Yes, sort of. This is normally a server setting. You should set X11Forwarding (no space) to yes in /etc/ssh/sshd_config (with the d) on the externalhost and restart SSHD.
ssh_config (no d) is a client setting and while it's possible X forwarding is disabled by your client, it would be rare.
Last edited by AlucardZero; 12-07-2010 at 08:07 AM.