Ubuntu – Is X11 forwarding over SSH possible in Ubuntu 15.10

To manually do this, once you have SSHed in, but before you reattach to screen, check your DISPLAY environment variable:

echo $DISPLAY

Once you have re-attached to screen, explicitly set the environment variable:

export DISPLAY=:N.0

where :N.0 is what the echo showed before the attach. This won't be perfect, since some application may be expecting to talk to the Session D-Bus, which is a bit more complex to send over the SSH connection.

Both options have something to do with X11 forwarding. This means if you enable this, you can use a graphical client through your SSH session (e.g., use Firefox or something else).

If you use ssh -X remotemachine the remote machine is treated as an untrusted client. So your local client sends a command to the remote machine and receives the graphical output. If your command violates some security settings you'll receive an error instead.

But if you use ssh -Y remotemachine the remote machine is treated as a trusted client. This last option can open security problems. Because other graphical (X11) clients could sniff data from the remote machine (make screenshots, do keylogging and other nasty stuff) and it is even possible to alter those data.

If you want to know more about those things, I suggest reading the Xsecurity manpage or the X Security extension spec. Furthermore, you can check the options ForwardX11 and ForwardX11Trusted in your /etc/ssh/ssh_config.