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.
mosh-client binary must be installed on the user's machine, and
mosh-server binary on the remote host.
The user runs:
$ mosh [user@]host
mosh-server binaries are installed outside the
user's PATH, mosh accepts the arguments
--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 high-port UDP datagrams to be passed between client and server. Sites that have these ports firewalled would not be able to use Mosh.
Mosh Web site: