13.04 and older

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

If you go to:

/etc/apache2/apache2.conf

and insert:

ServerName localhost   

and then restart apache by typing into the terminal:

sudo systemctl reload apache2

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).

13.10 or newer

As of Apache 2.4 - which is available by default as of 13.10 - you cannot use the method about adding a config file in the conf.d directory.

Apache no longer uses the conf.d directory. All the configuration files are stored inside /etc/apache2/conf-available directory and all the configuration files should now have a .conf extension.

In order to solve this message in Apache 2.4, we have to create a configuration file inside the conf-available directory. For example servername.conf.

sudo vi /etc/apache2/conf-available/servername.conf

And inside this we just need to add one line

ServerName localhost

You can combine the previous two commands in one with:

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

If you want to use a domain name or any other name depending on the requirement its fine, just replace localhost with whatever you need. Next you have to enable this configuration. For this, you need to run the following command:

sudo a2enconf servername

a2enconf is a command to enable a configuration file in Apaches 2.4. Also note that servername on the above command is from the name of the configuration file servername.conf. If your configuration file was ngenericserver.conf then you would have to write sudo a2enconf ngenericserver.

After this reload the server and the above message will no longer bug you.

sudo service apache2 reload

or

sudo apache2ctl graceful

Now after this you will see that the message will not be shown again and the problem will be fixed.

Apache2 can also get the FQDN from a properly configured system hostname rather than hardcoded into the apache config file. Hardcoding it will also cause confusion if the hostname changes. You actually don't need any ServerName directives in httpd.conf or any apache config files.

Put the following in /etc/hosts:

# IPv4 and IPv6 localhost aliases
127.0.0.1 hostname.domainname.com  hostname  localhost
::1       hostname.domainname.com  hostname  localhost

where hostname.domainname.com is the FQDN of your machine.

Along with a properly configured hostname in /etc/hostname/ or with hostnamectl, this will also help other services on your machine run properly (i.e. the login prompt, showing This is hostname.domainname.com instead of This is hostname.unknown_domain.)

• Open a terminal

• Open the /etc/apache2/httpd.conf file:

  sudo editor /etc/apache2/httpd.conf # [1]
  

• By default, it would be blank. Simply add the following line:

  ServerName localhost
  

• Save the file and exit

• Restart the server

  sudo service apache2 restart
  

[1] Launch default editor, see sudo update-alternatives --config editor

In new version of apache2 you just following command like this:

sudo nano /etc/apache2/apache2.conf

Add the following new line end of file:

ServerName localhost

Then restart apache2:

sudo service apache2 restart

It's done.

I find it is a bit better to create a new file in /etc/conf.d that to modify either apache2.conf or httpd.conf.

It's a personal preference that keeps my configuration changes separated from the distribution package. So updates are less complicated.

I create the file /etc/apache2/conf.d/AAserverName and it contains only:

ServerName myhost.mycomain.tld

The other suggestions certainly work also.

In Ubuntu 16.04:

sudo -i

echo 'ServerName localhost' > /etc/apache2/conf-available/server-name.conf
a2enconf server-name

On Ubuntu 16.04:

Add ServerName localhost to the file httpd.conf usin gthe following command.

sudo vi  /etc/apache2/httpd.conf

Then include this line Include httpd.conf at the end of this file

sudo vi /etc/apache2/apache2.conf

Check for syntax errors again

sudo apache2ctl configtest

You should now get

Syntax OK

You can now restart the server gracefully by reloading the configuration

sudo service apache2 reload

or kill the process and start again

sudo service apache2 restart

Add ServerName localhost

To

 sudo leafpad /etc/apache2/apache2.conf

 sudo leafpad /etc/apache2/httpd.conf

It's not an error.. It's just a friend reminder

Specifying ServerName localhost in your configuration files outside of the virtual host sections is the way to do this.

Some other answers suggest that you should modify /etc/apache2/httpd.conf. This file gets overwritten when apache gets upgraded from apt. For Apache configuration that you don't want to get overwritten, you should create a new file. Here is the "Debian way" to make this configuration change:

# create the configuration file in the "available" section
echo "ServerName localhost" | sudo tee /etc/apache2/conf-available/servername.conf
# enable it by creating a symlink to it from the "enabled" section
sudo a2enconf servername
# restart the server
sudo service apache2 restart

This is basically the same as Stark's answer, but in an easy to copy and paste script form. I had originally posted this in a question that was marked as duplicate: https://askubuntu.com/a/432408