Apache: AH00558: Could not reliably determine the server's FQDN

Quick fix

Read the commands before running them. Anything that restarts a service, deletes data or changes permissions should be tried on a non-production system first.

# Set it globally
echo 'ServerName example.com' | sudo tee /etc/apache2/conf-available/servername.conf
sudo a2enconf servername
sudo apachectl configtest && sudo systemctl reload apache2

# And per virtual host
<VirtualHost *:443>
    ServerName example.com
    ServerAlias www.example.com
</VirtualHost>

# Behind a proxy, trust the forwarded host instead of guessing
UseCanonicalName On
RemoteIPHeader X-Forwarded-For

How to diagnose WebServer errors

Web server errors are usually about permissions, binding, or configuration that was never loaded. A common trap: the server runs as an unprivileged user (www-data, nginx), so it needs execute permission on every directory in the path to a file, not just read permission on the file itself. Another: editing a config file changes nothing until the server is reloaded and the file is actually included.

If the quick fix above does not resolve it, work through these steps. They apply to this whole class of error, not just to this one message, which is usually what saves the time.

1. Validate configuration before reloading: nginx -t, apachectl configtest, caddy validate. This catches syntax errors without dropping traffic.
2. Dump the fully resolved configuration with nginx -T to see what is actually in effect, including every include.
3. Read the error log, not the access log, for 5xx causes. nginx names the failing upstream and the exact filesystem path it could not open.
4. For permission errors, test as the server user: sudo -u www-data cat /path/to/file. Check execute bits on every parent directory.
5. For binding failures, find the current listener with ss -tulpn | grep :80. Ports below 1024 need privileges or a capability such as CAP_NET_BIND_SERVICE.

Tools worth reaching for

• nginx -t / nginx -T
• apachectl configtest
• ss -tulpn
• tail -f error.log
• sudo -u www-data

Authoritative references

Primary documentation for this error, worth reading before applying any fix in production.

📖 httpd.apache.org

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See all 10 WebServer errors →

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Something missing or wrong?

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