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// Examples for using socat (and filan)
//"$" means normal user, "#" requires privileges, "//" starts a comment
// similar to netcat
// connect to on port 80 and relay to and from stdio
$ socat - TCP: # similar to "netcat 80"
// listen on port 25, wait for an incoming connection, use CR+NL on this
// connection, relay data to and from stdio;
// then emulate a mailserver by hand :-)
# socat - TCP-LISTEN:25,crlf
// listen on port 25, wait for an incoming connection, use CR+NL on this
// connection, relay data to and from stdio, but have line editing and history;
// then emulate a mailserver by hand :-)
# socat readline TCP-LISTEN:25,crlf
// provide a transient history enabled front end to stupid line based
// interactive programs
$ socat readline exec:"nslookup",pty,ctty,setsid,echo=0
// same works for ftp (but password is not hidden)
// you may also use a file based history list
$ socat readline,history=.nslookup_hist exec:"nslookup",pty,ctty,setsid,echo=0
// using ~ as abbreviation for $HOME does not work!
// poor mans 'telnetd' replacement
# socat tcp-l:2023,reuseaddr,fork exec:/bin/login,pty,setsid,setpgid,stderr,ctty
// and here an appropriate client:
$ socat -,raw,echo=0 tcp:
// use ssl with client and server certificate for improved security;
// replace /bin/login by /bin/bash when using SSL client authentication, can be
// run without root then
// this is a cool trick, proposed by Christophe Lohr, to dump communications to
// two files; it would also work for other manipulations (recode, compress...)
// and it might also work with netcat ;-)
$ socat TCP-LISTEN:5555 SYSTEM:'tee l2r | socat - "TCP:remote:5555" | tee r2l'
// emergence solution because usleep(1) is not always available
// this will "sleep" for 0.1s
$ socat -T 0.1 pipe pipe
// a very primitive HTTP/1.0 echo server (problems: sends reply headers before
// request; hangs if client does not shutdown - HTTP keep-alive)
// wait for a connection on port 8000; do not wait for request, but immediately
// start a shell that sends reply headers and an empty line; then echo all
// incoming data back to client
$ socat TCP-LISTEN:8000,crlf SYSTEM:"echo HTTP/1.0 200; echo Content-Type\: text/plain; echo; cat"
// a less primitive HTTP echo server that sends back not only the reqest but
// also server and client address and port. Might have portability issues with
// echo
./socat -T 1 -d -d tcp-l:10081,reuseaddr,fork,crlf system:"echo -e \"\\\"HTTP/1.0 200 OK\\\nDocumentType: text/html\\\n\\\n<html>date: \$\(date\)<br>server:\$SOCAT_SOCKADDR:\$SOCAT_SOCKPORT<br>client: \$SOCAT_PEERADDR:\$SOCAT_PEERPORT\\\n<pre>\\\"\"; cat; echo -e \"\\\"\\\n</pre></html>\\\"\""
// for communicating with an attached modem, I had reasonable results with
// following command line. Required privileges depend on device mode.
// after leaving socat, type "sane".
// replace /dev/ttyS0 by the correct serial line or with /dev/modem
$ socat readline /dev/ttyS0,raw,echo=0,crlf
// or
$ socat readline /dev/ttyS0,raw,echo=0,crlf,nonblock
// then enter "at$"
// relay TCP port 80 from everywhere (internet, intranet, dmz) through your
// firewall to your DMZ webserver (like plug-gw)
// listen on port 80; whenever a connection is made, fork a new process (parent
// process keeps accepting connections), su to nobody, and connect to
// on port 80.
// attention: this is a substitute for a reverse proxy without providing
// application level security.
# socat TCP-LISTEN:80,reuseaddr,fork,su=nobody
// Note: parent process keeps running as root, su after forking
// relay mail from your DMZ server through your firewall.
// accept connections only on dmz interface and allow connections only from
// the advantages over plug-gw and other relays are:
// * you can bind to an IP address (even an alias), therefore enhance security
// * in your OS you can create several IP aliases and bind another socat daemon
// to each, making several application servers addressable
// * lots of options, like switching user, chroot, IP performance tuning
// * no need for inetd
# socat -lm -d -d TCP-LISTEN:25,,fork,su=nobody,
// convert line terminator in ascii streams, stdin to stdout
// use unidirectional mode, convert nl to crnl
$ socat -u - -,crlf
// or cr to nl
$ socat -u -,cr -
// save piped data similar to 'tee':
// copies stdin to stdout, but writes everything to the file too
$ socat -,echo=0 open:/tmp/myfile,create,trunc,ignoreeof!!/tmp/myfile
// intrusion testing
// found an XWindow Server behind IP filters with FTP data hole? (you are
// lucky!)
// prepare your host:
# rm -f /tmp/.X11-unix/X1
// relay a pseudo display :1 on your machine to victim:0
# socat UNIX-LISTEN:/tmp/.X11-unix/X1,fork,sp=20 &
// and try to take a screendump (must be very lucky - when server has not even
// host based authentication!)
# xwd -root -display :1 -silent >victim.xwd
// you sit behind a socks firewall that has IP filters but lazily allows socks
// connections to loopback and has only host based X11 security.
// like above, but from your inside client:
# socat UNIX-LISTEN:/tmp/.X11-unix/X1,fork SOCKS4:firewall:loopback:6000
// or for the HTTP proxy:
# socat UNIX-LISTEN:/tmp/.X11-unix/X1,fork PROXY:firewall:loopback:6000
// forms of stdin with stdout, all equivalent
$ socat echo -
$ socat echo STDIO
$ socat echo STDIN!!STDOUT
$ socat echo STDIO!!STDIO
$ socat echo -!!-
$ socat echo FD:0!!FD:1
$ socat echo 0!!1
$ socat echo /dev/stdin!!/dev/stdout // if your OS provides these
// some echo address examples
$ socat - PIPE
$ socat - PIPE:/tmp/pipi // other version of echo
$ socat - PIPE:/tmp/pipi,nonblock!!/tmp/pipi // other version of echo
$ socat - EXEC:/bin/cat // another echo
$ socat - SYSTEM:/bin/cat // another echo
$ socat - TCP:loopback:7 // if inetd echo/TCP service activated
$ socat - UDP:loopback:7 // if inetd echo/UDP service activated
$ socat - /tmp/hugo,trunc,ignoreeof!!/tmp/hugo // with delay
$ socat - UDP:loopback:2000,bind=:2000 // self "connection"
$ socat - TCP:loopback:2000,bind=:2000 // Linux bug?
# socat - IP:loopback:222 // raw protocol, self "connected" (attention,
// Linux might drop packets with less than 8 bytes payload)
// unidirectional data transfer
$ socat -u - -
// like "tail -f", but start with showing all file contents
$ socat -u FILE:/var/log/syslog.debug,ignoreeof -
// like "tail -f", but do not show existing file contents
$ socat -u FILE:/var/log/syslog.debug,ignoreeof,seek-end -
// write to new file, create with given permission and group (must be member) - race condition with group!!!
$ socat -u - CREATE:/tmp/outfile1,group=floppy,perm=0640
// for an existing file /tmp/outfile1
# socat -u - FILE:/tmp/outfile1,group=floppy,perm=0700,user=4321
// file handling
$ socat - FILE:/tmp/outfile1,ignoreeof!!FILE:/tmp/outfile1,append // prints outfile1, then echoes input and protocols into file (appends to old data)
// unix socket handling
// create a listening unix socket
$ rm -f /tmp/mysocket; socat UNIX-LISTEN:/tmp/mysocket -
// from another terminal, connect to this socket
$ socat UNIX:/tmp/mysocket -
// then transfer data bidirectionally
// transport examples
// socks relay (externally socksify applications);
// your ssh client and OS are not socksified, but you want to pass a socks
// server with ssh:
$ socat TCP-LISTEN:10022,fork
$ ssh -p 10022 loopback
// or better define a ProxyCommand in ~/.ssh/config:
ProxyCommand socat -
// and with proxy:
ProxyCommand socat -,proxyport=8000
// application examples
// run sendmail daemon with your favorite network options
# socat TCP-LISTEN:25,fork,ip-ttl=4,ip-tos=7,tcp-maxseg=576 EXEC:"/usr/sbin/sendmail -bs",nofork
// local mail delivery over UNIX socket - no SUID program required
# socat UNIX-LISTEN:/tmp/postoffice,fork,perm-early=0666 EXEC:"/usr/sbin/sendmail -bs"
$ socat - /tmp/postoffice
// uses of filan
// see what your operating system opens for you
$ filan
// or if that was too detailled
$ filan -s
// see what file descriptors are passed via exec function
$ socat - EXEC:filan,nofork
$ socat - EXEC:filan
$ socat - EXEC:filan,pipes,stderr
$ socat - EXEC:filan,pipes
$ socat - EXEC:filan,pty
// see what's done by your shell and with option "pipes"
$ socat - SYSTEM:filan,pipes
// see if gdb gives you an equivalent environment or opens some files for your program
$ gdb ./filan
(gdb) r
(gdb) r -s
// want to use chat from the ppp package?
// note: some OS's do not need "-e" for echo to print control characters
// note: chat might send bytes one by one
// with AIX, a similar program is available under the name "pppdial"
$ socat -d -d tcp:localhost:25,crlf,nodelay exec:'/usr/sbin/chat -v -s "\"220 \"" "\"HELO loopback\"" "\"250 \"" "\"MAIL FROM: <hugo@localhost>\"" "\"250 \"" "\"RCPT TO: root\"" "\"250 \"" "\"DATA\"" "\"354 \"" "\"test1'$(echo -e "\r.")'\"" "\"250 \"" "\"QUIT\"" "\"221 \""',pty,echo=0,cr
// IP6
# socat readline TCP6:[::1]:21 # if your inetd/ftp is listening on ip6
// application server solutions
// run a program (here: /bin/sh) chrooted, unprivileged;
// parent process stays in real / running as root
# socat -d -d - EXEC:/bin/sh,chroot=/home/sandbox,su=sandbox,pty
// make a program available on the network chrooted, unprivileged;
// parent process stays in / running as root
// script path is already chrooted
# ./socat -lm -d -d TCP-LISTEN:5555,fork EXEC:/bin/myscript,chroot=/home/sandbox,su=sandbox,pty,stderr
// to avoid terminal problems, you might - instead of telnet - connect using
$ socat -,icanon=0,echo=0 tcp:target:5555; reset
// access local display from ssh server, when ssh port forwarding is disabled
// socat must be installed on ssh server host
// might have to use xauth...
// this example is one-shot because ssh can handle only one channel
xterm1$ socat -d -d exec:"ssh rm -f /tmp/.X11-unix/X9; ~/bin/socat -d -d unix-l\:/tmp/.X11-unix/X9\,fork -" unix:/tmp/.X11-unix/X0
xterm2$ ssh target
target$ DISPLAY=:9 myxapplication
// touch with perms:
// no race condition for perms (applied with creat() call)
$ socat -u /dev/null creat:/tmp/tempfile,perm=0600
// touch with owner and perms:
// race condition before changing owner, but who cares - only root may access
# socat -u /dev/null creat:/tmp/tempfile,user=user1,perm=0600
// invoke an interactive ssh with exec
// first example passes control chars (^C etc.) to remote server as usual
socat -,echo=0,raw exec:'ssh server',pty,setsid,ctty
// second example interprets control chars on local command line
socat -,echo=0,icanon=0 exec:'ssh server',pty,setsid,ctty
// afterwards, type "reset"!
// convince ssh to provide an "interactive" shell to your script
// three main versions for entering password:
// 1) from your TTY; have 10 seconds to enter password:
(sleep 10; echo "ls"; sleep 1) |socat - exec:'ssh server',pty
// 2) from XWindows (DISPLAY !); again 10 seconds
(sleep 10; echo "ls"; sleep 1) |socat - exec:'ssh server',pty,setsid
// 3) from script
(sleep 5; echo PASSWORD; echo ls; sleep 1) |./socat - exec:'ssh server',pty,setsid,ctty
// download with proxy CONNECT
// use echo -e if required for \n
$ (echo -e "CONNECT HTTP/1.0\n"; sleep 5; echo -e "GET
/download/file HTTP/1.0\n"; sleep 10) |socat -d -d -t 3600 - tcp:proxy:8080,crlf
// retrieve a file from an sshd site with sourceforge style entry menu;
// fill in your personal values; cat lets you enter your password (will be
// visible on screen)
$ (sleep 10; read pass; echo $pass; sleep 10; echo M; sleep 5; echo cat FILENAME; sleep 10) |./socat -d -d -ly - EXEC:'ssh -c 3des -l USER',pty,setsid,ctty |tee FILENAME
// multicast community on local network: start the following command on all
// participating hosts; like a conference call:
# socat -d -d -d -d - udp-datagram:,bind=:6666,ip-add-membership=,bindtodevice=eth0
// or
$ socat -d -d -d -d - udp-datagram:,bind=:6666,ip-add-membership=
// possible reasons for failure:
// iptables or other filters (open your filters as required)
// packets leave via wrong interface (set route: ...)
// socket bound to specific address
// ioctl(): open CD drive (given value valid on Linux)
// on my Linux system I find in /usr/include/linux/cdrom.h the define:
// #define CDROMEJECT 0x5309 /* Ejects the cdrom media */
// the following command makes something like ioctl(fd, CDROMEJECT, NULL)
// (don't care about the read error):
$ socat /dev/cdrom,o-nonblock,ioctl-void=0x5309 -
// setsockopt(): SO_REUSEADDR
// the following command performs - beyond lots of overhead - something like:
// myint=1; setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &myint, sizeof(myint))
$ socat -u udp-recv:7777,setsockopt-int=1:2:1 -
// setsockopt(): SO_BINDTODEVICE
// ways to apply SO_BINDTODEVICE without using the special socat address option
// so-bindtodevice:
// with string argument:
$ sudo ./socat tcp-l:7777,setsockopt-string=1:25:eth0 pipe
// with binary argument:
$ sudo ./socat tcp-l:7777,setsockopt-bin=1:25:x6574683000 pipe
// not tested, just ideas, or have problems
// traverse firewall for making internal telnet server accessible for outside
// telnet client, when only outbound traffic (syn-filter) is allowed:
// on external client run "double server". this process waits for a
// connection from localhost on port 10023, and, when it is established, waits
// for a connection from anywhere to port 20023:
ext$ socat -d TCP-LISTEN:10023,range=localhost TCP-LISTEN:20023
// on internal server run double client:
int$ socat -d TCP:localhost:23 TCP:extclient:10023
// or, with socks firewall:
int$ socat -d TCP:localhost:23 SOCKS:socksserver:extclient:10023
// login with:
ext$ telnet localhost 20023
// you can make a double server capable of handling multiple instances:
ext$ socat -d TCP-LISTEN:10023,range=localhost,fork TCP-LISTEN:20023,reuseaddr
// access remote display via ssh, when ssh port forwarding is disabled
$ socat -d -d EXEC:"ssh target socat - UNIX:/tmp/.X11-unix/X0" TCP-LISTEN:6030
$ xclock -display localhost:30
// relay multiple webserver addresses through your firewall into your DMZ:
// make IP aliases on your firewall, and then:
# socat -d -d TCP-L:80,bind=fw-addr1,fork TCP:dmz-www1:80
# socat -d -d TCP-L:80,bind=fw-addr2,fork TCP:dmz-www2:80
// and for improved security:
# socat -d -d TCP-L:80,bind=fw-addr3,su=nobody,fork TCP:dmz-www3:80
// proxy an arbitrary IP protocol over your firewall (answers won't work)
# socat -d -d IP:,bind=fwnonsec IP:sec-host:150,bind=fwsec
// proxy an unsupported IP protocol over your firewall, point to point
// end points see firewall interfaces as IP peers!
# socat -d -d IP:nonsec-host:150,bind=fwnonsec IP:sec-host:150,bind=fwsec
// note that, for IPsec, you might face problems that are known with NAT