[v13] Add missing Connection header for ALPN connection upgrade

api/client/alpn_conn_upgrade.go

@@ -195,6 +195,18 @@ func upgradeConnThroughWebAPI(conn net.Conn, api url.URL, upgradeType string) (n
 
 	req.Header.Add(constants.WebAPIConnUpgradeHeader, upgradeType)
 
+	// Set "Connection" header to meet RFC spec:
+	// https://datatracker.ietf.org/doc/html/rfc2616#section-14.42
+	// Quote: "the upgrade keyword MUST be supplied within a Connection header
+	// field (section 14.10) whenever Upgrade is present in an HTTP/1.1
+	// message."
+	//
+	// Some L7 load balancers/reverse proxies like "ngrok" and "tailscale"
+	// require this header to be set to complete the upgrade flow. The header
+	// must be set on both the upgrade request here and the 101 Switching
+	// Protocols response from the server.
+	req.Header.Add(constants.WebAPIConnUpgradeConnectionHeader, constants.WebAPIConnUpgradeConnectionType)
+
 	// Send the request and check if upgrade is successful.
 	if err = req.Write(conn); err != nil {
 		return nil, trace.Wrap(err)

api/client/alpn_conn_upgrade_test.go

@@ -240,6 +240,7 @@ func mockConnUpgradeHandler(t *testing.T, upgradeType string, write []byte) http
 	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		require.Equal(t, constants.WebAPIConnUpgrade, r.URL.Path)
 		require.Equal(t, upgradeType, r.Header.Get(constants.WebAPIConnUpgradeHeader))
+		require.Equal(t, constants.WebAPIConnUpgradeConnectionType, r.Header.Get(constants.WebAPIConnUpgradeConnectionHeader))
 
 		hj, ok := w.(http.Hijacker)
 		require.True(t, ok)

api/constants/constants.go

@@ -413,4 +413,11 @@ const (
 	// long-lived connections alive as L7 LB usually ignores TCP keepalives and
 	// has very short idle timeouts.
 	WebAPIConnUpgradeTypeALPNPing = "alpn-ping"
+	// WebAPIConnUpgradeConnectionHeader is the standard header that controls
+	// whether the network connection stays open after the current transaction
+	// finishes.
+	WebAPIConnUpgradeConnectionHeader = "Connection"
+	// WebAPIConnUpgradeConnectionType is the value of the "Connection" header
+	// used for connection upgrades.
+	WebAPIConnUpgradeConnectionType = "Upgrade"
 )

lib/web/conn_upgrade.go

@@ -130,6 +130,7 @@ func (h *Handler) startPing(ctx context.Context, pingConn *pingconn.PingConn) {
 func writeUpgradeResponse(w io.Writer, upgradeType string) error {
 	header := make(http.Header)
 	header.Add(constants.WebAPIConnUpgradeHeader, upgradeType)
+	header.Add(constants.WebAPIConnUpgradeConnectionHeader, constants.WebAPIConnUpgradeConnectionType)
 	response := &http.Response{
 		Status:     http.StatusText(http.StatusSwitchingProtocols),
 		StatusCode: http.StatusSwitchingProtocols,

lib/web/conn_upgrade_test.go

@@ -133,6 +133,8 @@ func sendConnUpgradeRequest(t *testing.T, h *Handler, upgradeType string, server
 	io.Copy(io.Discard, resp.Body)
 	_ = resp.Body.Close()
 
+	require.Equal(t, upgradeType, resp.Header.Get(constants.WebAPIConnUpgradeHeader))
+	require.Equal(t, constants.WebAPIConnUpgradeConnectionType, resp.Header.Get(constants.WebAPIConnUpgradeConnectionHeader))
 	require.Equal(t, http.StatusSwitchingProtocols, resp.StatusCode)
 }