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<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Mosh: the mobile shell</title>
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta name="description" content="Mobile shell that supports roaming and intelligent local echo. Like SSH secure shell, but allows mobility and more responsive and robust.">
<meta name="author" content="Keith Winstein <mosh-devel@mit.edu>">
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</head>
<body data-spy="scroll">
<div class="navbar navbar-fixed-top">
<div class="navbar-inner">
<div class="container">
<a class="btn btn-navbar" data-toggle="collapse" data-target=".nav-collapse">
<span class="icon-bar"></span>
<span class="icon-bar"></span>
<span class="icon-bar"></span>
</a>
<a class="brand" href="#">Mosh</a>
<div class="nav-collapse">
<ul class="nav">
<li><a href="#about">About</a></li>
<li><a href="#news">News</a></li>
<li><a href="#getting">Getting Mosh</a></li>
<li><a href="#usage">Usage</a></li>
<li><a href="#techinfo">Technical Info</a></li>
<li><a href="#faq">FAQ</a></li>
<li><a href="#contact">Contact</a></li>
<li><a href="https://github.com/keithw/mosh">Mosh on GitHub</a></li>
</ul>
</div><!--/.nav-collapse -->
</div>
</div>
</div>
<div class="container">
<section id="about">
<div class="hero-unit span10">
<div class="row">
<div class="span5">
<h1>Mosh</h1>
<h2>(mobile shell)</h2>
<p>Remote terminal application that
allows <strong class="callout">roaming</strong>, supports <strong class="callout">intermittent connectivity</strong>, and provides intelligent <strong class="callout">local echo</strong> and line editing of user keystrokes.</p>
<p>Mosh is a replacement for SSH. It's more robust andresponsive, especially over Wi-Fi, cellular, and long-distance links.</p>
<p>Mosh is free software, available for GNU/Linux, FreeBSD, Solaris, Mac OS X, and Android.</p>
</div>
<div class="span5"><img src="mosh.png" style="max-width:120%;" alt="">
</div>
</div>
<p><a class="btn btn-primary btn-large" href="#getting">Getting Mosh »</a>
<a class="btn btn-primary btn-large" href="#techinfo">Tech Video »</a></p>
</div>
<div id="features">
<div class="row-fluid">
<div class="feature span3">
<h2 class="callout">Change IP. Stay connected.</h2>
<p>Mosh automatically roams as you move between Internet
connections. Use Wi-Fi on the train, Ethernet in a hotel,
and LTE on a beach: you'll stay logged in. Most network
programs lose their connections after roaming,
including SSH and Web apps like Gmail. Mosh
is different.</p>
</div>
<div class="feature span3">
<h2 class="callout">Makes for sweet dreams.</h2>
<p>With Mosh, you can put your laptop to sleep and wake it
up later, keeping your connection intact. If your
Internet connection drops, Mosh will warn you — but
the connection resumes when network service
comes back.</p>
</div>
<div class="feature span3">
<h2 class="callout">Get rid of network lag.</h2>
<p>SSH waits for the server's reply before showing you your
own typing. That can make for a lousy user interface. Mosh
is different: it gives an instant response to typing,
deleting, and line editing. It does this adaptively and
works even in full-screen programs like emacs and vim. On
a bad connection, outstanding predictions are underlined
so you won't be misled.
</p>
</div>
<div class="feature span3">
<h2 class="callout">No privileged code. No daemon.</h2>
<p>You don't need to be the superuser to install or run
Mosh. The client and server are executables run by an
ordinary user and last only for the life of the
connection.</p>
</div>
</div>
<div class="row-fluid">
<div class="feature span3">
<h2 class="callout">Same login method.</h2>
<p>Mosh doesn't listen on network ports or authenticate
users. The <span style="font-family: monospace">mosh</span> client logs in to the server via
SSH, and users present the same credentials (e.g.,
password, public key) as before. Then Mosh runs the
<span style="font-family: monospace">mosh-server</span> remotely and connects to it over UDP.</p>
</div>
<div class="feature span3">
<h2 class="callout">Runs inside your terminal, but better.</h2>
<p>Mosh is a command-line program, like ssh. You can use it
inside xterm, gnome-terminal, urxvt, Terminal.app, iTerm,
emacs, screen, or tmux. But mosh was designed from scratch
and supports just one character set: UTF-8. It fixes Unicode
bugs in other terminals and in SSH.</p>
</div>
<div class="feature span3">
<h2 class="callout">Control-C works great.</h2>
<p>Unlike SSH, mosh's UDP-based protocol handles packet loss
gracefully, and sets the frame rate based on network conditions. Mosh
doesn't fill up network buffers, so Control-C always works
to halt a runaway process.</p>
</div>
</div>
</div>
</section>
<section id="news">
<div class="page-header">
<h1>Recent News</h1>
</div>
<ul id="news-list">
<li><b>March 29, 2013</b>: Mosh has applied to the <a href="http://www.google-melange.com">Google Summer of Code</a>! If you use Mosh and are eager to improve it, we're excited to work with you. Please see our <a href="https://docs.google.com/document/d/10o-TLA03bY4cZzEIZR-5wKmBecvYKpOEFRqSJFbIm3M/edit?usp=sharing">Ideas List</a> and apply.</li>
<li><b>March 27,
2013</b>: <a href="http://mailman.mit.edu/pipermail/mosh-users/2013-March/000167.html">Mosh
1.2.4 has been released</a>. Changes largely include bug
fixes, improved robustness, and added platform support (now
on AIX and stock Solaris!). This version will be in Ubuntu
13.04 (raring).</li>
<li><b>March 24, 2013</b>: The <a href="https://twitter.com/zacchiro/status/315893796695064576">Debian Project Leader switches to Mosh</a>. Welcome, Stefano! We're proud to have you.</li>
<li><b>March 12, 2013</b>: Mosh celebrates its first anniversary of
1.0. Hard to believe it's already been a year. We could not have done
it without the hard work of many of you, especially Hari Balakrishnan,
Keegan McAllister, Anders Kaseorg, Quentin Smith, Richard Tibbetts,
Nelson Elhage, Christine Spang, Stefie Tellex, Joseph Sokol-Margolis,
Waseem Daher, Bill McCloskey, Austin Roach, Greg Hudson, Karl Ramm,
Alexander Chernyakhovsky, Peter Iannucci, Evan Broder, Neha Narula,
Katrina LaCurts, Ramesh Chandra, Peter Jeremy, Ed Schouten, Ryan
Steinmetz, Jay Freeman, Dave Täht, Larry Doolittle, Daniel Drown, Timo
Juhani Lindfors, Timo Sirainen, Ira Cooper, Felix Gröbert, Luke
Mewburn, Anton Lundin, Kevin Ballard, and Axel Beckert!</li>
<li><b>November 2012</b>: Mosh on the cover of <a href="http://www.linux-magazine.com/Issues/2012/144/Mosh-and-AutoSSH">Linux Magazine</a>.
<li><b>Oct. 19,
2012</b>: <a href="http://mailman.mit.edu/pipermail/mosh-users/2012-October/000152.html">Mosh
1.2.3 has been released</a>. Changes include more resilience to
evil NATs, power savings for mobile clients, switching to OpenSSL's AES
implementation, and a licensing exception to allow Mosh on Apple's app store.
This version will be in Debian 7.0 (wheezy).
<li><b>Aug. 22, 2012</b>: Mosh (and its tolerance for high
packer loss) helps Iain Learmonth <a href="http://gopher.floodgap.com/gopher/gw.lite?gopher://sdf.org:70/0/users/irl/blog/2012-08-22-mosh-in-a-lift.md">escape from an elevator</a>.
</ul>
</section>
<section id="getting">
<div class="page-header">
<h1>Getting Mosh</h1>
</div>
<div class="row">
<div class="span4" style="vertical-align: top;">
<h3 class="callout"><a href="http://www.debian.org"><img class="logo" src="debian.png" alt=""></a>Debian <small>squeeze-backports and later</small><br /></h3>
<pre>$ sudo apt-get install mosh</pre>
</div>
<div class="span4" style="vertical-align: top;">
<h3 class="callout"><a href="http://www.fedoraproject.org"><img class="logo" src="fedora.png" alt=""></a>Fedora <small>15 or later</small></h3>
<pre>$ sudo yum install mosh</pre>
</div>
<div class="span4" style="vertical-align: top;">
<h3 class="callout"><a href="http://www.ubuntu.com"><img class="logo" src="ubuntu.png" alt=""></a>Ubuntu <small>10.04 and later</small></h3>
<pre>$ sudo apt-get install python-software-properties
$ sudo add-apt-repository ppa:keithw/mosh
$ sudo apt-get update
$ sudo apt-get install mosh</pre>
<p><small>Ubuntu also includes older versions of mosh in the universe repository (12.04 and later) and backports repository (10.04–11.10).</small></p>
<br />
</div>
</div>
<div class="row">
<div class="span4" style="vertical-align: top;">
<h3 class="callout"><a href="http://www.gentoo.org"><img class="logo" src="gentoo.png" alt=""></a> Gentoo</h3>
<pre># emerge net-misc/mosh</pre>
<br />
</div>
<div class="span4" style="vertical-align: top;">
<h3 class="callout"><a href="http://www.archlinux.org"><img class="logo" src="arch.png" alt=""></a> Arch Linux</h3>
<pre># pacman -S mosh</pre>
<br />
</div>
<div class="span4" style="vertical-align: top;">
<h3 class="callout"><a href="http://www.freebsd.org"><img class="logo" src="freebsd.png" alt=""></a> FreeBSD</h3>
<pre># portmaster net/mosh</pre>
<br />
</div>
</div>
<div class="row">
<div class="span4" style="vertical-align: top;">
<h3 class="callout"><a href="http://www.netbsd.org"><img class="logo" src="NetBSD-smaller-tb.png" alt=""></a>NetBSD <small>(pkgsrc)</small></h3>
<pre># cd net/mosh; make install clean</pre>
<br />
</div>
<div class="span4" style="vertical-align: top;">
<h3 class="callout"><a href="http://www.android.com/"><img class="logo" src="Android_Robot_100.png" alt=""></a>Android <small>(experimental port)</small></h3>
<div class="prelike">Install the <a href="http://dan.drown.org/android/mosh/">port by Daniel Drown</a>, built on Irssi ConnectBot.</div>
<br />
</div>
<div class="span4" style="vertical-align: top;">
<h3 class="callout"><a href="http://www.cygwin.com"><img class="logo" src="Cygwin_logo.svg" width="50" height="50" alt=""></a>Cygwin <small>(experimental port)</small></h3>
<div class="prelike">Install the <a href="http://cygwin.com/setup.exe">experimental package</a> from the Cygwin setup.exe.</div>
<br />
</div>
</div>
<div class="row">
<div class="span4" style="vertical-align: top;">
<h3 class="callout"><a href="http://www.apple.com"><img class="logo" src="macosx.png" alt=""></a>OS X <small>10.6 or later</small></h3>
<div class="prelike">Install <a href="https://github.com/downloads/keithw/mosh/mosh-1.2.1.pkg"><img src="dmg.png" alt=""> mosh-1.2.1.pkg</a>.</div>
<br />
</div>
<div class="span4" style="vertical-align: top;">
<h3 class="callout"><a href="http://mxcl.github.com/homebrew/"><img class="logo" src="homebrew2.png" alt=""></a>Homebrew <small>OS X 10.5 or later</small></h3>
<pre>$ brew install mobile-shell</pre>
<br />
</div>
<div class="span4" style="vertical-align: top;">
<h3 class="callout"><a href="http://www.macports.org"><img class="logo" src="macports.png" alt=""></a>MacPorts <small>OS X 10.5 or later</small></h3>
<pre>$ sudo port install mosh</pre>
<br />
</div>
</div>
<p><small>Operating system logos are trademarks or registered trademarks and are displayed for identification
only. The vendors shown aren't affiliated with and haven't endorsed Mosh.</small></p>
<h2>Building from source</h2>
<div class="row">
<div class="span4">
<h3 class="callout">Dependencies</h3><br>
<table class="table table-striped" style="width: 100%;">
<thead><tr><th>Name</th><th>Typical package</th></tr></thead>
<tr class="deps"><td><a href="http://code.google.com/p/protobuf/">Protocol Buffers</a></td><td>protobuf-compiler, libprotobuf-dev</td></tr>
<tr><td>ncurses</td><td>libncurses5-dev</td></tr>
<tr><td>zlib</td><td>zlib1g-dev</td></tr>
<tr><td>utempter (optional)</td><td>libutempter-dev</td></tr>
<tr><td>IO::Pty Perl module</td><td>libio-pty-perl</td></tr>
<tr><td>OpenSSL</td><td>libssl-dev</td></tr>
</table>
</div>
<div class="span3">
<h3 class="callout">Latest release</h3><br>
<p>Extract
<a href="//mosh.mit.edu/mosh-1.2.4.tar.gz">mosh-1.2.4.tar.gz</a>,
then</p>
<pre>$ cd mosh-1.2.4
$ ./configure
$ make
# make install</pre>
</div>
<div class="span5">
<h3 class="callout">Compiling from Git</h3><br>
<pre>$ git clone <a href="https://github.com/keithw/mosh">https://github.com/keithw/mosh</a>
$ cd mosh
$ ./autogen.sh
$ ./configure
$ make
# make install</pre>
</div>
<div class="span3">
<h3 class="callout">Security on new operating systems</h3><br>
<p>
Note that <code>mosh-client</code> receives an AES session key as an environment
variable. If you are porting Mosh to a new operating system, please make sure that a
running process's environment variables are not readable by other users. We have
confirmed that this is the case on GNU/Linux, OS X, and FreeBSD.
</p>
</div>
</div>
</section>
<section id="usage">
<div class="page-header">
<h1>Usage</h1>
</div>
<h3 class="callout">Replaces interactive SSH. Instant keystroke response, robust to roaming. <small>But you'll need working UDP.</small></h3>
<div class="row">
<div class="well span3 offset2">
<h2 class="callout" style="color: darkblue;">Typical usage</h2>
<pre>$ mosh <i>chewbacca.norad.mil</i></pre>
<p>Mosh will log the user in via SSH, then start a connection on a UDP port between 60000 and 61000.</p>
</div>
<div class="span4">
<h3 class="callout">Different username</h3>
<pre>$ mosh <b>potus@</b><i>ackbar.bls.gov</i></pre>
</div>
<div class="span4">
<h3 class="callout">Server binary outside path</h3>
<pre>$ mosh <b>--server=/tmp/mosh-server</b> <i>r2d2</i></pre>
<p>The user can specify an alternate path for the <code>mosh-server</code> on the remote host. The server binary can even
be installed in the user's home directory.</p>
</div>
</div>
<div class="row">
<div class="span4">
<h3 class="callout">Selecting Mosh UDP port</h3>
<pre>$ mosh <b>-p 1234</b> <i>darth</i></pre>
<p>Useful when the server is behind a port-forwarder or NAT.</p>
</div>
<div class="span4">
<h3 class="callout">Selecting SSH port</h3>
<pre>$ mosh <b>--ssh="ssh -p 2222"</b> <i>figrindan</i></pre>
</div>
<div class="span4">
<h3 class="callout">Other SSH options</h3>
<pre>$ mosh <b>--ssh="~/bin/ssh -i ./identity"</b> <i>fett</i></pre>
</div>
</div>
<div class="row">
<div class="span4">
<h3 class="callout">Disable instant echo</h3>
<pre>$ mosh <b>--predict=never</b> <i>niennunb</i></pre>
<p>The <code>-n</code> switch is a synonym. By contrast,
passing <code style="white-space: nowrap;">--predict=always</code> or <code>-a</code>
will enable instant local echo even on low-delay
links.</p>
</div>
<div class="span4">
<h3 class="callout">With a command</h3>
<pre>$ mosh <i>pello</i> <b>-- screen -dr</b></pre>
<p>This reattaches to a long-running screen session.</p>
</div>
</div>
<h3 class="callout">Ending the connection</h3>
<p>Normally, <span style="font-family: monospace">logout</span> or <span style="font-family: monospace">exit</span> on the remote host will close
the session. Mosh accepts the escape sequence <code>Ctrl-^
.</code> (typically typed with Control-Shift-6, then a
period) to end the connection forcibly. To send a
literal Ctrl-^, type <code>Ctrl-^ ^</code>.</p>
<h3 class="callout">Not yet supported, but on the roadmap</h3>
<ul>
<li>Forwarding of <a href="https://github.com/keithw/mosh/issues/41">X11</a>, <a href="https://github.com/keithw/mosh/issues/120">SSH agent</a>, etc.</li>
<li><a href="https://github.com/keithw/mosh/issues/81">IPv6</a>, with roaming between v4 and v6</li>
<li><a href="https://github.com/keithw/mosh/issues/32">Android client</a></li>
</ul>
<h3 class="callout">Manual</h3>
<p>More details can be found in
the <code>mosh(1)</code>, <code>mosh-client(1)</code>,
and <code>mosh-server(1)</code> manual pages.</p>
</section>
<section id="techinfo">
<div class="page-header">
<h1>Technical Info</h1>
</div>
<h3 class="callout">Mosh at USENIX ATC</h3>
<p>The <a href="mosh-paper.pdf"><img style="padding: 3px;"
src="pdf.png" alt=""> Mosh research paper</a> describes the
design and evaluation of Mosh in more detail than you may
want. The paper was presented at the
<a href="https://www.usenix.org/conference/atc12/tech-schedule/usenix-atc-12-technical-sessions">2012 USENIX
Annual Technical Conference</a>, held June 13–15, 2012, in
sunny Boston, Mass.</p>
<div style="text-align: center"><iframe width="560" height="315" src="https://www.youtube.com/embed/XsIxNYl0oyU?rel=0" style="border: none" allowfullscreen></iframe></div>
<div class="well"><h4 class="callout">“ISO 2022 locking escape
sequences oh flying spaghetti monster please kill me
now.” <small>— actual USENIX peer review on reading the
Mosh paper.</small>
</h4>
<p>(Why you should trust Mosh with your remote terminal needs: we
worry about details so obscure, even USENIX reviewers don't want to
hear about them.)</p>
</div>
<h3 class="callout">How Mosh works</h3>
<p>Remote-shell protocols traditionally work by conveying a
byte-stream from the server to the client, to be interpreted
by the client's terminal. (This includes TELNET, RLOGIN, and
SSH.)</p>
<p>Mosh works differently and at a different layer. With
Mosh, the server and client both maintain a snapshot of
the current screen state. The problem becomes one of
<i>state-synchronization</i>: getting the client to the
most recent server-side screen as efficiently as
possible.</p>
<p>This is accomplished using a new protocol called the
State Synchronization Protocol, for which Mosh is the
first application. SSP runs over UDP, synchronizing the
state of any object from one host to another. Datagrams
are encrypted and authenticated
using <a href="http://www.cs.ucdavis.edu/~rogaway/ocb/">AES-128
in OCB mode</a>. While SSP takes care of the networking
protocol, it is the implementation of the object being
synchronized that defines the ultimate semantics of the
protocol.</p>
<p>Roaming with SSP becomes easy: the client sends datagrams
to the server with increasing sequence numbers, including
a heartbeat at least once every three seconds. Every time
the server receives an authentic packet from the client
with a sequence number higher than any it has previously
received, the IP source address of that packet becomes the
server's new target for its outgoing packets. By doing
roaming “statelessly” in this manner, roaming works in and
out of NATs, even ones that may themselves be
roaming. Roaming works even when the client is not aware
that its Internet-visible IP address has changed. The
heartbeats allow Mosh to inform the user when it hasn't
heard from the server in a while (unlike SSH, where users
may be unaware of a dropped connection until they try to
type).</p>
<p>Mosh runs two copies of SSP, one in each direction of the
connection. The connection from client to server
synchronizes an object that represents the keys typed by
the user, and with TCP-like semantics. The connection from
server to client synchronizes an object that represent the
current screen state, and the goal is always to convey the
client to the most recent server-side state, possibly
skipping intermediate frames.</p>
<p>Because SSP works at the object layer and can control the
rate of synchronization (in other words, the frame rate),
it does not need to send every byte it receives from the
application. That means Mosh can regulate the frames so as
not to fill up network buffers, retaining the
responsiveness of the connection and making sure Control-C
always works quickly. Protocols that must send every byte
can't do this.</p>
<h3 class="callout">Careful terminal emulation</h3>
<p>One benefit of working at the terminal layer
was the opportunity to build a clean UTF-8 terminal
emulator from scratch. Mosh fixes several Unicode bugs in
existing terminals and in SSH, and was designed as a fresh
start to try to be robust and correct even for
pathological inputs.</p>
<h4 class="callout">Tricky Unicode</h4>
<p>Only Mosh and the OS X Terminal correctly handle a Unicode combining character in the first column.</p>
<div class="row">
<div class="span5 well"><img src="terminal-shots/firstcol-xterm.png.2.png" alt=""><br>xterm: circumflex on wrong letter.</div>
<div class="span5 well"><img src="terminal-shots/firstcol-gnome.png.2.png" alt=""><br>GNOME Terminal: no circumflex at all.</div>
</div>
<div class="row">
<div class="span5 well"><img src="terminal-shots/firstcol-osx.png.2.png" alt=""><br>OS X Terminal.app gets it right.</div>
<div class="span5 well"><img src="terminal-shots/firstcol-mosh.png.2.png" alt=""><br>Mosh gets it right too.</div>
</div>
<h4 class="callout">ISO 2022 locking escapes</h4>
<p>Only Mosh will never get stuck in hieroglyphs when a nasty program writes to the terminal. (See Markus Kuhn's discussion of the relationship between
<a href="http://www.cl.cam.ac.uk/~mgk25/unicode.html#term">ISO 2022 and UTF-8</a>.)</p>
<div class="row">
<div class="span5 well"><img src="terminal-shots/acs-xterm.png.2.png" alt=""><br>xterm</div>
<div class="span5 well"><img src="terminal-shots/acs-gnome.png.2.png" alt=""><br>GNOME Terminal</div>
</div>
<div class="row">
<div class="span5 well"><img src="terminal-shots/acs-osx.png.2.png" alt=""><br>OS X Terminal.app</div>
<div class="span5 well"><img src="terminal-shots/acs-mosh.png.2.png" alt=""><br>Mosh</div>
</div>
<h4 class="callout">Evil escape sequences</h4>
<p>Only Mosh and GNOME Terminal have a defensible rendering when
Unicode mixes with an ECMA-48/ANSI escape sequence. The OS X Terminal
unwisely tries to normalize its input before the vt500 state machine,
causing it to misinterpret and become unusable after receiving the
following input!* (This also means the OS X Terminal's interpretation
of the incoming octet stream <strong>varies</strong> depending on how
the incoming octets are split across TCP segments, because the
normalization only looks ahead to available bytes.)</p>
<p><small>* We earlier wrote that this misbehaving sequence "crashes"
the OS X Terminal.app. This was mistaken—instead, Terminal.app
interprets the escape sequence as shutting off keyboard input, and
because of an unrelated bug in Terminal.app, it is not possible for
the user to restore keyboard input by resetting the terminal from the
menu.</small></p>
<div class="row">
<div class="span5 well"><img src="terminal-shots/unicode-and-escape-xterm.png.2.png" alt=""><br>xterm: circumflex on wrong letter.</div>
<div class="span5 well"><img src="terminal-shots/unicode-and-escape-gnome.png.2.png" alt=""><br>GNOME Terminal's circumflex placement is defensible.</div>
</div>
<div class="row">
<div class="span5 well"><img src="terminal-shots/unicode-and-escape-osx.png.2.png" alt=""><br>OS X Terminal.app applies circumflex to part of escape sequence, then irretrievably shuts off keyboard input.</div>
<div class="span5 well"><img src="terminal-shots/unicode-and-escape-mosh.png.2.png" alt=""><br>Mosh gets this one right.</div>
</div>
<h4 class="callout">Mosh sets IUTF8</h4>
<p>In the POSIX framework, the kernel needs to know whether
the user is typing in an 8-bit character set or in UTF-8,
because in canonical mode (i.e. "cooked" mode), the kernel
needs to be able to delete a typed multibyte character
sequence from an input buffer. On OS X and Linux, this is
done with the "IUTF8" termios flag.)
(See <a href="http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=665757">diagnostic
explaining the need for this flag.</a>)</p>
<p>Mosh sets the IUTF8 flag when possible and stubbornly refuses to start up unless the user has a
UTF-8-clean environment. SSH does not set the IUTF8 flag, which can lead to garbage in input buffers.</p>
<h3 class="callout">Instant local echo and line editing</h3>
<p>The other major benefit of working at the
terminal-emulation layer is that the Mosh client is free
to scribble on the local screen without lasting
consequence. We use this to implement intelligent local
echo. The client runs a predictive model in the background
of the server's behavior, hypothesizing that each
keystroke will be echoed at the cursor location and that
the backspace and left- and right-arrow keys will have
their traditional effect. But only when a prediction is
confirmed by the server are these effects actually shown
to the user. (In addition, by default predictions are only
displayed on high-delay connections or during a network
“glitch.”) Predictions are done in epochs: when the
user does something that might alter the echo behavior
— like hit ESC or carriage return or an up- or
down-arrow — Mosh goes back into making background
predictions until a prediction from the new batch can be
confirmed as correct.</p>
<p>Thus, unlike previous attempts at local echo with TELNET
and RLOGIN, Mosh's local echo can be used everywhere, even
in full-screen programs like emacs and vi.</p>
<h4 class="callout">Real-world benefits</h4>
<p>We evaluated Mosh using traces contributed by six users, covering
about 40 hours of real-world usage and including 9,986 total
keystrokes. These traces included the timing and contents of all
writes from the user to the host and vice versa. The users were asked
to contribute "typical, real-world sessions." In practice, the traces
include use of popular programs such as the bash shell and zsh shells,
the alpine and mutt e-mail clients, the emacs and vim text editors,
the irssi and barnowl chat clients, the links text-mode Web browser,
and several programs unique to each user.</p>
<p>To evaluate typical usage of a "mobile" terminal, we replayed the
traces over an otherwise unloaded Sprint commercial EV-DO (3G)
cellular Internet connection in Cambridge, Mass. A client-side process
played the user portion of the traces, and a server-side process
waited for the expected user input and then replied (in time) with the
prerecorded server output. We speeded up long periods with no
activity. The average round-trip time on the link was about half a
second.</p>
<p>We replayed the traces over two different transports, SSH and Mosh,
and recorded the user interface response latency to each simulated
user keystroke. The Mosh predictive algorithm was frozen prior to
collecting the traces and was not adjusted in response to their
contents or results.</p>
<p><strong>The results:</strong> Mosh reduced the median keystroke response
time from 503 ms to nearly instant (because more than 70% of the
keystrokes could be immediately displayed), and reduced the mean
keystroke response time from 515 ms to 173 ms. Qualitatively, Mosh makes
remote servers "feel" more like the local machine!</p>
<div class="well">
<h4 class="callout">Cumulative distribution of keystroke response times with Sprint EV-DO (3G) Internet service</h4><br>
<img src="cdfs.png" alt="">
</div>
</section>
<section id="faq">
<div class="page-header">
<h1>Frequently Asked Questions</h1>
</div>
<h4 class="callout">Q: Who wrote Mosh?</h4>
<p>Mosh was written by Keith Winstein, along with Anders Kaseorg, Quentin Smith, Richard Tibbetts and Keegan McAllister.</p>
<h4 class="callout">Q: Why another remote-terminal protocol?</h4>
<p>Practical latency on the Internet is on the increase, with
the rise of bufferbloat and sophisticated wireless links
that optimize for throughput over delay. And roaming is more
common than ever, now that laptops and handheld devices have
largely displaced desktops. SSH is great, but frustrating to
use when you want to change IP addresses or have a
long-delay link or a dodgy connection.</p>
<p>Moreover, TELNET had some good things going for it — a
local-echo mode and a well-defined network virtual
terminal. Even today, SSH doesn't properly support UTF-8
end-to-end on a POSIX system.</p>
<h4 class="callout">Q: Are the mosh principles relevant to other network applications?</h4>
<p>We think so. The design principles that Mosh stands for are
conservative: warning the user if the state being displayed
is out of date, serializing and checkpointing all
transactions so that if there are no warnings, the user
knows every prior transaction has succeeded, and handling expected events (like roaming from one
WiFi network to another) gracefully.</p>
<p>Those don't seem too controversial, but fancy apps like
Gmail-in-Chromium or on Android still behave atrociously on
dodgy connections or after switching IP addresses. (Have you
ever had Gmail leave an e-mail message in "Sending..." for ten
hours while merrily retrieving new mail and not indicating any
kind of error? Us too.) We think there may be considerable
room for improvement in many network user interfaces from the
application of these values.</p>
<h4 class="callout">Q: I'm getting "mosh requires a UTF-8 locale."
How can I fix this?</h4>
<p>To diagnose the problem, run <code>locale</code> on the local
terminal, and <code>ssh <i>remotehost</i> locale</code>. To use Mosh,
both sides of the connection will need to show a UTF-8 locale, like
<code>LC_CTYPE="en_US.UTF-8"</code>.</p>
<p>On many systems, SSH will transfer the locale-related
environment variables, which are then inherited by
<code>mosh-server</code>. If this mechanism fails, Mosh (as of
version 1.2) will pass the variables itself. If neither
mechanism is successful, you can do something like</p>
<pre>mosh <i>remotehost</i> <b>--server="LANG=en_US.UTF-8 mosh-server"</b></pre>
<p>If <code>en_US.UTF-8</code> does not exist on the remote server,
you can replace this with a UTF-8 locale that does exist. You
may also need to set LANG locally for the benefit of
<code>mosh-client</code>. It is possible that the local and
remote machines will need different locale names. See also <a
href="https://github.com/keithw/mosh/issues/98">this GitHub
ticket</a>.</p>
<h4 class="callout">Q: Why do you insist on UTF-8 everywhere?</h4>
<p>We're really not UTF-8 zealots. But it's a lot easier to
correctly implement <strong>one</strong> terminal emulator
than to try to do the right thing in a variety of difficult
edge cases. (This is what GNU screen tries to do, and in our
experience it leads to some very tricky-to-debug situations.)
So mosh just won't start up until the user has everything
configured for a UTF-8-clean pathway. It may be annoying, but
it also probably reduces frustration down the
road. (Unfortunately an 8-bit vt220 and a UTF-8 vt220 are
different and incompatible terminal types; the UTF-8 goes
in <strong>underneath</strong> the vt220 state machine.)</p>
<h4 class="callout">Q: How do I use a different SSH port (not 22)?</h4>
<p>As of Mosh 1.2, you can pass arguments to <code>ssh</code> like so:</p>
<pre>mosh <i>remotehost</i> <b>--ssh="ssh -p 2222"</b></pre>
<p>Or configure a host alias in <code>~/.ssh/config</code> with a
<code>Port</code> directive. Mosh will respect that too.</p>
<h4 class="callout">Q: I'm getting 'mosh-server not found'.</h4>
<p>Please make sure that mosh is installed on the client, and
mosh (or at least mosh-server) is installed on the server you
are trying to connect to. If you install mosh-server in your
home directory, please see the "Server binary outside path"
instructions in the Usage section, above.</p>
<h4 class="callout">Q: SSH authenticates using Kerberos tickets, but Mosh asks me for a password.</h4>
<p>In some configurations, SSH canonicalizes the hostname
before passing it to the Kerberos GSSAPI plugin. This breaks
for Mosh, because the initial forward DNS lookup is done by
the Mosh wrapper script. To work around this, invoke Mosh as</p>
<pre>mosh <i>remotehost</i> <b>--ssh="ssh -o GSSAPITrustDns=no"</b></pre>
<p>This <a href="https://bugzilla.mindrot.org/show_bug.cgi?id=1008">will
often fail</a> on a round-robin DNS setup. In that case it is probably
best to pick a specific host from the round-robin pool.</p>
<h4 class="callout">Q: Why is my terminal's scrollback buffer incomplete?</h4>
<p>Mosh 1.2 synchronizes only the visible state of the terminal. Mosh
1.3 will have complete scrollback support; see <a
href="https://github.com/keithw/mosh/issues/2">this issue</a> and the
others which are linked from there. For now, the workaround is to use
<a href="http://www.gnu.org/software/screen/">screen</a> or <a
href="http://tmux.sourceforge.net/">tmux</a> on the remote side.</p>
<h4 class="callout">Q: How do I get 256 colors?</h4>
<p>Make sure you are running mosh in a terminal that
advertises itself as 256-color capable. (This generally means
TERM will be xterm-256color or screen-256color-bce.)</p>
<h4 class="callout">Q: Has your secure datagram protocol been audited by experts?</h4>
<p>No. Mosh is actively used and has been read over by
security-minded crypto nerds who think its design is
reasonable, but any novel datagram protocol is going to have
to prove itself, and SSP is no exception. We use the
reference implementations of AES-128 and OCB, and we welcome
your eyes on the code. We think the radical simplicity of
the design is an advantage, but of course others have
thought that and have been wrong. We don't doubt it will
(properly!) take time for the security community to get
comfortable with mosh.</p>
<h4 class="callout">Q: Does mosh work with Amazon EC2?</h4>
<p>Yes, it works great, but please remember to open up UDP ports 60000–61000 on the EC2 firewall.</p>
<h4 class="callout">Q: How do I run the mosh client and server separately?</h4>
<p>If the <code>mosh</code> wrapper script isn't working for you, you can try running
the <code>mosh-client</code> and <code>mosh-server</code> programs separately to
form a connection. This can be a useful debugging technique.</p>
<p>1. Log in to the remote host, and run <code>mosh-server</code>.</p>
<p>It will give output like:</p>
<pre>$ mosh-server
MOSH CONNECT <b>60004</b> <b>4NeCCgvZFe2RnPgrcU1PQw</b>
mosh-server (mosh 1.1.3)
Copyright 2012 Keith Winstein <mosh-devel@mit.edu>
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
[mosh-server detached, pid = 30261]
</pre>
<p>2. On the local host, run:</p>
<pre>$ MOSH_KEY=<b>key</b> mosh-client <b>remote-IP</b> <b>remote-PORT</b></pre>
<p>where "key" is the 22-byte string printed by mosh-server (in this
example, "4NeCCgvZFe2RnPgrcU1PQw"), "remote-PORT" is the port number
given by the server (60004 in this case), and "remote-IP" is the IP address of the server. You can look up the
server's IP address with "host remotehost".</p>
<p>3. If all goes well, you should have a working Mosh connection. Information about where the process fails can help us debug why Mosh isn't working for you.</p>
<h4 class="callout">Q: With the mosh-server on FreeBSD or OS X, I sometimes get weird color problems. What's wrong?</h4>
<p>This <a href="https://github.com/keithw/mosh/pull/129">bug</a> is fixed in Mosh 1.2. Thanks to Ed Schouten and Peter Jeremy for tracking this down.</p>
<h4 class="callout">Q: How do I contribute to mosh?</h4>
<p>We welcome your contribution! Please <a href="https://github.com/keithw/mosh">visit us on GitHub</a>
or email <code>mosh-devel@mit.edu</code>.</p>
<h4 class="callout">Q: Who helped with mosh?</h4>
<p>We're very grateful for assistance and support from:</p>
<ul>
<li>Hari Balakrishnan, who advised this work and came up with the name.
<li><a href="http://www.vt100.net">Paul Williams</a>, whose reverse-engineered vt500 state diagram is the basis for the Mosh parser.
<li>The anonymous users who contributed session logs for tuning and measuring Mosh's predictive echo.
<li>Nickolai Zeldovich for helpful comments on the Mosh research paper.
<li>Richard Stallman for helpful discussion about the capabilities of the <a href="http://hdl.handle.net/1721.1/5694">SUPDUP Local Editing Protocol</a>.
<li>Nelson Elhage
<li>Christine Spang
<li>Stefie Tellex
<li>Joseph Sokol-Margolis
<li>Waseem Daher
<li>Bill McCloskey
<li>Austin Roach
<li>Greg Hudson
<li>Karl Ramm
<li>Alexander Chernyakhovsky
<li>Peter Iannucci
<li>Evan Broder
<li>Neha Narula
<li>Katrina LaCurts
<li>Ramesh Chandra
</ul>
</section>
<section id="contact">
<div class="page-header">
<h1>Contact</h1>
</div>
<ul>
<li><p><code>mosh-devel@mit.edu</code><br>
Mosh development and discussion.
Sign up or view archives at <a href="http://mailman.mit.edu/mailman/listinfo/mosh-devel">http://mailman.mit.edu/mailman/listinfo/mosh-devel</a>.</p></li>
<li><p><code>mosh-users@mit.edu</code><br>
Mosh user discussion and site best practices.
Sign up or view archives at <a href="http://mailman.mit.edu/mailman/listinfo/mosh-users">http://mailman.mit.edu/mailman/listinfo/mosh-users</a>.</p></li>
<li><p><code>#mosh</code> channel on <a href="http://freenode.net/">Freenode</a> IRC<br>
You can <a
href="http://webchat.freenode.net/?channels=mosh">connect with a
Web client</a>, try an <a
href="irc://irc.freenode.net/mosh"><span style="font-family: monospace">irc://</span> URL</a>,
or manually configure your client for <code>irc.freenode.net</code>.</p></li>
<li><p>At the recommendation of the security community, <b>confidential security-related matters</b> may be sent to: <code>mosh-security@mit.edu</code><br></p>
<div class="callout">
Messages may optionally be encrypted with <a href="keithw-gpg.txt">Keith Winstein's public key</a>:
<pre>pub 4096R/FE254C69 2012-02-05 [expires: 2015-02-04]
Key fingerprint = B1A4 7069 121F 6642 BB3D 7F3E 20B7 283A FE25 4C69</pre>
</div></li>
</ul>
</section>
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