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  [Resolved] Running Client & Introducer behind same router  (Read 1232 times)
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Offline Alan_W

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Medals: 8
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Java tames rock!

« Posted 2005-07-31 21:17:37 »

I've added Net Play to my SharpShooter game.  It features UDP Punchthrough and Peer-Peer gameplay.  I've written an Introducer to help the clients find each other.

It's getting close to deployment, but I've hit a snag.  The client does a DNS lookup to find my home IP address via DynDns.  It then sends a UDP packet to port 15000 and waits for the reply (a list of players IP:Port addresses).

However if the client is behind the same router as the Introducer, the router does loopback and the query packet arriving at the Introducer has a source IP:Port on the routers Internal IP address.  Currently, it just adds that address to the client list.

However, an external client out on the internet now pings the Introducer for the Client list.  That list, when received has the unreachable private IP:Port combo on my router, rather than an external IP:Port.

After much gnashing of teeth, it occurred to me that I could forward another port to provide a fixed external IP:Port on my router.  The Introducer now checks the address of each incoming query packet.  If it is in the private address space range, it does a DNS lookup to obtain the router's external IP and assigns a fixed port (which has to be port forwarded).

All well and good.  However this means that the client must do all its Tx/Rx on a fixed port (which can't be the server port, as that is already port forwarded to the Introducer).  Port 15001 seemed reasonable.

Now comes the problem.  I am currently opening a UDP port with

            channel =;

There isn't any obvious way to get the channel opened on a specific port.

I could create a DatagramSocket directly, which I can bind to a specific port, but then I'm stuck with blocking I/O.

Hopefully there's a way round this, as writing a multiplayer game that I can't play myself sucks rather. Shocked

Alan  Huh


On looking afresh this morning, the answer appears to be:
            socketAddress = InetSocketAddress(InetAddress.getLocalHost(), 15001);
            channel =;

Still need to fix the introducer code.  Also the above modification stops me running two clients on the same computer during testing.  Maybe pass an argument to force a fixed socket.  Anyway getting there now Smiley

Time flies like a bird. Fruit flies like a banana.
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