrect tight zrle hextile zlib corre rre raw"

vncviewer -encodings "$enc" -listen &
pid=$!
ssh -t -R 5500:localhost:5500 $host "$cmd"
kill $pid

Note the use of the ssh option "-R" instead of "-L" to set up a remote port redirection.

#3. A third way is specific to the TightVNC vncviewer special option -via for gateways. The only tricky part is we need to start up x11vnc and give it some time (5 seconds in this example) to start listening for connections (so we cannot use the TightVNC default setting for VNC_VIA_CMD):

#!/bin/sh
# usage: x11vnc_ssh <host>:<xdisplay>
#  e.g.: x11vnc_ssh snoopy.peanuts.com:0

host=`echo $1 | awk -F: '{print $1}'`
disp=`echo $1 | awk -F: '{print $2}'`
if [ "x$disp" = "x" ]; then disp=0; fi

VNC_VIA_CMD="ssh -f -t -L %L:%H:%R %G x11vnc -localhost -rfbport 5900 -display
:$disp; sleep 5"
export VNC_VIA_CMD

vncviewer -via $host localhost:0      # must be TightVNC vncviewer.

Of course if you already have the x11vnc running waiting for connections (or have it started out of inetd(8)), you can simply use the TightVNC "vncviewer -via gateway host:port" in its default mode to provide secure ssh tunnelling.

VNC password file: Also note in the #1. example script that the option "-rfbauth .vnc/passwd" provides additional protection by requiring a VNC password for every VNC viewer that connects. The vncpasswd or storepasswd programs, or the x11vnc -storepasswd option can be used to create the password file. x11vnc also has the slightly less secure -passwdfile and "-passwd XXXXX" options to specify passwords.

Very Important: It is up to YOU to tell x11vnc to use password protection (-rfbauth or -passwdfile), it will NOT do it for you automatically or force you to (use -usepw if you want to be forced to.) The same goes for encrypting the channel between the viewer and x11vnc: it is up to you to use ssh, stunnel, -ssl mode, a VPN, etc. (use the Enhanced TightVNC Viewer (SSVNC) GUI if you want to be forced to use SSL or SSH.) For additional safety, also look into the -allow and -localhost options and building x11vnc with tcp_wrappers support to limit host access.

Tunnelling x11vnc via SSL/TLS:

One can also encrypt the VNC traffic using an SSL/TLS tunnel such as stunnel.mirt.net (also stunnel.org) or using the built-in (Mar/2006) -ssl openssl mode. A SSL-enabled Java applet VNC Viewer is also provided in the x11vnc package (and https can be used to download it.)

Although not as ubiquitous as ssh, SSL tunnelling still provides a useful alternative. See this FAQ on -ssl and -stunnel modes for details and examples.

The Enhanced TightVNC Viewer (SSVNC) bundles contain some convenient utilities to automatically set up an SSL tunnel from the viewer-side (i.e. to connect to "x11vnc -ssl ...".) And many other enhancements too.

Downloading x11vnc:

x11vnc is a contributed program to the LibVNCServer project at SourceForge.net. I use libvncserver for all of the VNC aspects; I couldn't have done without it. The full source code may be found and downloaded (either file-release tarball or GIT tree) from the above link. As of Sep 2010, the x11vnc-0.9.12.tar.gz source package is released (recommended download). The x11vnc 0.9.12 release notes.

The x11vnc package is the subset of the libvncserver package needed to build the x11vnc program. Also, you can get a copy of my latest, bleeding edge x11vnc-0.9.13-dev.tar.gz tarball to build the most up to date one.

Precompiled Binaries/Packages: See the FAQ below for information about where you might obtain a precompiled x11vnc binary from 3rd parties and some ones I create.

VNC Viewers: To obtain VNC viewers for the viewing side (Windows, Mac OS, or Unix) try these links:

More tools: Here is a ssh/rsh wrapper script rx11vnc that attempts to automatically do the above Steps 1-3 for you (provided you have ssh/rsh login permission on the machine x11vnc is to be run on.) The above example would be: "rx11vnc far-away.east:0" typed into a shell on sitting-here.west. Also included is an experimental script rx11vnc.pl that attempts to tunnel the vnc traffic through an ssh port redirection (and does not assume port 5900 is free.) Have a look at them to see what they do and customize as needed:

  • rx11vnc wrapper script
  • rx11vnc.pl wrapper script to tunnel traffic thru ssh

Building x11vnc:

Make sure you have all the needed build/compile/development packages installed. On a Debian-based distro you can simply do

sudo apt-get build-dep x11vnc

to install most of them.

To generate the build system, do a

autoreconf -fiv

After that, it's the usual

./configure
make

You might want to take a look at the dfferent configure options provided by configure --help to build without X11 or adapt the build to your needs in other ways.

Building on Solaris, FreeBSD, etc: Depending on your version of Solaris or other Unix OS the jpeg and/or zlib libraries may be in non-standard places (e.g. /usr/local, /usr/sfw, /opt/sfw, etc.)

Note: If configure cannot find these two libraries then TightVNC and ZRLE encoding support will be disabled, and you don't want that!!! The TightVNC encoding gives very good compression and performance, it even makes a noticeable difference over a fast LAN.

Shortcuts: On Solaris 10 you can pick up almost everything just by insuring that your PATH has /usr/sfw/bin (for gcc) and /usr/ccs/bin (for other build tools), e.g.:

env PATH=/usr/sfw/bin:/usr/ccs/bin:$PATH sh -c './configure; make'

(The only thing this misses is /usr/X11/lib/libXrandr.so.2, which is for the little used -xrandr option, see the script below to pick it up as well.)

libjpeg is included in Solaris 9 and later (/usr/sfw/include and /usr/sfw/lib), and zlib in Solaris 8 and later (/usr/include and /usr/lib.) So on Solaris 9 you can pick up everything with something like this:

env PATH=/usr/local/bin:/usr/ccs/bin:$PATH sh -c './configure --with-jpeg=/us
r/sfw; make'

assuming your gcc is in /usr/local/bin and x11vnc 0.7.1 or later. These are getting pretty long, see those assignments split up in the build script below.

If your system does not have these libraries at all you can get the source for the libraries to build them: libjpeg is available at ftp://ftp.uu.net/graphics/jpeg/ and zlib at http://www.gzip.org/zlib/. See also http://www.sunfreeware.com/ for Solaris binary packages of these libraries as well as for gcc. Normally they will install into /usr/local but you can install them anywhere with the --prefix=/path/to/anywhere, etc.

Here is a build script that indicates one way to pass the library locations information to the libvncserver configuration via the CPPFLAGS and LDFLAGS environment variables.

#!/bin/sh

# Build script for Solaris, etc, with gcc, libjpeg and libz in
# non-standard locations.

# set to get your gcc, etc:
#
PATH=/path/to/gcc/bin:/usr/ccs/bin:/usr/sfw/bin:$PATH

JPEG=/path/to/jpeg      # set to maybe "/usr/local", "/usr/sfw", or "/opt/sfw"
ZLIB=/path/to/zlib      # set to maybe "/usr/local", "/usr/sfw", or "/opt/sfw"

# Below we assume headers in $JPEG/include and $ZLIB/include and the
# shared libraries are in $JPEG/lib and $ZLIB/lib.  If your situation
# is different change the locations in the two lines below.
#
CPPFLAGS="-I $JPEG/include -I $ZLIB/include"
LDFLAGS="-L$JPEG/lib -R $JPEG/lib -L$ZLIB/lib -R $ZLIB/lib"

# These two lines may not be needed on more recent Solaris releases:
#
CPPFLAGS="$CPPFLAGS -I /usr/openwin/include"
LDFLAGS="$LDFLAGS -L/usr/openwin/lib -R /usr/openwin/lib"

# These are for libXrandr.so on Solaris 10:
#
CPPFLAGS="$CPPFLAGS -I /usr/X11/include"
LDFLAGS="$LDFLAGS -L/usr/X11/lib -R /usr/X11/lib"

# Everything needs to built with _REENTRANT for thread safe errno:
#
CPPFLAGS="$CPPFLAGS -D_REENTRANT"

export PATH CPPFLAGS LDFLAGS

./configure
make

ls -l ./x11vnc/x11vnc

Then do make install or copy the x11vnc binary to your desired destination.

BTW, To run a shell script, just cut-and-paste the above into a file, say "myscript", then modify the "/path/to/..." items to correspond to your system/environment, and then type: "sh myscript" to run it.

Note that on Solaris make is /usr/ccs/bin/make, so that is why the above puts /usr/ccs/bin in PATH. Other important build utilities are there too: ld, ar, etc. Also, it is probably a bad idea to have /usr/ucb in your PATH while building.

Starting with the 0.7.1 x11vnc release the "configure --with-jpeg=DIR --with-zlib=DIR" options are handy if you want to avoid making a script.

If you need to link OpenSSL libssl.a on Solaris see this method.

If you need to build on Solaris 2.5.1 or earlier or other older Unix OS's, see this workaround FAQ.

Building on FreeBSD, OpenBSD, ...: The jpeg libraries seem to be in /usr/local or /usr/pkg on these OS's. You won't need the openwin stuff in the above script (but you may need /usr/X11R6/....) Also starting with the 0.7.1 x11vnc release, this usually works:

./configure --with-jpeg=/usr/local
make

Building on HP-UX: For jpeg and zlib you will need to do the same sort of thing as described above for Solaris. You set CPPFLAGS and LDFLAGS to find them (see below for an example.) You do not need to do any of the above /usr/openwin stuff. Also, HP-UX does not seem to support -R, so get rid of the -R items in LDFLAGS. Because of this, at runtime you may need to set LD_LIBRARY_PATH or SHLIB_PATH to indicate the directory paths so the libraries can be found. It is a good idea to have static archives, e.g. libz.a and libjpeg.a for the nonstandard libraries so that they get bolted into the x11vnc binary (and so won't get "lost".)

Here is what we recently did to build x11vnc 0.7.2 on HP-UX 11.11

./configure --with-jpeg=$HOME/hpux/jpeg --with-zlib=$HOME/hpux/zlib
make

Where we had static archives (libjpeg.a, libz.a) only and header files in the $HOME/hpux/... directories as discussed for the build script.

On HP-UX 11.23 and 11.31 we have had problems compiling with gcc. "/usr/include/rpc/auth.h:87: error: field 'syncaddr' has incomplete type". As a workaround for x11vnc 0.9.4 and later set your CPPFLAGS to include:

CPPFLAGS="-DIGNORE_GETSPNAM"
export CPPFLAGS

This disables a very rare usage mode for -unixpw_nis by not trying getspnam(3).

Using HP-UX's C compiler on 11.23 and 11.31 we have some severe compiler errors that have not been worked around yet. If you need to do this, contact me and I will give you a drastic recipe that will produce a working binary.

Building on AIX: AIX: one user had to add the "X11.adt" package to AIX 4.3.3 and 5.2 to get build header files like XShm.h, etc. You may also want to make sure that /usr/lpp/X11/include, etc is being picked up by the configure and make.

For a recent build on AIX 5.3 we needed to add these CFLAGS to be able to build with gcc: env CFLAGS='-maix64 -Xlinker -bbigtoc' ./configure ...

we also built our own libjpeg and libz using -maix64.

BTW, one way to run an Xvfb-like virtual X server for testing on AIX is something like "/usr/bin/X11/X -force -vfb -ac :1".

Building on Mac OS X: There is now native Mac OS X support for x11vnc by using the raw framebuffer feature. This mode does not use or need X11 at all. To build you may need to disable X11:

./configure --without-x ...
make

However, if your system has the Mac OS X build package for X11 apps you will not need to supply the "--without-x" option (in this case the resulting x11vnc would be able to export both the native Mac OS X display and windows displayed in the XDarwin X server.) Be sure to include the ./configure option to find libjpeg on your system.

OpenSSL: Starting with version 0.8.3 x11vnc can now be built with SSL/TLS support. For this to be enabled the libssl.so library needs to be available at build time. So you may need to have additional CPPFLAGS and LDFLAGS items if your libssl.so is in a non-standard place. As of x11vnc 0.9.4 there is also the --with-ssl=DIR configure option.

Note that from OpenSSL 1.1.0 on SSLv2 support has been dropped and SSLv3 deactivated at build time per default. This means that unless explicitly enabled, OpenSSL builds only support TLS (any version). Since there is a reason for dropping SSLv3 (heard of POODLE?), most distributions do not enable it for their OpenSSL binary. In summary this means compiling x11vnc against OpenSSL 1.1.0 or newer is no problem, but using encryption will require a viewer with TLS support.

On Solaris using static archives libssl.a and libcrypto.a instead of .so shared libraries (e.g. from www.sunfreeware.com), we found we needed to also set LDFLAGS as follows to get the configure to work:

env LDFLAGS='-lsocket -ldl' ./configure --with-ssl=/path/to/openssl ...
make

Beta Testing:

I don't have any formal beta-testers for the releases of x11vnc, so I'd appreciate any additional testing very much.

Thanks to those who suggested features and helped beta test x11vnc 0.9.12 released in Sep 2010!

Please help test and debug the 0.9.13 version for release sometime in Winter 2010.

The version 0.9.13 beta tarball is kept here: x11vnc-0.9.13-dev.tar.gz

There are also some Linux, Solaris, Mac OS X, and other OS test binaries here. Please kick the tires and report bugs, performance regressions, undesired behavior, etc. to me.

To aid testing of the built-in SSL/TLS support for x11vnc, a number of VNC Viewer packages for Unix, Mac OS X, and Windows have been created that provide SSL Support for the TightVNC Viewer (this is done by wrapper scripts and a GUI that starts STUNNEL.) It should be pretty convenient for automatic SSL and SSH connections. It is described in detail at and can be downloaded from the Enhanced TightVNC Viewer (SSVNC) page. The SSVNC Unix viewer also supports x11vnc's symmetric key encryption ciphers (see the 'UltraVNC DSM Encryption Plugin' settings panel.)

Here are some features that will appear in the 0.9.13 release:

 * Improved support for non-X11 touchscreen devices (e.g. handheld or
   cell phone) via Linux uinput input injection. Additional tuning
   parameters are added. TSLIB touchscreen calibration is supported.
   Tested on Qtmoko Neo Freerunner. A tool, misc/uinput.pl, is
   provided to diagnose uinput behavior on new devices. The env.
   vars. X11VNC_UINPUT_BUS and X11VNC_UINPUT_VERSION are available if
   leaving them unset does not work.
 * The Linux uinput non-X11 input injection can now be bypassed:
   events can be directly written to the /dev/input/event devices
   specified by the user (direct_abs=..., etc.) A -pipeinput input
   injection helper script, misc/qt_tslib_inject.pl is provided as a
   tweakable non-builtin direct input injection method.
 * The list of new uinput parameters for the above two features is:
   pressure, tslib_cal, touch_always, dragskip, btn_touch;
   direct_rel, direct_abs, direct_btn, direct_key.
 * The MacOSX native server can now use OpenGL for the screen
   capture. In nearly all cases this is faster than the raw
   framebuffer capture method. There are build and run time flags,
   X11VNC_MACOSX_NO_DEPRECATED, etc. to disable use of deprecated
   input injection and screen access interfaces. Cursor shape now
   works for 64bit binaries.
 * The included SSL enabled Java VNC Viewers now handle Mouse Wheel
   events.
 * miscellaneous new features and changes:
 * In -reflect mode, the libvncclient connection can now have the
   pixel format modified via the environment variables
   X11VNC_REFLECT_bitsPerSample, X11VNC_REFLECT_samplesPerPixel, and
   X11VNC_REFLECT_bytesPerPixel
 * In -create mode the following environment variables are added to
   fine tune the behavior: FIND_DISPLAY_NO_LSOF: do not use lsof(1)
   to try to determine the Linux VT, FIND_DISPLAY_NO_VT_FIND: do not
   try to determine the Linux VT at all, X11VNC_CREATE_LC_ALL_C_OK:
   do not bother undoing the setting LC_ALL=C that the create_display
   script sets. The performance of the -create script has been
   improved for large installations (100's of user sessions on one
   machine.)
 * In -unixpw mode, one can now Tab from login: to Password.
 * An environment variable, X11VNC_SB_FACTOR, allows one to scale the
   -sb screenblank sleep time from the default 2 secs.
 * An experimental option -unixsock is available for testing. Note,
   however, that it requires a manual change to
   libvncserver/rfbserver.c for it to work.
 * Documented that -grabkbd is no longer working with some/most
   window managers (it can prevent resizing and menu posting.)

Here are some features that appeared in the 0.9.12 release (Sep/2010):

 * One can now specify the maximum number of displays that can be
   created in -create mode via the env. var.
   X11VNC_CREATE_MAX_DISPLAYS
 * The X11VNC_NO_LIMIT_SHM env. var. is added to skip any automatic
   shared memory reduction.
 * The kdm display manager is now detected when trying not to get
   killed by the display manager.
 * A compile time bug is fixed so that configuring using
   --with-system-libvncserver pointing to LibVNCServer 0.9.7 works
   again. A bug from forced use of Xdefs.h is worked around.

Here are some features that appeared in the 0.9.11 release (Aug/2010):

 * The source tree is synchronized with the most recent libvncclient
   (this only affects -reflect mode.) Build is fixed for
   incompatibilities when using an external LibVNCServer (e.g.
   ./configure --with-system-libvncserver...) Please help test these
   build and runtime aspects and report back what you find, thanks.
 * The SSL enabled Java VNC Viewer Makefile has been modified so that
   the jar files that are built are compatible back to Java 1.4.
 * In -create/-unixpw mode, the env. var. FD_USERPREFS may be set to
   a filename in the user's home directory that includes default
   username:options values (so the options do not need to be typed
   every time at the login prompt.)
 * In -reflect mode cursor position updates are now handled
   correctly.

Here are some features that appeared in the 0.9.10 release (May/2010):

 * The included SSL enabled Java applet viewer now supports Chained
   SSL Certificates. The debugCerts=yes applet parameter aids
   troubleshooting certificate validation. The x11vnc -ssl mode has
   always supported chained SSL certificates (simply put the
   intermediate certificates, in order, after the server certificate
   in the pem file.)
 * A demo CGI script desktop.cgi shows how to create an SSL
   encrypted, multi-user x11vnc web login desktop service. The script
   requires x11vnc version 0.9.10. The user logs into a secure web
   site and gets his/her own virtual desktop (Xvfb.) x11vnc's SSL
   enabled Java Viewer Applet is launched by the web browser for
   secure viewing (and so no software needs to be installed on the
   viewer-side.) One can use the desktop.cgi script for ideas to
   create their own fancier or customized web login desktop service
   (e.g. user-creation, PHP, SQL, specialized desktop application,
   etc.) More info here. There is also an optional 'port redirection'
   mode that allows redirection to other SSL enabled VNC servers
   running inside the firewall.
 * Built-in support for IPv6 (128 bit internet addresses) is now
   provided. See the -6 and -connect options for details.
   Additionally, in case there are still problems with built-in IPv6
   support, a transitional tool is provided in inet6to4 that allows
   x11vnc (or any other IPv4 application) to receive connections over
   IPv6.
 * The Xdummy wrapper script for Xorg's dummy driver is updated and
   no longer requires being run as root. New service options are
   provided to select Xdummy over Xvfb as the virtual X server to be
   created.
 * The "%" unix password verification tricks for the -unixpw option
   are now documented. They have also been extended to run a command
   as the user if one sets the environment variable UNIXPW_CMD. The
   desktop.cgi demo script takes advantage of this new feature.
 * A bug has been fixed that would prevent the Java applet viewer
   from being downloaded successfully in single-port HTTPS/VNC inetd
   mode. The env. var. X11VNC_HTTPS_DOWNLOAD_WAIT_TIME can be used to
   adjust for how many seconds a -inetd or -https httpd download is
   waited for (default 15 seconds.) The applet will now autodetect
   x11vnc and use GET=1 for faster connecting. Many other
   improvements and fixes.
 * The TightVNC security type (TightVNC features enabler) now works
   for RFB version 3.8.
 * The X property X11VNC_TRAP_XRANDR can be set on a desktop to force
   x11vnc to use the -xrandr screen size change trapping code.
 * New remote control query options: pointer_x, pointer_y,
   pointer_same, pointer_root, and pointer_mask. A demo script using
   them misc/panner.pl is provided.
 * The -sslScripts option prints out the SSL certificate management
   scripts.

Here are some features that appeared in the 0.9.9 release (Dec/2009):

 * The -unixpw_system_greeter option, when used in combined unixpw
   and XDMCP FINDCREATEDISPLAY mode (for example: -xdmsvc), enables
   the user to press Escape to jump directly to the XDM/GDM/KDM login
   greeter screen. This way the user avoids entering his unix
   password twice at X session creation time. Also, the unixpw login
   panel now has a short help displayed if the user presses 'F1'.
 * x11vnc now tries to be a little bit more aggressive in keeping up
   with VNC client's framebuffer update requests. Some broken VNC
   clients like Eggplant and JollysFastVNC continuously spray these
   requests at VNC servers (regardless of whether they have received
   any updates or not.) Under some circumstances this could lead to
   x11vnc falling behind. The -extra_fbur option allows one to fine
   tune the setting. Additionally, one may also dial down delays:
   e.g. "-defer 5" and "-wait 5" (or to 1 or even 0) or -nonap or
   -allinput to keep up with these VNC clients at the expense of
   increased system load.
 * Heuristics are applied to try to determine if the X display is
   currently in a Display Manager Greeter Login panel (e.g. GDM) If
   so, x11vnc's creation of any windows and use of XFIXES are
   delayed. This is to try to avoid x11vnc being killed after the
   user logs in if the GDM KillInitClients=true is in effect. So one
   does not need to set KillInitClients=false. Note that in recent
   GDM the KillInitClients option has been removed. Also delayed is
   the use of the XFIXES cursor fetching functionality; this avoids
   an Xorg bug that causes Xorg to crash right after the user logs
   in.
 * A new option -findauth runs the FINDDISPLAY script that applies
   heuristics that try to determine the XAUTHORITY file. The use of
   '-auth guess' will use the XAUTHORITY that -findauth reveals. This
   can be handy in with the lastest GDM where the ability to store
   cookies in ~/.Xauthority has been removed. If x11vnc is running as
   root (e.g. inetd) and you add -env FD_XDM=1 to the above -findauth
   or -auth guess command lines, it will find the correct XAUTHORITY
   for the given display (this works for XDM/GDM/KDM if the login
   greeter panel is up or if someone has already logged into an X
   session.)
 * The FINDDISPLAY and FINDCREATEDISPLAY modes (i.e. "-display
   WAIT:cmd=...", -find, -create) now work correctly for the
   user-supplied login program scheme "-unixpw_cmd ...", as long as
   the login program supports running commands specified in the
   environment variable "RFB_UNIXPW_CMD_RUN" as the logged-in user.
   The mode "-unixpw_nis ..." has also been made more consistent.
 * The -stunnel option (like -ssl but uses stunnel as an external
   helper program) now works with the -ssl "SAVE" and "TMP" special
   certificate names. The -sslverify and -sslCRL options now work
   correctly in -stunnel mode. Single port HTTPS connections are also
   supported for this mode.
 * There is an experimental Application Sharing mode that improves
   upon the -id/-sid single window sharing: -appshare (run "x11vnc
   -appshare -help" for more info.) It is still very primitive and
   approximate, but at least it displays multiple top-level windows.
 * The remote control command -R can be used to instruct x11vnc to
   resend its most recent copy of the Clipboard, Primary, or
   Cutbuffer selections: "x11vnc -R resend_clipboard", "x11vnc -R
   resend_primary", and "x11vnc -R resend_cutbuffer".
 * The fonts in the GUI (-gui) can now by set via environment
   variables, e.g. -env X11VNC_FONT_BOLD='Helvetica -16 bold' and
   -env X11VNC_FONT_FIXED='Courier -14'.
 * The XDAMAGE mechanism is now automatically disabled for a period
   of time if a game or screensaver generates too many XDAMAGE
   rectangles per second. This avoids the X11 event queue from
   soaking up too much memory.
 * There is an experimental workaround: "-env X11VNC_WATCH_DX_DY=1"
   that tries to avoid problems with poorly constructed menu themes
   that place the initial position of the mouse cursor inside a menu
   item's active zone. More information can be found here.

Here are some features that appeared in the 0.9.8 release (Jul/2009):

 * Stability improvements to -threads mode. Running x11vnc this way
   is more reliable now. Threaded operation sometimes gives better
   interactive response and faster updates: try it out. The threaded
   mode now supports multiple VNC viewers using the same VNC
   encoding. The threaded mode can also yield a performance
   enhancement in the many client case (e.g. class-room broadcast.)
   We have tested with 30 to 50 simultaneous clients. See also
   -reflect.
   For simultaneous clients: the ZRLE encoding is thread safe on all
   platforms, and the Tight and Zlib encodings are currently only
   thread safe on Linux where thread local storage, __thread, is
   used. If your non-Linux system and compiler support __thread one
   can supply -DTLS=__thread to enable it. When there is only one
   connected client, all encodings are safe on all platforms. Note
   that some features (e.g. scroll detection and -ncache) may be
   disabled or run with reduced functionality in -threads mode.
 * Automatically tries to work around an Xorg server and GNOME bug
   involving infinitely repeating keys when turning off key
   repeating. Use -repeat if the automatic workaround fails.
 * Improved reliability of the Single Port SSL VNC and HTTPS java
   viewer applet delivery mechanism.
 * The -clip mode works under -rawfb.

Here are some features that appeared in the 0.9.7 release (Mar/2009):

 * Support for polling Linux Virtual Terminals (also called virtual
   consoles) directly instead of using /dev/fb. The option to use is,
   for example, "-rawfb vt2" for Virtual Terminal 2, etc. In this
   case the special file /dev/vcsa2 is used to retrieve vt2's current
   text. Text and colors are shown, but no graphics.
 * Support for less than 8 bits per pixel framebuffers (e.g. 4 or 1
   bpp) in the -rawfb mode.
 * The SSL enabled UltraVNC Java viewer applet now has a [Home] entry
   in the "drives" drop down menu. This menu can be configured with
   the ftpDropDown applet parameter. All of the applet parameters are
   documented in classes/ssl/README.
 * Experimental support for VirtualGL's TurboVNC (an enhanced
   TightVNC for fast LAN high framerate usage.)
 * The CUPS Terminal Services helper mode has been improved.
 * Improvements to the -ncache_cr that allows smooth opaque window
   motions using the 'copyrect' encoding when using -ncache mode.
 * The -rmflag option enables a way to indicate to other processes
   x11vnc has exited.
 * Reverse connections using anonymous Diffie Hellman SSL encryption
   now work.

Here are some features that appeared in the 0.9.6 release (Dec/2008):

 * Support for VeNCrypt SSL/TLS encrypted connections. It is enabled
   by default in the -ssl mode. VNC Viewers like vinagre,
   gvncviewer/gtk-vnc, the vencrypt package, SSVNC, and others
   support this encryption mode. It can also be used with the -unixpw
   option to enable Unix username and password authentication
   (VeNCrypt's "*Plain" modes.) A similar but older VNC security type
   "ANONTLS" (used by vino) is supported as well. See the -vencrypt
   and -anontls options for additional control. The difference
   between x11vnc's normal -ssl mode and VeNCrypt is that the former
   wraps the entire VNC connection in SSL (like HTTPS does for HTTP,
   i.e. "vncs://") while VeNCrypt switches on the SSL/TLS at a
   certain point during the VNC handshake. Use -sslonly to disable
   both VeNCrypt and ANONTLS (vino.)
 * The "-ssl ANON" option enables Anonymous Diffie-Hellman (ADH) key
   exchange for x11vnc's normal SSL/TLS operation. Note that
   Anonymous Diffie-Hellman uses encryption for privacy, but provides
   no authentication and so is susceptible to Man-In-The-Middle
   attacks (and so we do not recommend it: we prefer you use "-ssl
   SAVE", etc. and have the VNC viewer verify the cert.) The ANONTLS
   mode (vino) only supports ADH. VeNCrypt mode supports both ADH and
   regular X509 SSL certificates modes. For these ADH is enabled by
   default. See -vencrypt and -anontls for how to disable ADH.
 * For x11vnc's SSL/TLS modes, one can now specify a Certificate
   Revocation List (CRL) with the -sslCRL option. This will only be
   useful for wide deployments: say a company-wide x11vnc SSL access
   deployment using a central Certificate Authority (CA) via
   -sslGenCA and -sslGenCert. This way if a user has his laptop lost
   or stolen, you only have to revoke his key instead of creating a
   new Certificate Authority and redeploying new keys to all users.
 * The default SSL/TLS mode, "-ssl" (no pem file parameter supplied),
   is now the same as "-ssl SAVE" and will save the generated
   self-signed cert in "~/.vnc/certs/server.pem". Previously "-ssl"
   would create a temporary self-signed cert that was discarded when
   x11vnc exited. The reason for the change is to at least give the
   chance for the VNC Viewer side (e.g. SSVNC) to remember the cert
   to authenticate subsequent connections to the same x11vnc server.
   Use "-ssl TMP" to regain the previous behavior. Use "-ssl
   SAVE_NOPROMPT" to avoid being prompted about using passphrase when
   the certificate is created.
 * The option -http_oneport enables single-port HTTP connections via
   the Java VNC Viewer. So, for example, the web browser URL
   "http://myhost.org:5900" works the same as
   "http://myhost.org:5800", but with the convenience of only
   involving one port instead of two. This works for both unencrypted
   connections and for SSH tunnels (see -httpsredir if the tunnel
   port differs.) Note that HTTPS single-port operation in -ssl SSL
   encrypted mode has been available since x11vnc version 0.8.3.
 * For the -avahi/-zeroconf Service Advertizing mode, if x11vnc was
   not compiled with the avahi-client library, then an external
   helper program, either avahi-publish(1) (on Unix) or dns-sd(1) (on
   Mac OS X), is used instead.
 * The "-rfbport PROMPT" option will prompt the user via the GUI to
   select the VNC port (e.g. 5901) to listen on, and a few other
   basic settings. This enables a handy GUI mode for naive users:

x11vnc -gui tray=setpass -rfbport PROMPT -logfile $HOME/.x11vnc.log.%VNCDISP LAY suitable for putting in a launcher or menu, e.g. x11vnc.desktop. The -logfile expansion is new too. In the GUI, the tray=setpass Properties panel has been improved. * The -solid solid background color option now works for the Mac OS X console. * The -reopen option instructs x11vnc to try to reopen the X display if it is prematurely closed by, say, the display manager (e.g. GDM.)

Here are some features that appeared in the 0.9.5 release (Oct/2008):

 * Symmetric key encryption ciphers. ARC4, AES-128, AES-256,
   blowfish, and 3des are supported. Salt and initialization vector
   seeding is provided. These compliment the more widely used SSL and
   SSH encryption access methods. SSVNC also supports these
   encryption modes.
 * Scaling differently along the X- and Y-directions. E.g. "-scale
   1280x1024" or "-scale 0.8x0.75"    Also, "-geometry WxH" is an
   alias for "-scale WxH"
 * By having SSVNC version 1.0.21 or later available in your $PATH,
   the -chatwindow option allows a UltraVNC Text Chat window to
   appear on the local X11 console/display (this way the remote
   viewer can chat with the person at the physical display; e.g.
   helpdesk mode.) This also works on the Mac OS X console if the
   Xquartz X11 server (enabled by default on leopard) is running for
   the chatwindow.
 * The HTTP Java viewer applet jar, classes/VncViewer.jar, has been
   updated with an improved implementation based on the code used by
   the classes/ssl applets.

Here are some features that appeared in the 0.9.4 release (Sep/2008):

 * Improvements to the -find and -create X session finding or
   creating modes: new desktop types and service redirection options.
   Personal cupsd daemon and SSH port redirection helper for use with
   SSVNC's Terminal Services feature.
 * Reverse VNC connections via -connect work in the -find, -create
   and related -display WAIT:... modes.
 * Reverse VNC connections (either normal or SSL) can use a Web Proxy
   or a SOCKS proxy, or a SSH connection, or even a CGI URL to make
   the outgoing connection. See: -proxy. Forward connections can also
   use: -ssh.
 * Reverse VNC connections via the UltraVNC repeater proxy (either
   normal or SSL) are supported. Use either the "-connect
   repeater=ID:NNNN+host:port" or "-connect
   repeater://host:port+ID:NNNN" notation. The SSVNC VNC viewer also
   supports the UltraVNC repeater. Also, a perl repeater implemention
   is here: ultravnc_repeater.pl
 * Support for indexed colormaps (PseudoColor) with depths other than
   8 (from 1 to 16 now work) for non-standard hardware. Option
   "-advertise_truecolor" to handle some workaround in this mode.
 * Support for the ZYWRLE encoding, this is the RealVNC ZRLE encoding
   extended to do motion video and photo regions more efficiently by
   way of a Wavelet based transformation.
 * The -finddpy and -listdpy utilities help to debug and configure
   the -find, -create, and -display WAIT:... modes.
 * Some automatic detection of screen resizes are handled even if the
   -xrandr option is not supplied.
 * The -autoport options gives more control over the VNC port x11vnc
   chooses.
 * The -ping secs can be used to help keep idle connections alive.
 * Pasting of the selection/clipboard into remote applications (e.g.
   Java) has been improved.
 * Fixed a bug if a client disconnects during the 'speed-estimation'
   phase.
 * To unset Caps_Lock, Num_Lock and raise all keys in the X server
   use -clear_all.
 * Usage with dvorak keyboards has been improved. See also: -xkb.
 * The Java Viewer applet source code is now included in the
   x11vnc-0.9.*.tar.gz tarball. This means you can now build the Java
   viewer applet jar files from source. If you stopped shipping the
   Java viewer applet jar files due to lack of source code, you can
   start again.

Here are some features that appeared in the 0.9.3 release (Oct/2007):

 * Viewer-side pixmap caching. A large area of pixels (at least 2-3
   times as big as the framebuffer itself; the bigger the better...
   default is 10X) is placed below the framebuffer to act as a
   buffer/cache area for pixel data. The VNC CopyRect encoding is
   used to move it around, so any viewer can take advantage of it.
   Until we start modifying viewers you will be able to see the cache
   area if you scroll down (this makes it easier to debug!) For
   testing the default is "-ncache 10". The unix Enhanced TightVNC
   Viewer ssvnc has a nice -ycrop option to help hide the pixel cache
   area from view.

Here are some features that appeared in the 0.9.2 release (Jun/2007):

 * Building with no OpenSSL libssl available (or with --without-ssl)
   has been fixed.
 * One can configure x11vnc via "./configure
   --with-system-libvncserver" to use a system installed libvncserver
   library instead of the one bundled in the release tarball.
 * If UltraVNC file transfer or chat is detected, then VNC clients
   are "pinged" more often to prevent these side channels from
   becoming serviced too infrequently.
 * In -unixpw mode in the username and password dialog no text will
   be echoed if the first character sent is "Escape". This enables a
   convenience feature in SSVNC to send the username and password
   automatically.

Here are some features that appeared in the 0.9.1 release (May/2007):

 * The UltraVNC Java viewer has been enhanced to support SSL (as the
   TightVNC viewer had been previously.) The UltraVNC Java supports
   ultravnc filetransfer, and so can be used as a VNC viewer on Unix
   that supports ultravnc filetransfer. It is in the
   classes/ssl/UltraViewerSSL.jar file (that is pointed to by
   ultra.vnc.) The signed applet SignedUltraViewerSSL.jar version
   (pointed to by ultrasigned.vnc) will be needed to access the local
   drive if you are using it for file transfer via a Web browser.
   Some other bugs in the UltraVNC Java viewer were fixed and a few
   improvements to the UI made.
 * A new Unix username login mode for VNC Viewers authenticated via a
   Client SSL Certificate: "-users sslpeer=". The emailAddress
   subject field is inspected for username@hostname and then acts as
   though "-users +username" has been supplied. This way the Unix
   username is identified by (i.e. simply extracted from) the Client
   SSL Certificate. This could be useful with -find, -create and -svc
   modes if you are also have set up and use VNC Client SSL
   Certificate authentication.
 * For external display finding/creating programs (e.g. WAIT:cmd=...)
   if the VNC Viewer is authenticated via a Client SSL Certificate,
   then that Certificate is available in the environment variable
   RFB_SSL_CLIENT_CERT.

Here are some features that appeared in the 0.9 release (Apr/2007):

 * VNC Service advertising via mDNS / ZeroConf / BonJour with the
   Avahi client library. Enable via "-avahi" or "-zeroconf".
 * Implementations of UltraVNC's TextChat, SingleWindow, and
   ServerInput extensions (requires ultravnc viewer or ssvnc Unix
   viewer.) They toggle the selection of a single window (-id), and
   disable (friendly) user input and viewing (monitor blank) at the
   VNC server.
 * Short aliases "-find", "-create", "-svc", and "-xdmsvc" for
   commonly used FINDCREATEDISPLAY usage modes.
 * Reverse VNC connections (viewer listening) now work in SSL (-ssl)
   mode.
 * New options to control the Monitor power state and keyboard/mouse
   grabbing: -forcedpms, -clientdpms, -noserverdpms, and -grabalways.
 * A simple way to emulate inetd(8) to some degree via the "-loopbg"
   option.
 * Monitor the accuracy of XDAMAGE and apply "-noxdamage" if it is
   not working well. OpenGL applications like like beryl and MythTv
   have been shown to make XDAMAGE not work properly.
 * For Java SSL connections involving a router/firewall port
   redirection, an option -httpsredir to spare the user from needing
   to include &PORT=NNN in the browser URL.

Here are some features that appeared in the 0.8.4 release (Feb/2007):

 * Native Mac OS X Aqua/Quartz support. (i.e. OSXvnc alternative;
   some activities are faster)
 * A new login mode: "-display WAIT:cmd=FINDCREATEDISPLAY -unixpw
   ..." that will Create a new X session (either virtual or real and
   with or without a display manager, e.g. kdm) for the user if it
   cannot find the user's X session display via the FINDDISPLAY
   method. See the -svc and the -xdmsvc aliases.
 * x11vnc can act as a VNC reflector/repeater using the "-reflect
   host:N" option. Instead of polling an X display, the remote VNC
   Server host:N is connected to and re-exported via VNC. This is
   intended for use in broadcasting a display to many (e.g. > 16;
   classroom or large demo) VNC viewers where bandwidth and other
   resources are conserved by spreading the load over a number of
   repeaters.
 * Wireframe copyrect detection for local user activity (e.g. someone
   sitting at the physical display moving windows) Use
   -nowireframelocal to disable.
 * The "-N" option couples the VNC Display number to the X Display
   number. E.g. if your X DISPLAY is :2 then the VNC display will be
   :2 (i.e. using port 5902.) If that port is taken x11vnc will exit.
 * Option -nodpms to avoid problems with programs like KDE's
   kdesktop_lock that keep restarting the screen saver every few
   seconds.
 * To automatically fix the common mouse motion problem on XINERAMA
   (multi-headed) displays, the -xwarppointer option is enabled by
   default when XINERAMA is active.

If you have a Mac please try out the native Mac OS X support, build with "./configure --without-x", or download a binary mentioned above, (even if you don't plan on ever using it in this mode!), and let me know how it went. Thanks.

Here are some features that appeared in the 0.8.3 release (Nov/2006):

 * The -ssl option provides SSL encryption and authentication
   natively via the www.openssl.org library. One can use from a
   simple self-signed certificate server certificate up to full CA
   and client certificate authentication schemes.
 * Similar to -ssl, the -stunnel option starts up a SSL tunnel server
   stunnel (that must be installed separately on the system:
   stunnel.mirt.net ) to allow only encrypted SSL connections from
   the network.
 * The -sslverify option allows for authenticating VNC clients via
   their certificates in either -ssl or -stunnel modes.
 * Certificate creation and management tools are provide in the
   -sslGenCert, -sslGenCA, and related options.
 * An SSL enabled Java applet VNC Viewer applet is provided by x11vnc
   in classes/ssl/VncViewer.jar. In addition to normal HTTP, the
   applet may be loaded into the web browser via HTTPS (HTTP over
   SSL.) (one can use the VNC port, e.g. https://host:5900/, or also
   the separate -https port option.) A wrapper shell script
   ss_vncviewer is also provided that sets up a stunnel client-side
   tunnel on Unix systems. See Enhanced TightVNC Viewer (SSVNC) for
   other SSL/SSH viewer possibilities.
 * The -unixpw option supports Unix username and password
   authentication (a simpler variant is the -unixpw_nis option that
   works in environments where the encrypted passwords are readable,
   e.g. NIS.) The -ssl or -localhost + -stunnel options are enforced
   in this mode to prevent password sniffing. As a convenience, these
   requirements are lifted if a SSH tunnel can be deduced (but
   -localhost still applies.)
 * Coupling -unixpw with "-display WAIT:cmd=FINDDISPLAY" or "-display
   WAIT:cmd=FINDCREATEDISPLAY" provides a way to allow a user to
   login with their UNIX password and have their display connected to
   automatically. See the -svc and the -xdmsvc aliases.
 * Hooks are provided in the -unixpw_cmd and "-passwdfile
   cmd:,custom:..." options to allow you to supply your own
   authentication and password lookup programs.
 * x11vnc can be configured and built to not depend on X11 libraries
   "./configure --without-x" for -rawfb only operation (e.g. embedded
   linux console devices.)
 * The -rotate option enables you to rotate or reflect the screen
   before exporting via VNC. This is intended for use on handhelds
   and other devices where the rotation orientation is not "natural".
 * The "-ultrafilexfer" alias is provided and improved UltraVNC
   filetransfer rates have been achieved.
 * Under the "-connect_or_exit host" option x11vnc will exit
   immediately unless the reverse connection to host succeeds. The
   "-rfbport 0" option disables TCP listening for connections (useful
   for this mode.)
 * The "-rawfb rand" and "-rawfb none" options are useful for testing
   automation scripts, etc., without requiring a full desktop.
 * Reduced spewing of information at startup, use "-verbose" (also
   "-v") to turn it back on for debugging or if you are going to send
   me a problem report.

Here are some Previous Release Notes

Some Notes:

Both a client and a server: It is sometimes confusing to people that x11vnc is both a client and a server at the same time. It is an X client because it connects to the running X server to do the screen polls. Think of it as a rather efficient "screenshot" program running continuously. It is a server in the sense that it is a VNC server that VNC viewers on the network can connect to and view the screen framebuffer it manages.

When trying to debug problems, remember to think of both roles. E.g. "how is x11vnc connecting to the X server?", "how is the vncviewer connecting to x11vnc?", "what permits/restricts the connection?". Both links may have reachability, permission, and other issues.

Network performance: Whether you are using Xvnc or x11vnc it is always a good idea to have a solid background color instead of a pretty background image. Each and every re-exposure of the background must be resent over the network: better to have that background be a solid color that compresses very well compared to a photo image. (This is one place where the X protocol has an advantage over the VNC protocol.) I suggest using xsetroot, dtstyle or similar utility to set a solid background while using x11vnc. You can turn the pretty background image back on when you are using the display directly. Update: As of Feb/2005 x11vnc has the -solid [color] option that works on recent GNOME, KDE, and CDE and also on classic X (background image is on the root window.) Update: As of Oct/2007 x11vnc has the -ncache option that does a reasonable job caching the background (and other) pixmap data on the viewer side.

I also find the TightVNC encoding gives the best response for my usage (Unix <-> Unix over cable modem.) One needs a tightvnc-aware vncviewer to take advantage of this encoding.

TCP port issues: Notice the lines

18/07/2003 14:36:31 Autoprobing selected port 5900
PORT=5900

in the output. 5900 is the default VNC listening port (just like 6000 is X11's default listening port.) Had port 5900 been taken by some other application, x11vnc would have next tried 5901. That would mean the viewer command above should be changed to vncviewer far-away.east:1. You can force the port with the "-rfbport NNNN" option where NNNN is the desired port number. If that port is already taken, x11vnc will exit immediately. The "-N" option will try to match the VNC display number to the X display. (also see the "SunRay Gotcha" note below)

Options: x11vnc has (far too) many features that may be activated via its command line options. Useful options are, e.g., -scale to do server-side scaling, and -rfbauth passwd-file to use VNC password protection (the vncpasswd or storepasswd programs, or the x11vnc -storepasswd option can be used to create the password file.)

Algorithm: How does x11vnc do it? Rather brute-forcedly: it continuously polls the X11 framebuffer for changes using XShmGetImage(). When changes are discovered, it instructs libvncserver which rectangular regions of the framebuffer have changed, and libvncserver compresses the changes and sends them off to any connected VNC viewers. A number of applications do similar things, such as x0rfbserver, krfb, x0vncserver, vino. x11vnc uses a 32 x 32 pixel tile model (the desktop is decomposed into roughly 1000 such tiles), where changed tiles are found by pseudo-randomly polling 1 pixel tall horizontal scanlines separated vertically by 32 pixels. This is a surprisingly effective algorithm for finding changed regions. For keyboard and mouse user input the XTEST extension is used to pass the input events to the X server. To detect XBell "beeps" the XKEYBOARD extension is used. If available, the XFIXES extension is used to retrieve the current mouse cursor shape. Also, if available the X DAMAGE extension is used to receive hints from the X server where modified regions on the screen are. This greatly reduces the system load when not much is changing on the screen and also improves how quickly the screen is updated.

Barbershop mirrors effect: What if x11vnc is started up, and vncviewer is then started up on the same machine and displayed on the same display x11vnc is polling? One might "accidentally" do this when first testing out the programs. You get an interesting recursive/feedback effect where vncviewer images keep popping up each one contained in the previous one and slightly shifted a bit by the window manager decorations. There will be an even more interesting effect if -scale is used. Also, if the XKEYBOARD is supported and the XBell "beeps" once, you get an infinite loop of beeps going off. Although all of this is mildly exciting it is not much use: you will normally run and display the viewer on a different machine!

Sun Ray Notes:

You can run x11vnc on your (connected or disconnected) SunRay session. Here are some notes on SunRay usage with x11vnc.

Limitations:

 * Due to the polling nature, some activities (opaque window moves,
   scrolling), can be pretty choppy/ragged and others (exposures of
   large areas) slow. Experiment with interacting a bit differently
   than you normally do to minimize the effects (e.g. do fullpage
   paging rather than line-by-line scrolling, and move windows in a
   single, quick motion.) Recent work has provided the
   -scrollcopyrect and -wireframe speedups using the CopyRect VNC
   encoding and other things, but they only speed up some activities,
   not all.
 * A rate limiting factor for x11vnc performance is that graphics
   hardware is optimized for writing, not reading (x11vnc reads the
   video framebuffer for the screen image data.) The difference can
   be a factor of 10 to 1000, and so it usually takes about 0.5-1 sec
   to read in the whole video hardware framebuffer (e.g. 5MB for
   1280x1024 at depth 24 with a read rate of 5-10MB/sec.) So whenever
   activity changes most of the screen (e.g. moving or iconifying a
   large window) there is a delay of 0.5-1 sec while x11vnc reads the
   changed regions in.
   A slow framebuffer read rate will often be the performance
   bottleneck on a fast LAN (whereas on slower links the reduced
   network bandwidth becomes the bottleneck.)
   Note: A quick way to get a 2X speedup of this for x11vnc is to
   switch your X server from depth 24 (32bpp) to depth 16 (16bpp.)
   You get a 4X speedup going to 8bpp, but the lack of color cells is
   usually unacceptable.
   To get a sense of the read and write speeds of your video card,
   you can run benchmarks like: "x11perf -getimage500",  "x11perf
   -putimage500",  "x11perf -shmput500" and for XFree86 displays with
   direct graphics access the "dga" command (press "b" to run the
   benchmark and then after a few seconds press "q" to quit.) Even
   this "dd if=/dev/fb0 of=/dev/null" often gives a good estimate.
   x11vnc also prints out its estimate:

28/02/2009 11:11:07 Autoprobing TCP port 28/02/2009 11:11:07 Autoprobing selected port 5900 28/02/2009 11:11:08 fb read rate: 10 MB/sec 28/02/2009 11:11:08 screen setup finished. We have seen a few cases where the hardware fb read speed is greater than 65 MB/sec: on high end graphics workstations from SGI and Sun, and also from a Linux user using nvidia proprietary drivers for his nvidia video card. Update 2008: thankfully, these sped up drivers are becoming more common on Linux and *BSD systems and that makes x11vnc run somewhat more quickly. Sometimes they have a read rate of over 400 MB/sec. On XFree86/Xorg it is actually possible to increase the framebuffer read speed considerably (10-100 times) by using the Shadow Framebuffer (a copy of the framebuffer is kept in main memory and this can be read much more quickly.) To do this one puts the line Option "ShadowFB" "true" in the Device section of the /etc/X11/XF86Config or /etc/X11/xorg.conf file. Note that this disables 2D acceleration at the physical display and so that might be unacceptable if one plays games, etc. on the machine's local display. Nevertheless this could be handy in some circumstances, e.g. if the slower speed while sitting at the physical display was acceptable (this seems to be true for most video cards these days.) Unfortunately it does not seem shadowfb can be turned on and off dynamically... Another amusing thing one can do is use Xvfb as the X server, e.g. "xinit $HOME/.xinitrc -- /usr/X11R6/bin/Xvfb :1 -screen 0 1024x768x16" x11vnc can poll Xvfb efficiently via main memory. It's not exactly clear why one would want to do this instead of using vncserver/Xvnc, (perhaps to take advantage of an x11vnc feature, such as framebuffer scaling or built-in SSL encryption), but we mention it because it may be of use for special purpose applications. You may need to use the "-cc 4" option to force Xvfb to use a TrueColor visual instead of DirectColor. See also the description of the -create option that does all of this automatically for you (be sure to install the Xvfb package, e.g. apt-get install xvfb.) Also, a faster and more accurate way is to use the "dummy" Xorg/XFree86 device driver (or our Xdummy wrapper script.) See this FAQ for details. * Somewhat surprisingly, the X11 mouse (cursor) shape is write-only and cannot be queried from the X server. So traditionally in x11vnc the cursor shape stays fixed at an arrow. (see the "-cursor X" and "-cursor some" options, however, for a partial hack for the root window, etc.) However, on Solaris using the SUN_OVL overlay extension, x11vnc can show the correct mouse cursor when the -overlay option is also supplied. A similar thing is done on IRIX as well when -overlay is supplied. More generally, as of Dec/2004 x11vnc supports the new XFIXES extension (in Xorg and Solaris 10) to query the X server for the exact cursor shape, this works pretty well except that cursors with transparency (alpha channel) need to approximated to solid RGB values (some cursors look worse than others.) * Audio from applications is of course not redirected (separate redirectors do exist, e.g. esd, see the FAQ on this below.) The XBell() "beeps" will work if the X server supports the XKEYBOARD extension. (Note that on Solaris XKEYBOARD is disabled by default. Passing +kb to Xsun enables it.) * The scroll detection algorithm for the -scrollcopyrect option can give choppy or bunched up transient output and occasionally painting errors. * Using -threads can expose some bugs/crashes in libvncserver.

Please feel free to contact me if you have any questions, problems, or comments about x11vnc, etc. Please be polite, thorough, and not demanding (sadly, the number of people contacting me that are rude and demanding is increasing dramatically.) Also, some people ask if they can make a donation, see this link for that.

http://www.karlrunge.com/x11vnc/faq.html:

x11vnc FAQ:

[Building and Starting]

Q-1: I can't get x11vnc to start up. It says "XOpenDisplay failed (null)" or "Xlib: connection to ":0.0" refused by server Xlib: No protocol specified" and then exits. What do I need to do?

Q-2: I can't get x11vnc and/or libvncserver to compile.

Q-3: I just built x11vnc successfully, but when I use it my keystrokes and mouse button clicks are ignored (I am able to move the mouse though.)

Q-4: Help, I need to run x11vnc on Solaris 2.5.1 (or other old Unix/Linux) and it doesn't compile!

Q-5: Where can I get a precompiled x11vnc binary for my Operating System?

Q-6: Where can I get a VNC Viewer binary (or source code) for the Operating System I will be viewing from?

Q-7: How can I see all of x11vnc's command line options and documentation on how to use them?

Q-8: I don't like typing arcane command line options every time I start x11vnc. What can I do? Is there a config file? Or a GUI?

Q-9: How can I get the GUI to run in the System Tray, or at least be a smaller, simpler icon?

Q-10: How can I get x11vnc to listen on a different port besides the default VNC port (5900)?

Q-11: My Firewall/Router doesn't allow VNC Viewers to connect to x11vnc.

Q-12: Is it possible for a VNC Viewer and a VNC Server to connect to each other even though both are behind Firewalls that block all incoming connections?

Q-13: Can I make x11vnc more quiet and also go into the background after starting up?

Q-14: Sometimes when a VNC viewer dies abruptly, x11vnc also dies with the error message like: "Broken pipe". I'm using the -forever mode and I want x11vnc to keep running.

Q-15: The Windows TightVNC 1.3.9 Viewer cannot connect to x11vnc.

Q-16: KDE's krdc VNC viewer cannot connect to x11vnc.

Q-17: When I start x11vnc on an Alpha Tru64 workstation the X server crashes!

Q-18: When running x11vnc on an IBM AIX workstation after a few minutes the VNC connection freezes.

Q-19: Are there any build-time customizations possible, e.g. change defaults, create a smaller binary, etc?

[Win2VNC Related]

Q-20: I have two separate machine displays in front of me, one Windows the other X11: can I use x11vnc in combination with Win2VNC in dual-screen mode to pass the keystrokes and mouse motions to the X11 display?

Q-21: I am running Win2VNC on my Windows machine and "x11vnc -nofb" on Unix to pass keyboard and mouse to the Unix monitor. Whenever I start Win2VNC it quickly disconnects and x11vnc says: rfbProcessClientNormalMessage: read: Connection reset by peer

Q-22: Can I run "x11vnc -nofb" on a Mac OS X machine to redirect mouse and keyboard input to it from Windows and X11 machines via Win2VNC and x2vnc, respectively?

[Color Issues]

Q-23: The X display I run x11vnc on is only 8 bits per pixel (bpp) PseudoColor (i.e. only 256 distinct colors.) The x11vnc colors may start out OK, but after a while they are incorrect in certain windows.

Q-24: Color problems: Why are the colors for some windows incorrect in x11vnc? BTW, my X display has nice overlay/multi-depth visuals of different color depths: e.g. there are both depth 8 and 24 visuals available at the same time.

Q-25: I am on a high color system (depth >= 24) but I seem to have colormap problems. They either flash or everything is very dark.

Q-26: How do I figure out the window id to supply to the -id windowid option?

Q-27: Why don't menus or other transient windows come up when I am using the -id windowid option to view a single application window?

Q-28: My X display is depth 24 at 24bpp (instead of the normal depth 24 at 32bpp.) I'm having lots of color and visual problems with x11vnc and/or vncviewer. What's up?

[Xterminals]

Q-29: Can I use x11vnc to view and interact with an Xterminal (e.g. NCD) that is not running UNIX and so x11vnc cannot be run on it directly?

Q-30: How do I get my X permissions (MIT-MAGIC-COOKIE file) correct for a Unix/Linux machine acting as an Xterminal?

[Sun Rays]

Q-31: I'm having trouble using x11vnc with my Sun Ray session.

[Remote Control]

Q-32: How do I stop x11vnc once it is running in the background?

Q-33: Can I change settings in x11vnc without having to restart it? Can I remote control it?

[Security and Permissions]

Q-34: How do I create a VNC password for use with x11vnc?

Q-35: Can I make it so -storepasswd doesn't show my password on the screen?

Q-36: Can I have two passwords for VNC viewers, one for full access and the other for view-only access to the display?

Q-37: Can I have as many full-access and view-only passwords as I like?

Q-38: Does x11vnc support Unix usernames and passwords? Can I further limit the set of Unix usernames who can connect to the VNC desktop?

Q-39: Can I supply an external program to provide my own custom login method (e.g. Dynamic/One-time passwords or non-Unix (LDAP) usernames and passwords)?

Q-40: Why does x11vnc exit as soon as the VNC viewer disconnects? And why doesn't it allow more than one VNC viewer to connect at the same time?

Q-41: Can I limit which machines incoming VNC clients can connect from?

Q-42: How do I build x11vnc/libvncserver with libwrap (tcp_wrappers) support?

Q-43: Can I have x11vnc only listen on one network interface (e.g. internal LAN) rather than having it listen on all network interfaces and relying on -allow to filter unwanted connections out?

Q-44: Now that -localhost implies listening only on the loopback interface, how I can occasionally allow in a non-localhost via the -R allowonce remote control command?

Q-45: Can I fine tune what types of user input are allowed? E.g. have some users just be able to move the mouse, but not click or type anything?

Q-46: Can I prompt the user at the local X display whether the incoming VNC client should be accepted or not? Can I decide to make some clients view-only? How about running an arbitrary program to make the decisions?

Q-47: I start x11vnc as root because it is launched via inetd(8) or a display manager like gdm(1). Can I have x11vnc later switch to a different user?

Q-48: I use a screen-lock when I leave my workstation (e.g. xscreensaver or xlock.) When I remotely access my workstation desktop via x11vnc I can unlock the desktop fine, but I am worried people will see my activities on the physical monitor. What can I do to prevent this, or at least make it more difficult?

Q-49: Can I have x11vnc automatically lock the screen when I disconnect the VNC viewer?

[Encrypted Connections]

Q-50: How can I tunnel my connection to x11vnc via an encrypted SSH channel between two Unix machines?

Q-51: How can I tunnel my connection to x11vnc via an encrypted SSH channel from Windows using an SSH client like Putty?

Q-52: How can I tunnel my connection to x11vnc via an encrypted SSL channel using an external tool like stunnel?

Q-53: Does x11vnc have built-in SSL tunneling?

Q-54: How do I use VNC Viewers with built-in SSL tunneling?

Q-55: How do I use the Java applet VNC Viewer with built-in SSL tunneling when going through a Web Proxy?

Q-56: Can Apache web server act as a gateway for users to connect via SSL from the Internet with a Web browser to x11vnc running on their workstations behind a firewall?

Q-57: Can I create and use my own SSL Certificate Authority (CA) with x11vnc?

[Display Managers and Services]

Q-58: How can I run x11vnc as a "service" that is always available?

Q-59: How can I use x11vnc to connect to an X login screen like xdm, GNOME gdm, KDE kdm, or CDE dtlogin? (i.e. nobody is logged into an X session yet.)

Q-60: Can I run x11vnc out of inetd(8)? How about xinetd(8)?

Q-61: Can I have x11vnc advertise its VNC service and port via mDNS / Zeroconf (e.g. Avahi) so VNC viewers on the local network can detect it automatically?

Q-62: Can I have x11vnc allow a user to log in with her UNIX username and password and then have it find her X session display on that machine and then attach to it? How about starting an X session if one cannot be found?

Q-63: Can I have x11vnc restart itself after it terminates?

Q-64: How do I make x11vnc work with the Java VNC viewer applet in a web browser?

Q-65: Are reverse connections (i.e. the VNC server connecting to the VNC viewer) using "vncviewer -listen" and vncconnect(1) supported?

Q-66: Can reverse connections be made to go through a Web or SOCKS proxy or SSH?

Q-67: Can x11vnc provide a multi-user desktop web login service as an Apache CGI or PHP script?

Q-68: Can I use x11vnc as a replacement for Xvnc? (i.e. not for a real display, but for a virtual one I keep around.)

Q-69: How can I use x11vnc on "headless" machines? Why might I want to?

[Resource Usage and Performance]

Q-70: I have lots of memory, but why does x11vnc fail with shmget: No space left on device or Minor opcode of failed request: 1 (X_ShmAttach)?

Q-71: How can I make x11vnc use less system resources?

Q-72: How can I make x11vnc use MORE system resources?

Q-73: I use x11vnc over a slow link with high latency (e.g. dialup modem or broadband), is there anything I can do to speed things up?

Q-74: Does x11vnc support the X DAMAGE Xserver extension to find modified regions of the screen quickly and efficiently?

Q-75: My OpenGL application shows no screen updates unless I supply the -noxdamage option to x11vnc.

Q-76: When I drag windows around with the mouse or scroll up and down things really bog down (unless I do the drag in a single, quick motion.) Is there anything to do to improve things?

Q-77: Why not do something like wireframe animations to avoid the windows "lurching" when being moved or resized?

Q-78: Can x11vnc try to apply heuristics to detect when a window is scrolling its contents and use the CopyRect encoding for a speedup?

Q-79: Can x11vnc do client-side caching of pixel data? I.e. so when that pixel data is needed again it does not have to be retransmitted over the network.

Q-80: Does x11vnc support TurboVNC?

[Mouse Cursor Shapes]

Q-81: Why isn't the mouse cursor shape (the little icon shape where the mouse pointer is) correct as I move from window to window?

Q-82: When using XFIXES cursorshape mode, some of the cursors look really bad with extra black borders around the cursor and other cruft. How can I improve their appearance?

Q-83: In XFIXES mode, are there any hacks to handle cursor transparency ("alpha channel") exactly?

[Mouse Pointer]

Q-84: Why does the mouse arrow just stay in one corner in my vncviewer, whereas my cursor (that does move) is just a dot?

Q-85: Can I take advantage of the TightVNC extension to the VNC protocol where Cursor Positions Updates are sent back to all connected clients (i.e. passive viewers can see the mouse cursor being moved around by another viewer)?

Q-86: Is it possible to swap the mouse buttons (e.g. left-handed operation), or arbitrarily remap them? How about mapping button clicks to keystrokes, e.g. to partially emulate Mouse wheel scrolling?

[Keyboard Issues]

Q-87: How can I get my AltGr and Shift modifiers to work between keyboards for different languages?

Q-88: When I try to type a "<" (i.e. less than) instead I get ">" (i.e. greater than)! Strangely, typing ">" works OK!!

Q-89: Extra Character Inserted, E.g.: When I try to type a "<" (i.e. less than) instead I get "<," (i.e. an extra comma.)

Q-90: I'm using an "international" keyboard (e.g. German "de", or Danish "dk") and the -modtweak mode works well if the VNC viewer is run on a Unix/Linux machine with a similar keyboard. But if I run the VNC viewer on Unix/Linux with a different keyboard (e.g. "us") or Windows with any keyboard, I can't type some keys like: "@", "$", "<", ">", etc. How can I fix this?

Q-91: When typing I sometimes get double, triple, or more of my keystrokes repeated. I'm sure I only typed them once, what can I do?

Q-92: The x11vnc -norepeat mode is in effect, but I still get repeated keystrokes!!

Q-93: After using x11vnc for a while, I find that I cannot type some (or any) characters or my mouse clicks and drags no longer have any effect, or they lead to strange effects. What happened?

Q-94: The machine where I run x11vnc has an AltGr key, but the local machine where I run the VNC viewer does not. Is there a way I can map a local unused key to send an AltGr? How about a Compose key as well?

Q-95: I have a Sun machine I run x11vnc on. Its Sun keyboard has just one Alt key labelled "Alt" and two Meta keys labelled with little diamonds. The machine where I run the VNC viewer only has Alt keys. How can I send a Meta keypress? (e.g. emacs needs this)

Q-96: Running x11vnc on HP-UX I cannot type "#" I just get a "3" instead.

Q-97: Can I map a keystroke to a mouse button click on the remote machine?

Q-98: How can I get Caps_Lock to work between my VNC viewer and x11vnc?

[Screen Related Issues and Features]

Q-99: The remote display is larger (in number of pixels) than the local display I am running the vncviewer on. I don't like the vncviewer scrollbars, what I can do?

Q-100: Does x11vnc support server-side framebuffer scaling? (E.g. to make the desktop smaller.)

Q-101: Does x11vnc work with Xinerama? (i.e. multiple monitors joined together to form one big, single screen.)

Q-102: Can I use x11vnc on a multi-headed display that is not Xinerama (i.e. separate screens :0.0, :0.1, ... for each monitor)?

Q-103: Can x11vnc show only a portion of the display? (E.g. for a special purpose application or a very large screen.)

Q-104: Does x11vnc support the XRANDR (X Resize, Rotate and Reflection) extension? Whenever I rotate or resize the screen x11vnc just seems to crash.

Q-105: Independent of any XRANDR, can I have x11vnc rotate and/or reflect the screen that the VNC viewers see? (e.g. for a handheld whose screen is rotated 90 degrees.)

Q-106: Why is the view in my VNC viewer completely black? Or why is everything flashing around randomly?

Q-107: I use Linux Virtual Terminals (VT's) to implement 'Fast User Switching' between users' sessions (e.g. Betty is on Ctrl-Alt-F7, Bobby is on Ctrl-Alt-F8, and Sid is on Ctrl-Alt-F1: they use those keystrokes to switch between their sessions.) How come the view in a VNC viewer connecting to x11vnc is either completely black or otherwise all messed up unless the X session x11vnc is attached to is in the active VT?

Q-108: I am using x11vnc where my local machine has "popup/hidden taskbars" and the remote display where x11vnc runs also has "popup/hidden taskbars" and they interfere and fight with each other. What can I do?

Q-109: Help! x11vnc and my KDE screensaver keep switching each other on and off every few seconds.

Q-110: I am running the compiz 3D window manager (or beryl, MythTv, Google Earth, or some other OpenGL app) and I do not get screen updates in x11vnc.

Q-111: Can I use x11vnc to view my VMWare session remotely?

[Exporting non-X11 devices via VNC]

Q-112: Can non-X devices (e.g. a raw framebuffer) be viewed (and even controlled) via VNC with x11vnc?

Q-113: Can I export the Linux Console (Virtual Terminals) via VNC using x11vnc?

Q-114: Can I export via VNC a Webcam or TV tuner framebuffer using x11vnc?

Q-115: Can I connect via VNC to a Qt-embedded/Qt-enhanced/Qtopia application running on my handheld, cell phone, or PC using the Linux console framebuffer (i.e. not X11)?

Q-116: How do I inject touch screen input into an Qt-embedded/Qt-enhanced/Qtopia cell phone such as openmoko/qtmoko Neo Freerunner?

Q-117: Now that non-X11 devices can be exported via VNC using x11vnc, can I build it with no dependencies on X11 header files and libraries?

Q-118: How do I cross compile x11vnc for a different architecture than my Linux i386 or amd64 PC?

Q-119: Does x11vnc support Mac OS X Aqua/Quartz displays natively (i.e. no X11 involved)?

Q-120: Can x11vnc be used as a VNC reflector/repeater to improve performance for the case of a large number of simultaneous VNC viewers (e.g. classroom broadcasting or a large demo)?

Q-121: Can x11vnc be used during a Linux, Solaris, etc. system Installation so the Installation can be done remotely?

[Misc: Clipboard, File Transfer/Sharing, Printing, Sound, Beeps, Thanks, etc.]

Q-122: Does the Clipboard/Selection get transferred between the vncviewer and the X display?

Q-123: Can I use x11vnc to record a Shock Wave Flash (or other format) video of my desktop, e.g. to record a tutorial or demo?

Q-124: Can I transfer files back and forth with x11vnc?

Q-125: Which UltraVNC extensions are supported?

Q-126: Can x11vnc emulate UltraVNC's Single Click helpdesk mode for Unix? I.e. something very simple for a naive user to initiate a reverse vnc connection from their Unix desktop to a helpdesk operator's VNC Viewer.

Q-127: Can I (temporarily) mount my local (viewer-side) Windows/Samba File share on the machine where x11vnc is running?

Q-128: Can I redirect CUPS print jobs from the remote desktop where x11vnc is running to a printer on my local (viewer-side) machine?

Q-129: How can I hear the sound (audio) from the remote applications on the desktop I am viewing via x11vnc?

Q-130: Why don't I hear the "Beeps" in my X session (e.g. when typing tput bel in an xterm)?

Q-131: Does x11vnc work with IPv6?

Q-132: Thanks for your program or for your help! Can I make a donation? _________________________________________________________________

[Building and Starting]

Q-1: I can't get x11vnc to start up. It says "XOpenDisplay failed (null)" or "Xlib: connection to ":0.0" refused by server Xlib: No protocol specified" and then exits. What do I need to do?

For the former error, you need to specify the X display to connect to (it also needs to be on the same machine the x11vnc process is to run on.) Set your DISPLAY environment variable (or use the -display option) to specify it. Nearly always the correct value will be ":0" (in fact, x11vnc will now assume :0 if given no other information.)

For the latter error, you need to set up the X11 permissions correctly.

To make sure X11 permissions are the problem do this simple test: while sitting at the physical X display open a terminal window (gnome-terminal, xterm, etc.) You should be able to run x11vnc successfully without any need for special steps or command line options in that terminal (i.e. just type "x11vnc".) If that works OK then you know X11 permissions are the only thing preventing it from working when you try to start x11vnc via, say, a remote shell.

How to Solve: See the xauth(1), Xsecurity(7), and xhost(1) man pages or this Howto for much info on X11 permissions. For example, you may need to set your XAUTHORITY environment variable (or use the -auth option) to point to the correct MIT-MAGIC-COOKIE file (e.g. /home/joe/.Xauthority or /var/gdm/:0.Xauth or /var/lib/kdm/A:0-crWk72K or /tmp/.gdmzndVlR, etc, etc.), or simply be sure you run x11vnc as the correct user (i.e. the user who is logged into the X session you wish to view.)

Note: The MIT cookie file contains the secret key that allows x11vnc to connect to the desired X display.

If, say, sshd has set XAUTHORITY to point to a random file it has created for X forwarding that will cause problems. (Under some circumstances even su(1) and telnet(1) can set XAUTHORITY. See also the gdm parameter NeverPlaceCookiesOnNFS that sets XAUTHORITY to a random filename in /tmp for the whole X session.)

Running x11vnc as root is often not enough: you need to know where the MIT-MAGIC-COOKIE file for the desired X display is.

Example solution: x11vnc -display :0 -auth /var/gdm/:0.Xauth

(this is for the display manager gdm and requires root permission to read the gdm cookie file, see this faq for other display manager cookie file names.)

Note as of Feb/2007 you can also try the -find option instead of "-display ..." and see if that finds your display and Xauthority.

Less safe, but to avoid figuring out where the correct XAUTHORITY file is, if the person sitting at the physical X session types "xhost +localhost" then one should be able to attach x11vnc to the session (from the same machine.) The person could then type "xhost -localhost" after x11vnc has connected to go back to the default permissions. Also, for some situations the "-users lurk=" option may soon be of use (please read the documentation on the -users option.)

To test out your X11 permissions from a remote shell, set DISPLAY and possibly XAUTHORITY (see your shell's man page, bash(1), tcsh(1), on how to set environment variables) and type xdpyinfo in the same place you will be typing (or otherwise running) x11vnc. If information is printed out about the X display (screen sizes, supported extensions, color visuals info) that means the X11 permissions are set up properly: xdpyinfo successfully connected to DISPLAY! You could also type xclock and make sure no errors are reported (a clock should appear on the X display, press Ctrl-C to stop it.) If these work, then typing "x11vnc" in the same environment should also work.

Important: if you cannot get your X11 permissions so that the xdpyinfo or xclock tests work, x11vnc also will not work (all of these X clients must be allowed to connect to the X server to function properly.)

Firewalls: Speaking of permissions, it should go without saying that the host-level firewall will need to be configured to allow connections in on a port. E.g. 5900 (default VNC port) or 22 (default SSH port for tunnelling VNC.) Most systems these days have firewalls turned on by default, so you will actively have to do something to poke a hole in the firewall at the desired port number. See your system administration tool for Firewall settings (Yast, Firestarter, etc.)

Q-2: I can't get x11vnc and/or libvncserver to compile.

Make sure you have gcc (or other C compiler) and all of the required libraries and the corresponding -dev/-devel packages installed. These include Xorg/XFree86, libX11, libjpeg, libz, libssl, ... and don't forget the devs: libjpeg-dev, libssl-dev ...

The most common build problem that people encounter is that the necessary X11 libraries are installed on their system however it does not have the corresponding -dev/-devel packages installed. These dev packages include C header files and build-time .so symlink. It is a shame the current trend in distros is to not install the dev package by default when the the library runtime package is installed... (it diminishes the power of open source)

As of Nov/2006 here is a list of libraries that x11vnc usually likes to use: libc.so libX11.so libXtst.so libXext.so libXfixes.so libXdamage.so libXinerama.so libXrandr.so libz.so libjpeg.so libpthread.so libssl.so libcrypto.so libcrypt.so

although x11vnc will be pretty usable with the subset: libc.so, libX11.so, libXtst.so, libXext.so, libz.so, and libjpeg.so.

After running the libvncserver configure, carefully examine the output and the messages in the config.log file looking for missing components. For example, if the configure output looks like: checking how to run the C preprocessor... gcc -E checking for X... no checking for XkbSelectEvents in -lX11... no checking for XineramaQueryScreens in -lXinerama... no checking for XTestFakeKeyEvent in -lXtst... no

or even worse: checking for C compiler default output file name... configure: error: C compiler cannot create executables See `config.log' for more details.

there is quite a bit wrong with the build environment. Hopefully simply adding -dev packages and/or gcc or make will fix it.

For Debian the list seems to be: gcc make libc6-dev libjpeg8-dev (formerly libjpeg62-dev) libx11-dev x11proto-core-dev (formerly x-dev) libxext-dev libxtst-dev libxdamage-dev libxfixes-dev libxrandr-dev libxinerama-dev libxss-dev (formerly xlibs-static-dev) zlib1g-dev libssl-dev libavahi-client-dev linux-libc-dev (only needed for linux console rawfb support)

Note that depending on your OS version the above names may have been changed and/or additional packages may be needed.

For Redhat the list seems to be: gcc make glibc-devel libjpeg-devel libX11-devel xorg-x11-proto-devel libXdamage-devel libXfixes-devel libXrandr-devel zlib-devel openssl-devel avahi-devel kernel-headers (only needed for linux console rawfb support)

For other distros or OS's the package names may not be the same but will look similar. Also, distros tend to rename packages as well so the above list may be out of date. So only use the above lists as hints for the package names that are needed.

Have a look at Misc. Build Problems for additional fixes.

Note: there is growing trend in Linux and other distros to slice up core X11 software into more and smaller packages. So be prepared for more headaches compiling software...

Q-3: I just built x11vnc successfully, but when I use it my keystrokes and mouse button clicks are ignored (I am able to move the mouse though.)

This is most likely due to you not having a working build environment for the XTEST client library libXtst.so. The library is probably present on your system, but the package installing the build header file is missing.

If you were watching carefully while configure was running you would have seen: checking for XTestFakeKeyEvent in -lXtst... no

The solution is to add the necessary build environment package (and the library package if that is missing too.) On Debian the build package is libxtst-dev. Other distros/OS's may have it in another package.

x11vnc will build without support for this library (e.g. perhaps one wants a view-only x11vnc on a stripped down or embedded system...) And at runtime it will also continue to run even if the X server it connects to does not support XTEST. In both cases it cannot inject keystrokes or button clicks since XTEST is needed for that (it can still move the mouse pointer using the X API XWarpPointer().)

You will see a warning message something like this at run time: 20/03/2005 22:33:09 WARNING: XTEST extension not available (either missing fr om 20/03/2005 22:33:09 display or client library libXtst missing at build time .) 20/03/2005 22:33:09 MOST user input (pointer and keyboard) will be DISCARDE D. 20/03/2005 22:33:09 If display does have XTEST, be sure to build x11vnc wit h 20/03/2005 22:33:09 a working libXtst build environment (e.g. libxtst-dev, 20/03/2005 22:33:09 or other packages.) 20/03/2005 22:33:09 No XTEST extension, switching to -xwarppointer mode for 20/03/2005 22:33:09 pointer motion input.

Also, as of Nov/2006 there will be a configure build time warning as well: ... checking for XFixesGetCursorImage in -lXfixes... yes checking for XDamageQueryExtension in -lXdamage... yes configure: WARNING:

A working build environment for the XTEST extension was not found (libXtst). An x11vnc built this way will be only barely usable. You will be able to move the mouse but not click or type. There can also be deadlocks if an application grabs the X server.

It is recommended that you install the necessary development packages for XTEST (perhaps it is named something like libxtst-dev) and run configure again.

Q-4: Help, I need to run x11vnc on Solaris 2.5.1 (or other old Unix/Linux) and it doesn't compile!

We apologize that x11vnc does not build cleanly on older versions of Solaris, Linux, etc.: very few users are on these old releases.

We have heard that since Dec/2004 a Solaris 2.6 built x11vnc will run on Solaris Solaris 2.5 and 2.5.1 (since a workaround for XConvertCase is provided.)

In any event, here is a workaround for Solaris 2.5.1 (and perhaps earlier and perhaps non-Solaris):

First use the environment settings (CPPFLAGS, LDFLAGS, etc.) in the above Solaris build script to run the configure command. That should succeed without failure. Then you have to hand edit the autogenerated rfb/rfbconfig.h file in the source tree, and just before the last #endif at the bottom of that file insert these workaround lines: struct timeval _tmp_usleep_tv; #define usleep(x)
_tmp_usleep_tv.tv_sec = (x) / 1000000;
_tmp_usleep_tv.tv_usec = (x) % 1000000;
select(0, NULL, NULL, NULL, &_tmp_usleep_tv); int gethostname(char *name, int namelen); long random(); int srandom(unsigned int seed); #undef LIBVNCSERVER_HAVE_LIBPTHREAD #define SHUT_RDWR 2 typedef unsigned int in_addr_t; #define snprintf(a, n, args...) sprintf((a), ## args)

Then run make with the Solaris build script environment, everything should compile without problems, and the resulting x11vnc binary should work OK. If some non-x11vnc related programs fail (e.g. test programs) and the x11vnc binary is not created try "make -k" to have it keep going. Similar sorts of kludges in rfb/rfbconfig.h can be done on other older OS (Solaris, Linux, ...) releases.

Here are some notes for similar steps that need to be done to build on SunOS 4.x

Please let us know if you had to use the above workaround (and whether it worked or not.) If there is enough demand we will try to push clean compilations back to earlier Solaris, Linux, etc, releases.

Q-5: Where can I get a precompiled x11vnc binary for my Operating System?

Hopefully the build steps above and FAQ provide enough info for a painless compile for most environments. Please report problems with the x11vnc configure, make, etc. on your system (if your system is known to compile other GNU packages successfully.)

There are precompiled x11vnc binaries built by other groups that are available at the following locations: Slackware: (.tgz) http://www.linuxpackages.net/

SuSE: (.rpm) http:/software.opensuse.org/ Gentoo: (info) http://gentoo-wiki.com/ and http://gentoo-portage.com/ FreeBSD: (.tbz) http://www.freebsd.org/ http://www.freshports.org/net/x11vnc NetBSD: (src) http://pkgsrc.se/x11/x11vnc OpenBSD: (.tgz) http://openports.se/ Arch Linux: (.tgz) http://www.archlinux.org/ Nokia 770 (.deb) http://mike.saunby.googlepages.com/x11vncfornokia7702 Sharp Zaurus http://www.focv.com/ Debian: (.deb) http://packages.debian.org/x11vnc Redhat/Fedora: (.rpm) http://packages.sw.be/x11vnc RPMforge http://dag.wieers.com/rpm/packages/x11vnc/ (N.B.: unmaintained after 0.9.3) Solaris: (pkg) http://www.sunfreeware.com/

If the above binaries don't work and building x11vnc on your OS fails (and all else fails!) you can try one of My Collection of x11vnc Binaries for various OS's and x11vnc releases.

As a general note, the x11vnc program is simple enough you don't really need to install a package: the binary will in most cases work as is and from any location (as long as your system libraries are not too old, etc.) So, for Linux distributions that are not one of the above, the x11vnc binary from the above packages has a good chance of working. You can "install" it by just copying the x11vnc binary to the desired directory in your PATH. Tip on extracting files from a Debian package: extract the archive via a command like: "ar x x11vnc_0.6-2_i386.deb" and then you can find the binary in the resulting data.tar.gz tar file. Also, rpm2cpio(1) is useful in extracting files from rpm packages.

If you use a standalone binary like this and also want x11vnc to serve up the Java VNC Viewer jar file (either SSL enabled or regular one), then you will need to extract the classes subdirectory from the source tarball and point x11vnc to it via the -httpdir option. E.g.: x11vnc -httpdir /path/to/x11vnc-0.9.9/classes/ssl ...

Alternatively, you can also go for a more up-to-date approach and use the very decent noVNC viewer (https://kanaka.github.io/noVNC/) that is purely HTML5 and does not need any plugins at all.

Q-6: Where can I get a VNC Viewer binary (or source code) for the Operating System I will be viewing from?

To obtain VNC viewers for the viewing side (Windows, Mac OS, or Unix) try here: * http://www.tightvnc.com/download.html * http://www.realvnc.com/download-free.html * http://sourceforge.net/projects/cotvnc/ * http://www.ultravnc.com/ * Our Enhanced TightVNC Viewer (SSVNC)

   [ssvnc.gif]

Q-7: How can I see all of x11vnc's command line options and documentation on how to use them?

Run: x11vnc -opts to list just the option names or run: x11vnc -help for long descriptions about each option. The output is listed here as well. Yes, x11vnc does have a lot of options, doesn't it...

Q-8: I don't like typing arcane command line options every time I start x11vnc. What can I do? Is there a config file? Or a GUI?

You could create a shell script that calls x11vnc with your options: #!/bin/sh

filename: X11vnc (i.e. not "x11vnc")

It resides in a directory in $PATH. "chmod 755 X11vnc" has been run on it.

x11vnc -wait 50 -localhost -rfbauth $HOME/.vnc/passwd -display :0 $*

a similar thing can be done via aliases in your shell (bash, tcsh, csh, etc..)

Or as of Jun/2004 you can use the simple $HOME/.x11vncrc config file support. If that file exists, each line is taken as a command line option. E.g. the above would be:

this is a comment in my ~/.x11vncrc file

wait 50 # this is a comment to the end of the line. -localhost # note: the leading "-" is optional. rfbauth /home/fred/.vnc/passwd display :0

As of Dec/2004 there is now a simple Tcl/Tk GUI based on the remote-control functionality ("-R") that was added. The /usr/bin/wish program is needed for operation. The gui is not particularly user-friendly, it just provides a point and click mode to set all the many x11vnc parameters and obtain help on them. It is also very useful for testing. See the -gui option for more info. Examples: "x11vnc ... -gui" and "x11vnc ... -gui other:0" in the latter case the gui is displayed on other:0, not the X display x11vnc is polling. There is also a "-gui tray" system tray mode.

[tkx11vnc.gif]

NOTE: You may need to install the "wish" or "tk" or "tk8.4" package for the gui mode to work (the package name depends on your OS/distro.) The tcl/tk "wish" interpreter is used. In debian (and so ubuntu too) one would run "apt-get install tk" or perhaps "apt-get install tk8.4"

Q-9: How can I get the GUI to run in the System Tray, or at least be a smaller, simpler icon?

As of Jul/2005 the gui can run in a more friendly small icon mode "-gui icon" or in the system tray: "-gui tray". It has balloon status, a simple menu, and a Properities dialog. The full, complicated, gui is only available under "Advanced". Other improvements were added as well. Try "Misc -> simple_gui" for a gui with fewer esoteric menu items.

If the gui fails to embed itself in the system tray, do a retry via "Window View -> icon" followed by "Window View -> tray" with the popup menu.

For inexperienced users starting up x11vnc and the GUI while sitting at the physical X display (not remotely), using something like "x11vnc -display :0 -gui tray=setpass" might be something for them that they are accustomed to in a Desktop environment (it prompts for an initial password, etc.) This is a basic "Share My Desktop" usage mode.

As of Nov/2008 in x11vnc 0.9.6 there is a desktop menu item (x11vnc.desktop) that runs this command: x11vnc -gui tray=setpass -rfbport PROMPT -logfile %HOME/.x11vnc.log.%VNCDISP LAY

which also prompts for which VNC port to use and a couple other parameters.

Q-10: How can I get x11vnc to listen on a different port besides the default VNC port (5900)?

Use something like, e.g., "x11vnc -rfbport 5901" to force it to use port 5901 (this is VNC display :1.) If something else is using that port x11vnc will exit immediately. If you do not supply the -rfbport option, it will autoprobe starting at 5900 and work its way up to 5999 looking for a free port to listen on. In that case, watch for the PORT=59xx line to see which port it found, then subtract 5900 from it for the VNC display number to enter into the VNC Viewer(s).

The "-N" option will try to match the VNC display number to the X display (e.g. X11 DISPLAY of :5 (port 6005) will have VNC display :5 (port 5905).)

Also see the "-autoport n" option to indicated at which value the auto probing should start at.

Q-11: My Firewall/Router doesn't allow VNC Viewers to connect to x11vnc.

See the Firewalls/Routers discussion.

Q-12: Is it possible for a VNC Viewer and a VNC Server to connect to each other even though both are behind Firewalls that block all incoming connections?

This is very difficult or impossible to do unless a third machine, reachable by both, is used as a relay. So we assume a third machine is somehow being used as a relay.

(Update: It may be possible to do "NAT-2-NAT" without a relay machine by using a UDP tunnel such as http://samy.pl/pwnat/. All that is required is that both NAT firewalls allow in UDP packets from an IP address to which a UDP packet has recently been sent to. If you try it out let us know how it went.)

In the following discussion, we will suppose port 5950 is being used on the relay machine as the VNC port for the rendezvous.

A way to rendezvous is to have the VNC Server start a reverse connection to the relay machine: x11vnc -connect third-machine.net:5950 ...

and the VNC viewer forward connects as usual: vncviewer third-machine.net:50

Or maybe two ports would be involved, e.g. the viewer goes to display :51 (5951.) It depends on the relay software being used.

What software to run on third-machine? A TCP relay of some sort could be used... Try a google search on "tcp relay" or "ip relay". However, note that this isn't a simple redirection because it hooks up two incoming connections. You can look at our UltraVNC repeater implementation ultravnc_repeater.pl for ideas and possibly to customize.

Also, if you are not the admin of third-machine you'd have to convince the owner to allow you to install this software (and he would likely need to open his server's firewall to allow the port through.)

It is recommended that SSL is used for encryption (e.g. "-ssl SAVE") when going over the internet.

We have a prototype for performing a rendezvous via a Web Server acting as the relay machine. Download the vncxfer CGI script and see the instructions at the top.

Once that CGI script is set up on the website, both users go to, say, http://somesite.com/vncxfer (or maybe a "/cgi-bin" directory or ".cgi" suffix must be used.) Previously, both have agreed on the same session name (say by phone or email) , e.g. "5cows", and put that into the entry form on the vncxfer starting page (hopefully separated by a few seconds, so the relay helper can fully start up at the first request.)

The page returned tells them the hostname and port number and possible command to use for forward (VNC Viewer) and reverse (VNC Server, i.e. x11vnc) connections as described above.

Also since Oct/2007, x11vnc can connect directly (no web browser), like this: x11vnc ... -connect localhost:0 -proxy 'http://somesite.com/vncxfer?session= 5cows&'

Unfortunately the prototype requires that the Web server's firewall allow in the port (e.g. 5950) used for the rendezvous. Most web servers are not configured to do this, so you would need to ask the admin to do this for you. Nearly all free webspace sites, e.g. www.zendurl.com, will not allow your CGI script to be an open relay like this. (If you find one that does allow this, let me know!)

Maybe someday a clever trick will be thought up to relax the listening port requirement (e.g. use HTTP/CGI itself for the transfer... it is difficult to emulate a full-duplex TCP connection with them.)

See also the Firewalls/Routers discussion and Reverse Connection Proxy discussion.

SSH method: If both users (i.e. one on Viewer-side and the other on x11vnc server side) have SSH access to a common machine on the internet (or otherwise mutually reachable), then SSH plumbing can be used to solve this problem. The users create SSH tunnels going through the SSH login machine.

Instead of assuming port 5900 is free on the SSH machine, we will assume both users agreed to use 5933. This will illustrate how to use a different port for the redir. It could be any port, what matters is that both parties refer to the same one.

Set up the Tunnel from the VNC Server side: ssh -t -R 5933:localhost:5900 user@third-machine.net

Set up the Tunnel from the VNC Viewer side: ssh -t -L 5900:localhost:5933 user@third-machine.net

Run Server on the VNC Server side: x11vnc -rfbport 5900 -localhost ...

Run Viewer on the VNC Viewer side: vncviewer -encodings "copyrect tight zrle hextile" localhost:0

(we assume the old-style -encodings option needs to be used. See here for details.)

If the SSH machine has been configured (see sshd_config(5)) with the option GatewayPorts=yes, then the tunnel set up by the VNC Server will be reachable directly by the VNC viewer (as long as the SSH machine's firewall does not block the port, 5933 in this example.) So in that case the Viewer side does not need to run any ssh command, but rather only runs: vncviewer third-machine.net:33

In this case we recommend SSL be used for encryption.

The creation of both tunnels can be automated. As of Oct/2007 the -ssh x11vnc option is available and so only this command needs to be run on the VNC Server side: x11vnc -ssh user@third-machine.net:33 ...

(the SSH passphrase may need to be supplied.)

To automate on the VNC Viewer side, the user can use the Enhanced TightVNC Viewer (SSVNC) by: * Clicking on 'Use SSH' * Entering user@third-machine.net:33 into 'VNC Host:Display' entry box * Clicking on 'Connect'

As above, if the SSH GatewayPorts=yes setting is configured the Viewer side doesn't need to create a SSH tunnel. In SSVNC the Viewer user could instead select 'Use SSL' and then, e.g., on the Server side supply "-ssl SAVE" to x11vnc. Then end-to-end SSL encryption would be used (in addition to the SSH encryption on the Server-side leg.)

Q-13: Can I make x11vnc more quiet and also go into the background after starting up?

Use the -q and -bg options, respectively. (also: -quiet is an alias for -q)

Note that under -bg the stderr messages will be lost unless you use the "-o logfile" option.

Q-14: Sometimes when a VNC viewer dies abruptly, x11vnc also dies with the error message like: "Broken pipe". I'm using the -forever mode and I want x11vnc to keep running.

As of Jan/2004 the SIGPIPE signal is ignored. So if a viewer client terminates abruptly, libvncserver will notice on the next I/O operation and will close the connection and continue on.

Up until of Apr/2004 the above fix only works for BSD signal systems (Linux, FreeBSD, ...) For SYSV systems there is a workaround in place since about Jun/2004.

Q-15: The Windows TightVNC 1.3.9 Viewer cannot connect to x11vnc.

This appears to be fixed in x11vnc version 0.9 and later. If you need to use an earlier version of x11vnc, try using the "-rfbversion 3.7" option. In general sometimes one can get a misbehaving viewer to work by supplying rfb versions 3.7 or 3.3.

Q-16: KDE's krdc VNC viewer cannot connect to x11vnc.

This has been fixed in x11vnc version 0.8.4. More info here, here, and here.

Q-17: When I start x11vnc on an Alpha Tru64 workstation the X server crashes!

This is a bug in the X server obviously; an X client should never be able to crash it.

The problem seems to be with the RECORD X extension and so a workaround is to use the "-noxrecord" x11vnc command line option.

Q-18: When running x11vnc on an IBM AIX workstation after a few minutes the VNC connection freezes.

One user reports when running x11vnc on AIX 5.3 in his CDE session after a few minutes or seconds x11vnc will "freeze" (no more updates being sent, etc.) The freezing appeared to be worse for versions later than 0.9.2.

The problem seems to be with the RECORD X extension on AIX and so a workaround is to use the "-noxrecord" x11vnc command line option. The user found no freezes occurred when using that option.

Q-19: Are there any build-time customizations possible, e.g. change defaults, create a smaller binary, etc?

There are some options. They are enabled by adding something like -Dxxxx=1 to the CPPFLAGS environment variable before running configure (see the build notes for general background.) /*

  • Mar/2006
  • Build-time customization via CPPFLAGS.
  • Summary of options to include in CPPFLAGS for custom builds:
  • -DVNCSHARED to have the vnc display shared by default.
  • -DFOREVER to have -forever on by default.
  • -DNOREPEAT=0 to have -repeat on by default.
  • -DADDKEYSYMS=0 to have -noadd_keysyms the default.
  • -DREMOTE_DEFAULT=0 to disable remote-control on by default (-yesremote.)
  • -DREMOTE_CONTROL=0 to disable remote-control mechanism completely.
  • -DEXTERNAL_COMMANDS=0 to disable the running of all external commands.
  • -DFILEXFER=0 disable filexfer.
  • -DHARDWIRE_PASSWD=... hardwired passwords, quoting necessary.
  • -DHARDWIRE_VIEWPASSWD=...
  • -DNOPW=1 make -nopw the default (skip warning)
  • -DUSEPW=1 make -usepw the default
  • -DPASSWD_REQUIRED=1 exit unless a password is supplied.
  • -DPASSWD_UNLESS_NOPW=1 exit unless a password is supplied and no -nopw.
  • -DWIREFRAME=0 to have -nowireframe as the default.
  • -DWIREFRAME_COPYRECT=0 to have -nowirecopyrect as the default.
  • -DWIREFRAME_PARMS=... set default -wirecopyrect parameters.
  • -DSCROLL_COPYRECT=0 to have -noscrollcopyrect as the default.
  • -DSCROLL_COPYRECT_PARMS=... set default -scrollcopyrect parameters.
  • -DSCALING_COPYRECT=0
  • -DXDAMAGE=0 to have -noxdamage as the default.
  • -DSKIPDUPS=0 to have -noskip_dups as the default or vice versa.
  • -DPOINTER_MODE_DEFAULT={0,1,2,3,4} set default -pointer_mode.
  • -DBOLDLY_CLOSE_DISPLAY=0 to not close X DISPLAY under -rawfb.
  • -DSMALL_FOOTPRINT=1 for smaller binary size (no help, no gui, etc)
  •                  use 2 or 3 for even smaller footprint.
    
  • -DNOGUI do not include the gui tkx11vnc.
  • -DPOLL_8TO24_DELAY=N
  • -DDEBUG_XEVENTS=1 enable printout for X events.
  • Set these in CPPFLAGS before running configure. E.g.:
  • % env CPPFLAGS="-DFOREVER -DREMOTE_CONTROL=0" ./configure
  • % make */

If other things (e.g. "-I ...") are needed in CPPFLAGS add them as well.

On some systems is seems you need to set LC_ALL=C for configure to work properly...

Be careful the following two variables: HARDWIRE_PASSWD and HARDWIRE_VIEWPASSWD. If set (remember to include the double quotes around the string), they will be used as default values for the -passwd and -viewpasswd options. Of course the strings will exist unobscured in the x11vnc binary: it better not be readable by unintendeds. Perhaps this is of use in remote access for an embedded application, etc...

Let us know if more build-time customizations would be useful.

[Win2VNC Related]

Q-20: I have two separate machine displays in front of me, one Windows the other X11: can I use x11vnc in combination with Win2VNC in dual-screen mode to pass the keystrokes and mouse motions to the X11 display?

Yes, for best response start up x11vnc with the "-nofb" option (disables framebuffer polling, and does other optimizations) on the secondary display (X11) machine. Then start up Win2VNC on the primary display (Windows) referring it to the secondary display.

This will also work X11 to X11 using x2vnc, however you would probably just want to avoid VNC and use x2x for that.

For reference, here are some links to Win2VNC-like programs for multiple monitor setups: * Original Win2VNC * Enhanced Win2VNC (broken?) and sourceforge link * x2vnc * x2x * zvnc (MorphOS)

All of them will work with x11vnc (except x2x where it is not needed.)

Q-21: I am running Win2VNC on my Windows machine and "x11vnc -nofb" on Unix to pass keyboard and mouse to the Unix monitor. Whenever I start Win2VNC it quickly disconnects and x11vnc says: rfbProcessClientNormalMessage: read: Connection reset by peer

Is the default visual of the X display you run x11vnc on low color (e.g. 8 bit per pixel PseudoColor)? (you can run xdpyinfo to check, look in the "screen" section.) There seems to be a bug in Win2VNC in that it cannot deal correctly with colormaps (PseudoColor is the most common example of a visual with a colormap.)

If so, there are a couple options. 1) Can you set the default visual on your display to be depth 24 TrueColor? Sun machines often have 8+24 overlay/multi-depth visuals, and you can make the default visual depth 24 TrueColor (see fbconfig(1) and Xsun(1).) 2) As of Feb/2004 x11vnc has the -visual option to allow you to force the framebuffer visual to whatever you want (this usually messes up the colors unless you are very clever.) In this case, the option provides a convenient workaround for the Win2VNC bug: x11vnc -nofb -visual TrueColor -display :0 ...

So the visual will be set to 8bpp TrueColor and Win2VNC can handle this. Since Win2VNC does not use the framebuffer data there should be no problems in doing this.

Q-22: Can I run "x11vnc -nofb" on a Mac OS X machine to redirect mouse and keyboard input to it from Windows and X11 machines via Win2VNC and x2vnc, respectively?

Yes, as of Nov/2006 you can. There may be a trick or two you'll need to do to get the Clipboard exchange between the machines to work.

[Color Issues]

Q-23: The X display I run x11vnc on is only 8 bits per pixel (bpp) PseudoColor (i.e. only 256 distinct colors.) The x11vnc colors may start out OK, but after a while they are incorrect in certain windows.

Use the -flashcmap option to have x11vnc watch for changes in the colormap, and propagate those changes back to connected clients. This can be slow (since the whole screen must be updated over the network whenever the colormap changes.) This flashing colormap behavior often happens if an application installs its own private colormap when the mouse is in its window. "netscape -install" is a well-known historical example of this. Consider reconfiguring the system to 16 bpp or depth 24 TrueColor if at all possible.

Also note the option -8to24 (Jan/2006) can often remove the need for flashing the colormap. Everything is dynamically transformed to depth 24 at 32 bpp using the colormaps. There may be painting errors however (see the following FAQ for tips on reducing and correcting them.)

In some rare cases (SCO unixware) the -notruecolor option has corrected colors on 8bpp displays. The red, green, and blue masks were non-zero in 8bpp PseudoColor on an obscure setup, and this option corrected the problems.

Q-24: Color problems: Why are the colors for some windows incorrect in x11vnc? BTW, my X display has nice overlay/multi-depth visuals of different color depths: e.g. there are both depth 8 and 24 visuals available at the same time.

You may want to review the previous question regarding 8 bpp PseudoColor.

On some hardware (Sun/SPARC and SGI), the -overlay option discussed a couple paragraphs down may solve this for you (you may want to skip to it directly.) On other hardware the less robust -8to24 option may help (also discussed below.)

Run xdpyinfo(1) to see what the default visual is and what the depths of the other visuals are. Does the default visual have a depth of 8 but there are other visuals of depth 24? If it does, can you possibly re-configure your X server to make a depth 24 visual the default? If you can do it, this will save you a lot of grief WRT colors and x11vnc (and for general usage too!) Here is how I do this on an old Sparcstation 20 running Solaris 9 with SX graphics xinit -- -dev /dev/fb defclass TrueColor defdepth 24

and it works nicely (note: to log into console from the dtlogin window, select "Options -> Command Line Login", then login and enter the above command.) See the -dev section of the Xsun(1) manpage for a description of the above arguments. If you have root permission, a more permanent and convenient thing to do is to record the arguments in a line like: :0 Local local_uid@console root /usr/openwin/bin/Xsun -dev /dev/fb defclass TrueColor defdepth 24

in /etc/dt/config/Xservers (copy /usr/dt/config/Xservers.) Also look at the fbconfig(1) and related manpages (e.g. ffbconfig, m64config, pgxconfig, SUNWjfb_config, etc ...) for hardware framebuffer settings that may achieve the same effect.

In general for non-Sun machines, look at the "-cc class" and related options in your X server manpage (perhaps Xserver(1)), it may allow modifying the default visual (e.g. "-cc 4", see <X11/X.h> for the visual class numbers.) On XFree86 some video card drivers (e.g. Matrox mga) have settings like Option "Overlay" "24,8" to support multi-depth overlays. For these, use the "-cc 4" X server command line option to get a depth 24 default visual.

The -overlay mode: Another option is if the system with overlay visuals is a Sun system running Solaris or SGI running IRIX you can use the -overlay x11vnc option (Aug/2004) to have x11vnc use the Solaris XReadScreen(3X11) function to poll the "true view" of the whole screen at depth 24 TrueColor. XReadDisplay(3X11) is used on IRIX. This is useful for Legacy applications (older versions of Cadence CAD apps are mentioned by x11vnc users) that require the default depth be 8bpp, or the app will use a 8bpp visual even if depth 24 visuals are available, and so the default depth workaround described in the previous paragraph is not sufficient for these apps.

It seems that Xorg is working toward supporting XReadDisplay(3X11) as part of the RENDER extension work. When it does support it and provides a library API x11vnc will be modified to take advantage of the feature to support -overlay on Linux, *BSD, etc. Until then see the -8to24 mode below.

Misc. notes on -overlay mode: An amusing by-product of -overlay mode is that the mouse cursor shape is correct! (i.e. XFIXES is not needed.) The -overlay mode may be somewhat slower than normal mode due to the extra framebuffer manipulations that must be performed. Also, on Solaris there is a bug in that for some popup menus, the windows they overlap will have painting errors (flashing colors) while the popup is up (a workaround is to disable SaveUnders by passing -su to Xsun, e.g. in your /etc/dt/config/Xservers file.)

The -8to24 mode: The -8to24 x11vnc option (Jan/2006) is a kludge to try to dynamically rewrite the pixel values so that the 8bpp part of the screen is mapped onto depth 24 TrueColor. This is less robust than the -overlay mode because it is done by x11vnc outside of the X server. So only use it on OS's that do not support -overlay. The -8to24 mode will work if the default visual is depth 24 or depth 8. It scans for any windows within 3 levels of the root window that are 8bpp (i.e. legacy application), or in general ones that are not using the default visual. For the windows it finds it uses XGetSubImage() to retrieve the pixels values and uses the correct indexed colormap to create a depth 24 TrueColor view of the whole screen. This depth 24, 32bpp view is exported via VNC.

Even on pure 8bpp displays it can be used as an alternative to -flashcmap to avoid color flashing completely.

This scheme is approximate and can often lead to painting errors. You can manually correct most painting errors by pressing 3 Alt_L's in a row, or by using something like: -fixscreen V=3.0 to automatically refresh the screen every 3 seconds. Also -fixscreen 8=3.0 has been added to just refresh the non-default visual parts of the screen.

In general the scheme uses many resources and may give rise to sluggish behavior. If multiple windows are using different 8bpp indexed colormaps all but one window may need to be iconified for the colors to be correct. There are a number of tunable parameters to try to adjust performance and painting accuracy. The option -8to24 nogetimage can give a nice speedup if the default depth 24 X server supports hiding the 8bpp bits in bits 25-32 of the framebuffer data. On very slow machines -8to24 poll=0.2,cachewin=5.0 gives an useful speedup. See the -8to24 help description for information on tunable parameters, etc.

Colors still not working correctly? Run xwininfo on the application with the incorrect colors to verify that the depth of its visual is different from the default visual depth (gotten from xdpyinfo.) One possible workaround in this case is to use the -id option to point x11vnc at the application window itself. If the application is complicated (lots of toplevel windows and popup menus) this may not be acceptable, and may even crash x11vnc (but not the application.) See also -appshare.

It is theoretically possible to solve this problem in general (see xwd(1) for example), but it does not seem trivial or sufficiently fast for x11vnc to be able to do so in real time. The -8to24 method does this approximately and is somewhat usable. Fortunately the -overlay option works for Solaris machines with overlay visuals where most of this problem occurs.

Q-25: I am on a high color system (depth >= 24) but I seem to have colormap problems. They either flash or everything is very dark.

This can happen if the default Visual (use xdpyinfo to list them) is DirectColor instead of TrueColor. These are both usually used in high color modes, but whereas TrueColor uses static ramps for the Red, Green, and Blue components, DirectColor has arbitrary colormaps for the Red, Green, and Blue Components. Currently x11vnc cannot decode these colormaps and treats them just like TrueColor.

The only workaround so far is to restart the X server with the "-cc 4" option to force TrueColor as the default visual (DirectColor is "-cc 5"; see /usr/include/X11/X.h.) The only place we have seen this is with the virtual framebuffer server Xvfb on Xorg 7.2. So in that case you probably should restart it with something like this: "Xvfb :1 -cc 4 -screen 0 1280x1024x24". It should be possible for x11vnc to handle DirectColor, but this hasn't been implemented due to its rare usage.

You may also see this problem on an X display with a TrueColor default visual where an application chooses a DirectColor visual for its window(s). It seems the application also needs to install its own colormap for the visual for the colors to be messed up in x11vnc. One can make xwud do this for example.

Q-26: How do I figure out the window id to supply to the -id windowid option?

Run the xwininfo program in a terminal. It will ask you to click on the desired application window. After clicking, it will print out much information, including the window id (e.g. 0x6000010.) Also, the visual and depth of the window printed out is often useful in debugging x11vnc color problems.

Also, as of Dec/2004 you can use "-id pick" to have x11vnc run xwininfo(1) for you and after you click the window it extracts the windowid. Besides "pick" there is also "id:root" to allow you to go back to root window when doing remote-control.

Q-27: Why don't menus or other transient windows come up when I am using the -id windowid option to view a single application window?

This is related to the behavior of the XGetImage(3X11) and XShmGetImage() interfaces regarding backingstore, saveunders, etc. The way the image is retrieved depends on some aspects of how the X server maintains the display image data and whether other windows are clipping or obscuring it. See the XGetImage(3X11) man page for more details. If you disable BackingStore and SaveUnders in the X server you should be able to see these transient windows.

If things are not working and you still want to do the single window polling, try the -sid windowid option ("shifted" windowid.)

Update: as of Nov/2009 in the 0.9.9 x11vnc development tarball, there is an experimental Application Sharing mode that improves upon the -id/-sid single window sharing: -appshare (run "x11vnc -appshare -help" for more info.) It is still very primitive and approximate, but at least it displays multiple top-level windows.

Q-28: My X display is depth 24 at 24bpp (instead of the normal depth 24 at 32bpp.) I'm having lots of color and visual problems with x11vnc and/or vncviewer. What's up?

First off, depth 24 at 24bpp (bpp=bits-per-pixel) is fairly uncommon and can cause problems in general. It also can be slower than depth 24 at 32bpp. You might want to switch to 32bpp (for XFree86 see the "-fbbpp 32", DefaultFbBpp, FbBpp and related options.) Perhaps you have 24bpp because the video memory of the machine is low and the screen wouldn't fit in video RAM at 32bpp. For this case depth 16 at 16bpp might be an acceptable option.

In any event x11vnc should handle depth 24 at 24bpp (although performance may be slower, and you may need to use the ZRLE encoding instead of Tight.) There are some caveats involving the viewer however:

The RealVNC Unix viewer cannot handle 24bpp from the server, it will say: "main: setPF: not 8, 16 or 32 bpp?" and exit. I have not checked the RealVNC Windows viewer.

So you need to use the TightVNC Unix viewer. However there are some problems with that too. It seems libvncserver does not do 24bpp correctly with the Tight encoding. The colors and screen ultimately get messed up. So you have to use a different encoding with the TightVNC vncviewer, try "zlib", "hextile", or one of the other encodings (e.g. vncviewer -encodings "zlib hextile" ....) I have not checked the TightVNC or UltraVNC Windows viewers.

It appears the older RealVNC Unix viewers (e.g. 3.3.3 and 3.3.7) can handle 24bpp from the server, so you may want to use those. They evidently request 32 bpp and libvncserver obliges.

Update: as of Apr/2006 you can use the -24to32 option to have x11vnc dynamically transform the 24bpp pixel data to 32bpp. This extra transformation could slow things down further however.

Now coming the opposite direction if you are running the vncviewer on the 24bpp display, TightVNC will fail with "Can't cope with 24 bits-per-pixel. Sorry." and RealVNC will fail with "main: Error: couldn't find suitable pixmap format" so evidently you cannot use 24bpp for the vncviewers to work on that X display.

Note, however, that the Unix viewer in the Enhanced TightVNC Viewer (SSVNC) project can handle 24bpp X displays. It does this by requesting a 16bpp pixel format (or 8bpp if the -bgr233 option has been supplied) from the VNC server, and translates that to 24bpp locally. [Xterminals]

Q-29: Can I use x11vnc to view and interact with an Xterminal (e.g. NCD) that is not running UNIX and so x11vnc cannot be run on it directly?

You can, but it will likely be very wasteful of network bandwidth since you will be polling the X display over the network as opposed to over the local hardware. To do this, run x11vnc on a UNIX machine as close as possible network-wise (e.g. same switch) to the Xterminal machine. Use the -display option to point the display to that of the Xterminal (you'll of course need basic X11 permission to do that) and finally supply the -noshm option (this enables the polling over the network.)

If the Xterminal's X display is open to the network for connections, you might use something like "-display xterm123:0". If you are trying to do this via an SSH tunnel (assuming you can actually ssh into the Xterminal) it will be a little tricky (either use the ssh "-R" option or consider ssh-ing in the other direction.) In all cases the X11 permissions need to allow the connection.

The response will likely be sluggish (maybe only one "frame" per second.) This mode is not recommended except for "quick checks" of hard to get to X servers. Use something like "-wait 150" to cut down on the polling rate. You may also need -flipbyteorder if the colors get messed up due to endian byte order differences.

Q-30: How do I get my X permissions (MIT-MAGIC-COOKIE file) correct for a Unix/Linux machine acting as an Xterminal?

If the X display machine is a traditional Xterminal (where the X server process runs on the Xterminal box, but all of the X client applications (firefox, etc) run on a central server (aka "terminal server")), you will need to log into the Xterminal machine (i.e. get a shell running there) and then start the x11vnc program. If the Xterminal Linux/Unix machine is stripped down (e.g. no users besides root) that may be difficult.

The next problem is the login Display Manager (e.g. gdm, kdm), and hence the MIT-MAGIC-COOKIE auth files, are on the central server and not on the Xterminal box where the X server and x11vnc processes are.

So unless X permissions are completely turned off (e.g. "xhost +"), to run the x11vnc process on the Xterminal box the MIT-MAGIC-COOKIE auth file data (XAUTHORITY or $HOME/.Xauthority) must be accessible by or copied to the Xterminal. If $HOME/.Xauthority is exported via NFS (this is insecure of course, but has been going on for decades), then x11vnc can simply pick it up via NFS (you may need to use the -auth option to point to the correct file.) Other options include copying the auth file using scp, or something like: central-server> xauth nextract - xterm123:0 | ssh xterm123 xauth nmerge -

and then, say, ssh from central-server to xterm123 to start x11vnc. Here "xterm123" refers to the computer acting as the Xterminal and "central-server" is the terminal server. You can use "xauth -f /path/to/cookie-file list" to examine the contents of the cookie(s) in a file "/path/to/cookie-file". See the xauth(1) manpage for more details.

If the display name in the cookie file needs to be changed between the two hosts, see this note on the "xauth add ..." command.

A less secure option is to run something like "xhost +127.0.0.1" while sitting at the Xterminal box to allow cookie-free local access for x11vnc. You can run "xhost -127.0.0.1" after x11vnc connects if you want to go back to the original permissions.

If the Xterminal is really stripped down and doesn't have any user accounts, NFS, etc. you'll need to contact your system administrator to set something up. It can be done!!! Some Xterminal projects have actually enabled "run locally" facilities for the running of an occasional app more efficiently locally on the Xterminal box (e.g. realplayer.)

Not recommended, but as a last resort, you could have x11vnc poll the Xterminal Display over the network. For this you would run a "x11vnc -noshm ..." process on the central-server (and hope the network admin doesn't get angry...)

Note: use of Display Manager (gdm, kdm, ...) auth cookie files (i.e. from /var/..., /tmp/..., or elsewhere) may require modification via xauth(1) to correctly include the display x11vnc refers to (e.g. "xauth -f cookie-file add :0 . 45be51ae2ce9dfbacd882ab3ef8e96b1", where the "45be51..." cookie value was found from an "xauth -f /path/to/original/cookie-file list") or other reasons. See xauth(1) manpage for full details on how to transfer an MIT-MAGIC-COOKIE between machines and displays.

VNCviewer performance on Xterminals: This isn't related to x11vnc on Xterminals, but we mention it here anyway because of the similar issues. If you are on an Xterminal and want to use vncviewer to connect to a VNC server somewhere, then performance would be best if you ran the viewer on the Xterminal box. Otherwise, (i.e. running the viewer process on the central-server) all of the vncviewer screen drawing is done more inefficiently over the network. Something to consider, especially on a busy network. (BTW, this has all of the above permission, etc, problems: both vncviewer and x11vnc are X client apps desired to be run on the Xterminal box.)

[Sun Rays]

Q-31: I'm having trouble using x11vnc with my Sun Ray session.

The Sun Ray technology is a bit like "VNC done in hardware" (the Sun Ray terminal device, DTU, playing the role of the vncviewer.) Completely independent of that, the SunRay user's session is still an X server that speaks the X11 protocol and so x11vnc simply talks to the X server part to export the SunRay desktop to any place in the world (i.e. not only to a Sun Ray terminal device), creating a sort of "Soft Ray". Please see this discussion of Sun Ray issues for solutions to problems.

Also see the Sun Ray Remote Control Toolkit that uses x11vnc.

[Remote Control]

Q-32: How do I stop x11vnc once it is running in the background?

As of Dec/2004 there is a remote control feature. It can change a huge number of parameters on the fly: see the -remote and -query options. To shut down the running x11vnc server just type "x11vnc -R stop". To disconnect all clients do "x11vnc -R disconnect:all", etc.

If the -forever option has not been supplied, x11vnc will automatically exit after the first client disconnects. In general if you cannot use the remote control, then you will have to kill the x11vnc process This can be done via: "kill NNNNN" (where NNNNN is the x11vnc process id number found from ps(1)), or "pkill x11vnc", or "killall x11vnc" (Linux only.)

If you have not put x11vnc in the background via the -bg option or shell & operator, then simply press Ctrl-C in the shell where x11vnc is running to stop it.

Potential Gotcha: If somehow your Keypress of Ctrl-C went through x11vnc to the Xserver that then delivered it to x11vnc it is possible one or both of the Ctrl or C keys will be left stuck in the pressed down state in the Xserver. Tapping the stuck key (either via a new x11vnc or at the physical console) will release it from the stuck state. If the keyboard seems to be acting strangely it is often fixed by tapping Ctrl, Shift, and Alt. Alternatively, the -clear_mods option and -clear_keys option can be used to release pressed keys at startup and exit. The option -clear_all will also try to unset Caps_Lock, Num_Lock, etc.

Q-33: Can I change settings in x11vnc without having to restart it? Can I remote control it?

Look at the -remote (an alias is -R) and -query (an alias is -Q) options added in Dec/2004. They allow nearly everything to be changed dynamically and settings to be queried. Examples: "x11vnc -R shared", "x11vnc -R forever", "x11vnc -R scale:3/4", "x11vnc -Q modtweak", "x11vnc -R stop", "x11vnc -R disconnect:all", etc..

These commands do not start a x11vnc server, but rather communicate with one that is already running. The X display (X11VNC_REMOTE property) is used as the communication channel, so the X permissions and DISPLAY must be set up correctly for communication to be possible.

There is also a simple Tcl/Tk gui based on this remote control mechanism. See the -gui option for more info. You will need to have Tcl/Tk (i.e. /usr/bin/wish) installed for it to work. It can also run in the system tray: "-gui tray" or as a standalone small icon window: "-gui icon". Use "-gui tray=setpass" for a naive user "Share My Desktop" mode.

[Security and Permissions]

Q-34: How do I create a VNC password for use with x11vnc?

You may already have one in $HOME/.vnc/passwd if you have used, say, the vncserver program from the regular RealVNC or TightVNC packages (i.e. launching the Xvnc server.) Otherwise, you could use the vncpasswd(1) program from those packages.

As of Jun/2004 x11vnc supports the -storepasswd "pass" "file" option, which is the same functionality of storepasswd. Be sure to quote the "pass" if it contains shell meta characters, spaces, etc. Example: x11vnc -storepasswd 'sword*fish' $HOME/myvncpasswd

You then use the password via the x11vnc option: "-rfbauth $HOME/myvncpasswd"

As of Jan/2006 if you do not supply any arguments: x11vnc -storepasswd

you will be prompted for a password to save to ~/.vnc/passwd (your keystrokes when entering the password will not be echoed to the screen.) If you supply one argument, e.g. "x11vnc -storepasswd ~/.mypass", the password you are prompted for will be stored in that file.

x11vnc also has the -passwdfile and -passwd/-viewpasswd plain text (i.e. not obscured like the -rfbauth VNC passwords) password options.

You can use the -usepw option to automatically use any password file you have in ~/.vnc/passwd or ~/.vnc/passwdfile (the latter is used with the -passwdfile option.)

x11vnc -usepw -display :0 ...

If neither file exists you are prompted to store a password in ~/.vnc/passwd. If a password file cannot be found or created x11vnc exits immediately. An admin may want to set it up this way for users who do not know better.

Q-35: Can I make it so -storepasswd doesn't show my password on the screen?

You can use the vncpasswd program from RealVNC or TightVNC mentioned above. As of Jan/2006 the -storepasswd option without any arguments will not echo your password as you type it and save the file to ~/.vnc/passwd:

x11vnc -storepasswd

Enter VNC password: Verify password: Write password to /home/myname/.vnc/passwd? [y]/n Password written to: /home/myname/.vnc/passwd

You can also give it an alternate filename, e.g. "x11vnc -storepasswd ~/.mypass"

Q-36: Can I have two passwords for VNC viewers, one for full access and the other for view-only access to the display?

Yes, as of May/2004 there is the -viewpasswd option to supply the view-only password. Note the full-access password option -passwd must be supplied at the same time. E.g.: -passwd sword -viewpasswd fish.

To avoid specifying the passwords on the command line (where they could be observed via the ps(1) command by any user) you can use the -passwdfile option to specify a file containing plain text passwords. Presumably this file is readable only by you, and ideally it is located on the machine x11vnc is run on (to avoid being snooped on over the network.) The first line of this file is the full-access password. If there is a second line in the file and it is non-blank, it is taken as the view-only password. (use "EMPTY" to supply an empty one.)

View-only passwords currently do not work for the -rfbauth password option (standard VNC password storing mechanism.) FWIW, note that although the output (usually placed in $HOME/.vnc/passwd) by the vncpasswd or storepasswd programs (or from x11vnc -storepasswd) looks encrypted they are really just obscured to avoid "casual" password stealing. It takes almost no skill to figure out how to extract the plain text passwords from $HOME/.vnc/passwd since it is very straight-forward to work out what to do from the VNC source code.

Q-37: Can I have as many full-access and view-only passwords as I like?

Yes, as of Jan/2006 in the libvncserver CVS the -passwdfile option has been extended to handle as many passwords as you like. You put the view-only passwords after a line BEGIN_VIEWONLY.

You can also easily annotate and comment out passwords in the file. You can have x11vnc re-read the file dynamically when it is modified.

Q-38: Does x11vnc support Unix usernames and passwords? Can I further limit the set of Unix usernames who can connect to the VNC desktop? Update: as of Feb/2006 x11vnc has the -unixpw option that does this outside of the VNC protocol and libvncserver. The standard su(1) program is used to validate the user's password. A familiar "login:" and "Password:" dialog is presented to the user on a black screen inside the vncviewer. The connection is dropped if the user fails to supply the correct password in 3 tries or does not send one before a 25 second timeout. Existing clients are view-only during this period. A list of allowed Unix usernames may also be supplied along with per-user settings.

There is also the -unixpw_nis option for non-shadow-password (typically NIS environments, hence the name) systems where the traditional getpwnam() and crypt() functions are used instead of su(1). The encrypted user passwords must be accessible to the user running x11vnc in -unixpw_nis mode, otherwise the logins will always fail even when the correct password is supplied. See ypcat(1) and shadow(5).

Two settings are enforced in the -unixpw and -unixpw_nis modes to provide extra security: the 1) -localhost and 2) -stunnel or -ssl options. Without these one might send the Unix username and password data in clear text over the network which is a very bad idea. They can be relaxed if you want to provide encryption other than stunnel or -ssl (the constraint is automatically relaxed if SSH_CONNECTION is set and indicates you have ssh-ed in, however the -localhost requirement is still enforced.)

The two -unixpw modes have been tested on Linux, Solaris, Mac OS X, HP-UX, AIX, Tru64, FreeBSD, OpenBSD, and NetBSD. Additional testing is appreciated. For the last 4 it appears that su(1) will not prompt for a password if su-ing to oneself. Since x11vnc requires a password prompt from su, x11vnc forces those logins to fail even when the correct password is supplied. On *BSD it appears this can be corrected by removing the pam_self.so entry in /etc/pam.d/su.

Previous older discussion (prior to the -unixpw option):

Until the VNC protocol and libvncserver support this things will be approximate at best.

One approximate method involves starting x11vnc with the -localhost option. This basically requires the viewer user to log into the workstation where x11vnc is running via their Unix username and password, and then somehow set up a port redirection of his vncviewer connection to make it appear to emanate from the local machine. As discussed above, ssh is useful for this: "ssh -L 5900:localhost:5900 user@hostname ..." See the ssh wrapper scripts mentioned elsewhere on this page. stunnel does this as well.

Of course a malicious user could allow other users to get in through his channel, but that is a problem with every method. Another thing to watch out for is a malicious user on the viewer side (where ssh is running) trying to sneak in through the ssh port redirection there.

Regarding limiting the set of Unix usernames who can connect, the traditional way would be to further require a VNC password to supplied (-rfbauth, -passwd, etc) and only tell the people allowed in what the VNC password is. A scheme that avoids a second password involves using the -accept option that runs a program to examine the connection information to determine which user is connecting from the local machine. That may be difficult to do, but, for example, the program could use the ident service on the local machine (normally ident should not be trusted over the network, but on the local machine it should be accurate: otherwise root has been compromised and so there are more serious problems! Unfortunately recent Linux distros seem to provide a random string (MD5 hash?) instead of the username.) An example script passed in via -accept scriptname that deduces the Unix username and limits who can be accepted might look something like this: #!/bin/sh if [ "$RFB_CLIENT_IP" != "127.0.0.1" -o "$RFB_SERVER_IP" != "127.0.0.1" ]; then exit 1 # something fishy... reject it. fi user=echo "$RFB_CLIENT_PORT, $RFB_SERVER_PORT" | nc -w 1 $RFB_CLIENT_IP 113 \ | grep 'USERID.*UNIX' | head -n 1 | sed -e 's/[\r ]//g' | awk -F: '{pri nt $4}'

for okuser in fred barney wilma betty do if [ "X$user" = "X$okuser" ]; then exit 0 # accept it fi done exit 1 # reject it

For this to work with ssh port redirection, the ssh option UsePrivilegeSeparation must be enabled otherwise the userid will always be "root".

Here is a similar example based on Linux netstat(1) output: #!/bin/sh

accept_local_netstat: x11vnc -accept command to accept a local

vncviewer connection from acceptable users. Linux netstat -nte is used.

PATH=/bin:/usr/bin:$PATH; export PATH; # set to get system utils

allowed="id -u fred"; # add more user numbers if desired.

check required settings

ok=1 if [ "X$allowed" = "X" ]; then ok=0; # something wrong with allowed list fi if [ "X$RFB_CLIENT_IP" != "X127.0.0.1" -o "X$RFB_SERVER_IP" != "X127.0.0.1" ]; then ok=0; # connection not over localhost fi if [ "$RFB_CLIENT_PORT" -le 0 -o "$RFB_SERVER_PORT" -le 0 ]; then ok=0; # something wrong with tcp port numbers fi if [ "$ok" = 0 ]; then echo "$0: invalid setting:" 1>&2 env | grep ^RFB | sort 1>&2 exit 1 fi

Linux netstat -nte:

Proto Recv-Q Send-Q Local Address Foreign Address State

User Inode

0 0 0 RFB_CLIENT RFB_SERVER ESTABLISHED

nnnn ....

user=netstat -nte | grep ESTABLISHED \ | grep " $RFB_CLIENT_IP:$RFB_CLIENT_PORT *$RFB_SERVER_IP:$RFB_SERVER_P ORT "

echo "netstat match: $user" 1>&2 user=echo "$user" | head -n 1 | sed -e 's/^.*ESTABLISHED/ /' | awk '{print $1} '

ok=0 for u in $allowed do if [ "X$user" = "X$u" ]; then ok=1 break fi done

if [ "X$ok" = "X1" ]; then echo "$0: user accepted: '$user'" 1>&2 exit 0 else echo "$0: user '$user' invalid:" 1>&2 echo "$0: allowed: $allowed" 1>&2 env | grep ^RFB | sort 1>&2 exit 1 fi

Q-39: Can I supply an external program to provide my own custom login method (e.g. Dynamic/One-time passwords or non-Unix (LDAP) usernames and passwords)? Yes, there are several possibilities. For background see the FAQ on the -accept where an external program may be run to decide if a VNC client should be allowed to try to connect and log in. If the program (or local user prompted by a popup) answers "yes", then -accept proceeds to the normal VNC and x11vnc authentication methods, otherwise the connection is dropped.

To provide more direct coupling to the VNC client's username and/or supplied password the following options were added in Sep/2006: * -unixpw_cmd command * -passwdfile cmd:command * -passwdfile custom:command

In each case "command" is an external command run by x11vnc. You supply it. For example, it may couple to your LDAP system or other servers you set up.

For -unixpw_cmd the normal -unixpw Login: and Password: prompts are supplied to the VNC viewer and the strings the client returns are then piped into "command" as the first two lines of its standard input. If the command returns success, i.e. exit(0), the VNC client is accepted, otherwise it is rejected.

For "-passwdfile cmd:command" the command is run and it returns a password list (like a password file, see the -passwdfile read:filename mode.) Perhaps a dynamic, one-time password is retrieved from a server this way.

For "-passwdfile custom:command" one gets complete control over the VNC challenge-response dialog with the VNC client. x11vnc sends out a string of random bytes (16 by the VNC spec) and the client returns the same number of bytes in a way the server can verify only the authorized user could have created. The VNC protocol specifies DES encryption with a password. If you are willing to modify the VNC viewers, you can have it be anything you want, perhaps a less crackable MD5 hash scheme or one-time pad. Your program will read from its standard input the size of the challenge-response followed by a newline, then the challenge bytes followed by the response bytes. If your command then returns success, i.e. exit(0), the VNC client is accepted, otherwise it is rejected.

In all cases the "RFB_*" environment variables are set as under -accept. These variables can provide useful information for the externally supplied program to use.

Q-40: Why does x11vnc exit as soon as the VNC viewer disconnects? And why doesn't it allow more than one VNC viewer to connect at the same time?

These defaults are simple safety measures to avoid someone unknowingly leaving his X11 desktop exposed (to the internet, say) for long periods of time. Use the -forever option (aka -many) to have x11vnc wait for more connections after the first client disconnects. Use the -shared option to have x11vnc allow multiple clients to connect simultaneously.

Recommended additional safety measures include using ssh (see above), stunnel, -ssl, or a VPN to authenticate and encrypt the viewer connections or to at least use the -rfbauth passwd-file option to use VNC password protection (or -passwdfile) It is up to YOU to apply these security measures, they will not be done for you automatically.

Q-41: Can I limit which machines incoming VNC clients can connect from?

Yes, look at the -allow and -localhost options to limit connections by hostname or IP address. E.g. x11vnc -allow 192.168.0.1,192.168.0.2

for those two hosts or x11vnc -allow 192.168.0.

for a subnet. For individual hosts you can use the hostname instead of the IP number, e.g.: "-allow snoopy", and "-allow darkstar,wombat". Note that -localhost achieves the same thing as "-allow 127.0.0.1"

For more control, build libvncserver with libwrap support (tcp_wrappers) and then use /etc/hosts.allow See hosts_access(5) for complete details.

Q-42: How do I build x11vnc/libvncserver with libwrap (tcp_wrappers) support?

Here is one way to pass this information to the configure script: env CPPFLAGS=-DUSE_LIBWRAP LDFLAGS=-lwrap ./configure

then run make as usual. This requires libwrap and its development package (tcpd.h) to be installed on the build machine. If additional CPPFLAGS or LDFLAGS options are needed supply them as well using quotes.

The resulting x11vnc then uses libwrap/tcp_wrappers for connections. The service name you will use in /etc/hosts.allow and /etc/hosts.deny is "vnc", e.g.: vnc: 192.168.100.3 .example.com

Note that if you run x11vnc out of inetd you do not need to build x11vnc with libwrap support because the /usr/sbin/tcpd reference in /etc/inetd.conf handles the tcp_wrappers stuff.

Q-43: Can I have x11vnc only listen on one network interface (e.g. internal LAN) rather than having it listen on all network interfaces and relying on -allow to filter unwanted connections out?

As of Mar/2005 there is the "-listen ipaddr" option that enables this. For ipaddr either supply the desired network interface's IP address (or use a hostname that resolves to it) or use the string "localhost". For additional filtering simultaneously use the "-allow host1,..." option to allow only specific hosts in.

This option is useful if you want to insure that no one can even begin a dialog with x11vnc from untrusted network interfaces (e.g. ppp0.) The option -localhost now implies "-listen localhost" since that is what most people expect it to do.

Q-44: Now that -localhost implies listening only on the loopback interface, how I can occasionally allow in a non-localhost via the -R allowonce remote control command?

To do this specify "-allow localhost". Unlike -localhost this will leave x11vnc listening on all interfaces (but of course only allowing in local connections, e.g. ssh redirs.) Then you can later run "x11vnc -R allowonce:somehost" or use to gui to permit a one-shot connection from a remote host.

Q-45: Can I fine tune what types of user input are allowed? E.g. have some users just be able to move the mouse, but not click or type anything?

As of Feb/2005, the -input option allows you to do this. "K", "M", "B", "C", and "F" stand for Keystroke, Mouse-motion, Button-clicks, Clipboard, and File-Transfer, respectively. The setting: "-input M" makes attached viewers only able to move the mouse. "-input KMBC,M" lets normal clients do everything and enables view-only clients to move the mouse.

These settings can also be applied on a per-viewer basis via the remote control mechanism or the GUI. E.g. x11vnc -R input:hostname:M

Q-46: Can I prompt the user at the local X display whether the incoming VNC client should be accepted or not? Can I decide to make some clients view-only? How about running an arbitrary program to make the decisions?

Yes, look at the "-accept command" option, it allows you to specify an external command that is run for each new client. (use quotes around the command if it contains spaces, etc.) If the external command returns 0 (success) the client is accepted, otherwise with any other return code the client is rejected. See below how to also accept clients view-only.

The external command will have the RFB_CLIENT_IP environment variable set to the client's numerical IP address, RFB_CLIENT_PORT its port number. Similarly for RFB_SERVER_IP and RFB_SERVER_PORT to allow identification of the tcp virtual circuit. DISPLAY will be set to that of the X11 display being polled. Also, RFB_X11VNC_PID is set to the x11vnc process id (e.g. in case you decided to kill it), RFB_CLIENT_ID will be an id number, and RFB_CLIENT_COUNT the number of other clients currently connected. RFB_MODE will be "accept".

Built-in Popup Window: As a special case, "-accept popup" will instruct x11vnc to create its own simple popup window. To accept the client press "y" or click mouse on the "Yes" button. To reject the client press "n" or click mouse on the "No" button. To accept the client View-only, press "v" or click mouse on the "View" button. If the -viewonly option has been supplied, the "View" action will not be present: the whole display is view only in that case.

The popup window times out after 120 seconds, to change this behavior use "-accept popup:N" where N is the number of seconds (use 0 for no timeout.) More tricks: "-accept popupmouse" will only take mouse click responses, while "-accept popupkey" will only take keystroke responses (popup takes both.) After any of the 3 popup keywords you can supply a position of the window: +N+M, (the default is to center the window) e.g. -accept popupmouse+10+10.

Also as a special case "-accept xmessage" will run the xmessage(1) program to prompt the user whether the client should be accepted or not. This requires that you have xmessage installed and available via PATH. In case it is not already on your system, the xmessage program is available at ftp://ftp.x.org/ (End of Built-in Popup Window:)

To include view-only decisions for the external commands, prefix the command something like this: "yes:0,no:,view:3 mycommand ..." This associates the three actions: yes(accept), no(reject), and view(accept-view-only), with the numerical return (i.e. exit()) codes. Use "" instead of a number to set the default action (e.g. in case the external command returns an unexpected return code.)

Here is an example -accept script called accept_or_lock. It uses xmessage and xlock (replace with your screen lock command, maybe it is "xscreensaver-command -lock", or kdesktop_lock, or "dtaction LockDisplay".) It will prompt the user at the X display whether to accept, reject, or accept view-only the client, but if the prompt times out after 60 seconds the screen is locked and the VNC client is accepted. This allows the remote access when no one is at the display. #!/bin/sh

accept_or_lock: prompt user at X display whether to accept an incoming

VNC connection. If timeout expires, screen is locked

and the VNC viewer is accepted (allows remote access

when no one is sitting at the display.)

usage: x11vnc ... -forever -accept 'yes:0,no:*,view:4 accept_or_lock'

xmessage -buttons yes:2,no:3,view-only:4 -center
-timeout 60 "x11vnc: accept connection from $RFB_CLIENT_IP?" rc=$? if [ $rc = 0 ]; then xlock & # or "xlock -mode blank" for no animations. sleep 5 exit 0 elif [ $rc = 2 ]; then exit 0 elif [ $rc = 4 ]; then exit 4 fi exit 1

Stefan Radman has written a nice dtksh script dtVncPopup for use in CDE environments to do the same sort of thing. Information on how to use it is found at the top of the file. He encourages you to provide feedback to him to help improve the script.

Note that in all cases x11vnc will block while the external command or popup is being run, so attached clients will not receive screen updates, etc during this period.

To run a command when a client disconnects, use the "-gone command" option. This is for the user's convenience only: the return code of the command is not interpreted by x11vnc. The same environment variables are set as in "-accept command" (except that RFB_MODE will be "gone".)

As of Jan/2006 the "-afteraccept command" option will run the command only after the VNC client has been accepted and authenticated. Like -gone the return code is not interpreted. RFB_MODE will be "afteraccept".)

Q-47: I start x11vnc as root because it is launched via inetd(8) or a display manager like gdm(1). Can I have x11vnc later switch to a different user?

As of Feb/2005 x11vnc has the -users option that allows things like this. Please read the documentation on it (also in the x11vnc -help output) carefully for features and caveats. It's use can often decrease security unless care is taken.

BTW, a nice use of it is "-users +nobody" that switches to the Unix user nobody right after connections to the X display are established.

In any event, while running x11vnc as root, remember it comes with no warranty ;-).

Q-48: I use a screen-lock when I leave my workstation (e.g. xscreensaver or xlock.) When I remotely access my workstation desktop via x11vnc I can unlock the desktop fine, but I am worried people will see my activities on the physical monitor. What can I do to prevent this, or at least make it more difficult?

Probably most work environments would respect your privacy if you powered off the monitor. Also remember if people have physical access to your workstation they basically can do anything they want with it (e.g. install a backdoor for later use, etc.)

In any event, as of Jun/2004 there is an experimental utility to make it more difficult for nosey people to see your x11vnc activities. The source for it is blockdpy.c The idea behind it is simple (but obviously not bulletproof): when a VNC client attaches to x11vnc put the display monitor in the DPMS "off" state, if the DPMS state ever changes immediately start up the screen-lock program. The x11vnc user will notice something is happening and think about what to do next (while the screen is in a locked state.)

This works (or at least has a chance of working) because if the intruder moves the mouse or presses a key on the keyboard, the monitor wakes up out of the DPMS off state, and this induces the screen lock program to activate as soon as possible. Of course there are cracks in this, the eavesdropper could detach your monitor and insert a non-DPMS one, and there are race conditions. As mentioned above this is not bulletproof. A really robust solution would likely require X server and perhaps even video hardware support.

The blockdpy utility is launched by the -accept option and told to exit via the -gone option (the vnc client user should obviously re-lock the screen before disconnecting!) Instructions can be found in the source code for the utility at the above link. Roughly it is something like this: x11vnc ... -accept "blockdpy -bg -f $HOME/.bdpy" -gone "touch $HOME/.bdpy"

but please read the top of the file.

Update: As of Feb/2007 there is some builtin support for this: -forcedpms and -clientdpms however, they are probably less robust than the above blockdpy.c scheme, since if the person floods the physical machine with mouse or pointer input he can usually see flashes of the screen before the monitor is powered off again. See also the -grabkbd, -grabptr, and -grabalways options.

Q-49: Can I have x11vnc automatically lock the screen when I disconnect the VNC viewer?

Yes, a user mentions he uses the -gone option under CDE to run a screen lock program: x11vnc -display :0 -forever -gone 'dtaction LockDisplay'

Other possibilities are: x11vnc -display :0 -forever -gone 'xscreensaver-command -lock' x11vnc -display :0 -forever -gone 'kdesktop_lock' x11vnc -display :0 -forever -gone 'xlock &' x11vnc -display :0 -forever -gone 'xlock -mode blank &'

Here is a scheme using the -afteraccept option (in version 0.8) to unlock the screen after the first valid VNC login and to lock the screen after the last valid VNC login disconnects: x11vnc -display :0 -forever -shared -afteraccept ./myxlocker -gone ./myxlocke r

Where the script ./myxlocker is: #!/bin/sh

#/usr/bin/env | grep RFB_ | sort # for viewing RFB_* settings.

if [ "X$RFB_MODE" = "Xafteraccept" ]; then if [ "X$RFB_STATE" = "XNORMAL" ]; then # require valid login if [ "X$RFB_CLIENT_COUNT" = "X1" ]; then killall xlock # Linux only. fi fi elif [ "X$RFB_MODE" = "Xgone" ]; then if [ "X$RFB_STATE" = "XNORMAL" ]; then # require valid login if [ "X$RFB_CLIENT_COUNT" = "X0" ]; then xlock -mode blank & fi fi fi

Note the xlock option "-mode blank" to avoid animations.

There is a problem if you have x11vnc running this way in -forever mode and you hit Ctrl-C to stop it. The xlock (or other program) will get killed too. To work around this make a little script called setpgrp that looks like: #!/usr/bin/perl setpgrp(0, 0); exec @ARGV;

then use -gone "setpgrp xlock &", etc. [Encrypted Connections]

Q-50: How can I tunnel my connection to x11vnc via an encrypted SSH channel between two Unix machines?

See the description earlier on this page on how to tunnel VNC via SSH from Unix to Unix. A number of ways are described along with some issues you may encounter.

Other secure encrypted methods exists, e.g. stunnel, IPSEC, various VPNs, etc.

See also the Enhanced TightVNC Viewer (SSVNC) page where much of this is now automated.

Q-51: How can I tunnel my connection to x11vnc via an encrypted SSH channel from Windows using an SSH client like Putty?

Above we described how to tunnel VNC via SSH from Unix to Unix, you may want to review it. To do this from Windows using Putty it would go something like this: * In the Putty dialog window under 'Session' enter the hostname or IP number of the Unix machine with display to be viewed. * Make sure the SSH protocol is selected and the server port is correct. * Under 'Connections/SSH/Tunnels' Add a Local connection with 'Source port: 5900' and 'Destination: localhost:5900' * Log into the remote machine by pressing 'Open' and supplying username, password, etc. * In that SSH shell, start up x11vnc by typing the command: x11vnc -display :0 plus any other desired options (e.g. -localhost.) * Finally, start up your VNC Viewer in Windows and enter 'localhost:0' as the VNC server.

You can keep all of the settings in a Putty 'Saved Session'. Also, once everything is working, you can consider putting x11vnc -display :0 (plus other cmdline options) in the 'Remote command' Putty setting under 'Connections/SSH'.

See also the Enhanced TightVNC Viewer (SSVNC) page where much of this is now automated via the Putty plink utility.

For extra protection feel free to run x11vnc with the -localhost and -rfbauth/-passwdfile options.

If the machine you SSH into via Putty is not the same machine with the X display you wish to view (e.g. your company provides incoming SSH access to a gateway machine), then you need to change the above Putty dialog setting to: 'Destination: otherhost:5900', Once logged in, you'll need to do a second login (ssh or rsh) to the workstation machine 'otherhost' and then start up x11vnc on it. This can also be automated by Chaining SSH's.

As discussed above another option is to first start the VNC viewer in "listen" mode, and then launch x11vnc with the "-connect localhost" option to establish the reverse connection. In this case a Remote port redirection (not Local) is needed for port 5500 instead of 5900 (i.e. 'Source port: 5500' and 'Destination: localhost:5500' for a Remote connection.)

Q-52: How can I tunnel my connection to x11vnc via an encrypted SSL channel using an external tool like stunnel?

It is possible to use a "lighter weight" encryption setup than SSH or IPSEC. SSL tunnels such as stunnel (also stunnel.org) provide an encrypted channel without the need for Unix users, passwords, and key passphrases required for ssh (and at the other extreme SSL can also provide a complete signed certificate chain of trust.) On the other hand, since SSH is usually installed everywhere and firewalls often let its port through, ssh is frequently the path of least resistance (it also nicely manages public keys for you.)

Update: As of Feb/2006 x11vnc has the options -ssl, -stunnel, and -sslverify to provide integrated SSL schemes. They are discussed in the Next FAQ (you probably want to skip to it now.)

We include these non-built-in method descriptions below for historical reference. They are handy because can be used to create SSL tunnels to any VNC (or other type of) server.

Here are some basic examples using stunnel but the general idea for any SSL tunnel utility is the same: * Start up x11vnc and constrain it to listen on localhost. * Then start up the SSL tunnel running on the same machine to forward incoming connections to that x11vnc. * Set up and run a similar SSL tunnel for the outgoing connection on the VNC viewer machine pointing it to the SSL/x11vnc server. * Optionally, set up server (or even client) public/private keys for use in authenticating one side to the other. * Finally, start the VNC Viewer and tell it to connect to the local port (e.g. a vnc display localhost:0) where its outgoing SSL tunnel is listening.

We'll first use the stunnel version 3 syntax since it is the most concise and Unixy.

Start up x11vnc listening on port 5900: x11vnc -display :0 -rfbport 5900 -localhost -bg -passwdfile ~/mypass

Then start stunnel (version 3, not 4) with this command: stunnel -d 5901 -r 5900 -p /path/to/stunnel.pem

The above two commands are run on host "far-away.east". The stunnel.pem is the self-signed PEM file certificate created when stunnel is built. One can also create certificates signed by Certificate Authorities or self-signed if desired using the x11vnc utilities described there.

SSL Viewers: Next, on the VNC viewer side we need an SSL tunnel to encrypt the outgoing connection. The nice thing is any SSL tunnel can be used because the protocol is a standard. For this example we'll also use stunnel on the viewer side on Unix. First start up the client-side stunnel (version 3, not 4): stunnel -c -d localhost:5902 -r far-away.east:5901

Then point the viewer to the local tunnel on port 5902: vncviewer -encodings "copyrect tight zrle hextile" localhost:2

That's it. Note that the ss_vncviewer script can automate this easily, and so can the Enhanced TightVNC Viewer (SSVNC) package.

Be sure to use a VNC password because unlike ssh by default the encrypted SSL channel provides no authentication (only privacy.) With some extra configuration one could also set up certificates to provide authentication of either or both sides as well (and hence avoid man-in-the-middle attacks.) See the stunnel and openssl documentation and also the key management section for details.

stunnel has also been ported to Windows, and there are likely others to choose from for that OS. Much info for using it on Windows can be found at the stunnel site and in this article The article also shows the detailed steps to set up all the authentication certificates. (for both server and clients, see also the x11vnc utilities that do this.) The default Windows client setup (no certs) is simpler and only 4 files are needed in a folder: stunnel.exe, stunnel.conf, libssl32.dll, libeay32.dll. We used an stunnel.conf containing:

stunnel.conf:

client = yes options = ALL [myvncssl] accept = localhost:5902 connect = far-away.east:5901

then double click on the stunnel.exe icon to launch it (followed by pointing the VNC viewer to localhost:2).

stunnel inetd-like mode:

As an aside, if you don't like the little "gap" of unencrypted TCP traffic (and a localhost listening socket) on the local machine between stunnel and x11vnc it can actually be closed by having stunnel start up x11vnc in -inetd mode: stunnel -p /path/to/stunnel.pem -P none -d 5900 -l ./x11vnc_sh

Where the script x11vnc_sh starts up x11vnc: #!/bin/sh x11vnc -q -inetd -display :0 -passwdfile ~/mypass

Note that this creates a separate x11vnc process for each incoming connection (as any inetd x11vnc usage would), but for the case of normally just one viewer at a time it should not be a big problem.

stunnel 4 syntax:

Somewhat sadly, the stunnel version 4 syntax is not so amenable to the command line or scripts. You need to create a config file with the parameters. E.g.: stunnel x11vnc.cfg

Where the file x11vnc.cfg contains: foreground = yes pid = cert = /path/to/stunnel.pem [x11vnc_stunnel] accept = 5901 connect = 5900

One nice thing about version 4 is often the PEM file does not need to be specified because stunnel finds it in its installed area. One other gotcha the PEM file is usually only readable by root (it has the private key afterall), so you'll need to relax the permissions or make a copy that the user running x11vnc/stunnel can read.

SSL VNC Viewers:

Regarding VNC viewers that "natively" do SSL unfortunately there do not seem to be many. The SingleClick UltraVNC Java Viewer is SSL and is compatible with x11vnc's -ssl option and stunnel.) Commercial versions of VNC seem to have some SSL-like encryption built in, but we haven't tried those either and they probably wouldn't work since their (proprietary) SSL-like negotiation is likely embedded in the VNC protocol unlike our case where it is external.

Note: as of Mar/2006 libvncserver/x11vnc provides a SSL-enabled Java applet that can be served up via the -httpdir or -http options when -ssl is enabled. It will also be served via HTTPS via either the VNC port (e.g. https://host:5900/) or a 2nd port via the -https option.

In general current SSL VNC solutions are not particularly "seemless". But it can be done, and with a wrapper script on the viewer side and the -stunnel or -ssl option on the server side it works well and is convenient. Here is a simple script ss_vncviewer that automates running stunnel on the VNC viewer side on Unix a little more carefully than the commands printed above. (One could probably do a similar thing with a .BAT file on Windows in the stunnel folder.)

Update Jul/2006: we now provide an Enhanced TightVNC Viewer (SSVNC) package that starts up STUNNEL automatically along with some other features. All binaries (stunnel, vncviewer, and some utilities) are provided in the package. It works on Unix, Mac OS X, and Windows.

Q-53: Does x11vnc have built-in SSL tunneling?

You can read about non-built-in methods in the Previous FAQ for background.

SSL tunnels provide an encrypted channel without the need for Unix users, passwords, and key passphrases required for ssh (and at the other extreme SSL can also provide a complete signed certificate chain of trust.) On the other hand, since SSH is usually installed everywhere and firewalls often let its port through, ssh is frequently the path of least resistance.

Built-in SSL x11vnc options:

As of Feb/2006 the x11vnc -ssl option automates the SSL tunnel creation on the x11vnc server side. An SSL-enabled Java Viewer applet is also provided that can be served via HTTP or HTTPS to automate SSL on the client side.

The -ssl mode uses the www.openssl.org library if available at build time.

The mode requires an SSL certificate and key (i.e. .pem file.) These are usually created via the openssl(1) program (in fact in for "-ssl" (same as "-ssl SAVE") it will run openssl for you automatically.) So the SSL is not completely "built-in" since this external tool needs to be installed, but at least x11vnc runs it for you automatically.

An -ssl example: x11vnc -display :0 -ssl -passwdfile ~/mypass

You'll get output like this: 09/04/2006 19:27:35 Creating a self-signed PEM certificate... 09/04/2006 19:27:35 ...

The SSL VNC desktop is: far-away.east:0 PORT=5900 SSLPORT=5900

In this case openssl(1) was used to create a PEM automatically. It will prompt you if you want to protect it with with a passphrase. Use "-ssl SAVE_NOPROMPT" to not be prompted. Use "-ssl TMP" to create a temporary self-signed cert that will be discarded when x11vnc exits.

Update: As of Nov/2008 x11vnc also supports the VeNCrypt SSL/TLS tunnel extension to the VNC protocol. The older ANONTLS method (vino) is also supported. This support is on by default when the -ssl option is in use and can be fine-tuned using these options: -vencrypt, -anontls, and -sslonly.

The normal x11vnc -ssl operation is somewhat like a URL method vncs://hostname if vnc://hostname indicates a standard unencrypted VNC connection. Just as https://hostname is an SSL encrypted version of http://hostname. The entire VNC session goes through the SSL tunnel. VeNCrypt, on the other hand, switches to SSL/TLS early in the VNC protocol handshake. x11vnc 0.9.6 supports both simultaneously when -ssl is active.

Note: With the advent of OpenSSL 1.1.0, SSLv2 is dropped and SSLv3 deactivated per default. A couple broken ciphers have also gone, most importantly though is that clients trying to connect to x11vnc will now have to support TLS if encryption is to be used. You can of course always cook up your own build and run time OpenSSL 1.1.x if SSLv3 is absolutely required, but it isn't wise from a security point of view.

SSL VNC Viewers:. Viewer-side will need to use SSL as well. See the next FAQ and here for SSL enabled VNC Viewers, including SSVNC, to connect to the above x11vnc via SSL.

As seen above, the PEM (privacy enhanced mail) file does not need to be supplied if the openssl(1) command is available in PATH, in that case a self-signed, certificate good the current and subsequent x11vnc sessions is created (this may take a while on very slow machines.)

In general, the PEM file contains both the Certificate (i.e. public key) and the Private Key. Because of the latter, the file should be protected from being read by untrusted users. The best way to do this is to encrypt the key with a passphrase (note however this requires supplying the passphrase each time x11vnc is started up.)

See the discussion on x11vnc Key Management for some utilities provided for creating and managing certificates and keys and even for creating your own Certificate Authority (CA) for signing VNC server and client certificates. This may be done by importing the certificate into Web Browser or Java plugin keystores, or pointing stunnel to it. The wrapper script ss_vncviewer provides an example on unix (see the -verify option.)

Here are some notes on the simpler default (non-CA) operation. To have x11vnc save the generated certificate and key, use the "SAVE" keyword like this: x11vnc -ssl SAVE -display :0 ...

(this is the same as the default: "-ssl".) This way it will be saved in the default directory ~/.vnc/certs/ as server.crt (the certificate only) and server.pem (both certificate and private key.) This opens up the possibility of copying the server.crt to machines where the VNC Viewer will be run to enable authenticating the x11vnc SSL VNC server to the clients. When authentication takes place this way (or via the more sophisticated CA signing described here), then Man-In-The-Middle-Attacks are prevented. Otherwise, the SSL encryption only provides protection against passive network traffic "sniffing" (i.e. you are not protected against M-I-T-M attacks.) Nowadays, most people seem mostly concerned mainly about passive sniffing (and the default x11vnc SSL mode protects against it.) Note that there are hacker tools like dsniff/webmitm and cain that implement SSL Man-In-The-Middle attacks. They rely on the client not bothering to check the cert.

One can test to some degree that SSL is working after starting x11vnc with the -stunnel or -ssl option. From another machine one can use the openssl command something like this: openssl s_client -debug -msg -showcerts -connect far-away.east:5900

After all of the debugging output and informational messages you'll see the string "RFB 003.008" that came from x11vnc. Pointing a web browser connecting to: https://far-away.east:5900/ and then viewing the SSL certificate information about the connection in the panels will also work.

Note: If you serve up the SSL enabled Java VNC Viewer via something like: x11vnc -ssl -httpdir /usr/local/share/x11vnc/classes/ssl

(or just the -http option), you can test it out completely using that, including using https to download it into the browser and connect to x11vnc.

The older -stunnel option: Before the -ssl option there was a convenience option -stunnel that would start an external SSL tunnel for you using stunnel. The -ssl method is the preferred way, but for historical reference we keep the -stunnel info here.

The -stunnel mode requires the stunnel.mirt.net command stunnel(8) to be installed on the system.

Some -stunnel examples: x11vnc -display :0 -stunnel /path/to/stunnel.pem -passwdfile ~/mypass

x11vnc -display :0 -stunnel SAVE ...

You'll get output like this: The VNC desktop is: localhost:50 The SSL VNC desktop is: far-away.east:0 PORT=5950 SSLPORT=5900

That indicates stunnel is listening on port 5900 for incoming SSL-wrapped VNC connections from viewers. x11vnc is listening for local connections on port 5950 in this case (remote viewers cannot connect to it directly.) For -stunnel to work the stunnel command must be installed on the machine and available in PATH (note stunnel is often installed in sbin directories rather than bin.) Note that the default "-stunnel" by itself creates a temporary cert (as in "-ssl TMP".)

Q-54: How do I use VNC Viewers with built-in SSL tunneling?

Notes on using "native" VNC Viewers with SSL:

There aren't any native VNC Viewers that do SSL (ask your VNC viewer developer to add the feature.) So a tunnel must be setup that you point the VNC Viewer to. This is often STUNNEL. You can do this manually, or use the ss_vncviewer script on Unix, or our Enhanced TightVNC Viewer (SSVNC) package on Unix, Windows, or MacOSX. See the next section for Java Web browser SSL VNC Viewers (you only need a Java-enabled Web browser for it to work.)

Notes on the SSL enabled Java VNC Viewer provided in x11vnc classes/ssl/VncViewer.jar:

A Java applet VNC Viewer allows you to connect to a VNC Server from a Java-enabled Web browser.

The SSL enabled Java VNC Viewer (VncViewer.jar) in the x11vnc package supports only SSL based connections by default. As mentioned above the -httpdir can be used to specify the path to .../classes/ssl. A typical location might be /usr/local/share/x11vnc/classes/ssl. Or -http can be used to try to have it find the directory automatically.

Also note that the SingleClick UltraVNC Java Viewer is compatible with x11vnc's -ssl SSL mode. (We tested it this way: "java -cp ./VncViewer.jar VncViewer HOST far-away.east PORT 5900 USESSL 1 TRUSTALL 1")

The Java viewer uses SSL to communicate securely with x11vnc. Note that the applet can optionally also be downloaded into your web browser via HTTPS (which is HTTP over SSL.) This way the HTML page and the Java applet itself are also delivered securely with SSL (as opposed to only the VNC traffic being encrypted with SSL.)

For this case the output will be something like this: x11vnc -ssl SAVE -http ... The SSL VNC desktop is: far-away.east:0 Java SSL viewer URL: https://far-away.east:5900/ Java SSL viewer URL: http://far-away.east:5800/ PORT=5900 SSLPORT=5900

Indicating the two URLs (the first one encrypted, the second not) one could point the web browser at to get the VNC viewer applet. E.g. put this http://far-away.east:5800/

or: https://far-away.east:5900/

into your Java-enabled Web browser.

Note that KDE's Konqueror web browser seems to have problems with https Java applets, so you'll have to use the http/5800 with it (if you get https/5900 working let us know how you did it.)

If you are using a router/firewall with port-redirection, and you are redirecting ports other than the default ones (5800, 5900) listed above see here.

The https service provided thru the actual VNC port (5900 in the above example) can occasionally be slow or unreliable (it has to read some input and try to guess if the connection is VNC or HTTP.) If it is unreliable for you and you still want to serve the Java applet via https, use the -https option to get an additional port dedicated to https (its URL will also be printed in the output.)

Another possibility is to add the GET applet parameter: https://far-away.east:5900/?GET=1

This will have the VNC Viewer send a special HTTP GET string "GET /request.https.vnc.connection HTTP/1.0" that x11vnc will notice more quickly as a request for a VNC connection. Otherwise it must wait for a timeout to expire before it assumes a VNC connection.

You may also use "urlPrefix=somestring" to have /somestring prepended to /request.https.vnc.connection". Perhaps you are using a web server proxy scheme to enter a firewall or otherwise have rules applied to the URL. If you need to have any slashes "/" in "somestring" use "2F" (a deficiency in libvncserver prevents using the more natural "%2F".)

You apply multiple applet parameters in the regular URL way, e.g.: https://far-away.east:5900/?GET=1&urlPrefix=mysubdir&...

All of the x11vnc Java Viewer applet parameters are described in the file classes/ssl/README

Tips on Getting the SSL Java Applet Working the First Time: Unfortunately, it can be a little tricky getting the SSL VNC Java Viewer working with x11vnc. Here are some tips to getting working the first time (afterwards you can incrementally customize with more complex settings.) * First try it on the LAN: Do NOT try to have it work the first time going through firewalls, Web proxies, home router port redirections, or Apache portal. Just try a direct connection over your LAN first (if you only have 1 machine and no LAN, just do a direct connection to the same machine: localhost.) If the LAN machine you run x11vnc on has its own host-level firewall (most linux machine come with that on by default), disable it or at least let tcp ports 5800-6000 through. * First try HTTP to download the Java Applet: x11vnc can serve both the Java Applet jar file and VNC out of the same port (both tunneled through SSL, see below.) But it can lead to timing and other problems. So first try HTTP instead of HTTPS to download the Applet jar file (VncViewer.jar.) That is to say try http://hostname:5800 in your web browser first before trying https://hostname:5900. x11vnc will print out the ports and URLs it is using, so use the HTTP one it prints out. * Always Restart the Browser: If you are having failures and have to repeatedly retry things ALWAYS restart the browser (i.e. completely exit it and then start a new browser process) each time. Otherwise as you are changing things the browser may "remember" failed applet downloads, etc. and just add to the confusion and irreproducibility. If you see it trying to download VncViewer.class (instead of VncViewer.jar) you know it is really confused and needs to be restarted. * Step Lively: If you get Browser or Java VM or VNC Viewer applet dialog boxes saying things like "Do you want to trust this certificate?" or "The hostname does not match the one on the certificate", etc. just go through them as quickly as possible. x11vnc cannot wait forever for each SSL connection, and so if you dawdle too long inspecting the certs, etc it can lead to problems. Get it working first before taking your time to read the details in the dialogs, etc. * No inetd, Please: Even if you intend to deploy via inetd or xinetd eventually, get that working later (and remember do not use something like "-ssl TMP" that creates a new temporary SSL certificate for every new socket connection.) * Nothing Fancy: Do not try fancy stuff like -svc, -create, -unixpw, "-users unixpw=", "-users sslpeer=", -sslverify, etc. Just get the simplest connection working first and then incrementally add what you need.

So the recommended test command lines are: x11vnc -ssl SAVE -http x11vnc -ssl SAVE -httpdir /path/to/x11vnc/classes/ssl

Use the latter if x11vnc cannot automatically find the classes/ssl directory (this what the -http option instructs it to do.) Then point your browser to the HTTP (not HTTPS) URL it prints out.

Following the above guidelines, did it work? If so, Congratulations!! you created an SSL encrypted connection between the SSL Java applet running in your web browser and x11vnc. The fact that you used HTTP instead of HTTPS to download the applet is not the end of the world (some users do it this way), the main thing is that the VNC traffic is encrypted with SSL. If you are having trouble even with the above baseline test case feel free to contact me (please send the Full x11vnc output, not just part of it; the complete x11vnc command line; the URL(s) entered in the browser; the full Java Console output; and anything else you can think of.)

Next, you can add the features you want one by one testing it still works each time. I suggest first turning on the HTTPS applet download (https://hostname:5900) if that is what you intend to use. That one gives the most trouble because of the ambiguity of passing two different protocols (HTTP and VNC) through the same SSL service port.

Next, turn on inetd if you intend to use that (this can be tricky too, be sure to use -oa logfile and inspect it carefully if there are problems.) If you are going to use non-standard ports (e.g. "-rfbport 443" as root), work on that next. Then enable the firewall, router port redirection channel (you will somehow need to be outside to do that, maybe test that through another VNC session.)

Then, if you plan to use them, enable "fancy stuff" like "-svc" or "-unixpw", etc, etc. Be sure to add a password either "-rfbauth" or "-unixpw" or both. If you need to have the web browser use a corporate Web Proxy (i.e. it cannot connect directly) work on that last. Ditto for the Apache portal.

Router/Firewall port redirs: If you are doing port redirection at your router to an internal machine running x11vnc AND the internet facing port is different from the internal machine's VNC port, you will need to apply the PORT applet parameter to indicate to the applet the Internet facing port number (otherwise by default the internal machine's port, say 5900, is sent and that of course is rejected at the firewall/router.) For example: https://far-away.east:443/?GET=1&PORT=443

So in this example the user configures his router to redirect connections to port 443 on his Internet side to, say, port 5900 on the internal machine running x11vnc. See also the -httpsredir option that will try to automate this for you.

To configure your router to do port redirection, see its instructions. Typically, from the inside you point a web browser to a special URL (e.g. http://192.168.1.1) and you get a web interface to configure it. Look for something like "Port Redirection" or "Port Forwarding", probably under "Advanced" or something like that. If you have a Linux or Unix system acting as your firewall/router, see its firewall configuration.

You can also use x11vnc options -rfbport NNNNN and -httpport NNNNN to match the ports that your firewall will be redirecting to the machine where x11vnc is run.

Tedious Dialogs: If you do serve the SSL enabled Java viewer via https be prepared for quite a number of "are you sure you trust this site?" dialogs: * First from the Web browser that cannot verify the self-signed certificate when it downloads index.vnc. * From the Web browser again noting that the common name on the certificate does not match the hostname of the remote machine. * Next from the Java VM that cannot verify the self-signed certificate when it downloads VncViewer.jar. * And also from the Java VM again noting that the common name on the certificate does not match the hostname of the remote machine. * Finally from the Java VncViewer applet itself saying it cannot verify the certificate! (or a popup asking you if you want to see the certificate.)

Note that sometimes if you pause too long at one of the above dialogs then x11vnc may exceed a timeout and assume the current socket connection is VNC instead of the HTTPS it actually is (but since you have paused too long at the dialog the GET request comes too late.) Often hitting Reload and going through the dialogs more quickly will let you connect. The Java VM dialogs are the most important ones to NOT linger at. If you see in the x11vnc output a request for VncViewer.class instead of VncViewer.jar it is too late... you will need to completely restart the Web browser to get it to try for the jar again. You can use the -https option if you want a dedicated port for HTTPS connections instead of sharing the VNC port.

To see example x11vnc output for a successful https://host:5900/ connection with the Java Applet see This Page. And here is a newer example including the Java Console output.

All of the x11vnc Java Viewer applet parameters are described in the file classes/ssl/README

Notes on the VNC Viewer ss_vncviewer wrapper script:

If you want to use a native VNC Viewer with the SSL enabled x11vnc you will need to run an external SSL tunnel on the Viewer side. There do not seem to be any native SSL VNC Viewers outside of our x11vnc and SSVNC packages. The basic ideas of doing this were discussed for external tunnel utilities here.

The ss_vncviewer script provided with x11vnc and SSVNC can set up the stunnel tunnel automatically on unix as long as the stunnel command is installed on the Viewer machine and available in PATH (and vncviewer too of course.) Note that on a Debian based system you will need to install the package stunnel4 not stunnel. You can set the environment variables STUNNEL and VNCVIEWERCMD to point to the correct programs if you want to override the defaults.

Here are some examples:

  1. ss_vncviewer far-away.east:0

  2. ss_vncviewer far-away.east:0 -encodings "copyrect tight zrle hextile"

  3. ss_vncviewer -verify ./server.crt far-away.east:0

  4. ss_vncviewer -mycert ./client.pem far-away.east:0

  5. ss_vncviewer -proxy far-away.east:8080 myworkstation:0

The first one is the default mode and accepts the x11vnc certificate without question. The second one is as the first, but adds the -encodings options to the vncviewer command line.

The third one requires that the x11vnc server authenticate itself to the client against the certificate in the file ./server.crt (e.g. one created by "x11vnc -ssl SAVE" and safely copied to the VNC viewer machine.)

The fourth one is for VNC Viewer authentication, it uses ./client.pem to authenticate itself to x11vnc. One can supply both -verify and -mycert simultaneously.

The fifth one shows that Web proxies can be used if that is the only way to get out of the firewall. If the "double proxy" situation arises separate the two by commas. See this page for more information on how Web proxies come into play.

If one uses a Certificate Authority (CA) scheme described here, the wrapper script would use the CA cert instead of the server cert: 3') ss_vncviewer -verify ./cacert.crt far-away.east:0

Update Jul/2006: we now provide an Enhanced TightVNC Viewer (SSVNC) package that starts up STUNNEL automatically along with some other features. All binaries (stunnel, vncviewer, and some utilities) are provided in the package. It works on Unix, Mac OS X, and Windows.

Q-55: How do I use the Java applet VNC Viewer with built-in SSL tunneling when going through a Web Proxy? The SSL enabled Java VNC Viewer and firewall Proxies:

SSL and HTTPS aside, there is a general problem with Firewall Proxies and Java Applets that open sockets. The applet is downloaded successfully (through the browser) using HTTP and the proxy, but when the applet tries to reconnect to the originating host (the only one allowed by security) it does not use the proxy channel. So it cannot reconnect to the server the applet came from!

We have found a convenient workaround: in the directory where VncViewer.jar resides there is a digitally signed version of the same applet called SignedVncViewer.jar. Since the applet is digitally signed, there will be an additional dialog from the Java VM plugin asking you if you want to trust the applet fully.

You should say "Yes". If you do, the applet will be run in a mode where it can try to determine the firewall proxy host name and port (it will ask you for them if it cannot find them.) This way it can connect directly to the Proxy and then request the CONNECT method to be redirected to the originating host (the x11vnc VNC Server.) SSL is then layered over this socket.

To do this you should use the proxy.vnc HTML file like via this URL in your browser: https://yourmachine.com:5900/proxy.vnc

(instead of the unsigned one in https://yourmachine.com:5900/ that gives the default index.vnc)

Proxies that limit CONNECT to ports 443 and 563:

Things become trickier if the Web proxy restricts which CONNECT ports can be redirected to. For security, some (most?) proxies only allow port 443 (HTTPS) and 563 (SNEWS) by default. In this case, the only thing to do is run x11vnc on that low port, e.g. "-rfbport 443", (or use a port redirection on, say, a firewall or router port 443 to the internal machine.)

If you do such a redirection to an internal machine and x11vnc is not listening on port 443, you will probably need to edit proxy.vnc. Suppose the SSL x11vnc server was listening on port 5901. You should change the line in proxy.vnc from:

to:

Since otherwise $PORT will be expanded to 5901 by x11vnc and the viewer applet will fail to connect to that port on the firewall.

Another way to achieve the same thing is to use the applet PORT parameter: https://yourmachine.com/proxy.vnc?PORT=443

this is cleaner because it avoids editing the file, but requires more parameters in the URL. See also the -httpsredir x11vnc option that will try to automate this for you. To use the GET trick discussed above, do: https://yourmachine.com/proxy.vnc?GET=1&PORT=443

All of the x11vnc Java Viewer applet parameters are described in the file classes/ssl/README

Here is an example of Java Console and x11vnc output for the Web proxy case.

Note that both the ss_vncviewer stunnel Unix wrapper script and Enhanced TightVNC Viewer (SSVNC) can use Web proxies as well even though they do not involve a Web browser.

Q-56: Can Apache web server act as a gateway for users to connect via SSL from the Internet with a Web browser to x11vnc running on their workstations behind a firewall? Yes. You will need to configure apache to forward these connections. It is discussed here. This SSL VNC portal provides a clean alternative to the traditional method where the user uses SSH to log in through the gateway to create the encrypted port redirection to x11vnc running on her desktop.

Also see the desktop.cgi CGI script method that achieves much of what this Apache VNC SSL portal method does (as long as desktop.cgi's 'port redirection' mode is enabled.)

Q-57: Can I create and use my own SSL Certificate Authority (CA) with x11vnc? Yes, see this page for how to do this and the utility commands x11vnc provides to create and manage many types of certificates and private keys.

[Display Managers and Services]

Q-58: How can I run x11vnc as a "service" that is always available?

There are a number of ways to do this. The primary thing you need to decide is whether you want x11vnc to connect to the X session on the machine 1) regardless of who (or if anyone) has the X session, or 2) only if a certain user has the X session. Because X sessions are protected by X permissions (MIT-MAGIC-COOKIE files XAUTHORITY and $HOME/.Xauthority) the automatically started x11vnc will of course need to have sufficient permissions to connect to the X display.

Here are some ideas: * Use the description under "Continuously" in the FAQ on x11vnc and Display Managers * Use the description in the FAQ on x11vnc and inetd(8) * Use the description in the FAQ on Unix user logins and inetd(8) * Start x11vnc from your $HOME/.xsession (or $HOME/.xinitrc or autostart script or ...) * Although less reliable, see the x11vnc_loop rc.local hack below.

The display manager scheme will not be specific to which user has the X session unless a test is specifically put into the display startup script (often named Xsetup.) The inetd(8) scheme may or may not be specific to which user has the X session (and it may not be able to do all users via the XAUTHORITY permission issues.)

The .xsession/.xinitrc scheme is obviously is specific to a particular user and only when they are logged into X. If you do not know what a $HOME/.xsession script is or how to use one, perhaps your desktop has a "session startup commands" configuration option. The command to be run in the .xsession or .xinitrc file may look like this: x11vnc -logfile $HOME/.x11vnc.log -rfbauth $HOME/.vnc/passwd -forever -bg

plus any other options you desire.

Depending on your desktop and/or OS/distribution the automatically run X startup scripts (traditionally .xsession/.xinitrc) may have to be in a different directory or have a different basename. One user recommends the description under 'Running Scripts Automatically' at this link.

Firewalls: note all methods will require the host-level firewall to be configured to allow connections in on a port. E.g. 5900 (default VNC port) or 22 (default SSH port for tunnelling VNC.) Most systems these days have firewalls turned on by default, so you will actively have to do something to poke a hole in the firewall at the desired port number. See your system administration tool for Firewall settings (Yast, Firestarter, etc.)

Q-59: How can I use x11vnc to connect to an X login screen like xdm, GNOME gdm, KDE kdm, or CDE dtlogin? (i.e. nobody is logged into an X session yet.)

We describe two scenarios here. The first is called 'One time only' meaning you just need to do it quickly once and don't want to repeat; and the second is called 'Continuously' meaning you want the access to be available after every reboot and after every desktop logout. _________________________________________________________________

One time only: If the X login screen is running and you just want to connect to it once (i.e. a one-shot):

It is usually possible to do this by just adjusting the XAUTHORITY environment variable to point to the correct MIT-COOKIE auth file while running x11vnc as root, e.g. for the gnome display manager, GDM: x11vnc -auth /var/gdm/:0.Xauth -display :0

(the -auth option sets the XAUTHORITY variable for you.)

There will be a similar thing to do for xdm using however a different auth directory path (perhaps something like /var/lib/xdm/authdir/authfiles/A:0-XQvaJk) for the xdm greeter or /var/lib/kdm/A:0-crWk72 (or /var/run/xauth/A:0-qQPftr, etc. etc) for the kdm greeter. Of course, the random characters in the file basename will vary and you will need to use the actual filename on your system. Read your system docs to find out where the display manager cookie files are kept.

Trick: sometimes ps(1) can reveal the X server process -auth argument (e.g. "ps wwaux | grep auth") and hence the path to the auth file.

x11vnc must be run as root for this because the /var/gdm/:0.Xauth, /var/lib/kdm/A:0-crWk72, etc. auth files are only readable by root. If you do not want to run x11vnc as root, you can copy (as root or sudo) the auth file to some location and make it readable by your userid. Then run x11vnc as your userid with -auth pointed to the copied file.

Update Dec/2009: use "-auth guess" to have x11vnc try to guess the location of the auth file for you.

You next connect to x11vnc with a VNC viewer, give your username and password to the X login prompt to start your session.

Note: GDM: gdm seems to have an annoying setting that causes x11vnc (and any other X clients) to be killed after the user logs in. Setting KillInitClients=false in the [daemon] section of /etc/X11/gdm/gdm.conf (or /etc/gdm/gdm.conf, etc.) avoids this. Otherwise, just restart x11vnc and then reconnect your viewer. Other display managers (kdm, etc) may also have a similar problem. One user reports having to alter "gdm.conf-custom" as well.

Note: Solaris: For dtlogin in addition to the above sort of trick (BTW, the auth file should be in /var/dt), you'll also need to add something like Dtlogin*grabServer:False to the Xconfig file (/etc/dt/config/Xconfig or /usr/dt/config/Xconfig on Solaris, see the example at the end of this FAQ.) Then restart dtlogin, e.g.: /etc/init.d/dtlogin stop; /etc/init.d/dtlogin start or reboot.

Update Nov/2008: Regarding GDM KillInitClients: see the -reopen option for another possible workaround.

Update Oct/2009: Regarding GDM KillInitClients: starting with x11vnc 0.9.9 it will try to apply heuristics to detect if a window manager is not running (i.e. whether the Display Manager Greeter Login panel is still up.) If it thinks the display manager login is still up it will delay creating windows or using XFIXES. The former is what GDM uses to kill the initial clients, use of the latter can cause a different problem: an Xorg server crash. So with 0.9.9 and later it should all work without needing to set KillInitClients=false (which is a good because recent GDM, v2.24, has removed this option) or use -noxfixes. To disable the heuristics and delaying set X11VNC_AVOID_WINDOWS=never; to set the delay time explicitly use, e.g., X11VNC_AVOID_WINDOWS=120 (delays for 120 seconds after the VNC connection; you have that long to log in.) _________________________________________________________________

Continuously: Have x11vnc reattach each time the X server is restarted (i.e. after each logout and reboot):

To make x11vnc always attached to the X server including the login screen you will need to add a command to a display manager startup script.

Please consider the security implications of this! The VNC display for the X session always accessible (but hopefully password protected.) Add -localhost if you only plan to access via a SSH tunnel.

The name of the display manager startup script file depends on desktop used and seem to be: GDM (GNOME) /etc/X11/gdm/Init/Default /etc/gdm/Init/Default KDM (KDE) /etc/kde*/kdm/Xsetup XDM /etc/X11/xdm/Xsetup (or sometimes xdm/Xsetup_0) CDE /etc/dt/config/Xsetup

although the exact location can be operating system, distribution, and time dependent. See the documentation for your display manager: gdm(1), kdm(1), xdm(1), dtlogin(1) for additional details. There may also be display number specific scripts: e.g. Xsetup_0 vs. Xsetup, you need to watch out for.

Note: You should read and understand all of the Note's and Update's in the 'One time only' section above. All of the GDM topics apply here as well:

Note: GDM: The above (in 'One time only') gdm setting of KillInitClients=false in /etc/X11/gdm/gdm.conf (or /etc/gdm/gdm.conf, etc.) for GDM is needed here as well. Other display managers (KDM, etc) may also have a similar problem.

Also see the Update Oct/2009 above where x11vnc 0.9.9 and later automatically avoids being killed.

Note: DtLogin: The above (in 'One time only') Dtlogin*grabServer:False step for Solaris will be needed for dtlogin here as well.

In any event, the line you will add to the display manager script (Xsetup, Default, or whatever) will look something like: /usr/local/bin/x11vnc -rfbauth /path/to/the/vnc/passwd -o /var/log/x11vnc.log -forever -bg

where you should customize the exact command to your needs (e.g. -localhost for SSH tunnel-only access; -ssl SAVE for SSL access; etc.)

Happy, happy, joy, joy: Note that we do not need to specify -display or -auth because happily they are already set for us in the DISPLAY and XAUTHORITY environment variables for the Xsetup script!!!

You may also want to force the VNC port with something like "-rfbport 5900" (or -N) to avoid autoselecting one if 5900 is already taken. _________________________________________________________________

Fedora/gdm: Here is an example of what we did on a vanilla install of Fedora-C3 (seems to use gdm by default.) Add a line like this to /etc/X11/gdm/Init/:0 /usr/local/bin/x11vnc -rfbauth /etc/x11vnc.passwd -forever -bg -o /var/log/x1 1vnc.log

And then add this line to /etc/X11/gdm/gdm.conf (or /etc/gdm/gdm.conf, etc.) in the [daemon] section: KillInitClients=false

Then restart: /usr/sbin/gdm-restart (or reboot.) The KillInitClients=false setting is important: without it x11vnc will be killed immediately after the user logs in. Here are full details on how to configure gdm _________________________________________________________________

Solaris/dtlogin: Here is an example of what we did on a vanilla install of Solaris: Make the directory /etc/dt/config: mkdir -p /etc/dt/config

Copy over the Xconfig file for customization: cp /usr/dt/config/Xconfig /etc/dt/config/Xconfig

Edit /etc/dt/config/Xconfig and uncomment the line: Dtlogin*grabServer: False

Next, copy over Xsetup for customization: cp /usr/dt/config/Xsetup /etc/dt/config/Xsetup

Edit /etc/dt/config/Xsetup and at the bottom put a line like: /usr/local/bin/x11vnc -forever -o /var/log/x11vnc.log -bg

(tweaked to your local setup and preferences, a password via -rfbauth, etc. would be a very good idea.)

Restart the X server and dtlogin: /etc/init.d/dtlogin stop /etc/init.d/dtlogin start

(or reboot or maybe just restart the X session.) _________________________________________________________________

KDM: One user running the kdm display manager reports putting this line: x11vnc -forever -rfbauth /home/xyz/.vnc/passwd -bg -o /var/log/x11vnc.log

in /etc/kde/kdm/Xsetup. After rebooting the system it all seemed to work fine. _________________________________________________________________

If you do not want to deal with any display manager startup scripts, here is a kludgey script that can be run manually or out of a boot file like rc.local: x11vnc_loop It will need some local customization before running. Because the XAUTHORITY auth file must be guessed by this script, use of the display manager script method described above is greatly preferred. There is also the -loop option that does something similar.

If the machine is a traditional Xterminal you may want to read this FAQ.

Firewalls: note all methods will require the host-level firewall to be configured to allow connections in on a port. E.g. 5900 (default VNC port) or 22 (default SSH port for tunnelling VNC.) Most systems these days have firewalls turned on by default, so you will actively have to do something to poke a hole in the firewall at the desired port number. See your system administration tool for Firewall settings (Yast, Firestarter, etc.)

Q-60: Can I run x11vnc out of inetd(8)? How about xinetd(8)?

Yes, perhaps a line something like this in /etc/inetd.conf will do it for you:

5900 stream tcp nowait root /usr/sbin/tcpd /usr/local/bin/x11vnc_sh

where the shell script /usr/local/bin/x11vnc_sh uses the -inetd option and looks something like (you'll need to customize to your settings.) #!/bin/sh /usr/local/bin/x11vnc -inetd -display :0 -auth /home/fred/.Xauthority
-rfbauth /home/fred/.vnc/passwd -o /var/log/x11vnc_sh.log

Important: Note that you must redirect the standard error output to a log file (e.g. -o logfile) or "2>/dev/null" for proper operation via inetd (otherwise the standard error also goes to the VNC vncviewer, and that confuses it greatly, causing it to abort.) If you do not use a wrapper script as above but rather call x11vnc directly in /etc/inetd.conf and do not redirect stderr to a file, then you must specify the -q (aka -quiet) option: "/usr/local/bin/x11vnc -q -inetd ...". When you supply both -q and -inet and no "-o logfile" then stderr will automatically be closed (to prevent, e.g. library stderr messages leaking out to the viewer.) The recommended practice is to use "-o logfile" to collect the output in a file or wrapper script with "2>logfile" redirection because the errors and warnings printed out are very useful in troubleshooting problems.

Note also the need to set XAUTHORITY via -auth to point to the MIT-COOKIE auth file to get permission to connect to the X display (setting and exporting the XAUTHORITY variable accomplishes the same thing.) See the x11vnc_loop file in the previous question for more ideas on what that auth file may be, etc. The scheme described in the FAQ on Unix user logins and inetd(8) works around the XAUTHORITY issue nicely.

Note: On Solaris you cannot have the bare number 5900 in /etc/inetd.conf, you'll need to replace it with a word like x11vnc an then put something like "x11vnc 5900/tcp" in /etc/services.

Since the process runs as root, it might be a bad idea to have the logfile in a world-writable area like /tmp if there are untrustworthy users on the machine. Perhaps /var/log is a better place.

Be sure to look at your /etc/hosts.allow and /etc/hosts.deny settings to limit the machines that can connect to this service (your desktop!) For the above example with /etc/hosts.allow: x11vnc_sh : 123.45.67.89

A really safe way to do things is to limit the above inetd to localhost only (via /etc/hosts.allow) and use ssh to tunnel the incoming connection. Using inetd for this prevents there being a tiny window of opportunity between x11vnc starting up and your vncviewer connecting to it. Always use a VNC password to further protect against unwanted access.

For xinetd(8), one user reports he created the file /etc/xinetd.d/x11vncservice containing the following:

default: off

description:

service x11vncservice { flags = REUSE NAMEINARGS port = 5900 type = UNLISTED socket_type = stream protocol = tcp wait = no user = root server = /usr/sbin/tcpd server_args = /usr/local/bin/x11vnc_sh disable = no }

With the contents of /usr/local/bin/x11vnc_sh similar to the example given above. One user reports this works with avoiding the wrapper script: service x11vncservice { port = 5900 type = UNLISTED socket_type = stream protocol = tcp wait = no user = root server = /usr/local/bin/x11vnc server_args = -inetd -q -display :0 -auth /var/gdm/:0.Xauth disable = no }

(or one can replace the -q with say "-o /var/log/x11vnc.log" to capture a log)

The above works nicely for GDM because the -auth file is a fixed name. For KDM or XDM the filename varies. Here is one idea for a x11vnc_sh wrapper to try to guess the name: #!/bin/sh COLUMNS=256 export COLUMNS authfile=ps wwaux | grep '/X.*-auth' | grep -v grep | sed -e 's/^.*-auth *//' -e 's/ .*$//' | head -n 1

if [ -r "$authfile" ]; then exec /usr/local/bin/x11vnc -inetd -o /var/log/x11vnc.log -display :0 -a uth "$authfile" fi exit 1

Starting with x11vnc 0.9.3 this can be automated by: #!/bin/sh exec /usr/local/bin/x11vnc -inetd -o /var/log/x11vnc.log -find -env FD_XDM=1

Q-61: Can I have x11vnc advertise its VNC service and port via mDNS / Zeroconf (e.g. Avahi) so VNC viewers on the local network can detect it automatically?

Yes, as of Feb/2007 x11vnc supports mDNS / Zeroconf advertising of its service via the Avahi client library. Use the option -avahi (same as -mdns or -zeroconf) to enable it. Depending on your setup you may need to install Avahi (including the development/build packages), enable the server: avahi-daemon and avahi-dnsconfd, and possibly open up UDP port 5353 on your firewall.

If the Avahi client library or build environment is not available at build-time, then at run-time x11vnc will try to look for external helper programs, avahi-browse(1) or dns-sd(1), to do the work.

The service was tested with Chicken of the VNC ("Use Bonjour" selected) on a Mac on the same network and the service was noted and listed in the servers list. Clicking on it and then "Connect" connected automatically w/o having to enter any hostnames or port numbers.

It appears SuSE 10.1 comes with avahi (or you can add packages, e.g. avahi-0.6.5-27) but not the development package (you can use the OpenSuSE avahi-devel rpm.) Unfortunately, you may need to disable another Zeroconf daemon "/etc/init.d/mdnsd stop", before doing "/etc/init.d/avahi-daemon start" and "/etc/init.d/avahi-dnsconfd start". We also had to comment out the browse-domains line in /etc/avahi/avahi-daemon.conf. Hopefully there is "LessConf" to do on other distros/OS's...

Q-62: Can I have x11vnc allow a user to log in with her UNIX username and password and then have it find her X session display on that machine and then attach to it? How about starting an X session if one cannot be found?

The easiest way to do this is via inetd(8) using the -unixpw and -display WAIT options. The reason inetd(8) makes this easier is that it starts a new x11vnc process for each new user connection. Otherwise a wrapper would have to listen for connections and spawn new x11vnc's (see this example and also the -loopbg option.) inetd(8) is not required for this, but it makes some aspects more general.

Also with inetd(8) users always connect to a fixed VNC display, say hostname:0, and do not need to memorize a special VNC display number just for their personal use, etc.

Update: Use the -find, -create, -svc, and -xdmsvc options that are shorthand for common FINDCREATEDISPLAY usage modes (e.g. terminal services) described below. (i.e. simply use "-svc" instead of the cumbersome "-display WAIT:cmd=FINDCREATEDISPLAY-Xvfb -unixpw -users unixpw= -ssl SAVE")

The -display WAIT option makes x11vnc wait until a VNC viewer is connected before attaching to the X display.

Additionally it can be used to run an external command that returns the DISPLAY and XAUTHORITY data. We provide some useful builtin ones (FINDDISPLAY and FINDCREATEDISPLAY below), but in principle one could supply his own script: "-display WAIT:cmd=/path/to/find_display" where the script find_display might look something like this.

A default script somewhat like the above is used under "-display WAIT:cmd=FINDDISPLAY" (same as -find) The format for any such command is that it returns DISPLAY=:disp as the first line and any remaining lines are either XAUTHORITY=file or raw xauth data (the above example does the latter.) If applicable (-unixpw mode), the program is run as the Unix user name who logged in.

On Linux if the virtual terminal is known the program appends ",VT=n" to the DISPLAY line; a chvt n will be attempted automatically. Or if only the X server process ID is known it appends ",XPID=n" (a chvt will be attempted by x11vnc.)

Tip: Note that the -find option is an alias for "-display WAIT:cmd=FINDDISPLAY". Use it!

The -unixpw option allows UNIX password logins. It conveniently knows the Unix username whose X display should be found. Here are a couple /etc/inetd.conf examples of this usage: 5900 stream tcp nowait nobody /usr/sbin/tcpd /usr/local/bin/x11vnc -inetd -unixpw
-find -o /var/log/x11vnc.log -ssl SAVE -ssldir /usr/local/certs 5900 stream tcp nowait root /usr/sbin/tcpd /usr/local/bin/x11vnc -inetd -unixpw
-find -o /var/log/x11vnc.log -ssl SAVE -users unixpw=

Note we have used the -find alias and the very long lines have been split. An alternative is to use a wrapper script, e.g. /usr/local/bin/x11vnc.sh that has all of the options. (see also the -svc alias.)

In the first inetd line x11vnc is run as user "nobody" and stays user nobody during the whole session. The permissions of the log files and certs directory will need to be set up to allow "nobody" to use them.

In the second one x11vnc is run as root and switches to the user that logs in due to the "-users unixpw=" option.

Note that SSL is required for this mode because otherwise the Unix password would be passed in clear text over the network. In general -unixpw is not required for this sort of scheme, but it is convenient because it determines exactly who the Unix user is whose display should be sought. Otherwise the find_display script would have to use some method to work out DISPLAY, XAUTHORITY, etc (perhaps you use multiple inetd ports and hardwire usernames for different ports.)

If you really want to disable the SSL or SSH -localhost constraints (this is not recommended unless you really know what you are doing: Unix passwords sent in clear text is a very bad idea...) read the -unixpw documentation.

A inetd(8) scheme for a fixed user that doesn't use SSL or unix passwds could be: /usr/local/bin/x11vnc -inetd -users =fred -find -rfbauth /home/fred/.vnc/pass wd -o /var/log/x11vnc.log

The "-users =fred" option will cause x11vnc to switch to user fred and then find his X display. The VNC password (-rfbauth) as opposed to Unix password (-unixpw) is used to authenticate the VNC client.

Similar looking commands to the above examples can be run directly and do not use inetd (just remove the -inetd option and run from the cmdline, etc.)

X Session Creation: An added (Nov/2006) extension to FINDDISPLAY is FINDCREATEDISPLAY where if it does not find an X display via the FINDDISPLAY method it will create an X server session for the user (i.e. desktop/terminal server.) This is the only time x11vnc actually tries to start up an X server (normally it just attaches to an existing one.)

For virtual sessions you will need to install the Xvfb program (e.g. apt-get install xvfb) or our Xdummy program (see below.)

By default it will only try to start up virtual (non-hardware) X servers: first Xvfb and if that is not available then Xdummy (included in the x11vnc source code.) Note that Xdummy only works on Linux whereas Xvfb works just about everywhere (and in some situations Xdummy must be run as root.) An advantage of Xdummy over Xvfb is that Xdummy supports RANDR dynamic screen resizing, which can be handy if the user accesses the desktop from different sized screens (e.g. workstation and laptop.)

So an inetd(8) example might look like: 5900 stream tcp nowait root /usr/sbin/tcpd /usr/local/bin/x11vnc -inetd
-o /var/log/x11vnc.log -http -prog /usr/local/bin/x11vnc
-ssl SAVE -unixpw -users unixpw= -display WAIT:cmd=FINDCREATEDISPLAY

Where the very long lines have been split. See below where that long and cumbersome last line is replaced by the -svc alias.

The above mode will allow direct SSL (e.g. ss_vncviewer or SSVNC) access and also Java Web browers access via: https://hostname:5900/.

Tip: Note that the -create option is an alias for "-display WAIT:cmd=FINDCREATEDISPLAY-Xvfb".

Tip: Note that -svc is a short hand for the long "-ssl SAVE -unixpw -users unixpw= -display WAIT:cmd=FINDCREATEDISPLAY" part. Unlike -create, this alias also sets up SSL encryption and Unix password login.

The above inetd example then simplifies to: 5900 stream tcp nowait root /usr/sbin/tcpd /usr/local/bin/x11vnc -inetd
-o /var/log/x11vnc.log -http -prog /usr/local/bin/x11vnc
-svc

Tip: In addition to the usual unixpw parameters, inside the VNC viewer the user can specify after his username (following a ":" see -display WAIT for details) for FINDCREATEDISPLAY they can add "geom=WxH" or "geom=WxHxD" to specify the width, height, and optionally the color depth. E.g. "fred:geom=800x600" at the login: prompt. Also if the env. var X11VNC_CREATE_GEOM is set to the desired WxH or WxHxD that will be used by x11vnc.

You can set the env. var X11VNC_SKIP_DISPLAY to a comma separated list of displays to ignore in the FINDDISPLAY process (to force creation of new displays in some cases.) The user logging in via the vncviewer can also set this via username:nodisplay=...)

If you do not plan on using the Java Web browser applet you can remove the -http (and -prog) option since this will speed up logging-in by a few seconds (x11vnc will not have to wait to see if a connection is HTTPS or VNC.)

For reference, xinetd format in the file, say, /etc/xinetd.d/x11vnc: service x11vnc { type = UNLISTED port = 5900 socket_type = stream protocol = tcp wait = no user = root server = /usr/local/bin/x11vnc server_args = -inetd -o /var/log/x11vnc.log -http -prog /usr/local/ bin/x11vnc -svc disable = no }

To print out the script in this case use "-display WAIT:cmd=FINDCREATEDISPLAY-print". To change the preference of Xservers and which to try list them, e.g.: "-display WAIT:cmd=FINDCREATEDISPLAY-X,Xvfb,Xdummy" or use "-create_xsrv X,Xvfb,Xdummy". The "X" one means to try to start up a real, hardware X server, e.g. startx(1) (if there is already a real X server running this may only work on Linux and the chvt program may need to be run to switch to the correct Linux virtual terminal.) x11vnc will try to run chvt automatically if it can determine which VT should be switched to.

XDM/GDM/KDM Login Greeter Panel: If you want to present the user with a xdm/gdm/kdm display manager "greeter" login you can use Xvfb.xdmcp instead of Xvfb, etc in the above list. However, you need to configure xdm/gdm/kdm to accept localhost XDMCP messages, this can be done by (from -help output): If you want the FINDCREATEDISPLAY session to contact an XDMCP login manager (xdm/gdm/kdm) on the same machine, then use "Xvfb.xdmcp" instead of "Xvfb", etc. The user will have to supply his username and password one more time (but he gets to select his desktop type so that can be useful.) For this to work, you will need to enable localhost XDMCP (udp port 177) for the display manager. This seems to be:

   for gdm in gdm.conf:   Enable=true in section [xdmcp]
   for kdm in kdmrc:      Enable=true in section [Xdmcp]
   for xdm in xdm-config: DisplayManager.requestPort: 177

Unless you are also providing XDMCP service to xterminals or other machines, make sure that the host access list only allows local connections (the name of this file is often Xaccess and it is usually setup by default to do just that.) Nowadays, host level firewalling will also typically block UDP (port 177 for XDMCP) by default effectively limiting the UDP connections to localhost.

Tip: Note that -xdmsvc is a short hand alias for the long "-ssl SAVE -unixpw -users unixpw= -display WAIT:cmd=FINDCREATEDISPLAY-Xvfb.xdmcp". So we simply use: service x11vnc { type = UNLISTED port = 5900 socket_type = stream protocol = tcp wait = no user = root server = /usr/local/bin/x11vnc server_args = -inetd -o /var/log/x11vnc.log -xdmsvc disable = no }

(Note: use "-svc" instead of "-xdmsvc" for no XDMCP login greeter.)

Local access (VNC Server and VNC Viewer on the same machine): To access your virtual X display session locally (i.e. while sitting at the same machine it is running on) one can perhaps have something like this in their $HOME/.xinitrc #!/bin/sh x11vnc -create -rfbport 5905 -env WAITBG=1 vncviewer -geometry +0+0 -encodings raw -passwd $HOME/.vnc/passwd localhost:5

You may not need the -passwd. Recent RealVNC viewers might be this: #!/bin/sh x11vnc -create -rfbport 5905 -env WAITBG=1 vncviewer -FullScreen -PreferredEncoding raw -passwd $HOME/.vnc/passwd localhos t:5

This way a bare X server is run with no window manager or desktop; it simply runs only the VNC Viewer on the real X server. The Viewer then draws the virtual X session on to the real one. On your system it might not be $HOME/.xinitrc, but rather .xsession, .Xclients, or something else. You will need to figure out what it is for your system and configuration.

There may be a problem if the resolution (WxH) of the virtual X display does not match that of the physical X display.

If you do not want to or cannot figure out the X startup script name (.xinitrc, etc) you could save the above commands to a shell script, say "vnclocal", and the log in via the normal KDM or GDM greeter program using the "Failsafe" option. Then in the lone xterm that comes up type "vnclocal" to connect to your virtual X display via x11vnc and vncviewer.

 _________________________________________________________________

Summary: The "-display WAIT:cmd=FINDCREATEDISPLAY" scheme can be used to provide a "desktop service" (i.e. terminal service) on the server machine: you always get some desktop there, either a real hardware X server or a virtual one (depending on how you set things up.)

So it provides simple "terminal services" based on Unix username and password. The created X server sessions (virtual or real hardware) will remain running after you disconnect the VNC viewer and will be found again on reconnecting via VNC and logging in. To terminate them use the normal way to Exit/LogOut from inside your X session. The user does not have to memorize which VNC display number is his. They all go the same one (e.g. hostname:0) and it switches based on username.

Q-63: Can I have x11vnc restart itself after it terminates?

One could do this in a shell script, but now there is an option -loop that makes it easier. Of course when x11vnc restarts it needs to have permissions to connect to the (potentially new) X display. This mode could be useful if the X server restarts often. Use e.g. "-loop5000" to sleep 5000 ms between restarts. Also "-loop2000,5" to sleep 2000 ms and only restart 5 times.

One can also use the -loopbg to emulate inetd(8) to some degree, where each connected process runs in the background. It could be combined, say, with the -svc option to provide simple terminal services without using inetd(8).

Q-64: How do I make x11vnc work with the Java VNC viewer applet in a web browser?

To have x11vnc serve up a Java VNC viewer applet to any web browsers that connect to it, run x11vnc with this option: -httpdir /path/to/the/java/classes/dir

(this directory will contain the files index.vnc and, for example, VncViewer.jar) Note that libvncserver contains the TightVNC Java classes jar file for your convenience. (it is the file classes/VncViewer.jar in the source tree.)

You will see output something like this: 14/05/2004 11:13:56 Autoprobing selected port 5900 14/05/2004 11:13:56 Listening for HTTP connections on TCP port 5800 14/05/2004 11:13:56 URL http://walnut:5800 14/05/2004 11:13:56 screen setup finished. 14/05/2004 11:13:56 The VNC desktop is walnut:0 PORT=5900

then you can connect to that URL with any Java enabled browser. Feel free to customize the default index.vnc file in the classes directory.

As of May/2005 the -http option will try to guess where the Java classes jar file is by looking in expected locations and ones relative to the x11vnc binary.

Also note that if you wanted to, you could also start the Java viewer entirely from the viewer-side by having the jar file there and using either the java or appletviewer commands to run the program. java -cp ./VncViewer.jar VncViewer HOST far-away.east PORT 5900

Proxies: See the discussion here if the web browser must use a web proxy to connect to the internet. It is tricky to get Java applets to work in this case: a signed applet must be used so it can connect to the proxy and ask for the redirection to the VNC server. One way to do this is to use the signed SSL one referred to in classes/ssl/proxy.vnc and set disableSSL=yes (note that this has no encryption; please use SSL or SSH as discuss elsewhere on this page) in the URL or the file.

Q-65: Are reverse connections (i.e. the VNC server connecting to the VNC viewer) using "vncviewer -listen" and vncconnect(1) supported?

As of Mar/2004 x11vnc supports reverse connections. On Unix one starts the VNC viewer in listen mode: "vncviewer -listen" (see your documentation for Windows, etc), and then starts up x11vnc with the -connect option. To connect immediately at x11vnc startup time use the "-connect host:port" option (use commas for a list of hosts to connect to.) The ":port" is optional (default is VNC listening port is 5500.)

If a file is specified instead: -connect /path/to/some/file then that file is checked periodically (about once a second) for new hosts to connect to.

The -remote control option (aka -R) can also be used to do this during an active x11vnc session, e.g.: x11vnc -display :0 -R connect:hostname.domain

Use the "-connect_or_exit" option to have x11vnc exit if the reverse connection fails. Also, note the "-rfbport 0" option disables TCP listening for connections (potentially useful for reverse connection mode, assuming you do not want any "forward" connections.)

Note that as of Mar/2006 x11vnc requires password authentication for reverse connections as well as for forward ones (assuming password auth has been enabled, e.g. via -rfbauth, -passwdfile, etc.) Many VNC servers do not require any password for reverse connections. To regain the old behavior supply this option "-env X11VNC_REVERSE_CONNECTION_NO_AUTH=1" to x11vnc.

Vncconnect command: To use the vncconnect(1) program (from the core VNC package at www.realvnc.com) specify the -vncconnect option to x11vnc (Note: as of Dec/2004 -vncconnect is now the default.) vncconnect(1) must be pointed to the same X11 DISPLAY as x11vnc (since it uses X properties to communicate with x11vnc.) If you do not have or do not want to get the vncconnect(1) program, the following script (named "Vncconnect") may work if your xprop(1) supports the -set option: #!/bin/sh

usage: Vncconnect

Vncconnect host:port

note: not all xprop(1) support -set.

xprop -root -f VNC_CONNECT 8s -set VNC_CONNECT "$1"

Q-66: Can reverse connections be made to go through a Web or SOCKS proxy or SSH?

Yes, as of Oct/2007 x11vnc supports reverse connections through proxies: use the "-proxy host:port" option. The default is to assume the proxy is a Web proxy. Note that most Web proxies only allow proxy destination connections to ports 443 (HTTPS) and 563 (SNEWS) and so this might not be too useful unless the proxy has been modified (AllowCONNECT apache setting) or the VNC viewer listens on one of those ports (or the router does a port redir.) A web proxy may also be specified via "-proxy http://host:port"

For SOCKS4 and SOCKS4a proxies use this format "-proxy socks://host:port". If the reverse connection hostname is a numerical IP or "localhost" then SOCKS4 (no host lookup) is used, otherwise SOCKS4a will be used. For SOCKS5 (proxy will do lookup and many other things) use "-proxy socks5://host:port". Note that the SSH builtin SOCKS proxy "ssh -D port" only does SOCKS4 or SOCKS5, so use socks5:// for a ssh -D proxy.

The proxying works for both SSL encrypted and normal reverse connections.

An experimental mode is "-proxy http://host:port/..." where the URL (e.g. a CGI script) is retrieved via the GET method. See -proxy for more info.

Another experimental mode is "-proxy ssh://user@host" in which case a SSH tunnel is used for the proxying. See -proxy for more info.

Up to 3 proxies may be chained together by listing them by commas e.g.: "-proxy http://host1:port1,socks5://host2:port2" in case one needs to ricochet off of several machines to ultimately reach the listening viewer.

Q-67: Can x11vnc provide a multi-user desktop web login service as an Apache CGI or PHP script? Yes. See the example script desktop.cgi for ideas. It is in the source tree in the directory x11vnc/misc. It serves x11vnc's SSL enabled Java Applet to the web browser with the correct connection information for the user's virtual desktop (an Xvfb session via -create; be sure to add the Xvfb package.) HTTPS/SSL enabled Apache should be used to serve the script to avoid unix and vnc passwords from being sent in cleartext and sniffed.

By default it uses a separate VNC port for each user desktop (either by autoprobing in a range of ports or using a port based on the userid number.) The web server's firewall must allow incoming connections to these ports.

It is somewhat difficult to do all of this with x11vnc listening on a single port, however there is also a 'fixed port' scheme described in the script based on -loopbg that works fairly well (but more experience is needed to see what problems contention for the same port causes; however at worst one user may need to re-login.)

There is also an optional 'port redirection' mode for desktop.cgi that allows redirection to other machines inside the firewall already running SSL enabled VNC servers. This provides much of the functionality as the SSL Portal and is easier to set up.

Q-68: Can I use x11vnc as a replacement for Xvnc? (i.e. not for a real display, but for a virtual one I keep around.)

You can, but you would not be doing this for performance reasons (for virtual X sessions via VNC, Xvnc should give the fastest response.) You may want to do this because Xvnc is buggy and crashes, does not support an X server extension you desire, or you want to take advantage of one of x11vnc's unending number of options and features.

One way to achieve this is to have a Xvfb(1) virtual framebuffer X server running in the background and have x11vnc attached to it. Another method, faster and more accurate, is to use the "dummy" Device Driver in XFree86/Xorg (see below.)

For these virtual sessions you will need to install the Xvfb program (e.g. apt-get install xvfb) or our Xdummy program (see below.)

In either case, one can view this desktop both remotely and also locally using vncviewer. Make sure vncviewer's "-encodings raw" is in effect for local viewing (compression seems to slow things down locally.) For local viewing you set up a "bare" window manager that just starts up vncviewer and nothing else (See how below.)

Here is one way to start up Xvfb: xinit -- /usr/bin/Xvfb :1 -cc 4 -screen 0 1024x768x16

This starts up a 16bpp virtual display. To export it via VNC use x11vnc -display :1 ...

Then have the remote vncviewer attach to x11vnc's VNC display (e.g. :0 which is port 5900.)

The "-cc 4" Xvfb option is to force it to use a TrueColor visual instead of DirectColor (this works around a recent bug in the Xorg Xvfb server.)

One good thing about Xvfb is that the virtual framebuffer exists in main memory (rather than in the video hardware), and so x11vnc can "screen scrape" it very efficiently (more than, say, 100X faster than normal video hardware.)

Update Nov/2006: See the FINDCREATEDISPLAY discussion of the "-display WAIT:cmd=FINDDISPLAY" option where virtual (Xvfb or Xdummy, or even real ones by changing an option) X servers are started automatically for new users connecting. This provides a "desktop service" for the machine. You either get your real X session or your virtual (Xvfb/Xdummy) one whenever you connect to the machine (inetd(8) is a nice way to provide this service.) The -find, -create, -svc, and -xdmsvc aliases can also come in handy here.

There are some annoyances WRT Xvfb however. The default keyboard mapping seems to be very poor. One should run x11vnc with -add_keysyms option to have keysyms added automatically. Also, to add the Shift_R and Control_R modifiers something like this is needed: #!/bin/sh xmodmap -e "keycode any = Shift_R" xmodmap -e "add Shift = Shift_L Shift_R" xmodmap -e "keycode any = Control_R" xmodmap -e "add Control = Control_L Control_R" xmodmap -e "keycode any = Alt_L" xmodmap -e "keycode any = Alt_R" xmodmap -e "keycode any = Meta_L" xmodmap -e "add Mod1 = Alt_L Alt_R Meta_L"

(note: these are applied automatically in the FINDCREATEDISPLAY mode of x11vnc.) Perhaps the Xvfb options -xkbdb or -xkbmap could be used to get a better default keyboard mapping...

Dummy Driver: A user points out a faster and more accurate method is to use the "dummy" Device Driver of XFree86/Xorg instead of Xvfb. He uses this to create a persistent and resizable desktop accessible from anywhere. In the Device Section of the config file set Driver "dummy". You may also need to set VideoRam NNN to be large enough to hold the framebuffer. The framebuffer is kept in main memory like Xvfb except that the server code is closely correlated with the real XFree86/Xorg Xserver unlike Xvfb.

The main drawback to this method (besides requiring extra configuration and possibly root permission) is that it also does the Linux Virtual Console/Terminal (VC/VT) switching even though it does not need to (since it doesn't use a real framebuffer.) There are some "dual headed" (actually multi-headed/multi-user) patches to the X server that turn off the VT usage in the X server. Update: As of Jul/2005 we have an LD_PRELOAD script Xdummy that allows you to use a stock (i.e. unpatched) Xorg or XFree86 server with the "dummy" driver and not have any VT switching problems! An advantage of Xdummy over Xvfb is that Xdummy supports RANDR dynamic screen resizing.

The standard way to start the "dummy" driver would be: startx -- :1 -config /etc/X11/xorg.conf.dummy

where the file /etc/X11/xorg.conf.dummy has its Device Section modified as described above. To use the LD_PRELOAD wrapper script: startx -- /path/to/Xdummy :1

An xdm(1) example is also provided.

In general, one can use these sorts of schemes to use x11vnc to export other virtual X sessions, say Xnest or even Xvnc itself (useful for testing x11vnc.)

Local access (VNC Server and VNC Viewer on the same machine): You use a VNC viewer to access the display remotely; to access your virtual X display locally (i.e. while sitting at the same machine it is running on) one can perhaps have something like this in their $HOME/.xinitrc #!/bin/sh x11vnc -display :5 -rfbport 5905 -bg vncviewer -geometry +0+0 -encodings raw -passwd $HOME/.vnc/passwd localhost:5

The display numbers (VNC and X) will likely be different (you could also try -find), and you may not need the -passwd. Recent RealVNC viewers might be this: #!/bin/sh x11vnc -display :5 -rfbport 5905 -bg vncviewer -FullScreen -PreferredEncoding raw -passwd $HOME/.vnc/passwd localhos t:5

This way a bare X server is run with no window manager or desktop; it simply runs only the VNC Viewer on the real X server. The Viewer then draws the virtual X session on to the real one. On your system it might not be $HOME/.xinitrc, but rather .xsession, .Xclients, or something else. You will need to figure out what it is for your system and configuration.

XDM/GDM/KDM One-Shot X sessions: For the general replacement of Xvnc by Xvfb+x11vnc, one user describes a similar setup he created where the X sessions are one-shot's (destroyed after the vncviewer disconnects) and it uses the XDM/GDM/KDM login greeter here.

Q-69: How can I use x11vnc on "headless" machines? Why might I want to?

An interesting application of x11vnc is to let it export displays of "headless" machines. For example, you may have some lab or server machines with no keyboard, mouse, or monitor, but each one still has a video card. One can use x11vnc to provide a simple "desktop service" from these server machines.

An X server can be started on the headless machine (sometimes this requires configuring the X server to not fail if it cannot detect a keyboard or mouse, see the next paragraph.) Then you can export that X display via x11vnc (e.g. see this FAQ) and access it from anywhere on the network via a VNC viewer.

Some tips on getting X servers to start on machines without keyboard or mouse: For XFree86/Xorg the Option "AllowMouseOpenFail" "true" "ServerFlags" config file option is useful. On Solaris Xsun the +nkeyboard and +nmouse options are useful (put them in the server command line args in /etc/dt/config/Xservers.) There are patches available for Xsun at lease back to Solaris 8 that support this. See Xserver(1) for more info.

Although this usage may sound strange it can be quite useful for a GUI (or other) testing or QA setups: the engineers do not need to walk to lab machines running different hardware, OS's, versions, etc (or have many different machines in their office.) They just connect to the various test machines over the network via VNC. The advantage to testing this way instead of using Xvnc or even Xvfb is that the test is done using the real X server, fonts, video hardware, etc. that will be used in the field.

One can imagine a single server machine crammed with as many video cards as it can hold to provide multiple simultaneous access or testing on different kinds of video hardware.

See also the FINDCREATEDISPLAY discussion of the "-display WAIT:cmd=FINDDISPLAY" option where virtual Xvfb or Xdummy, or real X servers are started automatically for new users connecting. The -find, -create, -svc, and -xdmsvc aliases can also come in handy here.

[Resource Usage and Performance]

Q-70: I have lots of memory, but why does x11vnc fail with shmget: No space left on device or Minor opcode of failed request: 1 (X_ShmAttach)?

It is not a matter of free memory, but rather free shared memory (shm) slots, also known as shm segments. This often occurs on a public Solaris machine using the default of only 100 slots. You (or the owner or root) can clean them out with ipcrm(1). x11vnc tries hard to release its slots, but it, and other programs, are not always able to (e.g. if kill -9'd.)

Sometimes x11vnc will notice the problem with shm segments and tries to get by with fewer, only giving a warning like this: 19/03/2004 10:10:58 shmat(tile_row) failed. shmat: Too many open files 19/03/2004 10:10:58 error creating tile-row shm for len=4 19/03/2004 10:10:58 reverting to single_copytile mode

Here is a shell script shm_clear to list and prompt for removal of your unattached shm segments (attached ones are skipped.) I use it while debugging x11vnc (I use "shm_clear -y" to assume "yes" for each prompt.) If x11vnc is regularly not cleaning up its shm segments, please contact me so we can work to improve the situation.

Longer term, on Solaris you can put something like this in /etc/system: set shmsys:shminfo_shmmax = 0x2000000 set shmsys:shminfo_shmmni = 0x1000

to sweep the problem under the rug (4096 slots.) On Linux, examine /proc/sys/kernel/shmmni; you can modify the value by writing to that file.

Things are even more tight on Solaris 8 and earlier, there is a default maximum number of shm segments per process of 6. The error is the X server (not x11vnc) being unable to attach to the segments, and looks something like this: 30/04/2004 14:04:26 Got connection from client 192.168.1.23 30/04/2004 14:04:26 other clients: X Error of failed request: BadAccess (attempt to access private resource den ied) Major opcode of failed request: 131 (MIT-SHM) Minor opcode of failed request: 1 (X_ShmAttach) Serial number of failed request: 14 Current serial number in output stream: 17

This tight limit on Solaris 8 can be increased via: set shmsys:shminfo_shmseg = 100

in /etc/system. See the next paragraph for more workarounds.

To minimize the number of shm segments used by x11vnc try using the -onetile option (corresponds to only 3 shm segments used, and adding -fs 1.0 knocks it down to 2.) If you are having much trouble with shm segments, consider disabling shm completely via the -noshm option. Performance will be somewhat degraded but when done over local machine sockets it should be acceptable (see an earlier question discussing -noshm.)

Q-71: How can I make x11vnc use less system resources?

The -nap (now on by default; use -nonap to disable) and "-wait n" (where n is the sleep between polls in milliseconds, the default is 30 or so) option are good places to start. In addition, something like "-sb 15" will cause x11vnc to go into a deep-sleep mode after 15 seconds of no activity (instead of the default 60.)

Reducing the X server bits per pixel depth (e.g. to 16bpp or even 8bpp) will further decrease memory I/O and network I/O. The ShadowFB X server setting will make x11vnc's screen polling less severe. Using the -onetile option will use less memory and use fewer shared memory slots (add -fs 1.0 for one less slot.)

Q-72: How can I make x11vnc use MORE system resources?

You can try -threads (note this mode can be unstable and/or crash; and as of May/2008 is strongly discouraged, see the option description) or dial down the wait time (e.g. -wait 1) and possibly dial down -defer as well. Note that if you try to increase the "frame rate" too much you can bog down the server end with the extra work it needs to do compressing the framebuffer data, etc.

That said, it is possible to "stream" video via x11vnc if the video window is small enough. E.g. a 256x192 xawtv TV capture window (using the x11vnc -id option) can be streamed over a LAN or wireless at a reasonable frame rate. If the graphics card's framebuffer read rate is faster than normal then the video window size and frame rate can be much higher. The use of TurboVNC and/or TurboJPEG can make the frame rate somewhat higher still (but most of this hinges on the graphics card's read rate.)

Q-73: I use x11vnc over a slow link with high latency (e.g. dialup modem or broadband), is there anything I can do to speed things up?

Some things you might want to experiment with (many of which will help performance on faster links as well):

 X server/session parameters:
 * Configure the X server bits per pixel to be 16bpp or even 8bpp.
   (reduces amount of data needed to be polled, compressed, and sent)
 * Use a smaller desktop size (e.g. 1024x768 instead of 1280x1024)
 * Make sure the desktop background is a solid color (the background
   is resent every time it is re-exposed.) Consider using the -solid
   [color] option to try to do this automatically.
 * Configure your window manager or desktop "theme" to not use fancy
   images, shading, and gradients for the window decorations, etc.
   Disable window animations, etc. Maybe your desktop has a "low
   bandwidth" theme you can easily switch into and out of. Also in
   Firefox disable eye-candy, e.g.: Edit -> Preferences -> Advanced
   -> Use Smooth Scrolling (deselect it.)
 * Avoid small scrolls of large windows using the Arrow keys or
   scrollbar. Try to use PageUp/PageDown instead. (not so much of a
   problem in x11vnc 0.7.2 if -scrollcopyrect is active and detecting
   scrolls for the application.)
 * If the -wireframe option is not available (earlier than x11vnc
   0.7.2 or you have disabled it via -nowireframe) then Disable
   Opaque Moves and Resizes in the window manager/desktop.
 * However if -wireframe is active (on by default in x11vnc 0.7.2)
   then you should Enable Opaque Moves and Resizes in the window
   manager! This seems counter-intuitive, but because x11vnc detects
   the move/resize events early there is a huge speedup over a slow
   link when Opaque Moves and Resizes are enabled. (e.g. CopyRect
   encoding will be used.)
 * Turn off Anti-aliased fonts on your system, web browser, terminal
   windows, etc. AA fonts do not compress as well as traditional
   fonts (sometimes 10X less.)
 * On Firefox/Mozilla (and anything else) turn off "Smooth Scroll"
   animations. In Firefox put in the URL "about:config" and set
   general.smoothScroll to false.
 * On Xorg/XFree86 turn on the Shadow Framebuffer to speed up
   reading. (Option "ShadowFB" "true" in the Device section of
   /etc/X11/XF86Config) This disables 2D acceleration on the physical
   display and so may not be worth it (if you play games, etc), but
   could be of use in some situations. Note: If the network link is
   very slow, this speedup may not be noticed.

 VNC viewer parameters:
 * Use a TightVNC enabled viewer! (Actually, RealVNC 4.x viewer with
   ZRLE encoding is not too bad either; some claim it is faster.)
 * Make sure the tight (or zrle) encoding is being used (look at
   vncviewer and x11vnc outputs)
 * Request 8 bits per pixel using -bgr233 (up to 4X speedup over
   depth 24 TrueColor (32bpp), but colors will be off)
 * RealVNC 4.x viewer has some extremely low color modes (only 64 and
   even 8 colors.) SSVNC does too. The colors are poor, but it is
   usually noticeably faster than bgr233 (256 colors.)
 * Try increasing the TightVNC -compresslevel (compresses more on
   server side before sending, but uses more CPU)
 * Try reducing the TightVNC -quality (increases JPEG compression,
   but is lossy with painting artifacts)
 * Try other VNC encodings via -encodings (tight may be the fastest,
   but you should compare it to zrle and maybe some of the others)
 * On the machine where vncviewer is run, make sure Backing Store is
   enabled (Xorg/XFree86 disables it by default causing re-exposures
   of vncviewer to be very slow) Option "backingstore" in config
   file.

 x11vnc parameters:
 * Make sure the -wireframe option is active (it should be on by
   default) and you have Opaque Moves/Resizes Enabled in the window
   manager.
 * Make sure the -scrollcopyrect option is active (it should be on by
   default.) This detects scrolls in many (but not all) applications
   an applies the CopyRect encoding for a big speedup.
 * Enforce a solid background when VNC viewers are connected via
   -solid
 * Try x11vnc's client-side caching client-side caching scheme:
   -ncache
 * Specify -speeds modem to force the wireframe and scrollcopyrect
   heuristic parameters (and any future ones) to those of a dialup
   modem connection (or supply the rd,bw,lat numerical values that
   characterize your link.)
 * If wireframe and scrollcopyrect aren't working, try using the more
   drastic -nodragging (no screen updates when dragging mouse, but
   sometimes you miss visual feedback)
 * Set -fs 1.0 (disables fullscreen updates)
 * Try increasing -wait or -defer (reduces the maximum "frame rate",
   but won't help much for large screen changes)
 * Try the -progressive pixelheight mode with the block pixelheight
   100 or so (delays sending vertical blocks since they may change
   while viewer is receiving earlier ones)
 * If you just want to watch one (simple) window use -id or -appshare
   (cuts down extraneous polling and updates, but can be buggy or
   insufficient)
 * Set -nosel (disables all clipboard selection exchange)
 * Use -nocursor and -nocursorpos (repainting the remote cursor
   position and shape takes resources and round trips)
 * On very slow links (e.g. <= 28.8) you may need to increase the
   -readtimeout n setting if it sometimes takes more than 20sec to
   paint the full screen, etc.
 * Do not use -fixscreen to automatically refresh the whole screen,
   tap three Alt_L's then the screen has painting errors (rare
   problem.)

Example for the KDE desktop:

Launch the "KDE Control Center" utility. Sometimes this is called "Personal Settings".

Select "Desktop".

Then Select "Window Behavior". In the "Moving" Tab set these:
 * YES - Display content in moving windows
 * YES - Display content in resizing windows
 * NO   - Display window geometry when moving or resizing
 * NO   - Animate minimize and restore

In the "Translucency" Tab set:
 * NO   - Use translucency/shadows

Next hit "Back" and then select "Panels".

In the "Appearance" Tab set:
 * NO   - Enable icon mouseover effects
 * NO   - Enable transparency

Now go all the way back up to the top and Select "Appearance & Themes".

Select "Background" and set:
 * YES - No picture
 * Colors: Single Color

Select "Fonts" and disable anti-aliased fonts if you are bold enough.

Select "Launch Feedback" and set:
 * Busy Cursor: No Busy Cursor
 * NO   - Enable taskbar notification

Select "Screen Saver" and set:
 * Screen Saver: Blank Screen

Select "Style" and in the "Effects" Tab set:
 * NO   - Enable GUI effects

Example for the GNOME desktop: * TBD.

Q-74: Does x11vnc support the X DAMAGE Xserver extension to find modified regions of the screen quickly and efficiently?

Yes, as of Mar/2005 x11vnc will use the X DAMAGE extension by default if it is available on the display. This requires libXdamage to be available in the build environment as well (recent Linux distros and Solaris 10 have it.)

The DAMAGE extension enables the X server to report changed regions of the screen back to x11vnc. So x11vnc doesn't have to guess where the changes are (by polling every pixel of the entire screen every 2-4 seconds.) The use of X DAMAGE dramatically reduces the load when the screen is not changing very much (i.e. most of the time.) It also noticeably improves updates, especially for very small changed areas (e.g. clock ticking, cursor flashing, typing, etc.)

Note that the DAMAGE extension does not speed up the actual reading of pixels from the video card framebuffer memory, by, say, mirroring them in main memory. So reading the fb is still painfully slow (e.g. 5MB/sec), and so even using X DAMAGE when large changes occur on the screen the bulk of the time is still spent retrieving them. Not ideal, but use of the ShadowFB XFree86/Xorg option speeds up the reading considerably (at the cost of h/w acceleration.)

Unfortunately the current Xorg DAMAGE extension implementation can at times be overly conservative and report very large rectangles as "damaged" even though only a small portion of the pixels have actually been modified. This behavior is often the fault of the window manager (e.g. it redraws the entire, unseen, frame window underneath the application window when it gains focus), or the application itself (e.g. does large, unnecessary repaints.)

To work around this deficiency, x11vnc currently only trusts small DAMAGE rectangles to contain real damage. The larger rectangles are only used as hints to focus the traditional scanline polling (i.e. if a scanline doesn't intersect a recent DAMAGE rectangle, the scan is skipped.) You can use the "-xd_area A" option to adjust the size of the trusted DAMAGE rectangles. The default is 20000 pixels (e.g. a 140x140 square, etc.) Use "-xd_area 0" to disable the cutoff and trust all DAMAGE rectangles.

The option "-xd_mem f" may also be of use in tuning the algorithm. To disable using DAMAGE entirely use "-noxdamage".

Q-75: My OpenGL application shows no screen updates unless I supply the -noxdamage option to x11vnc. One user reports in his environment (MythTV using the NVIDIA OpenGL drivers) he gets no updates after the initial screen is drawn unless he uses the "-noxdamage" option.

This seems to be a bug in the X DAMAGE implementation of that driver. You may have to use -noxdamage as well. A way to autodetect this will be tried, probably the best it will do is automatically stop using X DAMAGE.

A developer for MiniMyth reports that the 'alphapulse' tag of the theme G.A.N.T. can also cause problems, and should be avoided when using VNC.

Update: see this FAQ too.

Q-76: When I drag windows around with the mouse or scroll up and down things really bog down (unless I do the drag in a single, quick motion.) Is there anything to do to improve things?

This problem is primarily due to slow hardware read rates from video cards: as you scroll or move a large window around the screen changes are much too rapid for x11vnc to keep up them (it can usually only read the video card at about 5-10 MB/sec, so it can take a good fraction of a second to read the changes induce from moving a large window, if this to be done a number of times in succession the window or scroll appears to "lurch" forward.) See the description in the -pointer_mode option for more info. The next bottleneck is compressing all of these changes and sending them out to connected viewers, however the VNC protocol is pretty much self-adapting with respect to that (updates are only packaged and sent when viewers ask for them.)

As of Jan/2004 there are some improvements to libvncserver. The default should now be much better than before and dragging small windows around should no longer be a huge pain. If for some reason these changes make matters worse, you can go back to the old way via the "-pointer_mode 1" option.

Also added was the -nodragging option that disables all screen updates while dragging with the mouse (i.e. mouse motion with a button held down.) This gives the snappiest response, but might be undesired in some circumstances when you want to see the visual feedback while dragging (e.g. menu traversal or text selection.)

As of Dec/2004 the -pointer_mode n option was introduced. n=1 is the original mode, n=2 an improvement, etc.. See the -pointer_mode n help for more info.

Also, in some circumstances the -threads option can improve response considerably. Be forewarned that if more than one vncviewer is connected at the same time then libvncserver may not be thread safe (try to get the viewers to use different VNC encodings, e.g. tight and ZRLE.) This option can be unstable and so as of Feb/2008 it is disabled by default. Set env. X11VNC_THREADED=1 to re-enable.

As of Apr/2005 two new options (see the wireframe FAQ and scrollcopyrect FAQ below) provide schemes to sweep this problem under the rug for window moves or resizes and for some (but not all) window scrolls. These are the preferred way of avoiding the "lurching" problem, contact me if they are not working. Note on SuSE and some other distros the RECORD X extension used by scrollcopyrect is not enabled by default, turn it on in xorg.conf: Section "Module" ... Load "record" ... EndSection

Q-77: Why not do something like wireframe animations to avoid the windows "lurching" when being moved or resized?

Nice idea for a hack! As of Apr/2005 x11vnc by default will apply heuristics to try to guess if a window is being (opaquely) moved or resized. If such a change is detected framebuffer polling and updates will be suspended and only an animated "wireframe" (a rectangle outline drawn where the moved/resized window would be) is shown. When the window move/resize stops, it returns to normal processing: you should only see the window appear in the new position. This spares you from interacting with a "lurching" window between all of the intermediate steps. BTW the lurching is due to slow video card read rates (see here too.) A displacement, even a small one, of a large window requires a non-negligible amount of time, a good fraction of a second, to read in from the hardware framebuffer.

Note that Opaque Moves/Resizes must be Enabled by your window manager for -wireframe to do any good.

The mode is currently on by default because most people are afflicted with the problem. It can be disabled with the -nowireframe option (aka -nowf.) Why might one want to turn off the wireframing? Since x11vnc is merely guessing when windows are being moved/resized, it may guess poorly for your window-manager or desktop, or even for the way you move the pointer. If your window-manager or desktop already does its own wireframing then this mode is a waste of time and could do the wrong thing occasionally. There may be other reasons the new mode feels unnatural. If you have very expensive video hardware (SGI, well now even proprietary Xorg drivers are fast at reading) or are using an in-RAM video framebuffer (SunRay, ShadowFB, Xvfb), the read rate from that framebuffer may be very fast (100's of MB/sec) and so you don't really see much lurching (at least over a fast LAN): opaque moves look smooth in x11vnc. Note: ShadowFB is often turned on when you are using the vesafb or fbdev XFree86 video driver instead of a native one so you might be using it already and not know.

The heuristics used to guess window motion or resizing are simple, but are not fool proof: x11vnc is sometimes tricked and so you'll occasionally see the lurching opaque move and rarely something even worse.

First it assumes that the move/resize will occur with a mouse button pressed, held down and dragged (of course this is only mostly true.) Next it will only consider a window for wireframing if the mouse pointer is initially "close enough" to the edges of the window frame, e.g. you have grabbed the title bar or a resizer edge (this requirement can be disabled and it also not applied if a modifier key, e.g. Alt, is pressed.) If these are true, it will wait an amount of time to see if the window starts moving or resizing. If it does, it starts drawing the wireframe "outline" of where the window would be. When the mouse button is released, or a timeout occurs, it goes back to the standard mode to allow the actual framebuffer changes to propagate to the viewers.

These parameters can be tweaked: * Color/Shade of the wireframe. * Linewidth of the outline frame. * Cutoff size of windows to not apply wireframing to. * Cutoffs for closeness to Top, Bottom, Left, and Right edges of window. * Modifier keys to enable interior window grabbing. * Maximum time to wait for dragging pointer events. * Maximum time to wait for the window to start moving/resizing. * Maximum time to show a wireframe animation. * Minimum time between sending wireframe outlines.

See the "-wireframe tweaks" option for more details. On a slow link, e.g. dialup modem, the parameters may be automatically adjusted for better response.

CopyRect encoding: In addition to the above there is the "-wirecopyrect mode" option. It is also on by default. This instructs x11vnc to not only show the wireframe animation, but to also instruct all connected VNC viewers to locally translate the window image data from the original position to the new position on the screen when the animation is done. This speedup is the VNC CopyRect encoding: the framebuffer update doesn't need to send the actual new image data. This is nice in general, and very convenient over a slow link, but since it is based on heuristics you may need to disable it with the -nowirecopyrect option (aka -nowcr) if it works incorrectly or unnaturally for you.

The -wirecopyrect modes are: "never" (same as -nowirecopyrect); "top", only apply the CopyRect if the window is appears to be on the top of the window stack and is not obstructed by other windows; and "always" to always try to apply the CopyRect (obstructed regions are usually clipped off and not translated.)

Note that some desktops (KDE and xfce) appear to mess with the window stacking in ways that are not yet clear. In these cases x11vnc works around the problem by applying the CopyRect even if obscuring windows' data is translated! Use -nowirecopyrect if this yields undesirable effects for your desktop.

Also, the CopyRect encoding may give incorrect results under -scale (depending on the scale factor the CopyRect operation is often only approximate: the correctly scaled framebuffer will be slightly different from the translated one.) x11vnc will try to push a "cleanup" update after the CopyRect if -scale is in effect. Use -nowirecopyrect if this or other painting errors are unacceptable.

Q-78: Can x11vnc try to apply heuristics to detect when a window is scrolling its contents and use the CopyRect encoding for a speedup?

Another nice idea for a hack! As of May/2005 x11vnc will by default apply heuristics to try to detect if the window that has the input focus is scrolling its contents (but only when x11vnc is feeding user input, keystroke or pointer, to the X server.) So, when detected, scrolls induced by dragging on a scrollbar or by typing (e.g. Up or Down arrows, hitting Return in a terminal window, etc), will show up much more quickly than via the standard x11vnc screen polling update mechanism.

There will be a speedup for both slow and fast links to viewers. For slow links the speedup is mostly due to the CopyRect encoding not requiring the image data to be transmitted over the network. For fast links the speedup is primarily due to x11vnc not having to read the scrolled framebuffer data from the X server (recall that reading from the hardware framebuffer is slow.)

To do this x11vnc uses the RECORD X extension to snoop the X11 protocol between the X client with the focus window and the X server. This extension is usually present on most X servers (but SuSE disables it for some reason.) On XFree86/Xorg it can be enabled via Load "record" in the Module section of the config file if it isn't already: Section "Module" ... Load "record" ... EndSection

Currently the RECORD extension is used as little as possible so as to not slow down regular use. Only simple heuristics are applied to detect XCopyArea and XConfigureWindow calls from the application. These catch a lot of scrolls, e.g. in mozilla/firefox and in terminal windows like gnome-terminal and xterm. Unfortunately the toolkits KDE applications use make scroll detection less effective (only rarely are they detected: i.e. Konqueror and Konsole don't work.) An interesting project, that may be the direction x11vnc takes, is to record all of the X11 protocol from all clients and try to "tee" the stream into a modified Xvfb watching for CopyRect and other VNC speedups. A potential issue is the RECORD stream is delayed from actual view on the X server display: if one falls too far behind it could become a mess...

The initial implementation of -scrollcopyrect option is useful in that it detects many scrolls and thus gives a much nicer working environment (especially when combined with the -wireframe -wirecopyrect options, which are also on by default; and if you are willing to enable the ShadowFB things are very fast.) The fact that there aren't long delays or lurches during scrolling is the primary improvement.

But there are some drawbacks: * Not all scrolls are detected. Some apps scroll windows in ways that cannot currently be detected, and other times x11vnc "misses" the scroll due to timeouts, etc. Sometimes it is more distracting that a speedup occasionally doesn't work as opposed to being consistently slow! * For rapid scrolling (i.e. sequence of many scrolls over a short period) there can be painting errors (tearing, bunching up, etc.) during the scroll. These will repair themselves after the scroll is over, but when they are severe it can be distracting. Try to think of the approximate window contents as a quicker and more useful "animation" compared to the slower polling scheme... * Scrolling inside shells in terminal windows (gnome-terminal, xterm), can lead to odd painting errors. This is because x11vnc did not have time to detect a screen change just before the scroll (most common is the terminal undraws the block cursor before scrolling the text up: in the viewer you temporarily see multiple block cursors.) Another issue is with things like more(1): scroll detection for 5-6 lines happens nicely, but then it can't keep up and so there is a long pause for the standard polling method to deliver the remaining updates. * More rarely sometimes painting errors are not repaired after the scroll is over. This may be a bug in x11vnc or libvncserver, or it may be an inescapable fact of the CopyRect encoding and the delay between RECORD callbacks and what is actually on the X display. One can tap the Alt_L key (Left "Alt" key) 3 times in a row to signal x11vnc to refresh the screen to all viewers. Your VNC-viewer may have its own screen refresh hot-key or button. See also: -fixscreen * Some applications, notably OpenOffice, do XCopyArea scrolls in weird ways that assume ancestor window clipping is taking place. See the -scr_skip option for ways to tweak this on a per-application basis. * Selecting text while dragging the mouse may be slower, especially if the Button-down event happens near the window's edge. This is because the scrollcopyrect scheme is watching for scrolls via RECORD and has to wait for a timeout to occur before it does the update. * For reasons not yet understood the RECORD extension can stop responding (and hence scrolls are missed.) As a workaround x11vnc attempts to reset the RECORD connection every 60 seconds or so. Another workaround is to type 4 Super_L (Left Super/Windows-Flag key) in a row to reset RECORD. Work is in progress to try to fix this bug. * Sometimes you need to "retrain" x11vnc for a certain window because it fails to detect scrolls in it. Sometimes clicking inside the application window or selecting some text in it to force the focus helps. * When using the -scale option there will be a quick CopyRect scroll, but it needs to be followed by a slower "cleanup" update. This is because for a fixed finite screen resolution (e.g. 75 dpi) scaling and copyrect-ing are not exactly independent. Scaling involves a blending of nearby pixels and if you translate a pixel the neighbor pixel weighting may be different. So you have to wait a bit for the cleanup update to finish. On slow links x11vnc may automatically decide to not detect scrolls when -scale is in effect. In general it will also try to defer the cleanup update if possible.

If you find the -scrollcopyrect behavior too approximate or distracting you can go back to the standard polling-only update method with the -noscrollcopyrect (or -noscr for short.) If you find some extremely bad and repeatable behavior for -scrollcopyrect please report a bug.

Alternatively, as with -wireframe, there are many tuning parameters to try to improve the situation. You can also access these parameters inside the gui under "Tuning". These parameters can be tweaked: * The minimum pixel area of a rectangle to be watched for scrolls. * A list if application names to skip scroll detection. * Which keystrokes should trigger scroll detection. * Which applications should have a "terminal" tweak applied to them. * When repeating keys (e.g. Up arrow) should be discarded to preserve a scroll. * Cutoffs for closeness to Top, Bottom, Left, and Right edges of window for mouse induced scrolls. * Set timeout parameters for keystroke induced scrolls. * Set timeout parameters for mouse pointer induced scrolls. * Have the full screen be periodically refreshed to fix painting errors.

Q-79: Can x11vnc do client-side caching of pixel data? I.e. so when that pixel data is needed again it does not have to be retransmitted over the network.

As of Dec/2006 in the 0.9 development tarball there is an experimental client-side caching implementation enabled by the "-ncache n" option. In fact, during the test period it was on by default with n set to 10. To disable it use "-noncache".

It is a simple scheme where a (very large) lower portion of the framebuffer (i.e. starting just below the user's actual desktop display) is used for storing pixel data. CopyRect; a fast, essentially local viewer-side VNC encoding; is used to swap the pixel data in and out of the actual display area. It gives an excellent speedup for iconifying/deiconifying and moving windows and re-posting of menus (often it doesn't feel like VNC at all: there is no delay waiting for the pixel data to fill in.)

This scheme is nice because it does all of this within the existing VNC protocol, and so it works with all VNC viewers.

A challenge to doing more sophisticated (e.g. compressed and/or shared) client-side caching is that one needs to extend the VNC protocol, modify a viewer and then also convince users to adopt your modified VNC Viewer (or get the new features to be folded into the main VNC viewers, patches accepted, etc... likely takes many years before they might be deployed in the field.) So it is convenient that the "-ncache n" works with any unaltered VNC viewer.

A drawback of the "-ncache n" method is that in the VNC Viewer you can scroll down and actually see the cached pixel data. So it looks like there is a bug: you can scroll down in your viewer and see a strange "history" of windows on your desktop. This is working as intended. One will need to try to adjust the size of his VNC Viewer window so the cache area cannot be seen. SSVNC (see below) can do this automatically.

At some point LibVNCServer may implement a "rfbFBCrop" pseudoencoding that viewers can use to learn which portion of the framebuffer to actually show to the users (with the hidden part used for caching, or perhaps something else, maybe double buffering or other offscreen rendering...)

The Enhanced TightVNC Viewer (SSVNC) Unix viewer has a nice -ycrop option to help hide the pixel cache area from view. It will turn on automatically if the framebuffer appears to be very tall (height more than twice the width), or you can supply the actual value for the height. If the screen is resized by scaling, etc, the ycrop value is scaled as well. In fullscreen mode you cannot scroll past the end of the actual screen, and in non-fullscreen mode the window manager frame is adjusted to fit the actual display (so you don't see the pixel cache region) and the scrollbars are very thin to avoid distraction and trouble fitting inside your display. Use the "-sbwidth n" viewer option to make the scrollbars thicker if you like.

Another drawback of the scheme is that it is VERY memory intensive, the n in "-ncache n" is the factor of increase over the base framebuffer size to use for caching. It is an even integer and should be fairly large, 6-12, to achieve good response. This usually requires about 50-100MB of additional RAM on both the client and server sides. For example with n=6 a 1280x1024 display will use a framebuffer that is 1280x7168: everything below row 1024 is the pixel buffer cache. If you are running on low memory machines or memory is tight because of other running applications you should not use -ncache.

The reason for so much memory is because the pixel data is not compressed and so the whole window to be saved must be stored "offscreen". E.g. for a large web browser window this can be nearly 1 million pixels, and that is only for a single window! One typically wants to cycle between 5-10 large active windows. Also because both backing-store (the window's actual contents) and save-unders (the pixels covered up by the window) are cached offscreen that introduces an additional factor of 2 in memory use.

However, even in the smallest usage mode with n equal 2 and -ncache_no_rootpixmap set (this requires only 2X additional framebuffer memory) there is still a noticable improvement for many activities, although it is not as dramatic as with, say n equal 12 and rootpixmap (desktop background) caching enabled.

The large memory consumption of the current implementation can be thought of as a tradeoff to providing caching and being compatible with all VNC viewers and also ease of implementing. Hopefully it can be tuned to use less, or the VNC community will extend the protocol to allow caching and replaying of compressed blobs of data.

Another option to experiment with is "-ncache_cr". By specifying it, x11vnc will try to do smooth opaque window moves instead of its wireframe. This can give a very nice effect (note: on Unix the realvnc viewer seems to be smoother than the tightvnc viewer), but can lead to some painting problems, and can be jerky in some circumstances.

Surprisingly, for very slow connections, e.g. modem, the -ncache_cr option can actually improve window drags. This is probably because no pixel data (only CopyRect instructions) are sent when dragging a window. Normally, the wireframe must be sent and this involves compressing and sending the lines that give rise to the moving box effect (note that real framebuffer data is sent to "erase" the white lines of the box.)

If you experience painting errors you can can tap the Alt_L key (Left "Alt" key) 3 times in a row to signal x11vnc to refresh the screen to all viewers. You may also need to iconify and then deiconify any damaged windows to correct their cache data as well. Note that if you change color viewer depth (e.g. 8bpp to full color) dynamically that will usually lead to the entire extended framebuffer being resent which can take a long time over very slow links: it may be better to reconnect and reset the format right after doing so. x11vnc will try to detect the format change and clear (make completely black) the cache region.

Gotcha for older Unix VNC Viewers: The older Unix VNC viewers (e.g. current TightVNC Unix Viewer) require X server backingstore to keep off-viewer screen data local. If the viewer-side X server has backingstore disabled (sadly, currently the default on Linux, etc), then to get the offscreen pixels the viewer has to ask for a refresh over the network, thereby defeating the caching. Use something like this in your viewer-side /etc/X11/xorg.conf file (or otherwise get your viewer-side system to do it) Section "Device" ... Option "backingstore" ... EndSection

No problems like this have been observed with Windows VNC Viewers: they all seem to keep their entire framebuffer in local memory.

Gotcha for KDE krdc VNC Viewer: One user found that KDE's krdc viewer has some sort of hardwired limit on the maximum size of the framebuffer (64MB?). It fails quickly saying "The connection to the host has been interrupted." The workaround for his 1280x1024 x11vnc-side display was to run with "-ncache 10", i.e. a smaller value to be under the krdc threshold.

Although this scheme is not as quick (nor as compressed) as nx/nomachine, say, it does provide a good step in the direction of improving VNC performance by client side caching.

Q-80: Does x11vnc support TurboVNC?

As of Feb/2009 (development tarball) there is an experimental kludge to let you build x11vnc using TurboVNC's modified TightVNC encoding. TurboVNC is part of the VirtualGL project. It does two main things to speed up the TightVNC encoding: * It eliminates bottlenecks, overheads, wait-times in the TightVNC encoding implementation and instead only worries about sending very well (and quickly) compressed JPEG data. * A fast proprietary JPEG implemention is used (Intel IPP on x86) instead of the usual libjpeg implementation. TurboJPEG is an interface library, libturbojpeg, provided by the project that achieves this.

TurboVNC works very well over LAN and evidently fast Broadband too. When using it with x11vnc in such a situation you may want to dial down the delays, e.g. "-wait 5" and "-defer 5" (or even a smaller setting) to poll and pump things out more quickly.

See the instructions in "x11vnc/misc/turbovnc/README" for how to build x11vnc with TurboVNC support. You will also need to download the TurboJPEG software.

In brief, the steps look like this: cd x11vnc-x.y.z/x11vnc/misc/turbovnc ./apply_turbovnc cd ../../.. env LDFLAGS='-L/DIR -Xlinker --rpath=/DIR' ./configure make AM_LDFLAGS='-lturbojpeg'

where you replace "/DIR" with the directory containing libturbojpeg.so you downloaded separately. If it works out well enough TurboVNC support will be integrated into x11vnc and more of its tuning features will be implemented. Support for TurboVNC in SSVNC viewer has been added as an experiment as well. If you try either one, let us know how it went.

There also may be some Linux.i686 and Darwin.i386 x11vnc binaries with TurboVNC support in the misc. bins directory. For other platforms you will need to compile yourself.

On relatively cheap and old hardware (Althon64 X2 5000+ / GeForce 6200) x11vnc and SSVNC, both TurboVNC enabled, were able to sustain 13.5 frames/sec (fps) and 15 Megapixels/sec using the VirtualGL supplied OpenGL benchmark program glxspheres. VirtualGL on higher-end hardware can sustain 20-30 fps with the glxspheres benchmark.

Potential Slowdown: As we describe elsewhere, unless you use x11vnc with an X server using, say, NVidia proprietary drivers (or a virtual X server like Xvfb or Xdummy, or in ShadowFB mode), then the read rate from the graphics card can be rather slow (e.g. 10 MB/sec) and becomes the bottleneck when using x11vnc over fast networks. Note that all of Xorg's drivers currently (2009) have slow read rates (only proprietary drivers appear to have optimized reads.)

So under these (more or less typical) conditions, the speed improvement provided by TurboVNC may only be marginal. Look for this output to see your read rate: 28/02/2009 11:11:07 Autoprobing TCP port 28/02/2009 11:11:07 Autoprobing selected port 5900 28/02/2009 11:11:08 fb read rate: 10 MB/sec 28/02/2009 11:11:08 screen setup finished.

A rate of 10 MB/sec means a 1280x1024x24 screen takes 0.5 seconds to read in. TurboVNC compresses that to JPEG in a much shorter time. On the other hand, an NVidia driver may have a read rate of 250 MB/sec and so only takes 0.02 seconds to read the entire screen in.

[Mouse Cursor Shapes]

Q-81: Why isn't the mouse cursor shape (the little icon shape where the mouse pointer is) correct as I move from window to window?

On X servers supporting XFIXES or Solaris/IRIX Overlay extensions it is possible for x11vnc to do this correctly. See a few paragraphs down for the answer.

Historically, the X11 mouse cursor shape (i.e. little picture: an arrow, X, I-beam, resizer, etc) is one of the few WRITE-only objects in X11. That is, an application can tell the X server what the cursor shape should be when the pointer is in a given window, but a program (like x11vnc) unfortunately cannot read this information. I believe this is because the cursor shape is often downloaded to the graphics hardware (video card), but I could be mistaken.

A simple kludge is provided by the "-cursor X" option that changes the cursor when the mouse is on the root background (or any window has the same cursor as the root background.) Note that desktops like GNOME or KDE often cover up the root background, so this won't work for those cases. Also see the "-cursor some" option for additional kludges.

Note that as of Aug/2004 on Solaris using the SUN_OVL overlay extension and IRIX, x11vnc can show the correct mouse cursor when the -overlay option is supplied. See this FAQ for more info.

Also as of Dec/2004 XFIXES X extension support has been added to allow exact extraction of the mouse cursor shape. XFIXES fixes the problem of the cursor-shape being write-only: x11vnc can now query the X server for the current shape and send it back to the connected viewers. XFIXES is available on recent Linux Xorg based distros and Solaris 10.

The only XFIXES issue is the handling of alpha channel transparency in cursors. If a cursor has any translucency then in general it must be approximated to opaque RGB values for use in VNC. There are some situations where the cursor transparency can also handled exactly: when the VNC Viewer requires the cursor shape be drawn into the VNC framebuffer or if you apply a patch to your VNC Viewer to extract hidden alpha channel data under 32bpp. Details can be found here.

Q-82: When using XFIXES cursorshape mode, some of the cursors look really bad with extra black borders around the cursor and other cruft. How can I improve their appearance?

This happens for cursors with transparency ("alpha channel"); regular X cursors (bitmaps) should be correct. Unfortunately x11vnc 0.7 was released with a very poor algorithm for approximating the transparency, which led to the ugly black borders.

The problem is as follows: XFIXES allows x11vnc to retrieve the current X server cursor shape, including the alpha channel for transparency. For traditional bitmap cursors the alpha value will be 0 for completely transparent pixels and 255 for completely opaque pixels; whereas for modern, eye-candy cursors an alpha value between 0 and 255 means to blend in the background colors to that degree with the cursor colors. The pixel color blending formula is something like this: Red = Red_cursor * a + Red_background * (1 - a), (where here 0 =< a =< 1), with similar for Green and Blue. The VNC protocol does not currently support an alpha channel in cursors: it only supports regular X bitmap cursors and Rich Cursors that have RGB (Red, Green, Blue) color data, but no "A" = alpha data. So in general x11vnc has to approximate a cursor with transparency to create a Rich Cursor. This is easier said than done: some cursor themes have cursors with complicated drop shadows and other forms of translucency.

Anyway, for the x11vnc 0.7.1 release the algorithm for approximating transparency is much improved and hopefully gives decent cursor shapes for most cursor themes and you don't have to worry about it.

In case it still looks bad for your cursor theme, there are (of course!) some tunable parameters. The "-alphacut n" option lets you set the threshold "n" (between 0 and 255): cursor pixels with alpha values below n will be considered completely transparent while values equal to or above n will be completely opaque. The default is 240. The "-alphafrac f" option tries to correct individual cursors that did not fare well with the default -alphacut value: if a cursor has less than fraction f (between 0.0 and 1.0) of its pixels selected by the default -alphacut, the threshold is lowered until f of its pixels are selected. The default fraction is 0.33.

Finally, there is an option -alpharemove that is useful for themes where many cursors are light colored (e.g. "whiteglass".) XFIXES returns the cursor data with the RGB values pre-multiplied by the alpha value. If the white cursors look too grey, specify -alpharemove to brighten them by having x11vnc divide out the alpha value.

One user played with these parameters and reported back: Of the cursor themes present on my system:

gentoo and gentoo-blue: alphacut:192 - noalpharemove

gentoo-silver: alphacut:127 and alpharemove

whiteglass and redglass (presumably also handhelds, which is based heavily on redglass) look fine with the apparent default of alphacut:255.

Q-83: In XFIXES mode, are there any hacks to handle cursor transparency ("alpha channel") exactly?

As of Jan/2005 libvncserver has been modified to allow an alpha channel (i.e. RGBA data) for Rich Cursors. So x11vnc can now send the alpha channel data to libvncserver. However, this data will only be used for VNC clients that do not support the CursorShapeUpdates VNC extension (or have disabled it.) It can be disabled for all clients with the -nocursorshape x11vnc option. In this case the cursor is drawn, correctly blended with the background, into the VNC framebuffer before being sent out to the client. So the alpha blending is done on the x11vnc side. Use the -noalphablend option to disable this behavior (always approximate transparent cursors with opaque RGB values.)

The CursorShapeUpdates VNC extension complicates matters because the cursor shape is sent to the VNC viewers supporting it, and the viewers draw the cursor locally. This improves response over slow links. Alpha channel data for these locally drawn cursors is not supported by the VNC protocol.

However, in the libvncserver CVS there is a patch to the TightVNC viewer to make this work for CursorShapeUpdates under some circumstances. This hack is outside of the VNC protocol. It requires the screens on both sides to be depth 24 at 32bpp (it uses the extra 8 bits to secretly hide the cursor alpha channel data.) Not only does it require depth 24 at 32bpp, but it also currently requires the client and server to be of the same endianness (otherwise the hidden alpha data gets reset to zero by a libvncserver translation function; we can fix this at some point if there is interest.) The patch is for the TightVNC 1.3dev5 Unix vncviewer and it enables the TightVNC viewer to do the cursor alpha blending locally. The patch code should give an example on how to change the Windows TightVNC viewer to achieve the same thing (send me the patch if you get that working.)

This patch is applied to the Enhanced TightVNC Viewer (SSVNC) package we provide.

[Mouse Pointer]

Q-84: Why does the mouse arrow just stay in one corner in my vncviewer, whereas my cursor (that does move) is just a dot?

This default takes advantage of a tightvnc extension (CursorShapeUpdates) that allows specifying a cursor image shape for the local VNC viewer. You may disable it with the -nocursor option to x11vnc if your viewer does not have this extension.

Note: as of Aug/2004 this should be fixed: the default for non-tightvnc viewers (or ones that do not support CursorShapeUpdates) will be to draw the moving cursor into the x11vnc framebuffer. This can also be disabled via -nocursor.

Q-85: Can I take advantage of the TightVNC extension to the VNC protocol where Cursor Positions Updates are sent back to all connected clients (i.e. passive viewers can see the mouse cursor being moved around by another viewer)?

Use the -cursorpos option when starting x11vnc. A VNC viewer must support the Cursor Positions Updates for the user to see the mouse motions (the TightVNC viewers support this.) As of Aug/2004 -cursorpos is the default. See also -nocursorpos and -nocursorshape.

Q-86: Is it possible to swap the mouse buttons (e.g. left-handed operation), or arbitrarily remap them? How about mapping button clicks to keystrokes, e.g. to partially emulate Mouse wheel scrolling?

You can remap the mouse buttons via something like: -buttonmap 13-31 (or perhaps 12-21.) Also, note that xmodmap(1) lets you directly adjust the X server's button mappings, but in some circumstances it might be more desirable to have x11vnc do it.

One user had an X server with only one mouse button(!) and was able to map all of the VNC client mouse buttons to it via: -buttonmap 123-111.

Note that the -debug_pointer option prints out much info for every mouse/pointer event and is handy in solving problems.

To map mouse button clicks to keystrokes you can use the alternate format where the keystrokes are enclosed between colons like this :: in place of the mouse button digit. For a sequence of keysyms separate them with "+" signs. Look in the include file <X11/keysymdef.h>, or use xev(1), or -debug_keyboard to find the keysym names. Button clicks can also be included in the sequence via the fake keysyms Button1, etc.

As an example, suppose the VNC viewer machine has a mouse wheel (these generate button 4 and 5 events), but the machine that x11vnc is run on only has the 3 regular buttons. In normal operation x11vnc will discard the button 4 and 5 events. However, either of the following button maps could possibly be of use emulating the mouse wheel events in this case: -buttonmap 12345-123:Prior::Next: -buttonmap 12345-123:Up+Up+Up::Down+Down+Down:

Exactly what keystroke "scrolling" events they should be bound to depends on one's taste. If this method is too approximate, one could consider not using -buttonmap but rather configuring the X server to think it has a mouse with 5 buttons even though the physical mouse does not. (e.g. 'Option "ZAxisMapping" "4 5"'.)

Note that when a keysym-mapped mouse button is clicked down this immediately generates the key-press and key-release events (for each keysym in turn if the mapping has a sequence of keysyms.) When the mouse button goes back up nothing is generated.

If you include modifier keys like Shift_L instead of key-press immediately followed by key-release the state of the modifier key is toggled (however the initial state of the modifier key is ignored.) So to map the right button to type my name 'Karl Runge' I could use this: -buttonmap 3-:Shift_L+k+Shift_L+a+r+l+space+Shift_L+r+Shift_L+u+n+g+e:

(yes, this is getting a little silly.)

BTW, Coming the other way around, if the machine you are sitting at does not have a mouse wheel, but the remote machine does (or at least has 5 buttons configured), this key remapping can be useful: -remap Super_R-Button4,Menu-Button5

you just tap those two keys to get the mouse wheel scrolls (this is more useful than the Up and Down arrow keys because a mouse wheel "click" usually gives a multi-line scroll.) [Keyboard Issues]

Q-87: How can I get my AltGr and Shift modifiers to work between keyboards for different languages?

The option -modtweak should help here. It is a mode that monitors the state of the Shift and AltGr Modifiers and tries to deduce the correct keycode to send, possibly by sending fake modifier key presses and releases in addition to the actual keystroke.

Update: As of Jul/2004 -modtweak is now the default (use -nomodtweak to get the old behavior.) This was done because it was noticed on newer XFree86 setups even on bland "us" keyboards like "pc104 us" XFree86 included a "ghost" key with both "<" and ">" it. This key does not exist on the keyboard (see this FAQ for more info.) Without -modtweak there was then an ambiguity in the reverse map keysym => keycode, making it so the "<" symbol could not be typed.

Also see the FAQ about the -xkb option for a more powerful method of modifier tweaking for use on X servers with the XKEYBOARD extension.

When trying to resolve keyboard mapping problems, note that the -debug_keyboard option prints out much info for every keystroke and so can be useful debugging things.

Note that one user had a strange setup and none of the above helped. His solution was to disable all of the above and use -nomodtweak. This is the simplest form of keystroke insertion and it actually solved the problem. Try it if the other options don't help.

Q-88: When I try to type a "<" (i.e. less than) instead I get ">" (i.e. greater than)! Strangely, typing ">" works OK!!

Does your keyboard have a single key with both "<" and ">" on it? Even if it doesn't, your X server may think your keyboard has such a key (e.g. pc105 in the XF86Config file when it should be something else, say pc104.)

Short Cut: Try the -xkb or -sloppy_keys options and see if that helps the situation. The discussion below is a bit outdated (e.g. -modtweak is now the default) but it is useful reference for various tricks and so is kept.

The problem here is that on the Xserver where x11vnc is run there are two keycodes that correspond to the "<" keysym. Run something like this to see:

xmodmap -pk | egrep -i 'KeyCode|less|greater' There are 4 KeySyms per KeyCode; KeyCodes range from 8 to 255. KeyCode Keysym (Keysym) ... 59 0x002c (comma) 0x003c (less) 60 0x002e (period) 0x003e (greater) 94 0x003c (less) 0x003e (greater)

That keycode 94 is the special key with both "<" and ">". When x11vnc receives the "<" keysym over the wire from the remote VNC client, it unfortunately maps it to keycode 94 instead of 59, and sends 94 to the X server. Since Shift is down (i.e. you are Shifting the comma key), the X server interprets this as Shifted-94, which is ">".

A workaround in the X server configuration is to "deaden" that special key:

xmodmap -e "keycode 94 = "

However, one user said he had to do this:

xmodmap -e "keycode 94 = 0x002c 0x003c"

(If the numerical values are different for your setup, substitute the ones that correspond to your display. The above xmodmap scheme can often be used to work around other ambiguous keysym to keycode mappings.)

Alternatively, here are some x11vnc options to try to work around the problem: -modtweak

and -remap less-comma

These are convenient in that they do not modify the actual X server settings. The former (-modtweak) is a mode that monitors the state of the Shift and AltGr modifiers and tries to deduce the correct keycode sequence to send. Since Jul/2004 -modtweak is now the default. The latter (-remap less-comma) is an immediate remapping of the keysym less to the keysym comma when it comes in from a client (so when Shift is down the comma press will yield "<".)

See also the FAQ about the -xkb option as a possible workaround using the XKEYBOARD extension.

Note that the -debug_keyboard option prints out much info for every keystroke to aid debugging keyboard problems.

Q-89: Extra Character Inserted, E.g.: When I try to type a "<" (i.e. less than) instead I get "<," (i.e. an extra comma.)

This is likely because you press "Shift" then "<" but then released the Shift key before releasing the "<". Because of a keymapping ambiguity the last event "< up" is interpreted as "," because that key unshifted is the comma.

This extra character insertion will happen for other combinations of characters: in general it can happen whenever the Shift key is released early.

This should not happen in -xkb mode, because it works hard to resolve the ambiguities. If you do not want to use -xkb, try the option -sloppy_keys to attempt a similar type of algorithm.

One user had this problem for Italian and German keyboards with the key containing ":" and "." When he typed ":" he would get an extra "." inserted after the ":". The solution was -sloppy_keys.

Q-90: I'm using an "international" keyboard (e.g. German "de", or Danish "dk") and the -modtweak mode works well if the VNC viewer is run on a Unix/Linux machine with a similar keyboard. But if I run the VNC viewer on Unix/Linux with a different keyboard (e.g. "us") or Windows with any keyboard, I can't type some keys like: "@", "$", "<", ">", etc. How can I fix this?

The problem with Windows is it does not seem to handle AltGr well. It seems to fake it up by sending Control_L+Alt_R to applications. The Windows VNC viewer sends those two down keystrokes out on the wire to the VNC server, but when the user types the next key to get, e.g., "@" the Windows VNC viewer sends events bringing the up the Control_L+Alt_R keys, and then sends the "@" keysym by itself.

The Unix/Linux VNC viewer on a "us" keyboard does a similar thing since "@" is the Shift of the "2" key. The keysyms Shift and "@" are sent to the VNC server.

In both cases no AltGr is sent to the VNC server, but we know AltGr is needed on the physical international keyboard to type a "@".

This all worked fine with x11vnc running with the -modtweak option (it figures out how to adjust the Modifier keys (Shift or AltGr) to get the "@".) However it fails under recent versions of XFree86 (and the X.org fork.) These run the XKEYBOARD extension by default and make heavy use of it to handle international keyboards.

To make a long story short, on these newer XFree86 setups the traditional X keymap lookup x11vnc uses is no longer accurate. x11vnc can't find the keysym "@" anywhere in the keymapping! (even though it is in the XKEYBOARD extended keymapping.)

How to Solve: As of Jul/2004 x11vnc has two changes: * -modtweak (tweak Modifier keys) is now the default (use -nomodtweak to go back to the old way) * there is a new option -xkb to use the XKEYBOARD extension API to do the Modifier key tweaking.

The -xkb option seems to fix all of the missing keys: "@", "<", ">", etc.: it is recommended that you try it if you have this sort of problem. Let us know if there are any remaining problems (see the next paragraph for some known problems.) If you specify the -debug_keyboard (aka -dk) option twice you will get a huge amount of keystroke debugging output (send it along with any problems you report.)

Update: as of Jun/2005 x11vnc will try to automatically enable -xkb if it appears that would be beneficial (e.g. if it sees any of "@", "<", ">", "[" and similar keys are mapped in a way that needs the -xkb to access them.) To disable this automatic check use -noxkb.

Known problems: * One user had to disable a "ghost" Mode_switch key that was causing problems under -xkb. His physical AltGr key was bound to ISO_Level3_Shift (which seems to be the XKEYBOARD way of doing things), while there was a ghost key Mode_switch (which seems to be obsolete) in the mapping as well. Both of these keysyms were bound to Mod5 and x11vnc was unfortunately choosing Mode_switch. From the x11vnc -xkb -dk -dk output it was noted that Mode_switch was attached to keycode 93 (no physical key generates this keycode) while ISO_Level3_Shift was attached to keycode 113. The keycode skipping option was used to disable the ghost key: -skip_keycodes 93 * In implementing -xkb we noticed that some characters were still not getting through, e.g. "" and "^". This is not really an XKEYBOARD problem. What was happening was the VNC viewer was sending the keysyms asciitilde and asciicircum to x11vnc, but on the X server with the international keyboard those keysyms were not mapped to any keys. So x11vnc had to skip them (Note: as of May/2005 they are added by default see -add_keysyms below.) The way these characters are typically entered on international keyboards is by "dead" (aka "mute") keys. E.g. to enter "" at the physical display the keysym dead_tilde is pressed and released (this usually involves holding AltGr down while another key is pressed) and then space is pressed. (this can also be used get characters with the "~" symbol on top, e.g. "\E3" by typing "a" instead of space.) What to do? In general the VNC protocol has not really solved this problem: what should be done if the VNC viewer sends a keysym not recognized by the VNC server side? Workarounds can possibly be created using the -remap x11vnc option: -remap asciitilde-dead_tilde,asciicircum-dead_circumflex etc. Use -remap filename if the list is long. Please send us your workarounds for this problem on your keyboard. Perhaps we can have x11vnc adjust automatically at some point. Also see the -add_keysyms option in the next paragraph. Update: for convenience "-remap DEAD" does many of these mappings at once. * To complement the above workaround using the -remap, an option -add_keysyms was added. This option instructs x11vnc to bind any unknown Keysyms coming in from VNC viewers to unused Keycodes in the X server. This modifies the global state of the X server. When x11vnc exits it removes the extra keymappings it created. Note that the -remap mappings are applied first, right when the Keysym is received from a VNC viewer, and only after that would -add_keysyms, or anything else, come into play. Update: -add_keysyms is now on by default. Use -noadd_keysyms to disable.

Q-91: When typing I sometimes get double, triple, or more of my keystrokes repeated. I'm sure I only typed them once, what can I do?

This may be due to an interplay between your X server's key autorepeat delay and the extra time delays caused by x11vnc processing.

Short answer: disable key autorepeating by running the command "xset r off" on the Xserver where x11vnc is run (restore via "xset r on") or use the new (Jul/2004) -norepeat x11vnc option. You will still have autorepeating because that is taken care of on your VNC viewer side.

Update: as of Dec/2004 -norepeat is now the default. Use -repeat to disable it.

Details: suppose you press a key DOWN and it generates changes in large regions of the screen. The CPU and I/O work x11vnc does for the large screen change could be longer than your X server's key autorepeat delay. x11vnc may not get to processing the key UP event until after the screen work is completed. The X server believes the key has been held down all this time, and applies its autorepeat rules.

Even without inducing changes in large regions of the screen, this problem could arise when accessing x11vnc via a dialup modem or otherwise high latency link (e.g. > 250 ms latency.)

Look at the output of "xset q" for the "auto repeat delay" setting. Is it low (e.g. < 300 ms)? If you turn off autorepeat completely: "xset r off", does the problem go away?

The workaround is to manually apply "xset r off" and "xset r on" as needed, or to use the -norepeat (which has since Dec/2004 been made the default.) Note that with X server autorepeat turned off the VNC viewer side of the connection will (nearly always) do its own autorepeating so there is no big loss here, unless someone is also working at the physical display and misses his autorepeating.

Q-92: The x11vnc -norepeat mode is in effect, but I still get repeated keystrokes!!

Are you using x11vnc to log in to an X session via display manager? (as described in this FAQ) If so, x11vnc is starting before your session and it disables autorepeat when you connect, but then after you log in your session startup (GNOME, KDE, ...) could be resetting the autorepeat to be on. Or it could be something inside your desktop trying to be helpful that decides to turn it back on.

x11vnc in -norepeat mode will by default reset autorepeat to off 2 times (to help get thru the session startup problem), but it will not continue to battle with things turning autorepeat back on. It will also turn autorepeat off whenever it goes from a state of zero clients to one client. You can adjust the number of resets via "-norepeat N", or use "-norepeat -1" to have it keep resetting it whenever autorepeat gets turned back on when clients are connected.

In general you can manually turn autorepeating off by typing "xset r off", or a using desktop utility/menu, or "x11vnc -R norepeat". If something in your desktop is automatically turning it back on you should figure out how to disable that somehow.

Q-93: After using x11vnc for a while, I find that I cannot type some (or any) characters or my mouse clicks and drags no longer have any effect, or they lead to strange effects. What happened?

Probably a modifier key, e.g. Control or Alt is "stuck" in a pressed down state.

This happens for VNC in general by the following mechanism. Suppose on the Viewer side desktop there is some hot-key to switch desktops/rooms/spaces, etc. E.g. suppose Alt+LeftArrow moves to the left desktop/room/space. Or suppose an Alt+hotkey combination iconifies a window. This can leave the Alt key pressed down on the remote side.

Consider the sequence that happens. The Alt_L key and then the LeftArrow key go down. Since you are inside the viewer the Alt_L key press is sent to the other side (x11vnc) and so it is pressed down in the remote desktop as well. (by "Alt_L" we mean the Alt key on the left-hand side of the keyboard.) Your local desktop (where the VNC Viewer is running) then warps to the new desktop/room/space: Leaving the Alt_L key still pressed down in the remote desktop.

If someone is sitting at the desktop, or when you return in the viewer it may be very confusing because the Alt_L is still pressed down but you (or the person sitting at the desktop) do not realize this. Depending on which remote desktop (x11vnc side) is used, it can act very strangely.

A quick workaround when you notice this is to press and release all of the Alt, Shift, Control, Windows-Flag, modifier keys to free the pressed one. You need to do this for both the left and right Shift, Alt, Control, etc. keys to be sure.

Note that many VNC Viewers try to guard against this when they are notified by the window system that the viewer app has "lost focus". When it receives the "lost focus" event, the viewer sends VNC Key-Release events for all modifier keys that are currently pressed down. This does not always work, however, since it depends on how the desktop manages these "warps". If the viewer is not notified it cannot know it needs to release the modifiers.

You can also use the -clear_mods option to try to clear all of the modifier keys at x11vnc startup. You will still have to be careful that you do not leave the modifier key pressed down during your session. It is difficult to prevent this problem from occurring (short of using -remap to prevent sending all of the problem modifier keys, which would make the destkop pretty unusable.)

During a session these x11vnc remote control commands can also help: x11vnc -R clear_mods x11vnc -R clear_keys x11vnc -R clear_locks x11vnc -R clear_all

A similar problem can occur if you accidentally press the Caps_Lock or Num_Lock down. When these are locked on the remote side it can sometimes lead to strange desktop behavior (e.g. cannot drag or click on windows.) As above you may not notice this because the lock isn't down on the local (Viewer) side. See this FAQ on lock keys problem. These options may help avoid the problem: -skip_lockkeys and -capslock. See also -clear_all.

Q-94: The machine where I run x11vnc has an AltGr key, but the local machine where I run the VNC viewer does not. Is there a way I can map a local unused key to send an AltGr? How about a Compose key as well?

Something like "-remap Super_R-Mode_switch" x11vnc option may work. Note that Super_R is the "Right Windoze(tm) Flaggie" key; you may want to choose another. The -debug_keyboard option comes in handy in finding keysym names (so does xev(1).)

For Compose how about "-remap Menu-Multi_key" (note that Multi_key is the official name for Compose.) To do both at the same time: "-remap Super_R-Mode_switch,Menu-Multi_key" or use "-remap filename" to specify remappings from a file.

Q-95: I have a Sun machine I run x11vnc on. Its Sun keyboard has just one Alt key labelled "Alt" and two Meta keys labelled with little diamonds. The machine where I run the VNC viewer only has Alt keys. How can I send a Meta keypress? (e.g. emacs needs this)

Here are a couple ideas. The first one is to simply use xmodmap(1) to adjust the Sun X server. Perhaps xmodmap -e "keysym Alt_L = Meta_L Alt_L" will do the trick. (there are other ways to do it, one user used: xmodmap -e "keycode 26 = Meta_L" for his setup.)

Since xmodmap(1) modifies the X server mappings you may not want to do this (because it affects local work on that machine.) Something like the -remap Alt_L-Meta_L to x11vnc may be sufficient for ones needs, and does not modify the X server environment. Note that you cannot send Alt_L in this case, maybe -remap Super_L-Meta_L would be a better choice if the Super_L key is typically unused in Unix.

Q-96: Running x11vnc on HP-UX I cannot type "#" I just get a "3" instead.

One user reports this problem on HP-UX Rel_B.11.23. The problem was traced to a strange keyboard mapping for the machine (e.g. xmodmap -pk output) that looked like: ... 039 2 at at at ... 047 3 numbersign numbersign numbersign

and similar triple mappings (with two in the AltGr/Mode_switch group) of a keysum to a single keycode.

Use the -nomodtweak option as a workaround. You can also use xmodmap to correct these mappings in the server, e.g.: xmodmap -e "keycode 47 = 3 numbersign"

Also, as of Feb/2007, set the environment variable MODTWEAK_LOWEST=1 (either in your shell or via "-env MODTWEAK_LOWEST=1" option) to handle these mappings better.

Q-97: Can I map a keystroke to a mouse button click on the remote machine?

This can be done directly in some X servers using AccessX and Pointer_EnableKeys, but is a bit awkward. It may be more convenient to have x11vnc do the remapping. This can be done via the -remap option using the fake "keysyms" Button1, Button2, etc. as the "to" keys (i.e. the ones after the "-")

As an example, consider a laptop where the VNC viewer is run that has a touchpad with only two buttons. It is difficult to do a middle button "paste" because (using XFree86/Xorg Emulate3Buttons) you have to click both buttons on the touch pad at the same time. This remapping: -remap Super_R-Button2

maps the Super_R "flag" key press to the Button2 click, thereby making X pasting a bit easier.

Note that once the key goes down, the button down and button up events are generated immediately on the x11vnc side. When the key is released (i.e. goes up) no events are generated.

Q-98: How can I get Caps_Lock to work between my VNC viewer and x11vnc?

This is a little tricky because it is possible to get the Caps_Lock state out of sync between your viewer-side machine and the x11vnc-side X server. For best results, we recommend not ever letting the Caps_Lock keypresses be processed by x11vnc. That way when you press Caps_Lock in the viewer your local machine goes into the Caps_Lock on state and sends keysym "A" say when you press "a". x11vnc will then fake things up so that Shift is held down to generate "A". The -skip_lockkeys option should help to accomplish this. For finer grain control use something like: "-remap Caps_Lock-None".

Also try the -nomodtweak and -capslock options.

Another useful option that turns off any Lock keys on the remote side at startup and end is the -clear_all option. During a session you can run these remote control commands to modify the Lock keys: x11vnc -R clear_locks x11vnc -R clear_all

the former will try to unset any Lock keys, the latter will do same and also try to make it so no key is pressed down (e.g. "stuck" Alt_L, etc.) [Screen Related Issues and Features]

Q-99: The remote display is larger (in number of pixels) than the local display I am running the vncviewer on. I don't like the vncviewer scrollbars, what I can do?

vncviewer has a option (usually accessible via F8 key or -fullscreen option) for vncviewer to run in full screen, where it will automatically scroll when the mouse is near the edge of the current view. For quick scrolling, also make sure Backing Store is enabled on the machine vncviewer is run on. (XFree86/Xorg disables it by default for some reason, add Option "backingstore" to XF86Config on the vncviewer side.)

BTW, contact me if you are having problems with vncviewer in fullscreen mode with your window manager (i.e. no keyboard response.) I have a workaround for vncviewer using XGrabServer().

There may also be scaling viewers out there (e.g. TightVNC or UltraVNC on Windows) that automatically shrink or expand the remote framebuffer to fit the local display. Especially for hand-held devices. See also the next FAQ on x11vnc scaling.

Q-100: Does x11vnc support server-side framebuffer scaling? (E.g. to make the desktop smaller.)

As of Jun/2004 x11vnc provides basic server-side scaling. It is a global scaling of the desktop, not a per-client setting. To enable it use the "-scale fraction" option. "fraction" can either be a floating point number (e.g. -scale 0.75) or the alternative m/n fraction notation (e.g. -scale 3/4.) Note that if fraction is greater than one the display is magnified.

Extra resources (CPU, memory I/O, and memory) are required to do the scaling. If the machine is slow where x11vnc is run with scaling enabled, the interactive response can be unacceptable. OTOH, if run with scaling on a fast machine the performance degradation is usually not a big issue or even noticeable.

It may help to compile x11vnc with compiler option -O3 or -O4 to speed up the scaling code. Set the CFLAGS env. var. before running configure.

Also, if you just want a quick, rough "thumbnail" of the display you can append ":nb" to the fraction to turn on "no blending" mode. E.g.: "-scale 1/3:nb" Fonts will be difficult to read, but the larger features will be recognizable. BTW, "no blending" mode is forced on when scaling 8bpp PseudoColor displays (because blending an indexed colormap is a bad idea and leads to random colors, use :fb to force it on.)

One can also use the ":nb" with an integer scale factor (say "-scale 2:nb") to use x11vnc as a screen magnifier for vision impaired applications. Since with integer scale factors the framebuffers become huge and scaling operations time consuming, be sure to use ":nb" for the fastest response.

In general for a scaled display if you are using a TightVNC viewer you may want to turn off jpeg encoding (e.g. vncviewer -nojpeg host:0.) There appears to be a noise enhancement effect, especially for regions containing font/text: the scaling can introduce some pixel artifacts that evidently causes the tight encoding algorithm to incorrectly detect the regions as image data and thereby introduce additional pixel artifacts due to the lossiness of the jpeg compression algorithm. Experiment to see if -nojpeg vncviewer option improves the readability of text when using -scale to shrink the display size. Also note that scaling may actually slow down the transfer of text regions because after being scaled they do not compress as well. (this can often be a significant slowdown, e.g. 10X.)

Another issue is that it appears VNC viewers require the screen width to be a multiple of 4. When scaling x11vnc will round the width to the nearest multiple of 4. To disable this use the ":n4" sub option (like ":nb" in the previous paragraph; to specify both use a comma: ":nb,n4", etc.)

If one desires per-client scaling for something like 1:1 from a workstation and 1:2 from a smaller device (e.g. handheld), currently the only option is to run two (or more) x11vnc processes with different scalings listening on separate ports (-rfbport option, etc.)

Update: As of May/2006 x11vnc also supports the UltraVNC server-side scaling. This is a per-client scaling by factors 1/2, 1/3, ... and so may be useful for PDA's ("-scale 1/2", etc. will give similar results except that it applies to all clients.) You may need to supply "-rfbversion 3.6" for this to be recognized by UltraVNC viewers.

BTW, whenever you run two or more x11vnc's on the same X display and use the GUI, then to avoid all of the x11vnc's simultaneously answering the gui you will need to use something like "-connect file1 -gui ..." with different connect files for each x11vnc you want to control via the gui (or remote-control.) The "-connect file1" usage gives separate communication channels between a x11vnc process and the gui process. Otherwise they all share the same X property channels: VNC_CONNECT and X11VNC_REMOTE.

Update: As of Mar/2005 x11vnc now scales the mouse cursor with the same scale factor as the screen. If you don't want that, use the "-scale_cursor frac" option to set the cursor scaling to a different factor (e.g. use "-scale_cursor 1" to keep the cursor at its natural unscaled size.)

Q-101: Does x11vnc work with Xinerama? (i.e. multiple monitors joined together to form one big, single screen.)

Yes, it should generally work because it simply polls the big effective screen.

If the viewing-end monitor is not as big as the remote Xinerama display, then the vncviewer scrollbars, etc, will have to be used to pan across the large area. However one user started two x11vnc's, one with "-clip 1280x1024+0+0" and the other with "-clip 1280x1024+1280+0" to split the big screen into two and used two VNC viewers to access them.

As of Jun/2008: Use "-clip xinerama0" to clip to the first xinerama sub-screen (if xinerama is active.) xinerama1 for the 2nd sub-screen, etc. This way you don't need to figure out the WxH+X+Y of the desired xinerama sub-screen. screens are sorted in increasing distance from the (0,0) origin (I.e. not the Xserver's order.)

There are a couple potential issues with Xinerama however. If the screen is not rectangular (e.g. 1280x1024 and 1024x768 monitors joined together), then there will be "non-existent" areas on the screen. The X server will return "garbage" image data for these areas and so they may be distracting to the viewer. The -blackout x11vnc option allows you to blacken-out rectangles by manually specifying their WxH+X+Y geometries. If your system has the libXinerama library, the -xinerama x11vnc option can be used to have it automatically determine the rectangles to be blackened out. (Note on 8bpp PseudoColor displays the fill color may not be black.) Update: -xinerama is now on by default.

Some users have reported that the mouse does not behave properly for their Xinerama display: i.e. the mouse cannot be moved to all regions of the large display. If this happens try using the -xwarppointer option. This instructs x11vnc to fake mouse pointer motions using the XWarpPointer function instead of the XTestFakeMotionEvent XTEST function. (This may be due to a bug in the X server for XTEST when Xinerama is enabled.) Update: As of Dec/2006 -xwarppointer will be applied automatically if Xinerama is detected. To disable use: -noxwarppointer

Q-102: Can I use x11vnc on a multi-headed display that is not Xinerama (i.e. separate screens :0.0, :0.1, ... for each monitor)?

You can, but it is a little bit awkward: you must start separate x11vnc processes for each screen, and on the viewing end start up separate VNC viewer processes connecting to them. e.g. on the remote end: x11vnc -display :0.0 -bg -q -rfbport 5900 x11vnc -display :0.1 -bg -q -rfbport 5901

(this could be automated in the display manager Xsetup for example) and then on the local machine where you are sitting: vncviewer somehost:0 & vncviewer somehost:1 &

Note: if you are running on Solaris 8 or earlier you can easily hit up against the maximum of 6 shm segments per process (for Xsun in this case) from running multiple x11vnc processes. You should modify /etc/system as mentioned in another FAQ to increase the limit. It is probably also a good idea to run with the -onetile option in this case (to limit each x11vnc to 3 shm segments), or even -noshm to use no shm segments.

Q-103: Can x11vnc show only a portion of the display? (E.g. for a special purpose application or a very large screen.)

As of Mar/2005 x11vnc has the "-clip WxH+X+Y" option to select a rectangle of width W, height H and offset (X, Y). Thus the VNC screen will be the clipped sub-region of the display and be only WxH in size. One user used -clip to split up a large Xinerama screen into two more managable smaller screens.

This also works to view a sub-region of a single application window if the -id or -sid options are used. The offset is measured from the upper left corner of the selected window.

Q-104: Does x11vnc support the XRANDR (X Resize, Rotate and Reflection) extension? Whenever I rotate or resize the screen x11vnc just seems to crash.

As of Dec/2004 x11vnc supports XRANDR. You enable it with the -xrandr option to make x11vnc monitor XRANDR events and also trap X server errors if the screen change occurred in the middle of an X call like XGetImage. Once it traps the screen change it will create a new framebuffer using the new screen.

If the connected vnc viewers support the NewFBSize VNC extension (Windows TightVNC viewer and RealVNC 4.0 windows and Unix viewers do) then the viewer will automatically resize. Otherwise, the new framebuffer is fit as best as possible into the original viewer size (portions of the screen may be clipped, unused, etc.) For these viewers you can try the -padgeom option to make the region big enough to hold all resizes and rotations. We have fixed this problem for the TightVNC Viewer on Unix: SSVNC

If you specify "-xrandr newfbsize" then vnc viewers that do not support NewFBSize will be disconnected before the resize. If you specify "-xrandr exit" then all will be disconnected and x11vnc will terminate.

Q-105: Independent of any XRANDR, can I have x11vnc rotate and/or reflect the screen that the VNC viewers see? (e.g. for a handheld whose screen is rotated 90 degrees.)

As of Jul/2006 there is the -rotate option allow this. E.g's: "-rotate +90", "-rotate -90", "-rotate x", etc.

Q-106: Why is the view in my VNC viewer completely black? Or why is everything flashing around randomly?

See the next FAQ for a possible explanation.

Q-107: I use Linux Virtual Terminals (VT's) to implement 'Fast User Switching' between users' sessions (e.g. Betty is on Ctrl-Alt-F7, Bobby is on Ctrl-Alt-F8, and Sid is on Ctrl-Alt-F1: they use those keystrokes to switch between their sessions.) How come the view in a VNC viewer connecting to x11vnc is either completely black or otherwise all messed up unless the X session x11vnc is attached to is in the active VT?

This seems to have to do with how applications (the X server processes in this case) must "play nicely" if they are not on the active VT (sometimes called VC for virtual console.) That is, they should not read from the keyboard or mouse or manage the video display unless they have the active VT. Given that it appears the XGetImage() call must ultimately retrieve the framebuffer data from the video hardware itself, it would make sense x11vnc's polling wouldn't work unless the X session had active control of the VT.

There does not seem to be an easy way to work around this. Even xwd(1) doesn't work in this case (try it.) Something would need to be done at a lower level, say in the XFree86/Xorg X server. Also, using the Shadow Framebuffer (a copy of the video framebuffer is kept in main memory) does not appear to fix the problem.

If no one is sitting at the workstation and you just want to remotely switch the VT over to the one associated with your X session (so x11vnc can poll it correctly), one can use the chvt(1) command, e.g. "chvt 7" for VT #7.

Q-108: I am using x11vnc where my local machine has "popup/hidden taskbars" and the remote display where x11vnc runs also has "popup/hidden taskbars" and they interfere and fight with each other. What can I do?

When you move the mouse to the edge of the screen where the popups happen, the taskbars interfere with each other in strange ways. This sometimes happens where the local machine is GNOME or Mac OS X and the remote machine is GNOME. Is there a way to temporarily disable one or both of these magic desktop taskbars?

One x11vnc user suggests: it should be straightforward to right mouse click on the task bar panel, and uncheck "enable auto-hide" from the panel properties dialog box. This will make the panel always visible.

Q-109: Help! x11vnc and my KDE screensaver keep switching each other on and off every few seconds.

This is a new (Jul/2006) problem seen, say, on the version of KDE that is shipped with SuSE 10.1. It is not yet clear what is causing this... If you move the mouse through x11vnc the screensaver shuts off like it should but then a second or two after you stop moving the mouse the screensaver snaps back on.

This may be a bug in kdesktop_lock. For now the only workaround is to disable the screensaver. You can try using another one such as straight xscreensaver (see the instructions here for how to disable kdesktop_lock.) If you have more info on this or see it outside of KDE please let us know.

Update: It appears this is due to kdesktop_lock enabling the screen saver when the Monitor is in DPMS low-power state (e.g. standby, suspend, or off.) In Nov/2006 the x11vnc -nodpms option was added as a workaround. Normally it is a good thing that the monitor powers down (since x11vnc can still poll the framebuffer in this state), but if you experience the kdesktop_lock problem you can specify the "-nodpms" option to keep the Monitor out of low power state while VNC clients are connected. This is basically the same as typing "xset dpms force on" periodically. (if you don't want to do these things just disable the screensaver.) Feel free to file a bug against kdesktop_lock with KDE.

Q-110: I am running the compiz 3D window manager (or beryl, MythTv, Google Earth, or some other OpenGL app) and I do not get screen updates in x11vnc.

This appears to be because the 3D OpenGL/GLX hardware screen updates do not get reported via the XDAMAGE mechanism. So this is a bug in compiz/beryl or XDAMAGE/Xorg or the (possibly 3rd party) video card driver.

As a workaround apply the -noxdamage option. As of Feb/2007 x11vnc will try to autodetect the problem and disable XDAMAGE if is appears to be missing a lot of updates. But if you know you are using compiz you might as well always supply -noxdamage. Thanks to this user who reported the problem and discovered the workaround.

A developer for MiniMyth reports that the 'alphapulse' tag of the theme G.A.N.T. can also cause problems, and should be avoided when using VNC.

Please report a bug or complaint to Beryl/Compiz and/or Xorg about this: running x11vnc with -noxdamage disables a nice improvement in responsiveness (especially for typing) and also leads to unnecessary CPU and memory I/O load due to the extra polling.

Update: as of May/2010 NVIDIA may have fixed this problem in their proprietary drivers. See the NVIDIA Release Notes. (look for 'x11vnc'.)

Q-111: Can I use x11vnc to view my VMWare session remotely?

Yes, since VMWare usually runs as an X application you can view it via x11vnc in the normal way.

Note that VMWare has several viewing modes: * Normal X application window (with window manager frame) * Quick-Switch mode (with no window manager frame) * Fullscreen mode

The way VMWare does Fullscreen mode on Linux is to display the Guest desktop in a separate Virtual Terminal (e.g. VT 8) (see this FAQ on VT's for background.) Unfortunately, this Fullscreen VT is not an X server. So x11vnc cannot access it (however, see this discussion of -rawfb for a possible workaround.) x11vnc works fine with "Normal X application window" and "Quick-Switch mode" because these use X.

Update: It appears the in VMWare 5.x the Fullscreen mode is X, so x11vnc access does work.

One user reports he left his machine with VMWare in the Fullscreen mode, and even though his X session wasn't in the active VT, he could still connect x11vnc to the X session and pass the keystrokes Ctrl-Alt (typing "blind") to the VMWare X app. This induced VMWare to switch out of Fullscreen into Normal X mode and he could continue working in the Guest desktop remotely.

Aside: Sometimes it is convenient (for performance, etc.) to start VMWare in its own X session using startx(1). This can be used to have a minimal window manger (e.g. twm or even no window manager), to improve response. One can also cut the display depth (e.g. to 16bpp) in this 2nd X session to improve video performance. This 2nd X session emulates Fullscreen mode to some degree and can be viewed via x11vnc as long as the VMWare X session is in the active VT.

Also note that with a little bit of playing with "xwininfo -all -children" output one can extract the (non-toplevel) window-id of the of the Guest desktop only when VMWare is running as a normal X application. Then one can export just the guest desktop (i.e. without the VMWare menu buttons) by use of the -id windowid option. The caveats are the X session VMWare is in must be in the active VT and the window must be fully visible, so this mode is not terribly convenient, but could be useful in some circumstances (e.g. running VMWare on a very powerful server machine in a server room that happens to have a video card, (but need not have a monitor, Keyboard or mouse).)

[Exporting non-X11 devices via VNC]

Q-112: Can non-X devices (e.g. a raw framebuffer) be viewed (and even controlled) via VNC with x11vnc?

As of Apr/2005 there is support for this. Two options were added: "-rawfb string" (to indicate the raw frame buffer device, file, etc. and its parameters) and "-pipeinput command" (to provide an external program that will inject or otherwise process mouse and keystroke input.) Some useful -pipeinput schemes, VID, CONSOLE, and UINPUT, have since been built into x11vnc for convenience.

This non-X mode for x11vnc is somewhat experimental because it is so removed in scope from the intended usage of the tool. Incomplete attempt is made to make all of the other options consistent with non-X framebuffer polling. So all of the X-related options (e.g. -add_keysyms, -xkb) are just ignored or may cause an error if used. Be careful applying such an option via remote control.

The format for the -rawfb string is: -rawfb :@xx[-][://][+]

There are also some useful aliases (e.g. "console".) Some examples: -rawfb shm:210337933@800x600x32:ff/ff00/ff0000

-rawfb map:/dev/fb0@1024x768x16

-rawfb map:/tmp/Xvfb_screen0@640x480x8+3232

-rawfb file:/tmp/my.pnm@250x200x24+37

-rawfb file:/dev/urandom@128x128x8

-rawfb snap:/dev/video0@320x240x24 -24to32

-rawfb console

-rawfb vt2

-rawfb video

-rawfb setup:mycmd.sh

So the type can be "shm" for shared memory objects, and "map" or "file" for file objects. "map" uses mmap(2) to map the file into memory and is preferred over "file" (that uses the slower lseek(2) access method.) Only use file if map isn't working. BTW, "mmap" is an alias for "map" and if you do not supply a type and the file exists, map is assumed (see the -help output and below for some exceptions to this.) The "snap:" setting applies the -snapfb option with "file:" type reading (this is useful for exporting webcams or TV tuner video; see the next FAQ for more info.)

Also, if the string is of the form "setup:cmd" then cmd is run and the first line of its output retrieved and used as the rawfb string. This allows initializing the device, determining WxHxB, etc.

The object will be the numerical shared memory id for the case of shm. The idea here is some other program has created this shared memory segment and periodically updates it with new framebuffer data. x11vnc polls the area for changes. See shmat(2) and ipcs(8) for more info. The ipcs command will list current shared memory segments on the system. Sometimes you can snoop on a program's framebuffer it did not expect you would be polling!

The object will be the path to the regular or character special file for the cases of map and file. The idea here is that in the case of a regular file some other program is writing/updating framebuffer image data to it. In the case of a character special (e.g. /dev/fb0) it is the kernel that is "updating" the framebuffer data.

In most cases x11vnc needs to be told the width, height, and number of bits per pixel (bpp) of the framebuffer. This is the @WxHxB field. For the case of the Linux framebuffer device, /dev/fb0, the fbset(8) may be of use (but may not always be accurate for what is currently viewable.) In general some guessing may be required, especially for the bpp. Update: in "-rawfb console" mode x11vnc will use the linuxfb API to try to guess (it is still not always accurate.) Also try "-rawfb vtN" (on x11vnc 0.9.7 and later) for the N-th Linux text console (aka virtual terminal.) If the number of Bytes Per Line is not WxHxB/8 (i.e. the framebuffer lines are padded) you can specify this information after WxHxB via "-BPL", e.g. @800x600x16-2048

Based on the bpp x11vnc will try to guess the red, green, and blue masks (these indicate which bits correspond to each color.) It if gets it wrong you can specify them manually via the optional ":R/G/B" field. E.g. ":0xff0000/0x00ff00/0x0000ff" (this is the default for 32bpp.)

Finally, the framebuffer may not begin at the beginning of the memory object, so use the optional "+offset" parameter to indicate where the framebuffer information starts. So as an example, the Xvfb virtual framebuffer has options -shmem and -fbdir for exporting its virtual screen to either shm or a mapped file. The format of these is XWD and so the initial header should be skipped. BTW, since XWD is not strictly RGB the view will only be approximate, but usable. Of course for the case of Xvfb x11vnc can poll it much better via the X API, but you get the idea.

By default in -rawfb mode x11vnc will actually close any X display it happened to open. This is basically to shake out bugs (e.g it will crash rather than mysteriously interacting with the X display.) If you want x11vnc to keep the X display open while polling the raw framebuffer prefix a "+" sign at the beginning of the string (e.g. +file:/dev/urandom@64x64x8) This could be convenient for keeping the remote control channel active (it uses X properties.) The "-connect /path/to/file" mechanism could also be used for remote control to avoid the X property channel. Rare usage, but if you also supply -noviewonly in this "+" mode then the mouse and keyboard input are still sent to the X display, presumably for doing something amusing with /dev/fb...

Interesting Devices:. Here are some aliases for interesting device files that can be polled via -rawfb: -rawfb console /dev/fb0 Linux Console -rawfb vt2 /dev/vcsa2 Linux Console (e.g. virtual ter minal #2) -rawfb video /dev/video0 Video4Linux Capture device -rawfb rand /dev/urandom Random Bytes -rawfb null /dev/zero Zero Bytes (black screen)

The Linux console, /dev/fb0, etc needs to have its driver enabled in the kernel. Some of the drivers are video card specific and accelerated. The console is either the Text consoles (usually tty1-tty6), or X graphical display (usually starting at tty7.) In addition to the text console other graphical ones may be viewed and interacted with as well, e.g. DirectFB or SVGAlib apps, VMWare non-X fullscreen, or Qt-embedded apps (PDAs/Handhelds.) By default the pipeinput mechanisms UINPUT and CONSOLE (keystrokes only) are automatically attempted in this mode under "-rawfb console".

The Video4Linux Capture device, /dev/video0, etc is either a Webcam or a TV capture device and needs to have its driver enabled in the kernel. See this FAQ for details. If specified via "-rawfb Video" then the pipeinput method "VID" is applied (it lets you change video parameters dynamically via keystrokes.)

The last two, /dev/urandom and /dev/zero are just for fun, but are also useful in testing.

All of the above -rawfb options are just for viewing the raw framebuffer (although some of the aliases do imply keystroke and mouse pipeinput methods.) That may be enough for certain applications of this feature (e.g. suppose a video camera mapped its framebuffer into memory and you just wanted to look at it via VNC.) To handle the pointer and keyboard input from the viewer users the "-pipeinput cmd" option was added to indicate a helper program to process the user input. The input is streamed to it and looks something like this: Pointer 1 205 257 0 None Pointer 1 198 253 0 None Pointer 1 198 253 1 ButtonPress-1 Pointer 1 198 253 0 ButtonRelease-1 Pointer 1 198 252 0 None Keysym 1 1 119 w KeyPress Keysym 1 0 119 w KeyRelease Keysym 1 1 65288 BackSpace KeyPress Keysym 1 0 65288 BackSpace KeyRelease Keysym 1 1 112 p KeyPress Keysym 1 0 112 p KeyRelease

Run "-pipeinput tee:/bin/cat" to get a description of the format. Note that the -pipeinput option is independent of -rawfb mode and so may have some other interesting uses. The "tee:" prefix means x11vnc will both process the user input and pipe it to the command. The default is to just pipe it to the -pipeinput command.

Note the -pipeinput helper program could actually control the raw framebuffer. In the libvncserver CVS a simple example program x11vnc/misc/slide.pl is provided that demonstrates a simple jpeg "slideshow" application. Also the builtin "-pipeinput VID" mode does this for webcams and TV capture devices (/dev/video0.)

The -pipeinput program is run with these environment variables set: X11VNC_PID, X11VNC_PROG, X11VNC_CMDLINE, X11VNC_RAWFB_STR to aid its knowing what is up.

Another example provided in libvncserver CVS is a script to inject keystrokes into the Linux console (e.g. the virtual consoles: /dev/tty1, /dev/tty2, etc) in x11vnc/misc/vcinject.pl. It is based on the vncterm/LinuxVNC.c program also in the libvncserver CVS. So to view and interact with VT #2 (assuming it is the active VT) one can run something like: x11vnc -rawfb map:/dev/fb0@1024x768x16 -pipeinput './vcinject.pl 2'

This assumes your Linux framebuffer device (/dev/fb0) is properly configured. See fbset(8) and other documentation. Try "file:/dev/fb0@WxHxB" as a last resort. Starting with x11vnc 0.8.1, the above VT injection is built in, as well as WxHxB determination. Just use something like: x11vnc -rawfb console

this will try to guess the active virtual console (via /dev/tty0) and also the /dev/fb0 WxHxB and rgb masks automatically. Use, e.g., "-rawfb console3" to force the VT number. This input method can be used generally via "-pipeinput CONSOLE". Also starting with x11vnc 0.8.2 the "-pipeinput UINPUT" mode is tried first (it does both keyboard and mouse input) and then falls back to CONSOLE mode if it is not available. Here is the -help output for this mode:

 If the rawfb string begins with "console" the framebuffer device
 /dev/fb0 is opened (this requires the appropriate kernel modules to
 be installed) and so is /dev/tty0. The latter is used to inject
 keystrokes (not all are supported, but the basic ones are.) You
 will need to be root to inject keystrokes. /dev/tty0 refers to the
 active VT, to indicate one explicitly, use "console2", etc. using
 the VT number.

 If the Linux version seems to be 2.6 or later and the "uinput"
 module appears to be present, then the uinput method will be used
 instead of /dev/ttyN. uinput allows insertion of BOTH keystrokes
 and mouse input and so it preferred when accessing graphical (e.g.
 Qt-embedded) linux console apps. See -pipeinput UINPUT below for
 more information on this mode (you may want to also use the
 -nodragging and -cursor none options.) Use "console0", etc or
 -pipeinput CONSOLE to force the /dev/ttyN method.

 Note you can change VT remotely using the chvt(1) command.
 Sometimes switching out and back corrects the framebuffer state.

 To skip input injecting entirely use "consolex".

 The string "/dev/fb0" (1, etc) can be used instead of "console".
 This can be used to specify a different framebuffer device, e.g.
 /dev/fb1. As a shortcut the "/dev/" can be dropped. If the name is
 something nonstandard, use "console:/dev/foofb"

 If you do not want x11vnc to guess the framebuffer's WxHxB and
 masks automatically (sometimes the kernel gives inaccurate
 information), specify them with a @WxHxB at the end of the string.

The above is just an example of what can be done. Note that if you really want to view and interact with the Linux Text console it is better to use the more accurate and faster LinuxVNC program. The advantage x11vnc -rawfb might have is that it can allow interaction with a non-text application, e.g. one based on SVGAlib or Qt-embedded Also, for example the VMWare Fullscreen mode is actually viewable under -rawfb and can be interacted with if uinput is enabled.

If the Linux uinput driver is available then full keystroke and mouse input into the Linux console can be performed. You may be able to enable uinput via commands like these: modprobe uinput mknod /dev/input/uinput c 10 223

The -rawfb and -pipeinput features are intended to help one creatively "get out of a jam" (say on a legacy or embedded device) where X is absent or doesn't work properly. Feedback and bug reports are welcome. For more control and less overhead use libvncserver in your own C program that passes the framebuffer to libvncserver.

Q-113: Can I export the Linux Console (Virtual Terminals) via VNC using x11vnc?

Yes, you may need to be root to access the devices that make up the linux console.

To access the active Linux console via the computer's framebuffer try something like: x11vnc -rawfb console x11vnc -rawfb console2

These will try to access the framebuffer through /dev/fb (or /dev/fb0, etc.) and if it succeeds it will show any text or graphics that is currently displayed. Keystrokes will be injected via the device /dev/tty0 (to force an explicit virtual terminal append a number, e.g. "console2" to select /dev/tty2.)

If your Linux system does not have a framebuffer device (/dev/fb) you can get one by adding, e.g., vga=0x31B boot parameter. This enables the VGA framebuffer device at 1280x1024x24. 0x317 gives 1024x768x16, etc. You can also enable a Linux framebuffer device by modprobing a framebuffer driver specific to your video card.

Note that this "-rawfb console" mode shows the contents of the hardware framebuffer, and so will show whatever is on the screen. It has no concept of Virtual Terminals WRT what there is to view, it always shows the active virtual terminal.

Another mode is specific to the Linux text Virtual Terminals, it shows their text and colors (but no graphics) regardless of whether it is the active VT or not. It is available on x11vnc 0.9.7 and later. Enable this mode like this: x11vnc -rawfb vt x11vnc -rawfb vt2

The former will select the active one, the latter the 2nd VT. x11vnc implements this mode by opening the current console text file "/dev/vcsa2" instead of "/dev/fb". In this way it provides the basic functionality of the LibVNCServer LinuxVNC program.

The vt mode can be a useful way to try to get a machine's X server working remotely, e.g. you edit /etc/X11/xorg.conf and then type startx (or similar, e.g. gdm) in the virtual terminal. A 2nd x11vnc could be used to see if the X server is now working correctly.

Q-114: Can I export via VNC a Webcam or TV tuner framebuffer using x11vnc?

Yes, this is possible to some degree with the -rawfb option. There is no X11 involved: snapshots from the video capture device are used for the screen image data. See the previous FAQ on -rawfb for background. For best results, use x11vnc version 0.8.1 or later.

Roughly, one would do something like this: x11vnc -rawfb snap:/dev/video@320x240x32

This requires that the system allows simple read(2) access to the video device. This is true for video4Linux on Linux kernel 2.6 and later (it won't work for 2.4, you'll need a separate program to snapshot to a file that you point -rawfb to; ask me if it is not clear what to do.)

The "snap:" enforces -snapfb mode which appears to be necessary. The read pointer for video capture devices cannot be repositioned (which would be needed for scanline polling), but you can read a full frame of data from the device.

On Linux, if the Video4Linux API is present or the v4l-info(1) program (related to xawtv) exists in in PATH, then x11vnc can be instructed to try it to determine the -rawfb WxHxB parameters for you automatically. In this case one would just type: x11vnc -rawfb video

or "-rawfb video1" for the 2nd video device, etc.

x11vnc has also been extended to use the Video4Linux API over v4l-info if it is available at build time. This enables setting parameters (e.g. size and brightness) via x11vnc. See the description below. Without Video4Linux you will need to initialize the settings of the video device using something like xawtv or spcaview (and then hope the settings persist until x11vnc reopens the device.)

Many video4linux drivers tend to set the framebuffer to be 24bpp (as opposed to 32bpp.) Since this can cause problems with VNC viewers, etc, the -24to32 option will be automatically imposed when in 24bpp.

Note that by its very nature, video capture involves rapid change in the framebuffer. This is especially true for cameras where slight wavering in brightness is always happening. This can lead to much network bandwidth consumption for the VNC traffic and also local CPU and I/O resource usage. You may want to experiment with "dialing down" the framerate via the -wait, -slow_fb, or -defer options. Decreasing the window size and bpp also helps.

Setting Camera/Tuner parameters via x11vnc:

There is also some support for setting parameters of the capture device. This is done via "-rawfb video:". This could be useful for unattended startup at boottime, etc. Here is the -help description:

 A more sophisticated video device scheme allows initializing the
 device's settings using:

       -rawfb video:<settings>

 The prefix could also be, as above, e.g. "video1:" to specify the
 device file. The v4l API must be available for this to work.
 Otherwise, you will need to try to initialize the device with an
 external program, e.g. xawtv, spcaview, and hope they persist when
 x11vnc re-opens the device.

 <settings> is a comma separated list of key=value pairs. The
 device's brightness, color, contrast, and hue can be set to
 percentages, e.g. br=80,co=50,cn=44,hu=60.

 The device filename can be set too if needed (if it does not start
 with "video"), e.g. fn=/dev/qcam.

 The width, height and bpp of the framebuffer can be set via, e.g.,
 w=160,h=120,bpp=16.

 Related to the bpp above, the pixel format can be set via the
 fmt=XXX, where XXX can be one of: GREY, HI240, RGB555, RGB565,
 RGB24, and RGB32 (with bpp 8, 8, 16, 16, 24, and 32 respectively.)
 See http://www.linuxtv.org for more info (V4L api.)

 For TV/rf tuner cards one can set the tuning mode via tun=XXX where
 XXX can be one of PAL, NTSC, SECAM, or AUTO.

 One can switch the input channel by the inp=XXX setting, where XXX
 is the name of the input channel (Television, Composite1, S-Video,
 etc.) Use the name that is in the information about the device that
 is printed at startup.

 For input channels with tuners (e.g. Television) one can change
 which station is selected by the sta=XXX setting. XXX is the
 station number. Currently only the ntsc-cable-us (US cable)
 channels are built into x11vnc. See the -freqtab option below to
 supply one from xawtv. If XXX is greater than 500, then it is
 interpreted as a raw frequency in KHz.

 Example:

 -rawfb video:br=80,w=320,h=240,fmt=RGB32,tun=NTSC,sta=47

 one might need to add inp=Television too for the input channel to
 be TV if the card doesn't come up by default in that one.

 Note that not all video capture devices will support all of the
 above settings.

 See the -pipeinput VID option below for a way to control the
 settings through the VNC Viewer via keystrokes.

 As above, if you specify a "@WxHxB..." after the <settings> string
 they are used verbatim: the device is not queried for the current
 values. Otherwise the device will be queried.

Also, if you supply the "-pipeinput VID" (or use "-rawfb Video") option you can control the settings to some degree via keystroke mappings, e.g. B to increase the brightness or Up arrow to change the TV station:

 For "-pipeinput VID" and you are using the -rawfb for a video
 capture device, then an internal list of keyboard mappings is used
 to set parameters of the video. The mappings are:

     "B" and "b" adjust the brightness up and down.
     "H" and "h" adjust the hue.
     "C" and "c" adjust the colour.
     "N" and "n" adjust the contrast.
     "S" and "s" adjust the size of the capture screen.
     "I" and "i" cycle through input channels.
     Up and Down arrows adjust the station (if a tuner)
     F1, F2, ..., F6 will switch the video capture pixel
     format to HI240, RGB565, RGB24, RGB32, RGB555, and
     GREY respectively. See -rawfb video for details.

See also the -freqtab option to supply your own xawtv channel to frequency mappings for your country (only ntsc-cable-us is built into x11vnc.)

Q-115: Can I connect via VNC to a Qt-embedded/Qt-enhanced/Qtopia application running on my handheld, cell phone, or PC using the Linux console framebuffer (i.e. not X11)?

Yes, the basic method for this is the -rawfb scheme where the Linux console framebuffer (usually /dev/fb0) is polled and the uinput driver is used to inject keystrokes and mouse input. Often you will just have to type: x11vnc -rawfb console

(you may need to enable the uinput driver on the system via "modprobe uinput; mknod /dev/input/uinput c 10 223") If this does not find the correct frame buffer properties figure them out or guess them and use something like: x11vnc -rawfb /dev/fb0@640x480x16

Also, to force usage of the uinput injection method use "-pipeinput UINPUT". See the -pipeinput description for tunable parameters, etc.

One problem with the x11vnc uinput scheme is that it cannot guess the mouse motion "acceleration" used by the windowing application (e.g. QWS or X11.) For X11 and Qt-embedded the acceleration is usually 2 (i.e. a dx of 1 from the mouse yields a 2 pixel displacement of the mouse cursor.) The default x11vnc uses is 2, since that is often used. However for one Qt-embedded system we needed to do: x11vnc -rawfb console -pipeinput UINPUT:accel=4.0

to get reasonable positioning of the mouse.

Even with the correct acceleration setting there is still some drift (probably because of the mouse threshold where the acceleration kicks in) and so x11vnc needs to reposition the cursor from 0,0 about 5 times a second. See the -pipeinput UINPUT option for tuning parameters that can be set (there are some experimental thresh=N tuning parameters as well)

Currently, one can expect mouse input to be a little flakey. All in all, the Linux framebuffer input mechanism for Qt-embedded framebuffer apps is not perfect, but it is usable.

If you need to create a smaller x11vnc binary for a handheld environment be sure to run strip(1) on it and also consider configuring with, e.g. "env CPPFLAGS='-DSMALL_FOOTPRINT=1' ./configure ..." to remove rarely used features and large texts (use 2 or 3 instead of 1 to remove more.) Currently (Jul/2006) this can lower the size of the x11vnc from 1.1MB to 0.6-0.7MB.

The x11vnc uinput method applies to nearly anything on the Linux framebuffer console, not just Qt-embedded/Qtopia. DirectFB, SDL using fbcon driver, SVGAlib applications can also be viewed and interacted with. Even a Linux X session can be viewed and interacted with without using X11 (and x11vnc does not have to terminate when the X server restarts!) The Linux Text consoles (F1-F6) also work.

Note that Qt-embedded supplies its own VNC graphics driver, but it cannot do both the Linux console framebuffer and VNC at the same time, which is often what is desired from VNC.

Update: We are finding some setups like Qtopia on the IPAQ do not allow mouse input via uinput. Please help us debug this problem by trying x11vnc on your device and letting us know what does and does not work. See the next FAQ for a possible workaround for touchscreens.

Q-116: How do I inject touch screen input into an Qt-embedded/Qt-enhanced/Qtopia cell phone such as openmoko/qtmoko Neo Freerunner?

The qtmoko project does not use X11 for the graphical display. Unfortunately the Linux uinput method described in the previous FAQ does not work because Qt is using TSLIB (touch screen library) to process the input and it only reads from one device (often /dev/input/event1) and not from the new UINPUT device that x11vnc creates (under -pipeinput UINPUT)

So something else needs to be done. It was discovered that by simply writing the touchscreen events directly to /dev/input/event1 then input can be injected into the system. There is no x11vnc builtin mode for this yet (until we understand it better), but there is a working script provided in x11vnc/misc/qt_tslib_inject.pl. So one could use it this way for example: x11vnc ... -rawfb console -pipeinput path/to/qt_tslib_inject.pl -env INJECT_O PTIONS=clickonly,cal=/etc/pointercal

Read the script for how to enable other options and what the above options mean (e.g. /etc/pointercal contains TSLIB's calibration parameters and are necessary to achieve accurate pointing.)

The x11vnc/misc/qt_tslib_inject.pl script can potentially be modified to handle other devices where the uinput method fails. It could also be modified to create 'hot keys', etc.

Please let us know how things go if you try this out; there is much to learn about synthetic input injection in handhelds and cell phones. As we learn more we can develop a builtin x11vnc mode for this sort of injection.

Update Dec/2010: There is experimental built-in UINPUT support in the x11vnc development tarball for qtmoko with touchpad managed by tslib. See -pipeinput UINPUT for more info. Here is an example: x11vnc -rawfb console -pipeinput UINPUT:touch,tslib_cal=/etc/pointercal,dire ct_abs=/dev/input/event1,nouinput,dragskip=3

Q-117: Now that non-X11 devices can be exported via VNC using x11vnc, can I build it with no dependencies on X11 header files and libraries?

Yes, as of Jul/2006 x11vnc enables building for -rawfb only support. Just do something like when building: ./configure --without-x (plus any other flags) make

You can then test via "ldd x11vnc" that the binary does not depend on libX11.so, etc. See the previous FAQ's for non-X11 framebuffer usage. If you use this for an interesting non-X11 application please let us know what you did.

Q-118: How do I cross compile x11vnc for a different architecture than my Linux i386 or amd64 PC?

You will need a cross-compiling toolchain. Perhaps your distro provides these or you can find a HOWTO for your distro. We found a nice one at qtmoko.org for building armel binaries on Debian Linux i386 machines. It includes most of the libraries that x11vnc needs. We use that example here.

We ran this script to set PATH, configure, and build: #!/bin/sh

toolchain from: qtmoko-debian-toolchain-armv4t-eabi.tar.gz

export PATH=/opt/toolchains/arm920t-eabi/bin:$PATH

env CC=arm-linux-gcc ./configure --host=arm-linux --without-avahi

make

arm-linux-strip ./x11vnc/x11vnc ls -l ./x11vnc/x11vnc

Note we had to include --without-avahi due to lack of libavahi-client.so.3 supplied by the toolchain we used. One would need to add it if it was desired on the target machine. We also stripped the binary to make it smaller.

For an embedded system one may also want to add --without-x if the embedded system does not use X11 and the -rawfb mechanism must be used.

Q-119: Does x11vnc support Mac OS X Aqua/Quartz displays natively (i.e. no X11 involved)?

Yes, since Nov/2006 in the development tree (x11vnc-0.8.4 tarball) there is support for native Mac OS X Aqua/Quartz displays using the -rawfb mechanism described above. The mouse and keyboard input is achieved via Mac OS X API's.

So you can use x11vnc as an alternative to OSXvnc (aka Vine Server), or Apple Remote Desktop (ARD). Perhaps there is some x11vnc feature you'd like to use on Mac OS X, etc. For a number of activities (e.g. window drags) it seems to be faster than OSXvnc.

Notes:

X11: x11vnc will also work (as it has for years) with a X11 server (XDarwin) running on Mac OS X (people often install this software to display remote X11 apps on their Mac OS X system, or use some old favorites locally such as xterm.) However in this case x11vnc will only work reasonably in single window -id windowid mode (and the window may need to have mouse focus.)

If you do not have the DISPLAY env. variable set, x11vnc will assume native Aqua/Quartz on Mac OS X, however if DISPLAY is set it will assume an X11 connection. Use "-rawfb console" to force the native display (or unset DISPLAY.)

Update: Leopard sets DISPLAY by default in all sessions. Since it starts with the string "/tmp/" x11vnc will use that to know if it should ignore it. Use "-display :0.0" to force it.

Building: If you don't have the X11 build and runtime packages installed you will need to build it like this: (cd to the e.g. x11vnc-0.9, source directory) ./configure --without-x make

Win2VNC/x2vnc: One handy use is to use the -nofb mode to redirect mouse and keyboard input to a nearby Mac (i.e. one to the side of your desk) via x2vnc or Win2VNC. See this FAQ for more info.

Options: Here are the Mac OS X specific x11vnc options: -macnodim For the native Mac OS X server, disable dimming. -macnosleep For the native Mac OS X server, disable display sleep . -macnosaver For the native Mac OS X server, disable screensaver. -macnowait For the native Mac OS X server, do not wait for the user to switch back to his display. -macwheel n For the native Mac OS X server, set the mouse wheel speed to n (default 5.) -macnoswap For the native Mac OS X server, do not swap mouse buttons 2 and 3. -macnoresize For the native Mac OS X server, do not resize or rese t the framebuffer even if it is detected that the scree n resolution or depth has changed. -maciconanim n For the native Mac OS X server, set n to the number of milliseconds that the window iconify/deiconify animation takes. In -ncache mode this value will be used to skip the animation if possible. (default 400) -macmenu For the native Mac OS X server, in -ncache client-sid e caching mode, try to cache pull down menus (not perfe ct because they have animated fades, etc.)

PasteBoard/Clipboard: There is a bug that the Clipboard (called PasteBoard on Mac it appears) exchange will not take place unless x11vnc was started from inside the Aqua display (e.g. started inside a Terminal app window.) Otherwise it cannot connect to the PasteBoard server. So Clipboard exchange won't work for our standard "ssh in" startup scheme.

Hopefully this deficiency can be removed, but until then for Clipboard exchange to work you will need to start x11vnc inside the desktop session (i.e. either start it running before you leave, or start up a 2nd x11vnc inside from a 1st one started outside, or use the apple script below)

Here also is a osascript trick that seems to work (it opens the Terminal app and instructs it to start x11vnc):

#!/bin/sh

start_x11vnc: start x11vnc in a Terminal window

(this will allow Clipboard/Pasteboard exchange to work)

tmp=/tmp/start_x11vnc.$$

cat > $tmp <<END

tell application "Terminal" activate do script with command "$HOME/x11vnc -rfbauth .vnc/passwd -ssl SAVE" end tell

END

osascript $tmp rm -f $tmp

where you should customize the x11vnc command line to your needs and the full path to the binary. Save it in a file e.g. "start_x11vnc" and then after you SSH in just type "./start_x11vnc" (or have ssh run the command for you.) Then once you are connected via VNC, iconify the Terminal windows (you can't delete them since that will kill x11vnc.)

Update Aug/2010: A user reports the following useful information: This is not a problem on Mac OS X 10.6.x (Snow Leopard) when connecting via ssh to start x11vnc. And, on Mac OS X 10.5.x (Leopard), the problem can be permanently eliminated by doing this:

sudo /usr/libexec/PlistBuddy -c 'delete :LimitLoadToSessionType'
-c 'add :LimitLoadToSessionType string Background'
/System/Library/LaunchAgents/com.apple.pboard.plist

ignore any 'Delete: Entry, ":LimitLoadToSessionType", Does Not Exist' message

and then restarting (yes, you must restart not just log off). But ONLY do that for Mac OS X 10.5.x and NOT for 10.6.x (which doesn't need it anyway).

We recently got access to a MacOSX 10.6.4 (Snow Leopard) macbook and have confirmed that the above is correct.

Q-120: Can x11vnc be used as a VNC reflector/repeater to improve performance for the case of a large number of simultaneous VNC viewers (e.g. classroom broadcasting or a large demo)?

Yes, as of Feb/2007 there is the "-reflect host:N" option to connect to the VNC server "host:N" (either another x11vnc or any other VNC server) and re-export it. VNC viewers then connect to the x11vnc(s) running -reflect.

The -reflect option is the same as: "-rawfb vnc:host:N". See the -rawfb description under "VNC HOST" for more details.

You can replace "host:N" with "listen" or "listen:port" for reverse connections.

One can set up a number of such reflectors/repeaters to spread the resource usage around, e.g.: C -------<-------| C -------<-------| C -------<-------|---- R -----| C -------<-------| | C -------<-------| | | C -------<-------| | C -------<-------| | C -------<-------|---- R -----| C -------<-------| | C -------<-------| | |====== S C -------<-------| | C -------<-------| | C -------<-------|---- R -----| C -------<-------| | C -------<-------| | | C -------<-------| | C -------<-------| | C -------<-------|---- R -----| C -------<-------| C -------<-------|

Where "S" is the original VNC Server, "C" denote VNC viewer clients, and "R" denotes an x11vnc running -reflect to "S".

Ideally, a client "C" will be fairly close network-wise to its "R". It is fine to run the "R" on the same machine as one of its "C's". A nice setup for a large, (e.g. 64-128) viewer classroom broadcast case would be to run R's on areas isolated by network switches, e.g. one R per switch.

In an extreme case (e.g. 1000 viewers) one might actually need a 2nd layer of R's in the tree. If you try something like that let us know!

There are many resource savings in doing something like the above. The first obvious one is network bandwidth savings. Another is less CPU load on "S" since it handles many fewer simultaneous connections. Also, if there are a few clients C on very slow links, their presence does not slow down every other client, just the clients on their "R". One way a slow client affects things is if there are some large framebuffer writes (e.g. jpeg image region) then the repeater may block waiting for that large write to finish before going onto the next client (however, if the write is small enough, the kernel will buffer it and the server can go on to service the next client.)

The x11vnc -reflect implementation uses the libvncclient library in the LibVNCServer project to handle the connection to "S". It is not currently very efficient since it simply does its normal framebuffer polling scheme on the libvncclient framebuffer (which it then re-exports via VNC to its clients C.) However, CopyRect and CursorShape encodings are preserved in the reflection and that helps. Dragging windows with the mouse can be a problem (especially if S is not doing wireframing somehow, consider -nodragging if the problem is severe) For a really fast reflector/repeater it would have to be implemented from scratch with performance in mind. See these other projects: http://sourceforge.net/projects/vnc-reflector/, http://www.tightvnc.com/projector/ (closed source?),

Automation via Reverse Connections: Instead of having the R's connect directly to S and then the C's connect directly to the R they should use, some convenience can be achieved by using reverse connections (the x11vnc ""-connect host1,host2,..." option.) Suppose all the clients "C" are started up in Listen mode: client1> vncviewer -listen client2> vncviewer -listen client3> vncviewer -listen ... client64> vncviewer -listen

(e.g. client1> is the cmdline prompt on machine client1 ... etc) and all the repeaters R are started like this: repeater1> x11vnc -reflect listen -connect client1,client2,...client8 repeater2> x11vnc -reflect listen -connect client9,client10,...client16 ... repeater8> x11vnc -reflect listen -connect client57,client58,...client64

and finally the main server is started to kick the whole thing into motion: vncserver> x11vnc -display :0 -connect repeater1,repeater2,...repeater8

(or instruct a non-x11vnc VNC server to reverse connect to the repeaters.) For a classroom broadcasting setup one might have the first two sets of commands start automatically at bootup or when someone logs in, and then start everything up with the S server. One may even be able to script the forward connection bootstrap case, let us know what you did. A really nice thing would be some sort of auto-discovery of your repeater, etc...

Q-121: Can x11vnc be used during a Linux, Solaris, etc. system Installation so the Installation can be done remotely?

This can be done, but it doesn't always work because it depends on how the OS does its install. We have to "sneak in" somehow. Note that some OS's have a remote install (ssh etc.) built in and so you might want to use that instead.

Usually the OS install will have to be a network-install in order to have networking up during the install. Otherwise, you may have a (slim) chance to configure the networking manually (ifconfig(8) and route(8).)

To avoid library dependencies problems in the typical minimal (e.g. busybox) installation OS it is a good idea to build a statically linked x11vnc binary. A way that often works is to do a normal build and then paste the final x11vnc link line into a shell script. Then change the "gcc" to "gcc -static" and run the shell script. You may need to disable features (e.g. "--without-xfixes") if there is not a static library for the feature available. You may also need to add extra link options (e.g. "-lXrender") to complete library dependencies manually.

Let's call the binary x11vnc.static. Place it on a webserver somewhere. It may be possible to retrieve it via scp(1) too.

During the install you need to get a shell to retreive x11vnc.static and run it.

If the Solaris install is an older X-based one, there will be a menu for you to get a terminal window. From that window you might be able to retrieve x11vnc.static via wget, scp, or ftp. Remember to do "chmod 755 ./x11vnc.static" and then find the -auth file as in this FAQ.

If it is a Linux install that uses an X server (e.g. SuSE and probably Fedora), then you can often get a shell by pressing Ctrl-Alt-F2 or similar. Then get the x11vnc binary via something like this: cd /tmp wget http://192.168.0.22/x11vnc.static chmod 755 ./x11vnc.static

Find the name of the auth file as in this FAQ. (maybe run "ps wwaux | grep auth".) Then run it like this: ./x11vnc.static -forever -nopw -display :0 -auth /tmp/wherever/the/authfile

then press Alt-F7 to go back to the X install. You should now be able to connect via a vnc viewer and continue the install. Watch out for the display being :1, etc.

If there is a firewall blocking incoming connections during the install, use the "-connect hostname" option option for a reverse connection to the hostname running the VNC viewer in listen mode.

Debian based installs are either console-text or console-framebuffer based. These are install (or expert) and installgui (or expertgui) boot lines, respectively. For the console-text based installs you probably need to add a boot cmd line option like vga=0x314 (which is 800x600x16) to get the console-text to use the linux framebuffer device properly.

For a Debian console-text based install after the network is configured press Ctrl-Alt-F2 to get a shell. Retrieve the binary via wget as above and chmod 755 it. Then run it something like this: sleep 10; ./x11vnc.static -forever -nopw -rawfb console

then before the sleep is over press Alt-F1 to get back to the install virtual console. You should be able to connect via a VNC viewer and continue with the install.

For a recent (2009) Debian install we booted with "expert vga=0x301" and "expert vga=0x311" to get console text based installs at 640x480x8 and 640x480x16, respectively (replace "expert" with "install" if you like.) Otherwise it was giving a 16 color 640x480x4 (4 bit per pixel) display which x11vnc could not handle.

For Debian console-framebuffer GUI based installs (installgui or expertgui) we have not be able to enter keystrokes or mouse motions. This may be resolved if the install had the Linux kernel module uinput, but it doesn't; one can wget uinput.ko and then run insmod on it, but the module must match the installation kernel. So, failing that, you can only do the GUI view-only, which can be handy to watch a long network install from your desk instead of in front of the machine being installed. For these, after the network is configured press Ctrl-Alt-F2 to get a shell. Retrieve the binary via wget as above and chmod 755 it. Then run it something like this: sleep 10; ./x11vnc.static -forever -nopw -rawfb console

then before the sleep is over press Alt-F5 to get back to the GUI install console. You should be able to connect via a VNC viewer and watch the install. [Misc: Clipboard, File Transfer/Sharing, Printing, Sound, Beeps, Thanks, etc.]

Q-122: Does the Clipboard/Selection get transferred between the vncviewer and the X display?

As of Jan/2004 x11vnc supports the "CutText" part of the RFB (aka VNC) protocol. When text is selected/copied in the X session that x11vnc is polling it will be sent to connected VNC viewers. And when CutText is received from a VNC viewer then x11vnc will set the X11 selections PRIMARY, CLIPBOARD, and CUTBUFFER0 to it. x11vnc is able to hold the PRIMARY and CLIPBOARD selections (Xvnc does not seem to do this.)

The X11 selections can be confusing, especially to those coming from Windows or MacOSX where there is just a single 'Clipboard'. The X11 CLIPBOARD selection is a lot like that of Windows and MacOSX, e.g. highlighted text is sent to the clipboard when the user activates "Edit -> Copy" or presses "Control+C" (and pasting it via "Edit -> Paste" or "Control+V".) The X11 PRIMARY selection has been described as 'for power users' or 'an Easter Egg'. As soon as text is highlighted it is set to the PRIMARY selection and so it is immediately ready for pasting, usually via the Middle Mouse Button or "Shift+Insert". See this jwz link for more information.

x11vnc's default behavior is to watch both CLIPBOARD and PRIMARY and whenever one of them changes, it sends the new text to connected viewers. Note that since the RFB protocol only has a single "CutText" then both selections are "merged" to some degree (and this can lead to confusing results.) One user was confused why x11vnc was "forgetting" his CLIPBOARD selection and the reason was he also changed PRIMARY some time after he copied text to the clipboard. Usually an app will set PRIMARY as soon as any text is highlighted so it easy to see how CLIPBOARD was forgotten. Use the -noprimary described below as a workaround. Similarly, by default when x11vnc receives CutText it sets both CLIPBOARD and PRIMARY to it (this is probably less confusing, but could possibly lead to some failure modes as well.)

You may not like these defaults. Here are ways to change the behavior: * If you don't want the Clipboard/Selection exchanged at all use the -nosel option. * If you want changes in PRIMARY to be ignored use the -noprimary option. * If you want changes in CLIPBOARD to be ignored use the -noclipboard option. * If you don't want x11vnc to set PRIMARY to the "CutText" received from viewers use the -nosetprimary option. * If you don't want x11vnc to set CLIPBOARD to the "CutText" received from viewers use the -nosetclipboard option.

You can also fine-tune it a bit with the -seldir dir option and also -input.

You may need to watch out for desktop utilities such as KDE's "Klipper" that do odd things with the selection, clipboard, and cutbuffers.

Q-123: Can I use x11vnc to record a Shock Wave Flash (or other format) video of my desktop, e.g. to record a tutorial or demo?

Yes, it is possible with a number of tools that record VNC and transform it to swf format or others. One such popular tool is pyvnc2swf. There are a number of tutorials (broken link?) on how to do this. Another option is to use the vnc2mpg that comes in the LibVNCServer package. An important thing to remember when doing this is that tuning parameters should be applied to x11vnc to speed up its polling for this sort of application, e.g. "-wait 10 -defer 10".

Q-124: Can I transfer files back and forth with x11vnc?

As of Oct/2005 and May/2006 x11vnc enables, respectively, the TightVNC and UltraVNC file transfer implementations that were added to libvncserver. This currently works with TightVNC and UltraVNC viewers (and Windows viewers only support filetransfer it appears... but they do work to some degree under Wine on Linux.)

The SSVNC Unix VNC viewer supports UltraVNC file transfer by use of a Java helper program.

TightVNC file transfer is off by default, if you want to enable it use the -tightfilexfer option.

UltraVNC file transfer is off by default, to enable it use something like "-rfbversion 3.6 -permitfiletransfer" options (UltraVNC incorrectly uses the RFB protocol version to determine if its features are available, so x11vnc has to pretend to be version 3.6.) As of Sep/2006 "-ultrafilexfer" is an alias for these two options. Note that running as RFB version 3.6 may confuse other VNC Viewers.

Sadly you cannot do both -tightfilexfer and -ultrafilexfer at the same time because the latter requires setting the version to 3.6 and tightvnc will not do filetransfer when it sees that version number.

Also, because of the way the LibVNCServer TightVNC file transfer is implemented, you cannot do Tightvnc file transfer in -unixpw mode. However, UltraVNC file transfer does work in -unixpw (but if a client tries it do a filetransfer during the login process it will be disconnected.)

IMPORTANT: please understand if -ultrafilexfer or -tightfilexfer is specified and you run x11vnc as root for, say, inetd or display manager (gdm, kdm, ...) access and you do not have it switch users via the -users option, then VNC Viewers that connect are able to do filetransfer reads and writes as root.

The UltraVNC and TightVNC settings can be toggled on and off inside the gui or by -R remote control. However for TightVNC the changed setting only applies for NEW clients, current clients retain their TightVNC file transfer ability. For UltraVNC it works better, however if an UltraVNC client has initiated a file transfer dialog it will remain in effect until the dialog is closed. If you want to switch between UltraVNC and TightVNC file transfer in the gui or by remote control you will probably be foiled by the "-rfbversion 3.6" issue.

Q-125: Which UltraVNC extensions are supported?

Some of them are supported. To get UltraVNC Viewers to attempt to use these extensions you will need to supply this option to x11vnc: -rfbversion 3.6

Or use -ultrafilexfer which is an alias for the above option and "-permitfiletransfer". UltraVNC evidently treats any other RFB version number as non-UltraVNC.

Here are a list of the UltraVNC extensions supported by x11vnc: * ServerInput: "Toggle Remote Input and Remote Blank Monitor" * FileTransfer: "Open File Transfer..." * SingleWindow: "Select Single Window..." * TextChat: "Open Chat..." * 1/n Server Scaling

The SSVNC Unix VNC viewer supports these UltraVNC extensions.

To disable SingleWindow and ServerInput use -noultraext (the others are managed by LibVNCServer.) See this option too: -noserverdpms.

Also, the UltraVNC repeater proxy is supported for use with reverse connections: "-connect repeater://host:port+ID:NNNN". Use it for both plaintext and SSL connections. This mode can send any string before switching to the VNC protocol, and so could be used with other proxy/gateway tools. Also, a perl repeater implemention is here: ultravnc_repeater.pl

Q-126: Can x11vnc emulate UltraVNC's Single Click helpdesk mode for Unix? I.e. something very simple for a naive user to initiate a reverse vnc connection from their Unix desktop to a helpdesk operator's VNC Viewer.

Yes, UltraVNC's Single Click (SC) mode can be emulated fairly well on Unix.

We use the term "helpdesk" below, but it could be any sort of remote assistance you want to set up, e.g. something for Unix-using friends or family to use. This includes Mac OS X.

Assume you create a helpdesk directory "hd" on your website: http://www.mysite.com/hd (any website that you can upload files to should work, although remember the user will be running the programs you place there.)

In that "hd" subdirectory copy an x11vnc binary to be run on the Unix user's machine (e.g. Linux, etc) and also create a file named "vnc" containing the following: #!/bin/sh

webhost="http://www.mysite.com/hd" # Your helpdesk dir URL.

vnchost="ip.someplace.net" # Your host running 'vncviewer -listen' # It could also be your IP number. If it is # a router/firewall, you will need to # configure it to redirect port 5500 to you r # workstation running 'vncviewer -listen'

dir=/tmp/vnc_helpdesk.$$ # Make a temporary working dir. mkdir $dir || exit 1 cd $dir || exit 1

trap "cd /tmp; rm -rf $dir" 0 2 15 # Cleans up on exit.

wget $webhost/x11vnc # Fetch x11vnc binary. If multi- chmod 755 ./x11vnc # platform, use $webhost/uname/x11vnc # or similar.

./x11vnc -connect_or_exit $vnchost -rfbport 0 -nopw

with the hostnames / IP addresses customized to your case.

On the helpdesk VNC viewer machine (ip.someplace.net in this example) you have the helpdesk operator running VNC viewer in listen mode: vncviewer -listen

or if on Windows, etc. somehow have the VNC viewer be in "listen" mode.

Then, when the naive user needs assistance you instruct him to open up a terminal window on his Unix desktop and paste the following into the shell: wget -qO - http://www.mysite.com/hd/vnc | sh -

and then press Enter. You could have this instruction on a web page or in an email you send him, etc. This requires that the wget is installed on the user's Unix machine (he might only have curl or lynx, see below for more info.)

So I guess this is about 3-4 clicks (start a terminal and paste) and pressing "Enter" instead of "single click"...

See this page for some variations on this method, e.g. how to add a password, SSL Certificates, etc.

If you don't have a website (there are many free ones) or don't want to use one you will have to email him all of the ingredients (x11vnc binary and a launcher script) and tell him how to run it. This could be easy or challenging depending on the skill of the naive unix user...

A bit of obscurity security could be put in with a -passwd, -rfbauth options, etc. (note that x11vnc will require a password even for reverse connections.) More info here.

Firewalls: If the helpdesk (you) with the vncviewer is behind a NAT/Firewall/Router the router will have to be configured to redirect a port (i.e. 5500 or maybe different one if you like) to the vncviewer machine. If the vncviewer machine also has its own host-level firewall, you will have to open up the port there as well.

NAT-2-NAT: There is currently no way to go "NAT-2-NAT", i.e. both User and Helpdesk workstations behind NAT'ing Firewall/Routers without configuring a router to do a port redirection (i.e. on your side, the HelpDesk.) To avoid modifying either firewall/router, one would need some public (IP address reachable on the internet) redirection/proxy service. Perhaps such a thing exists. http://sc.uvnc.com provides this service for their UltraVNC Single Click users.

Update: It may be possible to do "NAT-2-NAT" with a UDP tunnel such as http://samy.pl/pwnat/. All that is required is that both NAT firewalls allow in UDP packets from an IP address to which a UDP packet has recently been sent to. If you try it out let us know how it went.

Very Naive Users:

If it is beyond the user how to open a terminal window and paste in a command (you have my condolences...) you would have to somehow setup his Web browser to download the "vnc" file (or a script containing the above wget line) and prompt the user if he wants to run it. This may be tricky to set up (which is probably a good thing to not have the web browser readily run arbitrary programs downloaded from the internet...)

One command-line free way, tested with KDE, is to name the file vnc.sh and then instruct the user to right-click on the link and do "Save Link As" to his Desktop. It will appear as an icon, probably one that looks like a terminal or a command line prompt. He next should right-click on the icon and select "Properties" and go to the "Permissions" tab. Then in that dialog select the checkbox "Is executable". He should then be able to click on the icon to launch it. Another option is to right-click on the icon and select "Open With -> Other ..." and for the name of the application type in "/bin/sh". Unfortunately in both cases the command output is lost and so errors cannot be debugged as easily. A similar thing appears to work in GNOME if under "Properties -> Permissions" they click on "Execute" checkbox for "Owner". Then when they click on the icon, they will get a dialog where they can select "Run in Terminal". In general for such cases, if it is feasible, it might be easier to ssh to his machine and set things up yourself...

SSL Encrypted Helpdesk Connections:

As of Apr/2007 x11vnc supports reverse connections in SSL and so we can do this. On the Helpdesk side (Viewer) you will need STUNNEL or better use the Enhanced TightVNC Viewer (SSVNC) package we provide that automates all of the SSL for you.

To do this create a file named "vncs" in the website "hd" directory containing the following: #!/bin/sh

webhost="http://www.mysite.com/hd" # Your helpdesk dir URL.

vnchost="ip.someplace.net" # Your host running 'vncviewer -listen' # It could also be your IP number. If it is # a router/firewall, you will need to # configure it to redirect port 5500 to you r # workstation running 'vncviewer -listen'

dir=/tmp/vnc_helpdesk.$$ # Make a temporary working dir. mkdir $dir || exit 1 cd $dir || exit 1

trap "cd /tmp; rm -rf $dir" 0 2 15 # Cleans up on exit.

wget $webhost/x11vnc # Fetch x11vnc binary. If multi- chmod 755 ./x11vnc # platform, use $webhost/uname/x11vnc # or similar.

./x11vnc -connect_or_exit $vnchost -rfbport 0 -nopw -ssl # Note -ssl option.

with the hostnames or IP addresses customized to your case.

The only change from the "vnc" above is the addition of the -ssl option to x11vnc. This will create a temporary SSL cert: openssl(1) will need to be installed on the user's end. A fixed SSL cert file could be used to avoid this (and provide some authentication; more info here.)

The naive user will be doing this: wget -qO - http://www.mysite.com/hd/vncs | sh -

(or perhaps even use https:// if available.)

But before that, the helpdesk operator needs to have "vncviewer -listen" running as before, however he needs an SSL tunnel at his end. The easiest way to do this is use Enhanced TightVNC Viewer (SSVNC). Start it, and select Options -> 'Reverse VNC Connection (-listen)'. Then UN-select 'Verify All Certs' (this can be enabled later if you want; you'll need the x11vnc SSL certificate), and click 'Listen'.

If you don't want to use SSVNC for the viewer, but rather set up STUNNEL manually instead, make a file "stunnel.cfg" containing: foreground = yes pid =

[vnc] accept = 5500 connect = localhost:5501

and run: stunnel ./stunnel.cfg

and then start the "vncviewer -listen 1" (i.e. 1 to correspond to the 5501 port.) Note that this assumes the stunnel install created a Server SSL cert+key, usually /etc/stunnel/stunnel.pem (not all distros will do this.) Also, that file is by default only readable by root, so stunnel needs to be run as root. If your system does not have a key installed or you do not want to run stunnel as root (or change the permissions on the file), you can use x11vnc to create one for you for example: x11vnc -sslGenCert server self:mystunnel

answer the prompts with whatever you want; you can take the default for all of them if you like. The openssl(1) package must be installed. See this link and this one too for more info on SSL certs. This creates $HOME/.vnc/certs/server-self:mystunnel.pem, then you would change the "stunnel.cfg" to look something like: foreground = yes pid = cert = /home/myusername/.vnc/certs/server-self:mystunnel.pem

[vnc] accept = 5500 connect = localhost:5501

In any event, with stunnel having been setup, the naive user is instructed to paste in and run: wget -qO - http://www.mysite.com/hd/vncs | sh -

to pick up the vncs script this time.

Of course if a man-in-the-middle can alter what the user downloads then all bets are off!.

More SSL variations and info about certificates can be found here.

OpenSSL libssl.so.0.9.7 problems:

If you build your own stunnel or x11vnc for deployment, you may want to statically link libssl.a and libcrypto.a into it because Linux distros are currently a bit of a mess regarding which version of libssl is installed.

You will find the details here.

Q-127: Can I (temporarily) mount my local (viewer-side) Windows/Samba File share on the machine where x11vnc is running?

You will have to use an external network redirection for this. Filesystem mounting is not part of the VNC protocol.

We show a simple Samba example here.

First you will need a tunnel to redirect the SMB requests from the remote machine to the one you sitting at. We use an ssh tunnel: sitting-here> ssh -C -R 1139:localhost:139 far-away.east

Or one could combine this with the VNC tunnel at the same time, e.g.: sitting-here> ssh -C -R 1139:localhost:139 -t -L 5900:localhost:5900 far-away .east 'x11vnc -localhost -display :0'

Port 139 is the Windows Service port. For Windows systems instead of Samba, you may need to use the actual IP address of the Window machine instead of "localhost" in the -R option (since the Windows service does not listen on localhost by default.)

Note that we use 1139 instead of 139 on the remote side because 139 would require root permission to listen on (and you may have a samba server running on it already.)

The ssh -C is to enable compression, which might speed up the data transfers.

Depending on the remote system side configuration, it may or may not be possible to mount the SMB share as a non-root user. Try it first as a non-root user and if that fails you will have to become root.

We will assume the user name is "fred" and we will try to mount the viewer-side Windows SMB share "//haystack/pub" in /home/fred/smb-haystack-pub. far-away> mkdir -p /home/fred/smb-haystack-pub far-away> smbmount //haystack/pub /home/fred/smb-haystack-pub -o username=fre d,ip=127.0.0.1,port=1139

(The 2nd command may need to be run as root.) Then run "df" or "ls -l /home/fred/smb-haystack-pub" to see if it is mounted properly. Consult the smbmount(8) and related documentation (it may require some fiddling to get write permissions correct, etc.) To unmount: far-away> smbumount /home/fred/smb-haystack-pub

At some point we hope to fold some automation for SMB ssh redir setup into the Enhanced TightVNC Viewer (SSVNC) package we provide (as of Sep 2006 it is there for testing.)

Q-128: Can I redirect CUPS print jobs from the remote desktop where x11vnc is running to a printer on my local (viewer-side) machine?

You will have to use an external network redirection for this. Printing is not part of the VNC protocol.

We show a simple Unix to Unix CUPS example here. Non-CUPS port redirections (e.g. LPD) should also be possible, but may be a bit more tricky. If you are viewing on Windows SMB and don't have a local cups server it may be trickier still (see below.)

First you will need a tunnel to redirect the print requests from the remote machine to the one you sitting at. We use an ssh tunnel: sitting-here> ssh -C -R 6631:localhost:631 far-away.east

Or one could combine this with the VNC tunnel at the same time, e.g.: sitting-here> ssh -C -R 6631:localhost:631 -t -L 5900:localhost:5900 far-away .east 'x11vnc -localhost -display :0'

Port 631 is the default CUPS port. The above assumes you have a Cups server running on your viewer machine (localhost:631), if not, use something like my-cups-srv:631 (the viewer-side Cups server) in the -R instead.

Note that we use 6631 instead of 631 on the remote side because 631 would require root permission to listen on (and you likely have a cups server running on it already.)

Now the tricky part: to get applications to notice your cups server/printer on localhost:6631.

If you have administrative privilege (i.e. root password) on the x11vnc side where the desktop is running, it should be easy to add the printer through some configuration utility (e.g. in KDE: Utilities -> Printing -> Printing Manager, and then supply admin password, and then Add Printer/Class, and then fill in the inquisitive wizard. Most important is the "Remote IPP server" panel where you put in localhost for Host and 6631 for Port.) The main setting you want to convey is the host is localhost and the port is non-standard (e.g. 6631.) Some configuration utilities will take an Internet Printing Protocol (IPP) URI, e.g. http://localhost:6631/printers/, ipp://localhost:6631/printers/printer-name, ipp://localhost:6631/ipp/printer-name, etc. Check your CUPS documentation and admin interfaces to find what the syntax is and what the "printer name" is.

If you do not have root or print admin privileges, but are running a recent (version 1.2 or greater) of the Cups client software, then an easy way to temporarily switch Cups servers is to create the directory and file: $HOME/.cups/client.conf on the remote side with a line like: ServerName localhost:6631

When not using x11vnc for remote access you can comment the above line out with a '#' (or rename the client.conf file), to have normal cups operation.

Unfortunately, running applications may need to be restarted to notice the new printers (libcups does not track changes in client.conf.) Depending on circumstances, a running application may actually notice the new printers without restarting (e.g. no print dialog has taken place yet, or there are no CUPS printers configured on the remote side.)

Cups client software that is older (1.1) does not support appending the port number, and for newer ones there is a bug preventing it from always working (fixed in 1.2.3.) Kludges like these at the command line will work: far-away> env CUPS_SERVER=localhost IPP_PORT=6631 lpstat -p -d far-away> env CUPS_SERVER=localhost IPP_PORT=6631 lpr -P myprinter file.ps far-away> env CUPS_SERVER=localhost IPP_PORT=6631 firefox

but are somewhat awkward since you have to retroactively set the env. var IPP_PORT. Its value cannot be broadcast to already running apps (like the $HOME/.cups/client.conf trick sometimes does.) A common workaround for an already running app is to somehow get it to "Print To File", e.g. file.ps and then use something like the lpr example above. Also, the option "-h host:port" works with CUPS lp(1) and lpr(1).

You can also print to Windows shares printers in principle. You may do this with the smbspool(8) command, or configure the remote CUPS via lpadmin(8), etc, to use a printer URI something like smb://machine:port/printer (this may have some name resolution problems WRT localhost.) Also, as with SMB mounting, the port redir (-R) to the Windows machine must use the actual IP address instead of "localhost".

At some point we hope to fold some automation for CUPS ssh redir setup into the Enhanced TightVNC Viewer (SSVNC) package we provide (as of Sep 2006 it is there for testing.)

Q-129: How can I hear the sound (audio) from the remote applications on the desktop I am viewing via x11vnc?

You will have to use an external network audio mechanism for this. Audio is not part of the VNC protocol.

We show a simple Unix to Unix esd example here (artsd should be possible too, we have also verified the esd Windows port works for the method described below.)

First you will need a tunnel to redirect the audio from the remote machine to the one you sitting at. We use an ssh tunnel: sitting-here> ssh -C -R 16001:localhost:16001 far-away.east

Or one could combine this with the VNC tunnel at the same time, e.g.: sitting-here> ssh -C -R 16001:localhost:16001 -t -L 5900:localhost:5900 far-a way.east 'x11vnc -localhost -display :0'

Port 16001 is the default ESD uses. So when an application on the remote desktop makes a sound it will connect to this tunnel and be redirected to port 16001 on the local machine (sitting-here in this example.) The -C option is an attempt to compress the audio a little bit.

So we next need a local (sitting-here) esd daemon running that will receive those requests and play them on the local sound device: sitting-here> esd -promiscuous -port 16001 -tcp -bind 127.0.0.1

See the esd(1) man page for the meaning of the options (the above are not very secure.) (This method also works with the EsounD windows port esd.exe)

To test this sound tunnel, we use the esdplay program to play a simple .wav file: far-away> esdplay -s localhost:16001 im_so_happy.wav

If you hear the sound (Captain Kirk in this example), that means you are in great shape.

To run individual audio applications you can use the esddsp(1) command: far-away> esddsp -s localhost:16001 xmms

Then you could try playing some sounds inside xmms. You could also set the environment variable ESPEAKER=localhost:16001 to not need to supply the -s option all the time. (for reasons not clear, sometimes esddsp can figure it out on its own.) All the script esddsp does is to set ESPEAKER and LD_PRELOAD for you so that when the application opens the sound device (usually /dev/dsp) its interactions with the device will be intercepted and sent to the esd daemon running on sitting-here (that in turn writes them to the real, local /dev/dsp.)

Redirecting All sound:

It does not seem to be possible to switch all of the sound of the remote machine from its sound device to the above esd+ssh tunnel without some preparation. But it can be done reasonably well if you prepare (i.e. restart) the desktop with this in mind.

Here is one way to redirect all sound. The idea is we run the entire desktop with sound directed to localhost:16001. When we are sitting at far-away.east we run "esd -promiscuous -port 16001 -tcp -bind 127.0.0.1" on far-away.east (to be able to hear the sound.) However, when we are sitting at sitting-here.west we kill that esd process and run that same esd command on sitting-here.west and start up the above ssh tunnel. This is a little awkward, but with some scripts one would probably kill and restart the esd processes automatically when x11vnc is used.

So next we have to run the whole desktop pointing toward our esd. Here is a simple way to test. Log in to the machine via the "FailSafe" desktop. Then in the lone terminal type something like: esddsp -s localhost:16001 gnome-session or: esddsp -s localhost:16001 startkde

where the last part is whatever command starts your desktop (even fvwm2.) This causes the environment variables ESPEAKER and LD_PRELOAD to be set appropriately and every application (processes with the desktop as an ancestor) will use them. If this scheme works well you can make it less klunky by adding the command to your ~/.xsession, etc. file that starts your default desktop. Or you may be able to configure your desktop to use localhost:16001, or whatever is needed, via a gui configuration panel. Some Notes: * Not all audio applications are compatible with the esd and artsd mechanisms, but many are. * The audio is not compressed so you probably need a broadband or faster connection. Listening to music may not be very pleasant... (Although we found streaming music from across the US over cable modem worked OK, but took 200 KB/sec, to use less bandwidth consider something like "ssh far-away.east 'cat favorite.mp3' | mpg123 -b 4000 -") * Linux does not seem to have the concept of LD_PRELOAD_64 so if you run on a mixed 64- and 32-bit ABI system (e.g. AMD x86_64) some of the applications will fail to run because LD_PRELOAD will point to libraries of the wrong wordsize. * At some point we hope to fold some automation for esd or artsd ssh redir setup into the Enhanced TightVNC Viewer (SSVNC) package we provide (as of Sep/2006 it is there for testing.)

Q-130: Why don't I hear the "Beeps" in my X session (e.g. when typing tput bel in an xterm)?

As of Dec/2003 "Beep" XBell events are tracked by default. The X server must support the XKEYBOARD extension (this is not on by default in Solaris, see Xserver(1) for how to turn it on via +kb), and so you won't hear them if the extension is not present.

If you don't want to hear the beeps use the -nobell option. If you want to hear the audio from the remote applications, consider trying a redirector such as esd.

Q-131: Does x11vnc work with IPv6?

Update: as of Apr/2010 in the 0.9.10 x11vnc development tarball, there is now built-in support for IPv6 (128 bit internet addresses.) See the -6 and -connect options for details.

The remainder of this FAQ entry shows how to do with this with pre 0.9.10 x11vnc using IPv6 helper tools. _________________________________________________________________

Using an external IPv6 helper: A way to do this is via a separate helper program such as inetd (or for encrypted connections: ssh or stunnel.) For example, you configure x11vnc to be run from inetd or xinetd and instruct it to listen on an IPv6 address. For xinetd the setting "flags = IPv6" will be needed. For inetd.conf, an example is: 5900 stream tcp6 nowait root /usr/sbin/tcpd /usr/local/bin/x11vnc_wrapper.sh

We also provide a transitional tool in "x11vnc/misc/inet6to4" that acts as a relay for any IPv4 application to allow connections over IPv6. For example: inet6to4 5900 localhost:5900

where x11vnc is listening on IPv4 port 5900.

Also note that not all VNC Viewers are IPv6 enabled, so a redirector may also be needed for them. The tool "inet6to4 -r ..." can do this as well. SSVNC (see below) supports IPv6 without need for the helper.

./inet6to4 -help

inet6to4: Act as an ipv6-to-ipv4 relay for tcp applications that do not support ipv6.

Usage: inet6to4 inet6to4 -r

Examples: inet6to4 5900 localhost:5900 inet6to4 8080 web1:80 inet6to4 -r 5900 fe80::217:f2ff:fee6:6f5a%eth0:5900

The -r option reverses the direction of translation (e.g. for ipv4 clients that need to connect to ipv6 servers.) Reversing is the default if this script is named 'inet4to6' (e.g. by a symlink.)

Use Ctrl-C to stop this program.

You can also set env. vars INET6TO4_LOOP=1 or INET6TO4_LOOP=BG to have an outer loop restarting this program (BG means do that in the background), and INET6TO4_LOGFILE for a log file. Also set INET6TO4_VERBOSE to verbosity level and INET6TO4_WAITTIME and INET6TO4_PIDFILE (see below.)

The "INET6TO4_LOOP=BG" and "INET6TO4_LOGFILE=..." env. variables make the tool run reliably as a daemon for very long periods. Read the top part of the script for more information. _________________________________________________________________

Encrypted Tunnels with IPv6 Support: For SSH tunnelled encrypted VNC connections, one can of course use the IPv6 support in ssh(1).

For SSL encrypted VNC connections, one possibility is to use the IPv6 support in stunnel(1). This includes the built-in support via the -stunnel option. For example: x11vnc -stunnel SAVE -env STUNNEL_LISTEN=:: -env STUNNEL_DEBUG=1 ... _________________________________________________________________

SSH IPv6 Tricks: It is interesting to note that ssh(1) can do basically the same thing as inet6to4 above by: ssh -g -L 5900:localhost:5901 localhost "printf 'Press Enter to Exit: '; read x"

(where we have x11vnc running via "-rfbport 5901" in this case.)

Note that one can also make a home-brew SOCKS5 ipv4-to-ipv6 gateway proxy using ssh like this: ssh -D '*:1080' localhost "printf 'Press Enter to Exit: '; read x"

then specify a proxy like socks://hostname:1080 where hostname is the machine running the above ssh command (add -v to ssh for connection logging info.) _________________________________________________________________

IPv6 SSVNC Viewer: Our SSVNC VNC Viewer is basically a wrapper for ssh(1) and stunnel(1), and so it already has good IPv6 support because these two commands do. On Unix, MacOSX, and Windows nearly all of the the remaining parts of SSVNC (e.g. the built-in proxying and un-encrypted connections) have been modified to support IPv6 in SSVNC 1.0.26.

Contributions:

Q-132: Thanks for your program or for your help! Can I make a donation?

Please do (any amount is appreciated; very few have donated) and thank you for your support! Click on the PayPal button below for more info.

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======================================================================= http://www.karlrunge.com/x11vnc/chainingssh.html:

 _________________________________________________________________

Chaining ssh's: Note that for use of a ssh gateway and -L redirection to an internal host (e.g. "-L 5900:otherhost:5900") the VNC traffic inside the firewall is not encrypted and you have to manually log into otherhost to start x11vnc. Kyle Amon shows a method where you chain two ssh's together that encrypts all network traffic and also automatically starts up x11vnc on the internal workstation: #!/bin/sh

gateway="example.com" # or "user@example.com" host="labyrinth" # or "user@hostname" user="kyle"

Need to sleep long enough for all of the passwords and x11vnc to start up.

The </dev/null below makes the vncviewer prompt for passwd via popup window.

(sleep 10; vncviewer -encodings "copyrect tight zrle zlib hextile"
localhost:0 </dev/null >/dev/null) &

Chain the vnc connection thru 2 ssh's, and connect x11vnc to user's display:

exec /usr/bin/ssh -t -L 5900:localhost:5900 $gateway
/usr/bin/ssh -t -L 5900:localhost:5900 $host
sudo /usr/bin/x11vnc -localhost -auth /home/$user/.Xauthority
-rfbauth .vnc/passwd -display :0

Also note the use of sudo(1) to switch to root so that the different user's .Xauthority file can be accessed. See the visudo(8) manpage for details on how to set this up (remove the sudo if you do not want to do this). One can also chain together ssh's for reverse connections with vncviewers using the -listen option. For this case -R would replace the -L (and 5500 the 5900, see the #2 example script above). If the gateway machine's sshd is configured with GatewayPorts=no (the default) then the double chaining of "ssh -R ..." will be required for reverse connections to work.

======================================================================= http://www.karlrunge.com/x11vnc/miscbuild.html:

 _________________________________________________________________

Misc. Build problems: We collect here rare build problems some users have reported and the corresponding workarounds. See also the FAQ's on building. _________________________________________________________________

ENV parameter: One user had a problem where the build script below was failing because his work environment had the ENV variable set to a script that was resetting his PATH so that gcc could no longer be found. Make sure you do not have any ENV or BASH_ENV in your environment doing things like that. Typing "unset ENV", etc. before configuring and building should clear it. _________________________________________________________________

Bash xpg: One user had his bash shell compiled with --enable-xpg-echo-default that causes some strange behavior with things like echo "\1 ..." the configure script executes. In particular instead of getting "\1" the non-printable character "^A" is produced, and causes failures at compile time like: ../rfb/rfbconfig.h:9:22: warning: extra tokens at end of #ifndef directive

The workaround is to configure like this: env CONFIG_SHELL=/bin/sh /bin/sh ./configure

i.e. avoid using the bash with the misbehavior. A bug has been filed against autoconf to guard against this. _________________________________________________________________

AIX: one user had to add the "X11.adt" package to AIX to get build header files like XShm.h, etc. _________________________________________________________________

Ubuntu Feisty Fawn 7.04: In May/2007 one user said he needed to add these packages to compile x11vnc on that Linux distro and version: apt-get install build-essential make bin86 libjpeg62-dev libssl-dev libxtst-d ev

Note that Ubuntu is based on Debian, so perhaps this is the list needed on Debian (testing?) as well. To build in Avahi (mDNS service advertising) support it would appear that libavahi-client-dev is needed as well. _________________________________________________________________

Exceedingly slow compilation: x11vnc has a couple of files which contain very large "case statements" (over 100 cases) that on some platforms can take a very long time to compile (in extreme cases over an hour). However on 32bit Linux with intel/amd processor and gcc these files usually take less than 10 seconds to compile. For 64bit systems using gcc the problem appears to be much worse.

The two files with the large number of cases, remote.c and x11vnc.c, have no real need to be optimized (the code is used only very infrequently). So it is fine to supply "-O0" (disables optimization) to CFLAGS when compiling them. However, it is tricky with autoconf/automake to do this (especially since both the compiler and make versions have a big effect).

So if the compile times are getting too long for you for these two files you will need to manually change some things. First, run configure and when it has finished, edit the generated file x11vnc/Makefile and put these lines at the very top: x11vnc-x11vnc.o : CFLAGS += -O0 x11vnc-remote.o : CFLAGS += -O0

Those lines assume gnu make (gmake) is being used. If you are using another make, say Solaris make, insert these instead: x11vnc-x11vnc.o := CFLAGS += -O0 x11vnc-remote.o := CFLAGS += -O0

You could write a build shell script that modified the Makefile this way before running make.

The "-O0" (note it is "capital Oh" followed by "zero") assumes the gcc compiler. If you are using a different compiler you will need to find the command line option to disable optimization, or otherwise have the lines set CFLAGS to the empty string. _________________________________________________________________

Broken Thread Local Storage on SuSE 9.2: Starting with x11vnc 0.9.8 the bundled libvncserver uses the __thread keyword to make some of the encodings (i.e. tight) thread safe (multiple VNC clients can be using tight at the same time in x11vnc -threads mode.) Evidently on the old SuSE 9.2 system the compiler does not support the thread local storage properly. Here is an example build failure: tight.c:1126: error: unrecognizable insn: (insn:HI 11 10 13 0 (nil) (set (reg/f:SI 59) (const:SI (plus:SI (symbol_ref:SI ("%lpalette")) (const_int 2048 [0x800])))) -1 (nil) (expr_list:REG_EQUAL (const:SI (plus:SI (symbol_ref:SI ("%lpalette")) (const_int 2048 [0x800]))) (nil))) tight.c:1126: internal compiler error: in extract_insn, at recog.c:2175 Please submit a full bug report, with preprocessed source if appropriate. See URL:http://www.suse.de/feedback for instructions.

The workaround is to disable thread local storage at configure time like this: env CPPFLAGS="-DTLS=''" ./configure

and then build it. _________________________________________________________________

======================================================================= http://www.karlrunge.com/x11vnc/sunray.html:

Sun Ray Notes:

You can run x11vnc on your (connected or disconnected) SunRay session (Please remember to use settings like -wait 200, -sb 15, and not running a screensaver animation (blank instead) to avoid being a resource hog! x11vnc does induce a lot of memory I/O from polling the X server. It also helps to have a solid background color, e.g. -solid).

News: Sun Ray Remote Control Toolkit: See the nice set of tools in the Sun Ray Remote Control Toolkit that launch x11vnc automatically for you for certain usage modes.

You have to know the name of the machine your SunRay session X server is running on (so you can ssh into it and start x11vnc). You also need to know the X11 DISPLAY number for the session: on a SunRay it could be a large number, e.g. :137, since there are many people with X sessions (Xsun processes) on the same machine. If you don't know it, you can get it by running who(1) in a shell on the SunRay server and looking for the dtlocal entry with your username (and if you don't even know which server machine has your session, you could login to all possible ones looking at the who output for your username...).

I put some code in my ~/.dtprofile script that stores $DISPLAY (including the hostname) in a ~/.sunray_current file at session startup (and deletes it when the X session ends) to make it easy to get at the hostname and X11 display number info for my current X sessions when I ssh in and am about to start x11vnc.

SunRay Gotcha #1: Note that even though your SunRay X11 DISPLAY is something like :137, x11vnc still tries for port 5900 as its listening port if it can get it, in which case the VNC display (i.e. the information you supply to the VNC viewer) is something like sunray-server:0 (note the :0 corresponding to port 5900, it is not :137). If it cannot get 5900, it tries for 5901, and so on. You can also try to force the port (and thereby the VNC display) using the -rfbport NNNN option.

Especially on a busy Sun Ray server it is often difficult to find free ports for both VNC and the HTTP Java applet server to listen on. This script, vnc_findports may be of use for doing this automatically. It suggests x11vnc command line options based on netstat output that lists the occupied ports. It is even more difficult to start vncserver/Xvnc on a busy Sun Ray because then 3 ports (HTTP, VNC, and X11), all separated by 100 are needed! This script, findvncports may be helpful as well. Both scripts start at VNC display :10 and work their way up.

SunRay Gotcha #2: If you get an error like: shmget(tile) failed. shmget: No space left on device

when starting up x11vnc that most likely means all the shared memory (shm) slots are filled up on your machine. The Solaris default is only 100, and that can get filled up in a week or so on a SunRay server with lots of users. If the shm slot is orphaned (e.g. creator process dies) the slot is not reclaimed. You can view the shm slots with the "ipcs -mA" command. If there are about 100 then you've probably hit this problem. They can be cleaned out (by the owner or by root) using the ipcrm command. I wrote a script shm_clear that finds the orphans and lists or removes them. Longer term, have your SunRay sysadmin add something like this to /etc/system: set shmsys:shminfo_shmmax = 0x2000000 set shmsys:shminfo_shmmni = 0x1000

SunRay Gotcha #3: Some SunRay installations have implemented suspending certain applications when a SunRay session is in a disconnected state (e.g. Java Badge pulled out, utdetach, etc). This is a good thing because it limits hoggy or runaway apps from wasting the shared CPU resource. Think how much CPU and memory I/O is wasted by a bunch of Firefox windows running worthless Flash animations while your session is disconnected!

So some sites have implemented scripts to suspend (e.g. kill -STOP) certain apps when your badge is removed from the SunRay terminal. When you reattach, it kill -CONT them. This causes problems for viewing the detached SunRay session via x11vnc: those suspended apps will not respond (their windows will be blank or otherwise inactive).

What to do? Well, since you are going to be using the application you might as well unfreeze it rather than starting up a 2nd instance. Here is one way to do it using the kill -CONT mechanism: kill -CONT ps -ealf | grep ' T ' | grep $LOGNAME | awk '{print $4}'

If you want to be a good citizen and re-freeze them before you exit x11vnc this script could be of use: #!/bin/sh

kill -STOP/-CONT script for x11vnc (or other) SunRay usage ("freezes"

certain apps from hogging resources when disconnected).

Put here a pattern that matches the apps that are frozen:

appmatch="java_vm|jre|netscape-bin|firefox-bin|realplay|acroread|mozilla-bin"

if [ "X$1" = "Xfreeze" ]; then pkill -STOP -U $LOGNAME "$appmatch" elif [ "X$1" = "Xthaw" ]; then pkill -CONT -U $LOGNAME "$appmatch"

elif [ "$RFB_MODE" = "afteraccept" -a "$RFB_STATE" = "NORMAL" ]; then # a valid x11vnc login. if [ "$RFB_CLIENT_COUNT" = "1" ]; then # only one client present. pkill -CONT -U $LOGNAME "$appmatch" fi elif [ "$RFB_MODE" = "gone" -a "$RFB_STATE" = "NORMAL" ]; then # a valid x11vnc login. if [ "$RFB_CLIENT_COUNT" = "0" ]; then # last client present has just left. pkill -STOP -U $LOGNAME "$appmatch" fi fi exit 0

If you called the script "goodcitizen" you could type "goodcitizen thaw" to unfreeze them, and then "goodcitizen freeze" to refreeze them. One could also use these x11vnc options "-afteraccept goodcitizen -gone goodcitizen" to do it automatically.

SunRay Gotcha #4: Recent versions of the Sun Ray Server Software SRSS (seems to be version 3.0 or 3.1) have a "misfeature" that when the session is disconnected (i.e. badge/smartcard out) the screen locker (xscreensaver) will freeze the X server just when the "Enter Password" dialog box appears. So you cannot unlock the screen remotely via x11vnc!

Update: please see Bob Doolittle's detailed description of the this issue at the bottom of this section.

Here "freeze" means "stop other X clients from inserting keyboard and mouse input and from viewing the current contents of the screen". Or something like that; the upshot is x11vnc can't do its normal thing.

There are several workarounds for this.

  1. The easiest one by far is to put these lines in your $HOME/.dtprofile file: SUN_SUNRAY_UTXLOCK_PREF="/usr/openwin/bin/xlock -mode blank" export SUN_SUNRAY_UTXLOCK_PREF

One might argue that xlock isn't particularly "pretty". (Just IMHO, but if something like this not being pretty actually gets in the way of your work I think some introspection may be in order. :-)

  1. The problem has been traced to the pam_sunray.so PAM module. Evidently xscreensaver invokes this pam module and it communicates with utsessiond who in turn instructs the Xsun server to not process any synthetic mouse/keyboard input or to update the screen framebuffer. It is not clear if this is by design (security?) or something else.

In any event, the problem can be avoided, somewhat drastically, by commenting out the corresponding line in /etc/pam.conf: #xscreensaver auth sufficient /opt/SUNWut/lib/pam_sunray.so syncondisplay

Leave the other xscreensaver pam authentication lines unchanged. The dtsession-SunRay line may also need to be commented out to avoid the problem for CDE sessions. N.B. it is possible the application of a SSRS patch, etc, may re-enable that /etc/pam.conf line. It may be difficult to convince a sysadmin to make this change.

  1. A more forceful way is to kill the xscreensaver process from a shell prompt whenever you connect via x11vnc and the screen is in a locked state: pkill -U $LOGNAME '^xscreensaver$'

And then after you are in be sure to restart it by typing something like: xscreensaver &

You may want to avoid restarting it until you are about to disconnect your VNC viewer (since if it locks the screen while you are working you'll be stuck again).

3') The above idea can be done a bit more cleanly by having x11vnc do it. Suppose we called the following script xss_killer: #!/bin/sh

xss_killer: kill xscreensaver after a valid x11vnc client logs in.

Restart xscreensaver and lock it when the last client

disconnects.

PATH=/usr/openwin/bin:/usr/bin:$PATH export PATH

if [ "$RFB_MODE" = "afteraccept" -a "$RFB_STATE" = "NORMAL" ]; then # a valid x11vnc login. if [ "$RFB_CLIENT_COUNT" = "1" ]; then # only one client present. pkill -U $LOGNAME '^xscreensaver$' pkill -KILL -U $LOGNAME -f xscreensaver/hacks fi elif [ "$RFB_MODE" = "gone" -a "$RFB_STATE" = "NORMAL" ]; then # a valid x11vnc login. if [ "$RFB_CLIENT_COUNT" = "0" ]; then # last client present has just left. xscreensaver -nosplash & sleep 1 xscreensaver-command -lock & fi fi

Then we would run x11vnc with these options: "-afteraccept xss_killer -gone xss_killer". The -afteraccept option (introduced in version 0.8) is used to run a command after a vncviewer has successfully logged in (note that this is a VNC login, not a Unix login, so you may not want to do this if you are really paranoid...)

Note if you use the above script and also plan to Ctrl-C (SIGINT) x11vnc you have to run the xscreensaver in a new process group to avoid killing it as well. One way to do this is via this kludge: perl -e 'setpgrp(0,0); exec "xscreensaver -nosplash &"'

in the above script.

  1. There appears to be a bug in pam_sunray.so in that it doesn't seem to honor the convention that, say, DISPLAY=unix:3 means to use Unix sockets to connect to display 3 on the local machine (this is a bit faster than TCP sockets). Rather, it thinks the display is a non-local one to a machine named "unix" (that usually does not resolve to an IP address).

Amusingly, this can be used to bypass the pam_sunray.so blocking of Xsun that prevents one from unlocking the screen remotely via x11vnc. One could put something like this in $HOME/.dtprofile to kill any existing xscreensavers and then start up a fresh xscreensaver using DISPLAY=unix:N

stop/kill any running xscreensavers (probably not running yet, but to be sure

) xscreensaver-command -exit pkill -U $LOGNAME '^xscreensaver$' env DISPLAY=echo $DISPLAY | sed -e 's/^.*:/unix:/' xscreensaver &

Important: Note that all of the above workarounds side-step the pam_sunray.so PAM module in one way or another. You'll need to see if that is appropriate for your site's SunRay / smartcard usage. Also, these hacks may break other things and so you may want to test various scenarios carefully. E.g. check corner cases like XDMCP/dtremote, NSCM, etc.

Update May 2008: Here is a useful description of this issue from Bob Doolittle who is a developer for Sun Ray at Sun. I don't have the time to digest and distill it and then adjust the above methods to provide a clearer description, so I just include below the description he sent me with the hope that it will help some users:

 In SRSS 4.0 and earlier, the purpose of pam_sunray.so in the "auth"
 PAM stack of screensavers is to enable NSCM (and, although this is
 much less commonly used, "SC", which is configured when 3rd-party
 software is installed to allow smartcards to be used as part of the
 authentication process) to work. It should have no effect with
 smartcards. Currently, however, it does block the PAM stack for all
 sessions, which causes xscreensaver, when it locks a disconnected
 session, to not process any mouse or keyboard events as you
 describe (unless xscreensaver does an X server grab, however, other
 applications should still be able to draw in the session although
 xscreensaver may be playing tricks like putting a black window on
 top of everything). In both of the NSCM and SC models,
 authentication occurs in a separate session before SRSS will
 reconnect to the user session, in which case pam_sunray.so causes
 xscreensaver to just unlock the screen without prompting the user
 to enter their password again. To do this, pam_sunray.so has to
 block until the session becomes reconnected, so it can query SRSS
 at that time to determine whether the user has already
 authenticated or not. In SRSS 4.0 and earlier releases,
 pam_sunray.so could have been optimized to not block smartcard
 sessions, although since the session is disconnected this typically
 isn't important (except in the x11vnc case, as you've observed).

 In SRSS 4.1, however, for increased security the out-of-session
 authentication model has been extended to *all* session types, so
 pam_sunray.so will be required in all cases unless users are
 willing to authenticate twice upon hotdesking (e.g. when their card
 is inserted). In future, we may do away with pam_sunray.so, and in
 fact with any traditional screen locker in the user session, since
 SRSS itself will be providing better security than a screen locker
 running entirely within the user's X session is capable of
 providing.

 Your trick of setting DISPLAY to unix:DPY will effectively disable
 pam_sunray.so (I'm not sure I'd call that a bug - you're going out
 of your way to do something that wouldn't occur in the normal
 course of events, and really provides no useful value other than to
 tickle this behavior in pam_sunray.so). This will mean that, in
 SRSS 4.0 and earlier releases, users will be prompted for their
 passwords twice when reconnecting to their sessions for NSCM and SC
 session types. In 4.1, disabling pam_sunray.so in this way will
 cause this double-authentication to occur for *all* sessions,
 including simple smartcard sessions. Users may be willing to pay
 that price in order to be able to use x11vnc in disconnected
 sessions. I like this hack, personally. It's a little less
 convenient than some of the other approaches you describe, but it's
 lighter-weight and more secure than most of the other approaches,
 and provides the value of being able to use x11vnc in locked
 sessions.

 Here are some other minor notes: - I wouldn't recommend storing
 your display in your .dtprofile, unless you're willing to live with
 a single session at a time. Personally, I often find myself using
 several sessions, in several FoGs, for short periods of time so
 this would certainly break. IMO it's pretty easy to use $DISPLAY to
 do what you want on the fly, as needed, so I don't think the price
 of breaking multiple-session functionality would be worth the
 convenience, to me at least. Here's some ksh/bash syntax to extract
 the hostname and display number on the fly which you may find
 useful:

HOSTNAME=${DISPLAY%:} FULLDPY=${DISPLAY#:} DPYNUM=${FULLDPY%.*}

 A final note may give you some insight into other clever hacks in
 this area: - Check out utaction. It's a very handy little utility
 that can be run as a daemon in the user session which will invoke a
 specified command upon session connects and/or disconnects.
 Personally, I start one up in my .dtprofile as follows:

utaction -c $HOME/.srconnectrc -d $HOME/.srdisconnectrc &

 This then allows me to construct a .srconnectrc script containing
 useful commands I'd like to have run every time I insert my
 smartcard, and a .srdisconnectrc script of commands to be run every
 time I remove my smartcard (or, connect/disconnect to my session
 via NSCM or SC). This can be used for things like notifying a chat
 client of away status, as well as some of the hacks you've
 described on your page such as freeze/unfreeze, or perhaps to
 terminate an xscreensaver and start up a new one with the unix:DPY
 $DISPLAY specification as you describe (although it probably makes
 most sense to do this at login time, as opposed to every connect or
 disconnect event).

======================================================================= http://www.karlrunge.com/x11vnc/ssl.html:

 _________________________________________________________________

Notes on x11vnc SSL Certificates and Key Management:

The simplest scheme ("x11vnc -ssl TMP") is where x11vnc generates a temporary, self-signed certificate each time (automatically using openssl(1)) and the VNC viewer client accepts the certificate without question (e.g. user clicks "Yes" in a dialog box. Perhaps the dialog allows them to view the certificate too). Also note stunnel's default is to quietly accept all certificates.

The encryption this provides protects against all passive sniffing of the VNC traffic and passwords on the network and so it is quite good, but it does not prevent a Man-In-The-Middle active attack: e.g. an attacker intercepts the VNC client stream and sends it his own Public key for SSL negotiation (pretending to be the server). Then it makes a connection to SSL x11vnc itself and forwards the data back and forth. He can see all the traffic and modify it as well.

Most people don't seem to worry about Man-In-The-Middle attacks these days; they are more concerned about passive sniffing of passwords, etc. Perhaps someday that will change if attack tools are used more widely to perform the attack. NOTE: There are hacker tools like dsniff/webmitm and cain that implement SSL Man-In-The-Middle attacks. They all rely on the client not bothering to check that the cert is valid.

If you are not worried about Man-In-The-Middle attacks you do not have to read the techniques described in the rest of this document.

To prevent Man-In-The-Middle attacks, certificates must somehow be verified. This requires the VNC client side have some piece of information that can be used to verify the SSL x11vnc server. Alternatively, although rarely done, x11vnc can verify VNC Clients' certificates, see the -sslverify option that is discussed below.

There are a number of ways to have the client authenticate the SSL x11vnc server. The quickest way perhaps would be to copy (safely) the certificate x11vnc prints out: 26/03/2006 21:12:00 Creating a temporary, self-signed PEM certificate... ... -----BEGIN CERTIFICATE----- MIIC4TCCAkqgAwIBAgIJAMnwCaOjvEKaMA0GCSqGSIb3DQEBBAUAMIGmMQswCQYD VQQGEwJBVTEOMAwGA1UEBxMFTGludXgxITAfBgNVBAsTGGFuZ2VsYS0xMTQzNDI1 NTIwLjQxMTE2OTEPMA0GA1UEChMGeDExdm5jMS4wLAYDVQQDEyV4MTF2bmMtU0VM (more lines) ... -----END CERTIFICATE-----

to the client machine(s) and have the client's SSL machinery (e.g. stunnel, Web Browser, or Java plugin) import the certificate. That way when the connection to x11vnc is made the client can verify that is it the desired server on the other side of the SSL connection.

So, for example suppose the user is using the SSL enabled Java VNC Viewer and has incorporated the x11vnc certificate into his Web browser on the viewing side. If he gets a dialog that the certificate is not verified he knows something is wrong. It may be a Man-In-The-Middle attack, but more likely x11vnc certificate has changed or expired or his browser was reinstalled and/or lost the certificate, etc, etc.

As another example, if the user was using stunnel with his VNC viewer (this is mentioned in this FAQ), e.g. STUNNEL.EXE on Windows, then he would have to set the "CAfile = path-to-the-cert" and "verify = 2" options in the stunnel.conf file before starting up the tunnel. If a x11vnc certificate cannot be verified, stunnel will drop the connection (and print a failure message in its log file).

A third example, using the VNC viewer on Unix with stunnel the wrapper script can be used this way: "ss_vncviewer -verify ./x11vnc.crt far-away.east:0" where ./x11vnc.crt is the copied certificate x11vnc printed out.

As fourth example, our SSVNC enhanced tightvnc viewer can also use these certificate files for server authentication. You can load them via the SSVNC 'Certs...' dialog and set 'ServerCert' to the certificate file you safely copied there.

Note that in principle the copying of the certificate to the client machine(s) itself could be altered by a Man-In-The-Middle attack! You can't win; it is very difficult to be completely secure. It is unlikely the attacker could predict how you were going to send it unless you had, say, done it many times before the same way. SSH is a very good way to send it (but of course it too depends on public keys being sent unaltered between the two machines!).

If you are really paranoid, I'm sure you'll figure out a really good way to transport the certificates. See the Certificate Authority scheme below for a way to make this easier (you just have to do it once).

 _________________________________________________________________

Saving SSL certificates and keys:

Now, it would be very inconvenient to copy the new temporary certificate every time x11vnc is run in SSL mode. So for convenience there is the "SAVE" keyword to instruct x11vnc to save the certificate it creates: x11vnc -ssl SAVE -display :0 ...

This behavior is now the default, you must use "TMP" for a temporary one. It will save the certificate and private key in these files: ~/.vnc/certs/server.crt ~/.vnc/certs/server.pem

The ".crt" file contains only the certificate and should be safely copied to the VNC Viewer machine(s) that will be authenticating the x11vnc server. The ".pem" file contains both the certificate and the private key and should be kept secret. (If you don't like the default location ~/.vnc/certs, e.g. it is on an NFS share and you are worried about local network sniffing, use the -ssldir dir option to point to a different directory.)

So the next time you run "x11vnc -ssl SAVE ..." it will read the server.pem file directly instead of creating a new one.

You can manage multiple SSL x11vnc server keys in this simple way by using: x11vnc -ssl SAVE-key2 -display :0 ...

etc, where you put whatever name you choose for the key after "SAVE-". E.g. "-ssl SAVE-fred".

Also, if you want to be prompted to possibly change the made up names, etc. that x11vnc creates (e.g. "x11vnc-SELF-SIGNED-CERT-7762" for the CommonName) for the certificates distinguished name (DN), then use "x11vnc -ssl SAVE_PROMPT ...", "x11vnc -ssl SAVE_PROMPT-fred ..." etc. when you create the key the first time.

Tip: when prompting, if you choose the CommonName entry to be the full internet hostname of the machine the clients will be connecting to then that will avoid an annoying dialog box in their Web browsers that warn that the CommonName doesn't match the hostname.

 _________________________________________________________________

Passphrases for server keys:

Well, since now with the "SAVE" keyword the certificate and key will be longer lived, one can next worry about somebody stealing the private key and pretending to be the x11vnc server! How to guard against this?

The first is that the file is created with perms 600 (i.e. -rw-------) to make it harder for an untrusted user to copy the file. A better way is to also encrypt the private key with a passphrase. You are prompted whether you want to do this or not when the key is first created under "-ssl SAVE" mode ("Protect key with a passphrase? y/n"). It is suggested that you use a passphrase. The inconvenience is every time you run "x11vnc -ssl SAVE ..." you will need to supply the passphrase to access the private key: 06/04/2006 11:39:11 using PEM /home/runge/.vnc/certs/server.pem 0.000s

A passphrase is needed to unlock an OpenSSL private key (PEM file). Enter passphrase>

before x11vnc can continue.

 _________________________________________________________________

Being your own Certificate Authority:

A very sophisticated way that scales well if the number of users is large is to use a Certificate Authority (CA) whose public certificate is available to all of the VNC clients and whose private key has been used to digitally sign the x11vnc server certificate(s).

The idea is as follows: * A special CA cert and key is generated. * Its private key is always protected by a good passphrase since it is only used for signing. * The CA cert is (safely) distributed to all machines where VNC clients will run. * One or more x11vnc server certs and keys are generated. * The x11vnc server cert is signed with the CA private key. * x11vnc is run using the server key. (e.g. "-ssl SAVE") * VNC clients (viewers) can now authenticate the x11vnc server because they have the CA certificate.

The advantage is the CA cert only needs to be distributed once to the various machines, that can be done even before x11vnc server certs are generated.

As above, it is important the CA private key and the x11vnc server key are kept secret, otherwise someone could steal them and pretend to be the CA or the x11vnc server if they copied the key. It is recommended that the x11vnc server keys are also protected via a passphrase (see the previous section).

Optionally, VNC viewer certs and keys could also be generated to enable the x11vnc server to authenticate each client. This is not normally done (usually a simple viewer password scheme is used), but this can be useful in some situations. These optional steps go like this: * One or more VNC client certs and keys are generated. * These VNC client certs are signed with the CA private key. * The VNC client certs+keys are safely distributed to the corresponding client machines. * x11vnc is told to verify clients by using the CA cert. (e.g. "-sslverify CA") * When VNC clients (viewers) connect, they must authenticate themselves to x11vnc by using their client key.

Again, it is a good idea if the client private keys are protected with a passphrase, otherwise if stolen they could be used to gain access to the x11vnc server. Once distributed to the client machines, there is no need to keep the client key on the CA machine that generated and signed it. You can keep the client certs if you like because they are public.

 _________________________________________________________________

How to do the above CA steps with x11vnc:

Some utility commands are provided to ease the cert+key creation, signing, and management: -sslGenCA, -sslGenCert, -sslDelCert, -sslEncKey, -sslCertInfo. They basically run the openssl(1) command for you to manage the certs/keys. It is required that openssl(1) is installed on the machine and available in PATH. All commands can be pointed to an alternate toplevel certificate directory via the -ssldir option if you don't want to use the default ~/.vnc/certs.

  1. To generate your Certificate Authority (CA) cert and key run this: x11vnc -sslGenCA

Follow the prompts, you can modify any informational strings you care to. You will also be required to encrypt the CA private key with a passphrase. This generates these files: ~/.vnc/certs/CA/cacert.pem (the CA public certificate) ~/.vnc/certs/CA/private/cakey.pem (the encrypted CA private key)

If you want to use a different directory use -ssldir It must supplied with all subsequent SSL utility options to point them to the correct directory.

  1. To generate a signed x11vnc server cert and key run this: x11vnc -sslGenCert server

As with the CA generation, follow the prompts and you can modify any informational strings that you care to. This will create the files: ~/.vnc/certs/server.crt (the server public certificate) ~/.vnc/certs/server.pem (the server private key + public cert)

It is recommended to protect the server private key with a passphrase (you will be prompted whether you want to). You will need to provide it whenever you start x11vnc using this key.

  1. Start up x11vnc using this server key: x11vnc -ssl SAVE -display :0 ...

(SAVE corresponds to server.pem, see -sslGenCert server somename info on creating additional server keys, server-somename.crt ...)

  1. Next, safely copy the CA certificate to the VNC viewer (client) machine(s). Perhaps: scp ~/.vnc/CA/cacert.pem clientmachine:.

  2. Then the tricky part, make it so the SSL VNC Viewer uses this certificate! There are a number of ways this might be done, it depends on what your client and/or SSL tunnel is. Some examples:

For the SSL Java VNC viewer supplied with x11vnc in classes/ssl/VncViewer.jar or classes/ssl/SignedVncViewer.jar: * Import the cacert.pem cert into your Web Browser (e.g. Edit -> Preferences -> Privacy & Security -> Manage Certificates -> WebSites -> Import) * Or Import the cacert.pem cert into your Java Plugin (e.g. run ControlPanel, then Security -> Certificates -> Secure Site -> Import)

When importing, one would give the browser/java-plugin the path to the copied cacert.pem file in some dialog. Note that the Web browser or Java plugin is used for the server authentication. If the user gets a "Site not verified" message while connecting he should investigate further.

For the use of stunnel (e.g. on Windows) one would add this to the stunnel.conf:

stunnel.conf:

client = yes options = ALL CAfile = /path/to/cacert.pem # or maybe C:\path\to\cacert.pem [myvncssl] accept = 5901 connect = far-away.east:5900

(then point the VNC viewer to localhost:1).

Here is an example for the Unix stunnel wrapper script ss_vncviewer in our SSVNC package: ss_vncviewer -verify ./cacert.pem far-away.east:0

Our SSVNC enhanced tightvnc viewer GUI can also use the certificate file for server authentication. You can load it via the SSVNC 'Certs...' dialog and set 'ServerCert' to the cacert.pem file you safely copied there.

 _________________________________________________________________

Tricks for server keys:

To create additional x11vnc server keys do something like this: x11vnc -sslGenCert server myotherkey

and use it this way: x11vnc -ssl SAVE-myotherkey ...

The files will be ~/.vnc/certs/server-myotherkey.{crt,pem}

You can also create a self-signed server key: x11vnc -sslGenCert server self:third_key

and use it this way: x11vnc -ssl SAVE-self:third_key ...

This key is not signed by your CA. This can be handy to have a key set separate from your CA when you do not want to create a 2nd CA cert+key.

 _________________________________________________________________

Using external CA's:

You don't have to use your own CA cert+key, you can use a third party's instead. Perhaps you have a company-wide CA or you can even have your x11vnc certificate signed by a professional CA (e.g. www.thawte.com or www.verisign.com or perhaps the free certificate service www.startcom.org or www.cacert.org).

The advantage to doing this is that the VNC client machines will already have the CA certificates installed and you don't have to install it on each machine.

To generate an x11vnc server cert+key this way you should generate a "request" for a certicate signing something like this (we use the name "external" in this example, it could be anything you want): x11vnc -sslGenCert server req:external

This will create the request file: ~/.vnc/certs/server-req:external.req

Which you should send to the external CA. When you get the signed certificate back from them, save it in the file: ~/.vnc/certs/server-req:external.crt

and create the .pem this way: mv ~/.vnc/certs/server-req:external.key ~/.vnc/certs/server-req:external. pem chmod 600 ~/.vnc/certs/server-req:external.pem cat ~/.vnc/certs/server-req:external.crt >> ~/.vnc/certs/server-req:external. pem

You also rename the two files (.crt and .pem) to have a shorter basename if you like. E.g.: mv ~/.vnc/certs/server-req:external.pem ~/.vnc/certs/server-ext.pem mv ~/.vnc/certs/server-req:external.crt ~/.vnc/certs/server-ext.crt

and the use via "x11vnc -ssl SAVE-ext ...", etc.

On the viewer side make sure the external CA's certificate is installed an available for the VNC viewer software you plan to use.

 _________________________________________________________________

Using Client Keys for Authentication:

You can optionally create certs+keys for your VNC client machines as well. After distributing them to the client machines you can have x11vnc verify the clients using SSL. Here is how to do this:

x11vnc -sslGenCert client dilbert x11vnc -sslGenCert client wally x11vnc -sslGenCert client alice ...

As usual, follow the prompts if you want to change any of the info field values. As always, it is a good idea (although inconvenient) to protect the private keys with a passphrase. These files are created: ~/.vnc/certs/clients/dilbert.crt ~/.vnc/certs/clients/dilbert.pem ...

Note that these are kept in a clients subdirectory.

Next, safely copy the .pem files to each corresponding client machine and incorporate them into the VNC viewer / SSL software (see the ideas mentioned above for the CA and server keys). The only difference is these certificates might be referred to as "My Certificates" or "Client Certificates". They are used for client authentication (which is relatively rare for SSL).

After copying them you can delete the clients/.pem files for extra safety because the private keys are not needed by the x11vnc server. You don't really need the clients/.crt files either (because they have been signed by the CA). But they could come in handy for tracking or troubleshooting, etc.

Now start up x11vnc and instruct it to verify connecting clients via SSL and the CA cert: x11vnc -ssl SAVE -sslverify CA

The "CA" special token instructs x11vnc to use its CA signed certs for verification.

For arbitrary self-signed client certificates (no CA) it might be something like this: x11vnc -ssl SAVE -sslverify path/to/client.crt x11vnc -ssl SAVE -sslverify path/to/client-hash-dir x11vnc -ssl SAVE -sslverify path/to/certs.txt

Where client.crt would be an individual client certificate; client-hash-dir a directory of file names based on md5 hashes of the certs (see -sslverify); and certs.txt signifies a single file full of client certificates.

Finally, connect with your VNC viewer using the key. Here is an example for the Unix stunnel wrapper script ss_vncviewer: using client authentication (and the standard server authentication with the CA cert): ss_vncviewer -mycert ./dilbert.pem -verify ./cacert.pem far-away.east:0

Our SSVNC enhanced tightvnc viewer can also use these openssl .pem files (you can load them via Certs... -> MyCert dialog).

It is also possible to use -sslverify on a per-client key basis, and also using self-signed client keys (x11vnc -sslGenCert client self:dilbert)

Now a tricky part is to get Web browsers or Java Runtime to import and use the openssl .pem cert+key files. See the next paragraph on how to convert them to pkcs12 format. If you find a robust way to import them and and get them to use the cert please let us know!

Here is how to convert our openssl crt/pem files to pkcs12 format (contains both the client certificate and key) that can be read by Web browsers and Java for use in client authentication: openssl pkcs12 -export -in mycert.crt -inkey mycert.pem -out mycert.p12

it will ask for a passphrase to protect mycert.p12. Some software (e.g. Java ControlPanel) may require a non-empty passphrase. Actually, since our .pem contains both the certificate and private key, you could just supply it for the -in and remove the -inkey option. It appears that for certificates only importing, our .crt file is sufficient and can be read by Mozilla/Firefox and Java...

If you have trouble getting your Java Runtime to import and use the cert+key, there is a workaround for the SSL-enabled Java applet. On the Web browser URL that retrieves the VNC applet, simply add a "/?oneTimeKey=..." applet parameter (see ssl-portal for more details on applet parameters; you don't need to do the full portal setup though). The value of the oneTimeKey will be the very long string that is output of the onetimekey program found in the classes/ssl x11vnc directory. Or you can set oneTimeKey=PROMPT in which case the applet will ask you to paste in the long string. These scheme is pretty ugly, but it works. A nice application of it is to make one time keys for users that have already logged into a secure HTTPS site via password. A cgi program then makes a one time key for the logged in user to use: it is passed back over HTTPS as the applet parameter in the URL and so cannot be sniffed. x11vnc is run to use that key via -sslverify.

Update: as of Apr 2007 in the 0.9.1 x11vnc tarball there is a new option setting "-users sslpeer=" that will do a switch user much like -unixpw does, but this time using the emailAddress field of the Certificate subject of the verified Client. This mode requires -sslverify turned on to verify the clients via SSL. This mode can be useful in situations using -create or -svc where a new X server needs to be started up as the authenticated user (but unlike in -unixpw mode, the unix username is not obviously known).

 _________________________________________________________________

Revoking Certificates:

A large, scaled-up installation may benefit from being able to revoke certificates (e.g. suppose a user's laptop with a vnc client or server key is compromised.) You can use this option with x11vnc: -sslCRL. See the info at that link for a guide on what openssl(1) commands you will need to run to revoke a certificate.

 _________________________________________________________________

Additional utlities:

You can get information about your keys via -sslCertInfo. These lists all your keys: x11vnc -sslCertInfo list x11vnc -sslCertInfo ll

(the latter is long format).

These print long output, including the public certificate, for individual keys: x11vnc -sslCertInfo server x11vnc -sslCertInfo dilbert x11vnc -sslCertInfo all (every key, very long)

If you want to add a protecting passphrase to a key originally created without one: x11vnc -sslEncKey SAVE x11vnc -sslEncKey SAVE-fred

To delete a cert+key: x11vnc -sslDelCert SAVE x11vnc -sslDelCert SAVE-fred x11vnc -sslDelCert wally

(but rm(1) will be just as effective).

 _________________________________________________________________

Chained Certificates:

There is increasing interest in using chained CA's instead of a single CA. The merits of using chained CA's are not described here besides to say its use may make some things easier when a certificate needs to be revoked.

x11vnc supports chained CA certificates. We describe a basic use case here.

Background: Of course the most straight forward way to use SSL with x11vnc is to use no CA at all (see above): a self-signed certificate and key is used and its certificate needs to be safely copied to the client side. This is basically the same as the SSH style of managing keys. Next level up, one can use a single CA to sign server keys: then only the CA's certificate needs to be safely copied to the client side, this can happen even before any server certs are created (again, see all of the discussion above.)

With a certificate chain there are two or more CA's involved. Perhaps it looks like this: root_CA ---> intermediate_CA ---> server_cert

Where the arrow basically means "signs".

In this usage mode the client (viewer-side) will have root_CA's certificate available for verifying (and nothing else.) If the viewer only received server_cert's certificate, it would not have enough info to verify the server. The client needs to have intermediate_CA's cert as well. The way to do this with x11vnc (i.e. an OpenSSL using app) is to concatenate the server_cert's pem and the intermediate_CA's certificate together.

For example, suppose the file intermediate_CA.crt had intermediate_CA's certificate. And suppose the file server_cert.pem had the server's certificate and private key pair as described above on this page. We need to do this: cat intermediate_CA.crt >> server_cert.pem

(Note: the order of the items inside the file matters; intermediate_CA must be after the server key and cert) and then we run x11vnc like this: x11vnc -ssl ./server_cert.pem ...

Then, on the VNC viewer client side, the viewer authenticates the x11vnc server by using root_CA's certificate. Suppose that is in a file named root_CA.crt, then using the SSVNC wrapper script ss_vncviewer (which is also included in the SSVNC package) as our example, we have: ss_vncviewer -verify ./root_CA.crt hostname:0

(where "hostname" is the machine where x11vnc is running.) One could also use the SSVNC GUI setting Certs -> ServerCert to the root_CA.crt file. Any other SSL enabled VNC viewer would use root_CA.crt in a similar way. _________________________________________________________________

Creating Chained Certificates:

Here is a fun example using VeriSign's "Trial Certificate" program. Note that VeriSign has a Root CA and also an Intermediate CA and uses the latter to sign customers certificates. So this provides an easy way to test out the chained certificates mechanism with x11vnc.

First we created a test x11vnc server key: openssl genrsa -out V1.key 1024

then we created a certificate signing request (CSR) for it: openssl req -new -key V1.key -out V1.csr

(we followed the prompts and supplied information for the various fields.)

Then we went to VeriSign's page http://www.verisign.com/ssl/index.html and clicked on "FREE TRIAL" (the certificate is good for 14 days.) We filled in the forms and got to the point where it asked for the CSR and so we pasted in the contents of the above V1.csr file. Then, after a few more steps, VeriSign signed and emailed us our certificate.

The VeriSign Trial certificates were found here: http://www.verisign.com/support/verisign-intermediate-ca/Trial_Secure_Server_ Root/index.html http://www.verisign.com/support/verisign-intermediate-ca/trial-secure-server- intermediate/index.html

The former was pasted into a file V-Root.crt and the latter was pasted into V-Intermediate.crt

We pasted our Trial certificate that VeriSign signed and emailed to us into a file named V1.crt and then we typed: cat V1.key V1.crt > V1.pem cat V1.pem V-Intermediate.crt > V1-combined.pem chmod 600 V1.pem V1-combined.pem

So now the file V1-combined.pem has our private key and (VeriSign signed) certificate and VeriSign's Trial Intermediate certificate.

Next, we start x11vnc: x11vnc -ssl ./V1-combined.pem ...

and finally, on the viewer side (SSVNC wrapper script example): ss_vncviewer -verify ./V-Root.crt hostname:0

One will find that only that combination of certs and keys will work, i.e. allow the SSL connection to be established. Every other combination we tried failed (note that ss_vncviewer uses the external stunnel command to handle the SSL so we are really testing stunnel's SSL implementation on the viewer side); and so the system works as expected. _________________________________________________________________

VNC Client Authentication using Certificate Chains:

Now, going the other way around with the client authenticating himself via this chain of SSL certificates, x11vnc is run this way: x11vnc -ssl SAVE -sslverify ./V-Root.crt ...

(note since the server must always supply a cert, we use its normal self-signed, etc., one via "-ssl SAVE" and use the VeriSign root cert for client authentication via -sslverify. The viewer must now supply the combined certificates, e.g.: ss_vncviewer -mycert ./V1-combined.pem hostname:0 _________________________________________________________________

Using OpenSSL and x11vnc to create Certificate Chains:

Although the x11vnc CA mechanism (-sslGenCA and -sslGenCert; see above) was designed to only handle a single root CA (to sign server and/or client certs) it can be coerced into creating a certificate chain by way of an extra openssl(1) command.

We will first create two CA's via -sslGenCA; then use one of these CA to sign the other; create a new (non-CA) server cert; and append the intermediate CA's cert to the server cert to have everything needed in the one file.

Here are the commands we ran to do what the previous paragraph outlines.

First we create the two CA's, called CA_root and CA_Intermediate here, in separate directories via x11vnc: x11vnc -ssldir ~/CA_Root -sslGenCA (follow the prompts, we included "CA_Root", e.g. Common Name, to aid ident ifying it)

x11vnc -ssldir ~/CA_Intermediate -sslGenCA (follow the prompts, we included "CA_Intermediate", e.g. Common Name, to a id identifying it)

Next backup CA_Intermediate's cert and then sign it with CA_Root: mv ~/CA_Intermediate/CA/cacert.pem ~/CA_Intermediate/CA/cacert.pem.ORIG cd ~/CA_Root openssl ca -config ./CA/ssl.cnf -policy policy_anything -extensions v3_ca -no text -ss_cert ~/CA_Intermediate/CA/cacert.pem.ORIG -out ~/CA_Intermediate/CA/ca cert.pem

Note that it is required to cd to the ~/CA_Root directory and run the openssl command from there.

You can print out info about the cert you just modified by: openssl x509 -noout -text -in ~/CA_Intermediate/CA/cacert.pem

Now we create an x11vnc server cert named "test_chain" that is signed by CA_Intermediate: x11vnc -ssldir ~/CA_Intermediate -sslGenCert server test_chain (follow the prompts)

You can print out information about this server cert just created via this command: x11vnc -ssldir ~/CA_Intermediate -sslCertInfo SAVE-test_chain

This will tell you the full path to the server certificate, which is needed because we need to manually append the CA_Intermediate cert for the chain to work: cat ~/CA_Intermediate/CA/cacert.pem >> ~/CA_Intermediate/server-test_chain.pe m

Now we are finally ready to use it. We can run x11vnc using this server cert+key by either this command: x11vnc -ssldir ~/CA_Intermediate -ssl SAVE-test_chain ...

or this command: x11vnc -ssl ~/CA_Intermediate/server-test_chain.pem ...

since they are equivalent (both load the same pem file.)

Finally we connect via VNC viewer that uses CA_Root to verify the server. As before we use ss_vncviewer: ss_vncviewer -verify ~/CA_Root/CA/cacert.pem hostname:0

Client Certificates (see above) work in a similar manner.

So although it is a little awkward with the extra steps (e.g. appending the CA_Intermediate cert) it is possible. If you want to do this entirely with openssl(1) you will have to learn the openssl commands corresponding to -genCA and -genCert. You may be able to find guides on the Internet to do this. Starting with x11vnc 0.9.10, you can have it print out the wrapper scripts it uses via: -sslScripts (you will still need to fill in a few pieces of information; ask if it is not clear from the source code.)

 _________________________________________________________________

More info:

See also this article for some some general info and examples using stunnel and openssl on Windows with VNC. Also http://www.stunnel.org/faq/certs.html is a very good source of information on SSL certificate creation and management.

======================================================================= http://www.karlrunge.com/x11vnc/ssl-portal.html:

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Using Apache as an SSL Gateway to multiple x11vnc servers inside a firewall:

Background:

The typical way to allow access to x11vnc (or any other VNC server) running on multiple workstations inside a firewall is via SSH. The user somewhere out on the Internet logs in to the SSH gateway machine and uses port forwarding (e.g. ssh -t -L 5900:myworkstation:5900 user@gateway) to set up the encrypted channel that VNC is then tunneled through. Next he starts up the VNC viewer on the machine where he is sitting directed to the local tunnel port (e.g. localhost:0).

The SSH scheme is nice because it is a widely used and well tested login technique for users connecting to machines inside their company or home firewall. For VNC access it is a bit awkward, however, because SSH needs to be installed on the Viewer machine and the user usually has to rig up his own port redirection plumbing (however, see our other tool).

Also, some users have restrictive work environments where SSH and similar applications are prohibited (i.e. only outgoing connections to standard WWW ports from a browser are allowed, perhaps mediated by a proxy server). These users have successfully used the method described here for remote access.

With the SSL support in x11vnc and the SSL enabled Java VNC viewer applet, a convenient and secure alternative exists that uses the Apache webserver as a gateway. The idea is that the company or home internet connection is already running apache as a web server (either SSL or non-SSL) and we add to it the ability to act as a gateway for SSL VNC connections. The only thing needed on the Viewer side is a Java enabled Web Browser: the user simply enters a URL that starts the entire VNC connection process. No VNC or SSH specific software needs to be installed on the viewer side machine.

The stunnel VNC viewer stunnel wrapper script provided (ss_vncviewer) can also take advantage of the method described here with its -proxy option.

 _________________________________________________________________

Simpler Solutions: This apache SSL VNC portal solution may be too much for you. It is mainly intended for automatically redirecting to MULTIPLE workstations inside the firewall. If you only have one or two inside machines that you want to access, the method described here is overly complicated! See below for some simpler (and still non-SSH) encrypted setups.

Also see the recent (Mar/2010) desktop.cgi x11vnc desktop web login CGI script that achieves much of what the method describes here (especially if its 'port redirection' feature is enabled.) _________________________________________________________________

There are numerous ways to achieve this with Apache. We present one of the simplest ones here.

Important: these sorts of schemes allow incoming connections from anywhere on the Internet to fixed ports on machines inside the firewall. Care must be taken to implement and test thoroughly. If one is paranoid one can (and should) add extra layers of protection. (e.g. extra passwords, packet filtering, SSL certificate verification, etc).

Also, it is easy to miss the point that unless precautions are taken to verify SSL Certificates, then the VNC Viewer is vulnerable to man-in-the-middle attacks (but not to the more common passive sniffing attacks). Note that there are hacker tools like dsniff/webmitm and cain that implement SSL Man-In-The-Middle attacks. They rely on the client not bothering to check the cert. _________________________________________________________________

The Holy Grail: a single https port (443)

Before we discuss the self-contained apache examples here, we want to mention that many x11vnc users who read this page and implement the apache SSL VNC portal ask for something that (so far) seems difficult or impossible to do entirely inside apache: * A single port, 443 (the default https:// port), is open to the Internet * It is HTTPS/SSL encrypted * It handles both VNC traffic and Java VNC Applet downloads. * And the server can also serve normal HTTPS webpages, CGI, etc.

It is the last item that makes it tricky (otherwise the method described on this page will work). If you are interested in such a solution and are willing to run a separate helper program (connect_switch) look here. Also, see this apache patch. _________________________________________________________________

Example:

The scheme described here sets up apache on the firewall/gateway as a regular Web proxy into the intranet and allows connections to a single fixed port on a limited set of machines.

The configuration described in this section does not use the mod_ssl apache module (the optional configuration described in the section "Downloading the Java applet to the browser via HTTPS" does take advantage of mod_ssl)

In this example suppose the gateway machine running apache is named "www.gateway.east" (e.g. it may also provide normal web service). We also choose the Internet-facing port for this VNC service to be port 563. One could choose any port, including the default HTTP port 80.

Detail: We choose 563 because it is the rarely used SNEWS port that is often allowed by Web proxies for the CONNECT method. The idea is the user may be coming out of another firewall using a proxy (not the one we describe here, that is, the case when two proxies are involved, e.g. one at work and another Apache (described here) at home redirecting into our firewall; the "double proxy" or "double firewall" problem). Using port 563 simplifies things because CONNECT's to it are usually allowed by default.

We also assume all of the x11vnc servers on the internal machines are all listening on port 5915 ("-rfbport 5915") instead of the default 5900. This is to limit any unintended proxy redirections to a lesser used port, and also to stay out of the way of normal VNC servers on the same machines. One could obviously implement a scheme that handles different ports, but we just discuss this simple setup here.

So we basically assume x11vnc has been started this way on all of the workstations to be granted VNC access: x11vnc -ssl SAVE -http -display :0 -forever -rfbauth ~/.vnc/passwd -rfbport 5 915

i.e. we force SSL VNC connections, port 5915, serve the Java VNC viewer applet, and require a VNC password (another option would be -unixpw). The above command could also be run out of inetd(8). It can also be used to autodetect the user's display and Xauthority data.

These sections are added to the httpd.conf apache configuration file on www.gateway.east:

In the global section you need to enable these modules.

Note that the ORDER MATTERS! mod_rewrite must be before mod_proxy

(so that we can check the allowed host list via rewrite)

LoadModule rewrite_module modules/mod_rewrite.so LoadModule proxy_module modules/mod_proxy.so LoadModule proxy_connect_module modules/mod_proxy_connect.so LoadModule proxy_ftp_module modules/mod_proxy_ftp.so LoadModule proxy_http_module modules/mod_proxy_http.so LoadModule ssl_module modules/mod_ssl.so

Near the bottom of httpd.conf you put the port 563 virtual host:

Listen 563

<VirtualHost *:563>

Allow proxy CONNECT requests only to port 5915.

If the machines use different ports, e.g. 5916 list them here as well:

ProxyRequests On AllowCONNECT 5915

RewriteEngine On

Convenience rules to expand applet parameters. These do not have a traili

ng "/"

/vnc for http jar file downloading:

RewriteRule /vnc/([^/]+)$ /vnc/$1/index.vnc?CONNECT=$1+5915&PO RT=563&urlPrefix=2F_vnc_2F$1 [R,NE,L] RewriteRule /vnc/trust/([^/]+)$ /vnc/$1/index.vnc?CONNECT=$1+5915&PO RT=563&urlPrefix=2F_vnc_2F$1&trustAllVncCerts=yes [R,NE,L] RewriteRule /vnc/proxy/([^/]+)$ /vnc/$1/proxy.vnc?CONNECT=$1+5915&PO RT=563&urlPrefix=2F_vnc_2F$1&forceProxy=yes [R,NE,L] RewriteRule /vnc/trust/proxy/([^/]+)$ /vnc/$1/proxy.vnc?CONNECT=$1+5915&PO RT=563&urlPrefix=2F_vnc_2F$1&forceProxy=yes&trustAllVncCerts=yes [R,NE,L]

Read in the allowed host to vnc display mapping file. It looks like:

host1 15

host2 15

...

the display "15" means 5815 for http applet download, 5915 for SSL vnc.

RewriteMap vnchosts txt:/dist/apache/conf/vnc.hosts

Proxy: check for the CONNECT hostname and port being in the vnc.hosts list

.

RewriteCond %{THE_REQUEST} ^CONNECT [NC] RewriteCond %{REQUEST_URI} ^(.):(.)$ RewriteCond ${vnchosts:%1|NOTFOUND} NOTFOUND RewriteRule ^.*$ /VNCFAIL [F,L]

RewriteCond %{THE_REQUEST} ^CONNECT [NC] RewriteCond %{REQUEST_URI} ^(.):(.)$ RewriteCond 59${vnchosts:%1}=%2 !^(.)=(\1)$ RewriteRule ^.$ /VNCFAIL [F,L]

Remap /vnc to the proxy http download (e.g. http://host:5815)

First, fail if it starts with the string /vnc0:

RewriteRule ^/vnc0.* /VNCFAIL [F,L]

Next, map the prefix to /vnc0/host:protocol:port

RewriteRule ^/vnc/([^/]+)/(.*) /vnc0/$1:http:58${vnchosts:$1|NOTFOUND}/$2 [NE]

Drop any not found:

RewriteRule ^/vnc0.NOTFOUND. /VNCFAIL [F,L]

Construct the proxy URL and retrieve it:

RewriteRule ^/vnc0/([^/]+):([^/]+):([^/]+)/(.*) $2://$1:$3/$4 [P,NE,L]

Then restart apache (perhaps: "apachectl stop; apachectl start").

Note that the listing of allowed internal workstations is done in an external file (/dist/apache/conf/vnc.hosts in the example above), the format is like this:

allowed vnc hosts file:

hostname1 15 hostname2 15 ...

You list the hostname and the VNC display (always 15 in our example). Only to these hosts will the external VNC viewers be able to connect to (via the HTTP CONNECT method).

The above setup requires mod_rewrite and mod_proxy be enabled in the apache web server. In this example they are loaded as modules (and note that mod_rewrite must be listed before mod_proxy);

The user at the Java enabled Web browser would simply enter this URL into the browser: http://www.gateway.east:563/vnc/host2

to connect to internal workstation host2, etc.

Important: do not put a trailing "/" on the URL, since that will defeat the RewriteRules that look for the hostname at the very end.

There will be a number of SSL certificate, etc, dialogs he will have to respond to in addition to any passwords he is required to provide (this depends on how you set up user authentication for x11vnc).

If a second Web proxy is involved (i.e. the user's browser is inside another firewall and policy requires using a Web proxy server) then use this URL: http://www.gateway.east:563/vnc/proxy/host2

This will involve downloading a signed java viewer applet jar file that is able to interact with the internal proxy for the VNC connection. See this FAQ for more info on how this works. Note: sometimes with the Proxy case if you see 'Bad Gateway' error you will have to wait 10 or so seconds and then hit reload. This seems to be due to having to wait for a Connection Keepalive to terminate...

For completeness, the "trust" cases that skip a VNC certificate dialog (discussed below) would be entered as: http://www.gateway.east:563/vnc/trust/host2 http://www.gateway.east:563/vnc/trust/proxy/host2

You can of course choose shorter or more easy to remember URL formats. Just change the Convenience RewriteRules in httpd.conf.

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Port Variations:

Note that you can run this on the default HTTP port 80 instead of port 563. If you do not expect to have a browser connecting from inside a proxying firewall (where sometimes only connections to ports 443 and 563 are allowed) this should be fine. Use "80" instead of "563" in the httpd.conf config file (you may need to merge it with other default port 80 things you have there).

Then the URL's will be a bit simpler: http://www.gateway.east/vnc/host2 http://www.gateway.east/vnc/trust/host2

etc.

Besides 80 one could use any other random port number (since there are so many port scans on 80, a little obscurity might be useful).

One option is to use port "443" (the default https:// port) instead of "563". In this case Apache is not configured for mod_ssl; we just happen to use port "443" in the way any random port would be used. This could be handy if the Viewer side environment is restrictive in that it only allows outgoing connections to ports 80 and 443 (and, say, you didn't want to use port 80, or you wanted to use 80 for something else). Another reason for using 443 would be some web proxy environments only allow the CONNECT method to go to port 443 (and not even the case 563 we use above).

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Details:

Let's go through the httpd.conf additions in detail from the top.

The LoadModules directives load the necessary apache modules. Note that mod_rewrite must be listed first. If you are compiling from scratch something like this worked for us: ./configure --enable-proxy=shared --enable-proxy-connect=shared --enable-ssl= shared --enable-rewrite=shared --prefix=/dist/apache

Then the VirtualHost *:563 virtual host section starts.

The "ProxyRequests On" and "AllowCONNECT 5915" enable the web server to forward proxy requests to port 5915 (and only this port) INSIDE the firewall. Think about the implications of this thoroughly and test it carefully.

The RewriteRule's are for convenience only so that the URL entered into the Web browser does not need the various extra parameters, e.g.: http://www.gateway.east:563/vnc/host2/index.vnc?CONNECT=host2+5915&PORT=563, blah,blah...

(or otherwise make direct edits to index.vnc to set these parameters). The forceProxy=yes parameter is passed to the applet to force the use of a outgoing proxy socket connection. Use it only if the Web browser is inside a separate Web proxying environment (i.e. large corporation)

The rewrites with parameter urlPrefix are described under Tricks for Better Response. The "trust" ones (also described under Tricks) with trustAllVncCerts tell the Java VNC applet to skip a dialog asking about the VNC Certificate. They are a bit faster and more reliable than the original method. In the best situation they lead to being logged in 20 seconds or less (without them the time to login can be much longer since a number of connections must timeout).

All of the x11vnc Java Viewer applet parameters are described in the file classes/ssl/README

The external file /dist/apache/conf/vnc.hosts containing the allowed VNC server hostnames is read in. Its 2nd column contains the VNC display of the host (always 15 in our example; if you make it vary you will need to adjust some lines in the httpd.conf accordingly, e.g. AllowCONNECT). This list is used to constrain