To allow X connections to be forwarded from or through an SSH server, what configuration...

To allow X connections to be forwarded from or through an SSH server, the configuration keyword that must be set to yes in the sshd configuration file is XllForwarding. This keyword enables X11 forwarding, which is a mechanism that allows graphical applications to be run on a remote host and displayed on a local host. The sshd configuration file is usually located at /etc/ssh/sshd_config, and the XllForwarding keyword can be set globally or per host. For example:

XllForwarding yes

This will enable X11 forwarding for all hosts. To enable X11 forwarding for a specific host, use the Match directive, followed by the Host keyword and the host name or pattern. For example:

Match Host example.com XllForwarding yes

This will enable X11 forwarding only for the host example.com.

References:

X11 Forwarding using SSH

sshd_config - OpenSSH SSH daemon configuration file

How To Configure X11 Forwarding Using SSH In Linux