Summary

This PR introduces a quic matcher that enables Caddy to multiplex QUIC with anything else like OpenVPN and Wireguard on a single UDP port. This matcher supports all the currently existing tls matchers, including alpn, sni, local_ip, and remote_ip. Besides, it exposes the following placeholders: {l4.quic.tls.server_name}, {l4.quic.tls.version}, and {l4.quic.version}.

Syntax

{
	layer4 {
		udp/:8443 {
			@q1 quic sni one.com two.com
        		route @q1 {
        			proxy udp/{l4.quic.tls.server_name}:443
        		}
			@q2 quic {
				alpn custom
				sni example.com
			}
        		route @q2 {
        			proxy udp/localhost:6543
        		}
		}
	}
}

Discussion / Further steps

• I can see 3 major places where it would be great to implement QUIC support consistent with what we have now for TLS:
  1/ a quic handler, i.e. an intermediary unwrapper to handle incoming QUIC connections;
  2/ QUIC support in the proxy handler, i.e. an intermediary wrapper to handle outgoing QUIC connections;
  3/ QUIC, and generally UDP, wrapper in the mainline code, i.e. a listener_wrappers alternative for net.PacketConn connections.
• As far as the first 2 steps are concerned, I've found an example of QUIC proxy. However, as of now I don't fully understand how to make it work correctly within the existing caddy-l4 architecture. Any help would be appreciated.