Mastering Linux Security and Hardening - Second Edition

Disabling X11 forwarding

When you SSH into a server in the normal manner, as we've been doing, you can only run text-mode programs. If you try to remotely run any GUI-based program, such as Firefox, you'll get an error message. But, when you open the sshd_config file of pretty much any Linux distribution, you'll see this line:

X11Forwarding yes

This means that with the right option switch, you can remotely run GUI-based programs. Assuming that you're logging into a machine that has a graphical desktop environment installed, you can use either the -Y or the -X option when logging in, like so:

ssh -X donnie@192.168.0.12orssh -Y donnie@192.168.0.12

The problem here is that the X11 protocol, which powers graphical desktop environments on most ...