add parameter to limit max connections on WebSocket servers (fixes #84)

autobahn/websocket/protocol.py

@@ -643,7 +643,8 @@ class WebSocketProtocol:
                            'serveFlashSocketPolicy',
                            'flashSocketPolicy',
                            'allowedOrigins',
-                           'allowedOriginsPatterns']
+                           'allowedOriginsPatterns',
+                           'maxConnections']
     """
    Configuration attributes specific to servers.
    """
@@ -3037,22 +3038,30 @@ class WebSocketServerProtocol(WebSocketProtocol):
                 # noinspection PyUnboundLocalVariable
                 self._wskey = key
 
-            # WebSocket handshake validated => produce opening handshake response
-
-            # Now fire onConnect() on derived class, to give that class a chance to accept or deny
-            # the connection. onConnect() may throw, in which case the connection is denied, or it
-            # may return a protocol from the protocols provided by client or None.
+            # DoS protection
             #
-            request = ConnectionRequest(self.peer,
-                                        self.http_headers,
-                                        self.http_request_host,
-                                        self.http_request_path,
-                                        self.http_request_params,
-                                        self.websocket_version,
-                                        self.websocket_origin,
-                                        self.websocket_protocols,
-                                        self.websocket_extensions)
-            self._onConnect(request)
+            if self.maxConnections > 0 and self.factory.countConnections > self.maxConnections:
+
+                # maximum number of concurrent connections reached
+                #
+                self.failHandshake("maximum number of connections reached", code=http.SERVICE_UNAVAILABLE[0])
+
+            else:
+                # WebSocket handshake validated => produce opening handshake response
+                #
+                request = ConnectionRequest(self.peer,
+                                            self.http_headers,
+                                            self.http_request_host,
+                                            self.http_request_path,
+                                            self.http_request_params,
+                                            self.websocket_version,
+                                            self.websocket_origin,
+                                            self.websocket_protocols,
+                                            self.websocket_extensions)
+                # Now fire onConnect() on derived class, to give that class a chance to accept or deny
+                # the connection. onConnect() may throw, in which case the connection is denied, or it
+                # may return a protocol from the protocols provided by client or None.
+                self._onConnect(request)
 
         elif self.serveFlashSocketPolicy or self.debug:
             flash_policy_file_request = self.data.find(b"<policy-file-request/>\x00")
@@ -3522,6 +3531,9 @@ class WebSocketServerFactory(WebSocketFactory):
         self.allowedOrigins = ["*"]
         self.allowedOriginsPatterns = wildcards2patterns(self.allowedOrigins)
 
+        # maximum number of concurrent connections
+        self.maxConnections = 0
+
     def setProtocolOptions(self,
                            versions=None,
                            allowHixie76=None,
@@ -3544,7 +3556,8 @@ class WebSocketServerFactory(WebSocketFactory):
                            autoPingSize=None,
                            serveFlashSocketPolicy=None,
                            flashSocketPolicy=None,
-                           allowedOrigins=None):
+                           allowedOrigins=None,
+                           maxConnections=None):
         """
         Set WebSocket protocol options used as defaults for new protocol instances.
 
@@ -3595,6 +3608,8 @@ class WebSocketServerFactory(WebSocketFactory):
         :type flashSocketPolicy: str or None
         :param allowedOrigins: A list of allowed WebSocket origins (with '*' as a wildcard character).
         :type allowedOrigins: list or None
+        :param maxConnections: Maximum number of concurrent connections. Set to `0` to disable (default: `0`).
+        :type maxConnections: int or None
         """
         if allowHixie76 is not None and allowHixie76 != self.allowHixie76:
             self.allowHixie76 = allowHixie76
@@ -3671,6 +3686,11 @@ class WebSocketServerFactory(WebSocketFactory):
             self.allowedOrigins = allowedOrigins
             self.allowedOriginsPatterns = wildcards2patterns(self.allowedOrigins)
 
+        if maxConnections is not None and maxConnections != self.maxConnections:
+            assert(type(maxConnections) in six.integer_types)
+            assert(maxConnections >= 0)
+            self.maxConnections = maxConnections
+
     def getConnectionCount(self):
         """
         Get number of currently connected clients.

examples/twisted/websocket/echo/server.py

@@ -60,6 +60,7 @@ if __name__ == '__main__':
 
     factory = WebSocketServerFactory("ws://localhost:9000", debug=False)
     factory.protocol = MyServerProtocol
+    # factory.setProtocolOptions(maxConnections=2)
 
     reactor.listenTCP(9000, factory)
     reactor.run()