How to enable or disable X11 forwarding in an SSH server

X11Forwarding in an SSH server allows users to run graphical applications on a remote server and display them on their local machine. This feature is useful for accessing GUI-based programs from a remote location, but it also introduces potential security risks. Administrators often disable this feature to prevent exposing the client’s display server to potential attacks.

When X11Forwarding is enabled, the server forwards the display information to the client over the SSH connection. This process can make both the server and client more vulnerable, especially if not properly configured. The default configuration restricts some risks, but enabling X11 forwarding still requires careful consideration of the security implications.

Disabling X11Forwarding reduces the risk but does not completely prevent users from setting up their own forwarding methods. Deciding whether to enable or disable X11Forwarding should be based on a thorough assessment of the specific needs and security requirements of your environment.

1. Open the SSH server configuration file in a text editor.
2. Locate the X11Forwarding directive in the configuration file.

   $ sudo vi /etc/ssh/sshd_config
   [sudo] password for user:

3. Set X11Forwarding to yes to enable or no to disable it.

   X11Forwarding no

   Add the line if it doesn't already exist and remove # at the beginning of the line if it exists.

4. Save the changes to the configuration file.
5. Restart the SSH server service to apply the changes.

   $ sudo systemctl restart sshd

   Related: How to start, restart, and stop the SSH server service