SOLVED - apache2: Could not reliably determine the server's fully qualified domain name

This has been annoying me for a while. I have a Digital Ocean VPS (droplet) running Ubuntu. This provides me with some email accounts and a couple of WordPress sites.

I’ve been receiving this error for a few weeks since I setup VirtualMin:

AH00558: apache2: Could not reliably determine the server's 
fully qualified domain name, using blah.com. Set the 'ServerName' 
directive globally to suppress this message

N.B. For this demo my VirtualMin installation is installed on panel.virtualmin-domain.com and the machine name is set to panel.

The usual answer to this appeared to be that apache2 needs the appropriate ServerName configured. I checked through each apache2 domain conf file, and all had an appropriate ServerName correctly set in the conf. To check this do the following:

sudo nano /etc/apache2/sites-available/
ls

Your available VirtualMin sites should be listed, i.e.

000-default.conf
default-ssl.conf
domain.com.conf

Each conf file should have the appropriate ServerName entry. For example the 000-default.conf should look like this:

ServerName panel.virtualmin-domain.com

To open and edit the file you can use nano:

sudo nano /etc/apache2/sites-available/000-default.conf

If it doesn’t exist then enter the name and restart apache2:

sudo /etc/init.d/apache2 restart

If you still get the same error (like I did) something else is mis-configured. For me the problem lay in my hosts configuration.

To check your hosts configuration type:

sudo nano /etc/hosts

It should look like this:

127.0.0.1 localhost
0.0.0.0 panel.virtualmin-domain.com panel

Where 0.0.0.0 is your Digital Ocean external IP address. Mine was mis-configured:

127.0.0.1 localhost
127.0.0.1 panel.virtualmin-domain.com panel # this extra line was causing the problem
0.0.0.0 panel.virtualmin-domain.com panel

I removed the offending line, rest the hostname and restarted apache2:

sudo hostname -F /etc/hostname
sudo /etc/init.d/apache2 restart

The result is now nice and error free:

root@panel:/etc/apache2/sites-available# /etc/init.d/apache2 restart
 * Restarting web server apache2                                         [ OK ]

Job done.