Tools for CLI Road Warriors: Remote Shells //at 19:44 //by abe

Most of my private online life happens on netbooks and besides the web browser, SSH is my most used program — especially on netbooks. Accordingly I also have hosts on the net to which I connect via SSH. My most used program there is GNU Screen.

So yes, for things like e-mail, IRC, and Jabber I connect to a running screen session on some host with a permanent internet connection. On those hosts there is usually one GNU Screen instance running permanently with either mutt or irssi (which is also my Jabber client via a Bitlbee gateway).

But there are some other less well-known tools which I regard as useful in such a setup. The following two tools can both be seen as SSH for special occassions.

autossh

I already blogged about autossh, even twice, so I’ll just recap the most important features here:

autossh is a wrapper around SSH which regularily checks via two tunnels connect to each other on the remote side if the connection is still alive, and if not, it kills the ssh and starts a new one with the same parameters (i.e. tunnels, port forwardings, commands to call, etc.).

It’s quite obvious that this is perfect to be combined with screen’s -R and -d options.

I use autossh so often that I even adopted its Debian package.

mosh

Since last week there’s a new kid in town^WDebian Unstable: mosh targets the same problems as autossh (unreliable networks, roaming, suspending the computer, etc.) just with a completely different approach which partially even obsoletes the usage of GNU Screen or tmux:

While mosh uses plain SSH for authentication, authorization and key exchange the final connection is an AES-128 encrypted UDP connection on a random port and is independent of the client’s IP address.

This allows mosh to have the following advantages: The connection stays even if you’re switching networks or suspending your netbook. So if you’re just running a single text-mode application you don’t even need GNU Screen or tmux. (You still do if you want the terminal multiplexing feature of GNU Screen or tmux.)

Another nice feature, especially on unreliable WLAN connections or laggy GSM or UMTS connections is mosh’s output prediction based on its input (i.e. what is typed). Per line it tries to guess which server reaction a key press would cause and if it detects a lagging connection, it shows the predicted result underlined until it gets the real result from the server. This eases writing mails in a remote mutt or chatting in a remote irssi, especially if you noticed that you made a typo, but can’t remember how many backspaces you would have to type to fix it.

Mosh needs to be installed on both, client and server, but the server is only activated via SSH, so it has no port open unless a connection is started. And despite that (in Debian) mosh is currently just available in Unstable, the package builds fine on Squeeze, too. There’s also an PPA for Ubuntu and of course you can also get the source code, e.g. as git checkout from GitHub.

mosh is still under heavy development and new features and bug fixes get added nearly every day.

Thanks to Christine Spang for sponsoring and mentoring Keith’s mosh package in Debian.

Update: I gave a lightning talk about Mosh and AutoSSH in German at Easterhegg 2012. The slides are available online.