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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
------------
Mosh is available from an [Ubuntu PPA][] and has a [package in
Debian unstable][Debian]. Packages for other operating systems are
planned.
[Ubuntu PPA]: https://launchpad.net/~keithw/+archive/mosh
[Debian]: http://packages.debian.org/sid/mosh
Ubuntu installation instructions:
sudo add-apt-repository ppa:keithw/mosh
sudo apt-get update
sudo apt-get install mosh
On a Unix-like system you can build Mosh from source using the following
commands:
./autogen.sh
./configure
make
make install # as root
`configure` accepts standard options, like `--prefix` to set the installation
prefix. Pass `--help` for a full listing.
To build and use Mosh you will need
* [GNU Autotools][]
* the [Protocol Buffers][] library and compiler
* [Boost][]
* `ncurses`
* `libutempter`
* `zlib`
* the Perl module [IO::Pty][]
including development packages where applicable.
The file `debian/control` contains a list of the relevant Debian packages.
[GNU Autotools]: http://www.gnu.org/software/autoconf/
[Protocol Buffers]: http://code.google.com/p/protobuf/
[Boost]: http://www.boost.org/
[IO::Pty]: http://search.cpan.org/~toddr/IO-Tty/Pty.pm
Usage
-----
The `mosh-client` binary must be installed 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 are installed outside the
user's PATH, `mosh` accepts the arguments `--client=PATH` and
`--server=PATH` to select alternate locations.
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.
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.
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.
More info
---------
* Mosh Web site:
<http://mosh.mit.edu>
* `mosh-devel@mit.edu` mailing list:
<http://mailman.mit.edu/mailman/listinfo/mosh-devel>
* `mosh-users@mit.edu` mailing list:
<http://mailman.mit.edu/mailman/listinfo/mosh-users>
|
