Could not reliably determine the server's fully qualified domain name

This is just a friendly warning and not really a problem (as in that something does not work).

If you insert a

ServerName localhost   

in either httpd.conf or apache2.conf in /etc/apache2 and restart apache the notice will disappear.

If you have a name inside /etc/hostname you can also use that name instead of localhost.

---

And it uses 127.0.1.1 if it is inside your /etc/hosts:

127.0.0.1 localhost
127.0.1.1 myhostname

Preferred method

Troubleshooting Apache

If you get this error:

apache2: Could not determine the server's fully qualified domain name, 
using 127.0.0.1 for ServerName

then use a text editor such as "sudo nano" at the command line or "gksudo gedit" on the desktop to create a new file,

sudo nano /etc/apache2/conf.d/fqdn

or

gksu "gedit /etc/apache2/conf.d/fqdn"

then add

ServerName localhost

to the file and save. This can all be done in a single command with the following:

 echo "ServerName localhost" | sudo tee /etc/apache2/conf.d/fqdn

But on Ubuntu 14.04:

 echo "ServerName localhost" | sudo tee /etc/apache2/conf-available/fqdn.conf
 sudo a2enconf fqdn

Don't forget the ".conf" (without will not work).

Source : http://askubuntu.com/questions/256013/could-not-reliably-determine-the-servers-fully-qualified-domain-name