Reconciling Tmux and SSH Agent Forwarding.

SSH agent forwarding creates a socket file and stores the path in SSH_AUTH_SOCK. In between reconnections tmux panes or windows could be left with a stale value in SSH_AUTH_SOCK leading to ssh connection errors. For example, authentication errors when trying to push/pull to a git repo.

To make sure SSH_AUTH_SOCK always points to the right socket:

1. Create a symlink on ssh connection. In ~/.ssh/rc.

#!/bin/bash

# Fix SSH auth socket location so agent forwarding works with tmux
if test "$SSH_AUTH_SOCK" ; then
    ln -sf $SSH_AUTH_SOCK ~/.ssh/ssh_auth_sock
fi

1. Set SSH_AUTH_SOCK to the symlink in tmux. In ~/tmux.conf.

# Remove SSH_AUTH_SOCK to disable tmux automatically resetting the variable
set -g update-environment "DISPLAY SSH_ASKPASS SSH_AGENT_PID \
                             SSH_CONNECTION WINDOWID XAUTHORITY"

# Use a symlink to look up SSH authentication
setenv -g SSH_AUTH_SOCK $HOME/.ssh/ssh_auth_sock

Source: Reconciling Tmux and SSH Agent Forwarding.