Linux – X11 forwarding doesn’t work with multiple tmux sessions

Each time you ssh into bastion01, a different socket is opened to handle the key forwarding. You can see the filename in the environment variable SSH_AUTH_SOCK. When you start tmux, the value of that environment variable is included in tmux's global environment, which is inherited by any shells started in that session.

Now, when you reconnect to bastion01 later, a different socket is allocated to handle your key forwarding (since it's a new ssh session). You can see this by examining the value of SSH_AUTH_SOCK before you re-attach to your tmux session and after. In order for key forwarding to work inside tmux, you need to update the value of SSH_AUTH_SOCK inside tmux to the name of the socket being used by the current ssh session.

A quick-and-dirty way to do this is to write a short script that will save this new value to a file, and execute that inside any tmux window where you will be ssh-ing from.

#!/bin/bash

echo "export SSH_AUTH_SOCK=$SSH_AUTH_SOCK" > ~/.auth_ssh

Execute that script as soon as you ssh into bastion01, but before you re-attach to your tmux session. Then, before you try to ssh anywhere from inside tmux, run the following:

source ~/.auth_ssh

Each tmux window has its own environment, so you'll need to run that in each window where you try to run ssh. For simplicity, you can alias ssh to do it for you:

alias ssh="source ~/.auth_ssh; ssh"

Note: this is a gross oversimplification of a script we use at work to update the SSH authorization information. If it doesn't work quite right, I hope this at least gives you enough information to google a better solution (or someone else posts a better solution here).

My advice:

Use mosh to connect to the remote server, once the session starts, fire up tmux

mosh was built for the common, but less apocalyptic scenario of remote a session connecting through a cellular data link, before 3G was even a thing.

From the mosh man page:

mosh (mobile shell) is a remote terminal application that supports intermittent connectivity, allows roaming, and provides speculative local echo and line editing of user keystrokes.

Compared with ssh, mosh is more robust — its connections stay up across sleeps and changes in the client's IP address — and more responsive, because the protocol is tolerant of packet loss and the client can echo most keystrokes immediately, without waiting for a network round-trip.

mosh uses ssh to establish a connection to the remote host and authen‐ ticate with existing means (e.g., public-key authentication or a pass‐ word). mosh executes the unprivileged mosh-server helper program on the server, then closes the SSH connection and starts the mosh-client, which establishes a long-lived datagram connection over UDP.

Back in those days, if you used your laptop to log in to your ssh server, for example in a commuter train, using a CDMA "pc-card" modem on your blindingly fast compaq armada (omg pentium!), or using a serial cable to hook up that palm VII thingy that had some sort of data service; you would have your session disconnected every time you switched from one radio cell to the next, which in a commuter train could be every 3 to 5 minutes.

That would be an equivalent scenario to the old Soviet Union raining down plutonium along the train's track, from the connection's point of view...

so mosh to the rescue. It uses ssh to authenticate, but the rest of the session is handled by the mosh tunnel, which was specifically designed for session resilience on flaky links.

From a user's perspective, nowdays it's imperceptible. I still use it to ssh, er... mosh from my Android device using termux even though the links on 4G don't have this issue anymore.

Another frequent use case was ssh connections through flaky modem links over POTS, that would drop the session if your sister decided that she wanted to call her boyfriend and picked up the other FIXED phone in the house, even though you warned her that you'd be downloading U2's new album in MP3 format from a shady WaReZ site.

So if you would like to use this, install mosh using your distro's package manager on both server and client (no root needed on either, it will do a userland install if it can't get root, handy for Android devices) and then do:

terminus:~>> mosh trantor.mydoman.tld tmux
Last login: Wed Apr  4 21:27:38 2018 from XX.XXX.XXX.XXX
trantor:~>> 

Enjoy! =)