X11 forwarding needs to be enabled on both the client side and the server side.
On the client side, the
-X (capital X) option to
ssh enables X11 forwarding, and you can make this the default (for all connections or for a specific conection) with
ForwardX11 yes in
On the server side,
X11Forwarding yes must specified in
/etc/ssh/sshd_config. Note that the default is no forwarding (some distributions turn it on in their default
/etc/ssh/sshd_config), and that the user cannot override this setting.
xauth program must be installed on the server side. If there are any X11 programs there, it's very likely that
xauth will be there. In the unlikely case
xauth was installed in a nonstandard location, it can be called through
~/.ssh/rc (on the server!).
Note that you do not need to set any environment variables on the server.
XAUTHORITY will automatically be set to their proper values. If you run ssh and
DISPLAY is not set, it means ssh is not forwarding the X11 connection.
To confirm that ssh is forwarding X11, check for a line containing
Requesting X11 forwarding in the
ssh -v -X output. Note that the server won't reply either way.