Today I installed Apache2 on my Linux Mint 21.3 and was greeted with the following message when running sudo apachectl configtest, i.e. configuration test:
AH00558: apache2: Could not reliably determine the server's fully qualified domain name, using
127.0.0.1. Set the 'ServerName' directive globally to suppress this message
How to get it sorted?
Edit the following file with your favorite text editor as root, e.g.:
sudo vim /etc/apache2/apache2.conf
Go to the end of this file, and add this line:
ServerName 127.0.0.1
just before the VIM syntax specification:
# vim: syntax=apache ts=4 sw=4 sts=4 sr noet
Save the file, and run:
sudo apachectl configtest
it should now no longer say error AH00558, instead you should be greeted with Syntax OK.
There could be many more solutions, I guess this is more of a workaround - but maybe not. If you can do better than me, feel free to comment and answer, I will delete my answer in such a case.