Tuesday Jul 16, 2013

WebSocket via HTTP proxy

As you might know, WebSocket can be used for bi-directional "real-time" communication with multiple clients. What does that mean in proxy environments and how this even works? WebSocket uses HTTP upgrade mechanism specified in HTTP 1.1 and by design requires open (TCP) connection.

HTTP CONNECT is there for exactly these usecases. It is usually used for tunneling HTTPS via proxy, but it can be used for WebSocket as well.

I will describe complete "proxified" handshake using captured connection of Tyrus Client connecting to public echo service - echo.websocket.org. Please note that we are directly using Tyrus API - not WebSocket specification (JSR-356), because we need to set a proxy.

final ClientManager client = ClientManager.createClient();

client.getProperties().put(
  GrizzlyClientSocket.PROXY_URI, "http://my.proxy:8080"
);
final Session session = client.connectToServer(new Endpoint() {

  @Override
  public void onOpen(Session session, EndpointConfig config) {
    session.addMessageHandler(new MessageHandler.Whole<String>() {
      @Override
      public void onMessage(String message) {
        System.out.println("# Message received: " + message);
      }
    });
  }
}, ClientEndpointConfig.Builder.create().build(),
   URI.create("ws://echo.websocket.org"));

session.getBasicRemote().sendText("test message");

BTW, Tyrus Client proxy support can be improved, currently it does not support proxy authentication, JDK's ProxySelector and so on. Please vote/comment on TYRUS-204 if you lack some of the mentioned options or anything else related to proxy support.

Current modern browsers do support this out of the box, so all you need is to set your HTTP proxy and described handshake will be done automatically. There might be limitation for parallel open connection in browser tab/instance, but I don't have any exact data about this.

Also, you might ask whether there is some need to server-side support - simple answer is "no". HTTP containers will see regular connection (from proxy), there is no additional work or overhead on that side.

Lets see our dumped communication:

client > proxy
CONNECT echo.websocket.org:80 HTTP/1.1
Host: echo.websocket.org
Proxy-Connection: keep-alive
Connection: keep-alive

Firstly, client need to send a request to proxy for new "permanent" connection. As already mentioned, CONNECT method handles this. First argument is a hostname (echo.websocket.org) and standard HTTP version.

proxy > client
HTTP/1.0 200 Connection established

If you are lucky, your proxy does support CONNECT and allows you to create connection (HTTP 200 is returned).

client > proxy
GET / HTTP/1.1
Connection: Upgrade
Host: echo.websocket.org
Origin: echo.websocket.org
Sec-WebSocket-Key : sDD3Wk7PMRCPE9+C0VyOcQ==
Sec-WebSocket-Version: 13
Upgrade: websocket

This is standard WebSocket handshake request, which will be passed to target HTTP container.

proxy > client
HTTP/1.1 101 Web Socket Protocol Handshake
Upgrade: WebSocket
Connection: Upgrade
Sec-WebSocket-Accept: 8TNIHr7bJHqQadjXYvqLql6RFEA=
Date: Tue, 16 Jul 2013 15:30:53 GMT
...

And there is a valid response to our handshake request, connection is established and communication can be started; there is nothing else different than in proxy-less environment. Please note that proxies do have limited resources and your request may be turned down because proxy "CONNECT" pool is empty.

Conclusion here is that WebSocket can work via proxies without any limitation, it just introduces different kind of traffic than pure HTTP and might cause some additional requirements related to proxy performance in case you are going to use WebSocket for long-running client connections.

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Pavel Bucek

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