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.
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.
The Mosh web site has information about packages for many operating systems, as well as instructions for building from source.
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
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
mosh-server binaries live outside the user's
mosh accepts the arguments
select alternate locations. More options are documented in the mosh(1) manual
How it works
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
./configure. On GNU/Linux with
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
-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
script detects which flags are supported by your compiler, and enables
them automatically. To disable this detection, pass
./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
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.
Mosh Web site:
#moshchannel on Libera Chat