In Ubuntu, open a terminal window. Log into terminal as your username. Enter your password when requested. Switch to the root super user as follows:
sudo -s (enter)
input your password when requested (enter)
Change to the /etc directory as follows:
cd /etc
The ‘hostname’ file will contain your short hostname, such as mx1.
Check the current hostname by entering the hostname command, and the hostname might be something like ‘mx1’ — as follows:
hostname (enter)
mx1
Your fully qualified domain hostname should be something like mx1.yourdomain.com. Check the current FQDN hostname with the -f option, as follows:
hostname -f (enter)
mx1
The ‘hosts’ file should be edited to insert your FQDN as well. Using the PICO text editor, open the hosts file to be edited as follows:
pico hosts (enter)
Change the second line of the hosts file from this:
127.0.0.1 localhost
127.0.1.1 mx1
To this:
127.0.0.1 localhost
127.0.1.1 mx1.yourdomain.com mx1
Then save the hosts file as follows:
CTRL+X
(y)es
hosts (enter)
Now check the fully qualified domain hostname, as follows:
hostname -f (enter)
mx1.yourdomain.com
Check the hostname, which should still be ‘mx1’
hostnamemx1
Escape from the terminal window:
Exit (enter)
Exit (enter)