Sane SSH_AUTH_SOCK handling for Screen and Tmux, so that new SSH agents created by subsequent logons are still usable.

.bashrc

_ssh_auth_save() {
    ln -sf "$SSH_AUTH_SOCK" "$HOME/.ssh/ssh-auth-sock.$HOSTNAME"
}
alias screen='_ssh_auth_save ; export HOSTNAME=$(hostname) ; screen'
alias tmux='_ssh_auth_save ; export HOSTNAME=$(hostname) ; tmux'

.screenrc

unsetenv SSH_AUTH_SOCK
setenv SSH_AUTH_SOCK $HOME/.ssh/ssh-auth-sock.$HOSTNAME

.tmux.conf

set -g update-environment "DISPLAY SSH_ASKPASS SSH_AGENT_PID SSH_CONNECTION WINDOWID XAUTHORITY"
set-environment -g SSH_AUTH_SOCK $HOME/.ssh/ssh-auth-sock.$HOSTNAME