How to fix "Could not reliably determine the server’s fully qualified domain name" warning in Apache

When Apache cannot determine a fully qualified domain name (FQDN) at startup, it logs a warning and falls back to an address such as 127.0.1.1 for its default ServerName, which clutters logs and can confuse hostname-sensitive behavior like server-generated redirects.

Could not reliably determine the server’s fully qualified domain name, using 127.0.1.1 for ServerName.

On Ubuntu and Debian packaging, Apache reads its global configuration from /etc/apache2/apache2.conf and expects a canonical hostname via the ServerName directive. If no global ServerName is set, Apache asks the operating system for the host name and attempts to resolve it; if that resolution does not produce a usable FQDN, the warning is emitted and a loopback address is used as a default.

Setting a global ServerName suppresses the warning without changing virtual host routing, since per-site ServerName values in /etc/apache2/sites-enabled still take precedence. The chosen name does not create DNS records or fix name resolution, so it should match a stable hostname already resolvable via DNS or /etc/hosts, especially on TLS-enabled sites where hostnames must align with certificates.

1. Open the Apache main configuration file for editing.

   $ sudo nano /etc/apache2/apache2.conf

   Related: Location for Apache main configuration file

2. Add a global ServerName directive in the main context.

   ServerName server.example.com

   Place the directive outside any <VirtualHost> block; a wrong global ServerName can cause absolute redirects to point at the wrong hostname.

3. Save the file.

   nano writes changes with Ctrl+O.

4. Exit the editor.

   nano exits with Ctrl+X.

5. Test the configuration for syntax errors.

   $ sudo apache2ctl configtest
   Syntax OK

   A missing global ServerName often appears as 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 followed by Syntax OK.

   Related: How to test your Apache configuration

6. Reload the Apache service.

   $ sudo systemctl reload apache2

   RHEL-family systems use the httpd unit and /etc/httpd/conf/httpd.conf.

   Related: How to manage the Apache web server service

7. Verify the service is running cleanly.

   $ sudo systemctl status apache2
   ● apache2.service - The Apache HTTP Server
        Loaded: loaded (/lib/systemd/system/apache2.service; enabled; vendor preset: enabled)
        Active: active (running) since Sat 2025-12-13 10:15:01 UTC; 1min ago
          Docs: https://httpd.apache.org/docs/2.4/
   ##### snipped #####

   Related: How to manage the Apache web server service