tmux ssh-agent forwarding

ssh-rc

# this file goes in ~/.ssh/rc

if [ -S "$SSH_AUTH_SOCK" ]; then
  # This gets executed by sshd on every client login, symlinking the forwarded
  # agent socket into a predictable location to be picked up by tmux.
  ln -sf "$SSH_AUTH_SOCK" "$HOME/.ssh/ssh_auth_sock"
fi

tmux.conf

# disable auto updating of things like SSH_AUTH_SOCK
set -g update-environment ""
if-shell '[ -h ~/.ssh/ssh_auth_sock ]' "set-environment -g 'SSH_AUTH_SOCK' ~/.ssh/ssh_auth_sock"