How to fix Apache "Could not reliably determine the server's FQDN"

AH00558: apache2: Could not reliably determine the server's fully qualified domain name, using 127.0.1.1. Set the 'ServerName' directive globally to suppress this message

Common causes

• No ServerName directive in the main config (apache2.conf or httpd.conf)
• The server's hostname has no matching entry in /etc/hosts
• A fresh Apache install where only virtual hosts define ServerName
• A container image where the hostname is a random ID
• The hostname is not a fully qualified domain name

How to fix it

1. Set a global ServerName on Debian/Ubuntu. Create /etc/apache2/conf-available/servername.conf with the line ServerName localhost (or your real hostname). Enable it with sudo a2enconf servername.
2. Set it on RHEL, CentOS or Alma. Open /etc/httpd/conf/httpd.conf, find the commented #ServerName www.example.com:80 line, and set ServerName localhost or your server's FQDN.
3. Test and reload. Run sudo apachectl configtest. It should print Syntax OK without the AH00558 warning. Then run sudo systemctl reload apache2 (or httpd).
4. Check the hostname mapping. Run hostname -f. If it fails, add a line such as 127.0.1.1 server1.example.com server1 to /etc/hosts.
5. Look for the real error if Apache will not start. If Apache still fails, run sudo journalctl -u apache2 -n 50. Errors like port already in use or a syntax error are the actual problem.

Add a global ServerName on Debian/Ubuntu

echo 'ServerName localhost' | sudo tee /etc/apache2/conf-available/servername.conf
sudo a2enconf servername
sudo apachectl configtest
sudo systemctl reload apache2

How to stop it happening again

• Set a global ServerName in your provisioning script
• Give servers a proper FQDN hostname
• Keep ServerName set in every virtual host too
• Read the full configtest output, not just the first line