fully qualified domain name

moon.nilsu.org

Basically you need your server's name, your domain name, and then your tld. Setting it seems confusing, but only because there's like no fucking documentation anywhere.

On most systems you just need to edit two files. /etc/hostname and /etc/hosts.

/etc/hostname

moon

/etc/hosts

127.0.0.1 localhost
127.0.1.1 moon.nilsu.org localhost
xxx.xxx.xxx.xxx moon.nilsu.org moon

The xxx.xxx.xxx.xxx is your public (static) ip address.