ssh agent forwarding in tmux and gnu screen

.screenrc

# Fix agent forwarding
# https://gist.github.com/martijnvermaat/8070533
# http://techblog.appnexus.com/2011/managing-ssh-sockets-in-gnu-screen/
# See .ssh/rc for socket linking
unsetenv SSH_AUTH_SOCK
setenv SSH_AUTH_SOCK $HOME/.ssh/ssh_auth_sock.$HOSTNAME

.ssh rc

#!/bin/bash

# http://techblog.appnexus.com/2011/managing-ssh-sockets-in-gnu-screen/
# https://gist.github.com/martijnvermaat/8070533
# http://stackoverflow.com/questions/21378569/how-to-auto-update-ssh-agent-environment-variables-when-attaching-to-existing-tm

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

# Don't break x11 Forwarding:
# Taken from the sshd(8) manpage.
if read proto cookie && [ -n "$DISPLAY" ]; then
        if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then
                # X11UseLocalhost=yes
                echo add unix:`echo $DISPLAY |
                    cut -c11-` $proto $cookie
        else
                # X11UseLocalhost=no
                echo add $DISPLAY $proto $cookie
        fi | xauth -q -
fi

.tmux.config

# https://gist.github.com/admackin/4507371
# fix ssh agent when tmux is detached
# See .ssh/rc for socket linking
set -g update-environment -r
setenv -g SSH_AUTH_SOCK $HOME/.ssh/ssh_auth_sock.$HOSTNAME