Enable X25519MLKEM768 on HAProxy

HAProxy terminates TLS with the library it was built with. With OpenSSL 3.5 or later, X25519MLKEM768 is available; set it as the first curve on your TLS listeners.

Before you start

• HAProxy built with OpenSSL 3.5 or later: haproxy -vv | grep -i "SSL library".
• TLS 1.3 allowed on your bind lines (it is unless ssl-max-ver says otherwise).

Option 1: for every listener

1. In the global section, set the default curve list with X25519MLKEM768 first:

global
    ssl-default-bind-curves X25519MLKEM768:X25519:P-256

2. Check and reload: haproxy -c -f /etc/haproxy/haproxy.cfg && systemctl reload haproxy.

Option 2: on one listener

1. Add curves to the bind line that terminates TLS:

frontend web
    bind :443 ssl crt /etc/haproxy/certs/site.pem curves X25519MLKEM768:X25519:P-256

2. Check and reload as above.

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

Then scan your domain below.