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Mosh: the mobile shell

Mosh is a remote terminal application that supports intermittent connectivity, allows roaming, and provides speculative local echo and line editing of user keystrokes.

It aims to support the typical interactive uses of SSH, plus:

• Mosh keeps the session alive if the client goes to sleep and wakes up later, or temporarily loses its Internet connection.

• Mosh allows the client and server to "roam" and change IP addresses, while keeping the connection alive. Unlike SSH, Mosh can be used while switching between Wi-Fi networks or from Wi-Fi to cellular data to wired Ethernet.

• The Mosh client runs a predictive model of the server's behavior in the background and tries to guess intelligently how each keystroke will affect the screen state. When it is confident in its predictions, it will show them to the user while waiting for confirmation from the server. Most typing and uses of the left- and right-arrow keys can be echoed immediately.

  As a result, Mosh is usable on high-latency links, e.g. on a cellular data connection or spotty Wi-Fi. In distinction from previous attempts at local echo modes in other protocols, Mosh works properly with full-screen applications such as emacs, vi, alpine, and irssi, and automatically recovers from occasional prediction errors within an RTT. On high-latency links, Mosh underlines its predictions while they are outstanding and removes the underline when they are confirmed by the server.

Mosh does not support X forwarding or the non-interactive uses of SSH, including port forwarding.

Other features

• Mosh adjusts its frame rate so as not to fill up network queues on slow links, so "Control-C" always works within an RTT to halt a runaway process.

• Mosh warns the user when it has not heard from the server in a while.

• Mosh supports lossy links that lose a significant fraction of their packets.

• Mosh handles some Unicode edge cases better than SSH and existing terminal emulators by themselves, but requires a UTF-8 environment to run.

• Mosh leverages SSH to set up the connection and authenticate users. Mosh does not contain any privileged (root) code.

Getting Mosh

The Mosh web site has information about packages for many operating systems, as well as instructions for building from source.

Note that mosh-client 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.

Usage

The mosh-client binary must exist on the user's machine, and the mosh-server binary on the remote host.

The user runs:

$ mosh [user@]host

If the mosh-client or mosh-server binaries live outside the user's $PATH, mosh accepts the arguments --client=PATH and --server=PATH to select alternate locations. More options are documented in the mosh(1) manual page.

There are more examples and a FAQ on the Mosh web site.

How it works

The mosh program will SSH to user@host to establish the connection. SSH may prompt the user for a password or use public-key authentication to log in.

From this point, mosh runs the mosh-server process (as the user) on the server machine. The server process listens on a high UDP port and sends its port number and an AES-128 secret key back to the client over SSH. The SSH connection is then shut down and the terminal session begins over UDP.

If the client changes IP addresses, the server will begin sending to the client on the new IP address within a few seconds.

To function, Mosh requires UDP datagrams to be passed between client and server. By default, mosh uses a port number between 60000 and 61000, but the user can select a particular port with the -p option. Please note that the -p option has no effect on the port used by SSH.

Advice to distributors

A note on compiler flags: Mosh is security-sensitive code. When making automated builds for a binary package, we recommend passing the option --enable-compile-warnings=error to ./configure. On GNU/Linux with g++ or clang++, the package should compile cleanly with -Werror. Please report a bug if it doesn't.

Where available, Mosh builds with a variety of binary hardening flags such as -fstack-protector-all, -D_FORTIFY_SOURCE=2, etc. These provide proactive security against the possibility of a memory corruption bug in Mosh or one of the libraries it uses. For a full list of flags, search for HARDEN in configure.ac. The configure script detects which flags are supported by your compiler, and enables them automatically. To disable this detection, pass --disable-hardening to ./configure. Please report a bug if you have trouble with the default settings; we would like as many users as possible to be running a configuration as secure as possible.

Mosh ships with a default optimization setting of -O2. Some distributors have asked about changing this to -Os (which causes a compiler to prefer space optimizations to time optimizations). We have benchmarked with the included src/examples/benchmark program to test this. The results are that -O2 is 40% faster than -Os with g++ 4.6 on GNU/Linux, and 16% faster than -Os with clang++ 3.1 on Mac OS X. In both cases, -Os did produce a smaller binary (by up to 40%, saving almost 200 kilobytes on disk). While Mosh is not especially CPU intensive and mostly sits idle when the user is not typing, we think the results suggest that -O2 (the default) is preferable.

Our Debian and Fedora packaging presents Mosh as a single package. Mosh has a Perl dependency that is only required for client use. For some platforms, it may make sense to have separate mosh-server and mosh-client packages to allow mosh-server usage without Perl.

Notes for developers

Mosh supports producing code coverage reports by tests, but this feature is disabled by default. To enable it, make sure lcov is installed on your system. Then, configure and run tests:

$ ./configure --enable-code-coverage
$ make check-code-coverage

This will run all tests and produce a coverage report in HTML form that can be opened with your favorite browser. Ideally, newly added code should strive for 90% (or better) incremental test coverage.

More info

• Mosh Web site:

  https://mosh.org

• [email protected] mailing list:

  https://mailman.mit.edu/mailman/listinfo/mosh-devel

• [email protected] mailing list:

  https://mailman.mit.edu/mailman/listinfo/mosh-users

• #mosh channel on Libera Chat

  https://web.libera.chat/#mosh