Why is my SSH X Window forwarding broken?

SSH into your destination:

ssh -vv -X <hostname>

If you see a line near the end that says “debug1: Remote: No xauth program; cannot forward with spoofing.“, you need to install xauth and reconnect. In Debian, run:

apt-get install xbase-clients

Next, ensure that you’re getting a DISPLAY variable through:

echo $DISPLAY

If that command outputs just a blank line, X forwarding is probably being denied by the server. Edit your sshd_config (/etc/ssh/sshd_config on Debian) and change/add the line:

X11Forwarding yes

If you change this, you’ll need to restart your SSH server:

/etc/init.d/ssh restart

Once your DISPLAY is being passed correctly, some programs may run but act oddly (”Gdk-error: […] BadAtom”, etc.) SSH uses “SECURITY” extensions by default, and some programs don’t work correctly with them. Try:

ssh -Y <hostname>

Note that this command may expose your local machine to compromise if someone malicious is in control of the host you’re connecting to.