Updating SSH_AUTH_SOCK in an already established tmux session

ssh_auth_sock.md

I found this information somewhere on StackOverflow but I forgot exactly where. I'm paraphrasing what I learned here, for future reference.

SSH uses a Unix socket to communicate with other processes. The socket's path can be found by looking at the environment variable $SSH_AUTH_SOCK. When you re-connect to a tmux session that was started during a previous SSH session, this variable will contain the path of the previous SSH auth socket, and this will cause processes that try to connect to your authentication agent to fail. To fix this, we have to

1. Create a symlink from the auth socket to a fixed path somewhere, so that we can refer to it later on, and not to modify the symlink if current is still alive. In ~/.ssh/rc, add

if [ ! -S ~/.ssh/ssh_auth_sock ] && [ -S "$SSH_AUTH_SOCK" ]; then
    ln -sf $SSH_AUTH_SOCK ~/.ssh/ssh_auth_sock
fi

2. On attaching to a tmux session, set SSH_AUTH_SOCK to the path we just created. In ~/.tmux.conf, add

setenv -g SSH_AUTH_SOCK $HOME/.ssh/ssh_auth_sock
set -g update-environment -r