AH00558: apache2: Could not reliably determine the server's fully qualified domain name

 Troubleshooting using apachectl

An AH00558: Could not reliably determine the server's fully qualified domain name error can be detected using Apache’s apachectl utility. With apachectl you can catch messages like these before reloading or restarting Apache, and you can avoid having to search through systemctl and journalctl logs to locate errors.

To check your Apache configuration for an AH00558 message, run the following command:

sudo apachectl configtest

You should receive output like the following if your server is affected by an AH00558 error message:

Output
AH00558: apache2: Could not reliably determine the server's fully qualified domain name, using 172.17.0.2. Set the 'ServerName' directive globally to suppress this message
Syntax OK

As with the previous sections in this tutorial that used systemctl and journalctl to locate AH00558 messages, the line that contains the AH00558 message, highlighted in the previous example, is the important one. Again note that the IP address 172.17.0.2 in this example may be different on your server.

The next section of this tutorial explains how to set the ServerName directive to resolve AH00558 error messages.

Setting a Global ServerName Directive

To resolve an AH00558: Could not reliably determine the server's fully qualified domain name error message, you will need to add a ServerName directive to your Apache configuration. Apache uses the ServerName directive to map incoming HTTP requests to an IP address or DNS hostname using VirtualHost directives in order to handle requests for multiple sites using a single server.

The error message notes that a global ServerName directive should also be set. Doing so will ensure that Apache can gracefully handle incoming requests that do not map to a VirtualHost without generating additional errors.

For maximum compatibility with various Apache configurations, use the value of 127.0.0.1 for your global ServerName directive. You can use a different IP address or DNS name that corresponds to your server’s configuration if you need to, but it is safest to use 127.0.0.1.

On Ubuntu and Debian-derived systems, open the /etc/apache2/apache2.conf file with root privileges using nano or your preferred text editor:

sudo nano /etc/apache2/apache2.conf

Add a line containing ServerName 127.0.0.1 to the end of the file:

/etc/apache2/apache2.conf

. . .
# Include the virtual host configurations:
IncludeOptional sites-enabled/*.conf

# vim: syntax=apache ts=4 sw=4 sts=4 sr noet
ServerName 127.0.0.1

Once you have added the ServerName directive to your configuration, run apachectl to test that the configuration is valid.

sudo apachectl configtest

A successful apachectl configtest invocation should result in output like this:

Output
Syntax OK

On Ubuntu and Debian-derived systems, run the following:
sudo systemctl restart apache2.service

After you restart Apache, the AH00558 error message will no longer appear in your logs.

Conclusion
In this tutorial you learned about AH00558: Could not reliably determine the server's fully qualified domain name error messages. While these messages do not prevent Apache from running, they can be resolved by setting a global ServerName directive