httpserver: Prevent TLS client authentication bypass in 3 ways (#2099)

- Introduce StrictHostMatching mode for sites that require clientauth
- Error if QUIC is enabled whilst TLS clientauth is configured
  (Our QUIC implementation does not yet support TLS clientauth, but
  maybe it will in the future - fixes #2095)
- Error if one but not all TLS configs for the same hostname have a
  different ClientAuth CA pool

caddyhttp/httpserver/server.go

@@ -416,6 +416,18 @@ func (s *Server) serveHTTP(w http.ResponseWriter, r *http.Request) (int, error)
 		r.URL = trimPathPrefix(r.URL, pathPrefix)
 	}
 
+	// enforce strict host matching, which ensures that the SNI
+	// value (if any), matches the Host header; essential for
+	// sites that rely on TLS ClientAuth sharing a port with
+	// sites that do not - if mismatched, close the connection
+	if vhost.StrictHostMatching && r.TLS != nil &&
+		strings.ToLower(r.TLS.ServerName) != strings.ToLower(hostname) {
+		r.Close = true
+		log.Printf("[ERROR] %s - strict host matching: SNI (%s) and HTTP Host (%s) values differ",
+			vhost.Addr, r.TLS.ServerName, hostname)
+		return http.StatusForbidden, nil
+	}
+
 	return vhost.middlewareChain.ServeHTTP(w, r)
 }