HAProxy with SNI & Host-based rules

This is a proof of concept, enabling HAProxy to use either SNI to redirect to backends with their own HTTPS certificates (which are then fully exposed to the client; HAProxy only proxies on a TCP level in that case), as well as to terminate HTTPS and use the Host header to redirect to backends that use HTTP (or a new HTTPS connection).

How it works

1. The http_redirect_frontend is only there to listen on port 80 and redirect every request to HTTPS.
2. The https_sni_frontend listens on port 443 and chooses a backend based on the SNI hostname of the TLS connection.
3. The https_termination_backend passes all requests to a unix socket (using the plain TCP data).
4. The https_termination_frontend listens on said unix socket, terminates the HTTPS connections and then chooses a backend based on the Host header.

In the example (see haproxy.cfg), the pages_backend is listening via HTTPS and is providing its own HTTPS certificates, while the gitea_backend only provides HTTP.

How to test

docker-compose up & ./test.sh docker-compose down # For manual testing: all HTTPS URLs connect to localhost:443 & certificates are not verified. ./test.sh [curl-options...] <url>