Add ability to disable HTTP/2 in dynamic config

pkg/server/service/roundtripper.go

@@ -100,7 +100,7 @@ func (r *RoundTripperManager) Get(name string) (http.RoundTripper, error) {
 
 // createRoundTripper creates an http.RoundTripper configured with the Transport configuration settings.
 // For the settings that can't be configured in Traefik it uses the default http.Transport settings.
-// An exception to this is the MaxIdleConns setting as we only provide the option MaxIdleConnsPerHostin Traefik at this point in time.
+// An exception to this is the MaxIdleConns setting as we only provide the option MaxIdleConnsPerHost in Traefik at this point in time.
 // Setting this value to the default of 100 could lead to confusing behavior and backwards compatibility issues.
 func createRoundTripper(cfg *dynamic.ServersTransport) (http.RoundTripper, error) {
 	if cfg == nil {
@@ -127,15 +127,6 @@ func createRoundTripper(cfg *dynamic.ServersTransport) (http.RoundTripper, error
 		WriteBufferSize:       64 * 1024,
 	}
 
-	transport.RegisterProtocol("h2c", &h2cTransportWrapper{
-		Transport: &http2.Transport{
-			DialTLS: func(netw, addr string, cfg *tls.Config) (net.Conn, error) {
-				return net.Dial(netw, addr)
-			},
-			AllowHTTP: true,
-		},
-	})
-
 	if cfg.ForwardingTimeouts != nil {
 		transport.ResponseHeaderTimeout = time.Duration(cfg.ForwardingTimeouts.ResponseHeaderTimeout)
 		transport.IdleConnTimeout = time.Duration(cfg.ForwardingTimeouts.IdleConnTimeout)
@@ -150,6 +141,20 @@ func createRoundTripper(cfg *dynamic.ServersTransport) (http.RoundTripper, error
 		}
 	}
 
+	// Return directly HTTP/1.1 transport when HTTP/2 is disabled
+	if cfg.DisableHTTP2 {
+		return transport, nil
+	}
+
+	transport.RegisterProtocol("h2c", &h2cTransportWrapper{
+		Transport: &http2.Transport{
+			DialTLS: func(netw, addr string, cfg *tls.Config) (net.Conn, error) {
+				return net.Dial(netw, addr)
+			},
+			AllowHTTP: true,
+		},
+	})
+
 	return newSmartRoundTripper(transport)
 }
 

pkg/server/service/roundtripper_test.go

@@ -227,3 +227,69 @@ func TestMTLS(t *testing.T) {
 
 	assert.Equal(t, http.StatusOK, resp.StatusCode)
 }
+
+func TestDisableHTTP2(t *testing.T) {
+	testCases := []struct {
+		desc          string
+		disableHTTP2  bool
+		serverHTTP2   bool
+		expectedProto string
+	}{
+		{
+			desc:          "HTTP1 capable client with HTTP1 server",
+			disableHTTP2:  true,
+			expectedProto: "HTTP/1.1",
+		},
+		{
+			desc:          "HTTP1 capable client with HTTP2 server",
+			disableHTTP2:  true,
+			serverHTTP2:   true,
+			expectedProto: "HTTP/1.1",
+		},
+		{
+			desc:          "HTTP2 capable client with HTTP1 server",
+			expectedProto: "HTTP/1.1",
+		},
+		{
+			desc:          "HTTP2 capable client with HTTP2 server",
+			serverHTTP2:   true,
+			expectedProto: "HTTP/2.0",
+		},
+	}
+
+	for _, test := range testCases {
+		test := test
+		t.Run(test.desc, func(t *testing.T) {
+			t.Parallel()
+
+			srv := httptest.NewUnstartedServer(http.HandlerFunc(func(rw http.ResponseWriter, req *http.Request) {
+				rw.WriteHeader(http.StatusOK)
+			}))
+
+			srv.EnableHTTP2 = test.serverHTTP2
+			srv.StartTLS()
+
+			rtManager := NewRoundTripperManager()
+
+			dynamicConf := map[string]*dynamic.ServersTransport{
+				"test": {
+					DisableHTTP2:       test.disableHTTP2,
+					InsecureSkipVerify: true,
+				},
+			}
+
+			rtManager.Update(dynamicConf)
+
+			tr, err := rtManager.Get("test")
+			require.NoError(t, err)
+
+			client := http.Client{Transport: tr}
+
+			resp, err := client.Get(srv.URL)
+			require.NoError(t, err)
+
+			assert.Equal(t, http.StatusOK, resp.StatusCode)
+			assert.Equal(t, test.expectedProto, resp.Proto)
+		})
+	}
+}