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SSH authentication with Mosh

8 May 2013

I’ve been using Mosh for a while to work on remote development virtual machines. I do a lot of work on the train while commuting, and being able to have a persistent session which I can access wherever I am is great. The reduced latency and notifications of unsent content that mosh provides are also very useful features.

The only significant downside has been that Mosh doesn’t support authentication forwarding. This means that to perform actions such as git pulls over SSH, I need to log into the machine separately over raw SSH, and perform the action in a separate terminal.

I was just shown a blog post which solves this problem for tmux by pointing the SSH_AUTH_SOCK environment variable on the remote machine at a symlink, and updating that symlink on each reconnection to point to the real authentication socket. I’m now using a variant on this approach with mosh, as follows:

First, as suggested for tmux, I have added a $HOME/.ssh/rc file on the remote server with the following contents:

if test "$SSH_AUTH_SOCK" ; then
    ln -sf $SSH_AUTH_SOCK ~/.ssh/ssh_auth_sock

This causes a symlink at $HOME/.ssh/ssh_auth_sock to be updated to point to the SSH auth socket on each connection to the machine. Note that keeping this symlink inside the .ssh directory is a good idea, since that directory should always have nice restrictive permissions to stop any other user on your system accessing it.

Then, I added a line to my $HOME/.bashrc file to set the SSH_AUTH_SOCK environment variable on login to point to the symlink. This is run on login, after the .ssh/rc file:

export SSH_AUTH_SOCK=$HOME/.ssh/ssh_auth_sock

As a result, in my mosh session, the SSH authentication socket used comes from whichever ssh connection I last made to the machine.

Finally, I run a process in the background on my laptop, which connects to the remote server over plain SSH (and also makes a few useful SSH tunnels, while it’s at it). Whenever this process loses its connection, it sleeps a little bit and then tries to reconnect. As a result, I can always access the SSH authentication from my laptop in my mosh session if there’s a reasonable network connection between the two, without needing to leave my comfortable mosh session.