Allow use of custom port value in Alt-Svc header.

http3/server.go

@@ -92,7 +92,8 @@ type Server struct {
 	// See https://www.ietf.org/archive/id/draft-schinazi-masque-h3-datagram-02.html.
 	EnableDatagrams bool
 
-	port uint32 // used atomically
+	port             uint32 // used atomically
+	customAltSvcPort uint32 // custom port used for Alt-Svc response header
 
 	mutex     sync.Mutex
 	listeners map[*quic.EarlyListener]struct{}
@@ -439,16 +440,22 @@ func (s *Server) SetQuicHeaders(hdr http.Header) error {
 	port := atomic.LoadUint32(&s.port)
 
 	if port == 0 {
-		// Extract port from s.Server.Addr
-		_, portStr, err := net.SplitHostPort(s.Server.Addr)
-		if err != nil {
-			return err
+		if s.customAltSvcPort != 0 {
+			// Use customAltSvcPort if set
+			port = s.customAltSvcPort
+		} else {
+			// Extract port from s.Server.Addr
+			_, portStr, err := net.SplitHostPort(s.Server.Addr)
+			if err != nil {
+				return err
+			}
+			portInt, err := net.LookupPort("tcp", portStr)
+			if err != nil {
+				return err
+			}
+			port = uint32(portInt)
 		}
-		portInt, err := net.LookupPort("tcp", portStr)
-		if err != nil {
-			return err
-		}
-		port = uint32(portInt)
+
 		atomic.StoreUint32(&s.port, port)
 	}
 
@@ -486,6 +493,17 @@ func ListenAndServeQUIC(addr, certFile, keyFile string, handler http.Handler) er
 // http.DefaultServeMux is used when handler is nil.
 // The correct Alt-Svc headers for QUIC are set.
 func ListenAndServe(addr, certFile, keyFile string, handler http.Handler) error {
+	return ListenAndServeWithCustomAltSvcPort(addr, certFile, keyFile, handler, 0)
+}
+
+// ListenAndServeWithCustomAltSvcPort listens on the given network address for both, TLS and QUIC
+// connetions in parallel. It returns if one of the two returns an error.
+// http.DefaultServeMux is used when handler is nil.
+// The correct Alt-Svc headers for QUIC are set.
+// customAltSvcPort is used to override the default port value in the Alt-Svc response header.
+// This is useful when a Layer 4 firewall is redirecting UDP traffic and clients must use
+// a port different from the port the QUIC server itself is listening on.
+func ListenAndServeWithCustomAltSvcPort(addr, certFile, keyFile string, handler http.Handler, customAltSvcPort uint32) error {
 	// Load certs
 	var err error
 	certs := make([]tls.Certificate, 1)
@@ -530,7 +548,8 @@ func ListenAndServe(addr, certFile, keyFile string, handler http.Handler) error
 	}
 
 	quicServer := &Server{
-		Server: httpServer,
+		Server:           httpServer,
+		customAltSvcPort: customAltSvcPort,
 	}
 
 	if handler == nil {