Enable X25519MLKEM768 on Apache httpd

Apache gets post-quantum key exchange from the OpenSSL that mod_ssl links. With OpenSSL 3.5 or later, X25519MLKEM768 is in the default group list; you only need to make sure nothing narrows it.

Before you start

• Apache 2.4 with mod_ssl linked against OpenSSL 3.5 or later. Check the library: ldd $(which httpd) | grep ssl, or your package manager (for example apk info libssl3).
• TLS 1.3 allowed: post-quantum key exchange needs it.

Option 1: use OpenSSL's defaults (simplest)

1. Look for a group or curve setting: grep -rniE "SSLOpenSSLConfCmd +(Groups|Curves)|SSLECDHCurve" /etc/apache2 /etc/httpd /usr/local/apache2/conf.
2. Remove any line that leaves out X25519MLKEM768.
3. Make sure TLS 1.3 is allowed:

SSLProtocol -all +TLSv1.2 +TLSv1.3

4. Test and reload: apachectl configtest && apachectl graceful.

Option 2: list the groups yourself

1. In the virtual host (or the server config), set the group list with X25519MLKEM768 first:

SSLProtocol -all +TLSv1.2 +TLSv1.3
SSLOpenSSLConfCmd Groups X25519MLKEM768:X25519:prime256v1

2. Test and reload: apachectl configtest && apachectl graceful.

Keep X25519MLKEM768 first, or browsers that offer both get classical key exchange.

Then scan your domain below to check every host at once.