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
To start contributing to Mosh, install the following dependencies:
Debian, Windows Subsystem for Linux:
$ sudo apt install -y build-essential protobuf-compiler \
libprotobuf-dev pkg-config libutempter-dev zlib1g-dev libncurses5-dev \
libssl-dev bash-completion tmux less
Fedora, RHEL:
$ sudo dnf group install development-tools
$ sudo dnf install automake protobuf-compiler protobuf-devel libutempter-devel \
zlib-ng-compat-devel ncurses-devel openssl-devel bash-completion tmux less \
perl-diagnostics
MacOS:
$ brew install protobuf automake
Once you have forked the repository, run the following to build and test Mosh:
$ ./autogen.sh
$ ./configure
$ make
$ make check
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
