gateway: remove alt-svc header from response

gateway/tunnel.go

@@ -41,7 +41,10 @@ func (g *Gateway) tunnelHandler() http.Handler {
 		BufferPool:   newBufferPool(),
 		ErrorHandler: g.errorHandler,
 		ModifyResponse: func(r *http.Response) error {
-			// This is only used for metrics, no modification to the response will be done
+			// since visitor and client shares the same port, proxied target may have
+			// alt-svc set in their response, which will either break peer quic,
+			// or the visitor won't be able to visit
+			r.Header.Del("alt-svc")
 			profiler.GatewayRequests.WithLabelValues("success", "forward").Add(1)
 			return nil
 		},