Add websocket browser_redirect_url config option

If an HTTP(S) request is received without the Upgrade: header, then if this new
option is set then we'll assume it's a browser and issue a redirect.

There's no flexibility based upon the passed in URL; this should help with
redirecting people to dashboards or other tooling.

server/opts.go

@@ -263,6 +263,12 @@ type WebsocketOpts struct {
 	// The host:port to advertise to websocket clients in the cluster.
 	Advertise string
 
+	// If someone visits the NATS server with a browser, they will send a GET
+	// without the Upgrade header.
+	// For convenience, you can supply a URL to be used as an HTTP redirect,
+	// instead of erroring out.
+	BrowserRedirectURL string
+
 	// If no user name is provided when a client connects, will default to the
 	// matching user from the global list of users in `Options.Users`.
 	NoAuthUser string
@@ -3453,6 +3459,8 @@ func parseWebsocket(v interface{}, o *Options, errors *[]error, warnings *[]erro
 			o.Websocket.JWTCookie = mv.(string)
 		case "no_auth_user":
 			o.Websocket.NoAuthUser = mv.(string)
+		case "browser_redirect_url":
+			o.Websocket.BrowserRedirectURL = mv.(string)
 		default:
 			if !tk.IsUsedVariable() {
 				err := &unknownConfigFieldErr{

server/websocket.go

@@ -529,7 +529,13 @@ func (s *Server) wsUpgrade(w http.ResponseWriter, r *http.Request) (*wsUpgradeRe
 	}
 	// Point 3.
 	if !wsHeaderContains(r.Header, "Upgrade", "websocket") {
-		return nil, wsReturnHTTPError(w, http.StatusBadRequest, "invalid value for header 'Upgrade'")
+		_, haveUpgrade := r.Header["Upgrade"]
+		if s.opts.Websocket.BrowserRedirectURL != "" && !haveUpgrade {
+			// We still have to return an error, to prevent the connection upgrade.
+			return nil, wsHTTPRedirectAndReturnError(w, s.opts.Websocket.BrowserRedirectURL)
+		} else {
+			return nil, wsReturnHTTPError(w, http.StatusBadRequest, "invalid value for header 'Upgrade'")
+		}
 	}
 	// Point 4.
 	if !wsHeaderContains(r.Header, "Connection", "Upgrade") {
@@ -645,6 +651,15 @@ func wsReturnHTTPError(w http.ResponseWriter, status int, reason string) error {
 	return err
 }
 
+// Send a redirect to a browser and return an error
+func wsHTTPRedirectAndReturnError(w http.ResponseWriter, redirectURL string) error {
+	err := fmt.Errorf("not a websocket request, issued redirect")
+	w.Header().Set("Sec-Websocket-Version", "13")
+	w.Header().Set("Location", redirectURL)
+	http.Error(w, http.StatusText(http.StatusFound), http.StatusFound)
+	return err
+}
+
 // If the server is configured to accept any origin, then this function returns
 // `nil` without checking if the Origin is present and valid.
 // Otherwise, this will check that the Origin matches the same origine or
@@ -761,6 +776,13 @@ func validateWebsocketOptions(o *Options) error {
 			return fmt.Errorf("trusted operators or trusted keys configuration is required for JWT authentication via cookie %q", wo.JWTCookie)
 		}
 	}
+	// Ensure that we're not given random junk which might have us do
+	// non-sensical things later.
+	if wo.BrowserRedirectURL != "" {
+		if _, err := url.Parse(wo.BrowserRedirectURL); err != nil {
+			return fmt.Errorf("unable to parse get redirect URL: %v", err)
+		}
+	}
 	return nil
 }