How to do a UDP multicast across the local network in c#?

Question

I am trying to get some simple UDP communication working on my local network.

All i want to do is do a multicast to all machines on the network

Here is my sending code

    public void SendMessage(string message)
    {
        var data = Encoding.Default.GetBytes(message);
        using (var udpClient = new UdpClient(AddressFamily.InterNetwork))
        {
            var address = IPAddress.Parse("224.100.0.1");
            var ipEndPoint = new IPEndPoint(address, 8088);
            udpClient.JoinMulticastGroup(address);
            udpClient.Send(data, data.Length, ipEndPoint);
            udpClient.Close();
        }
    }

and here is my receiving code

    public void Start()
    {
        udpClient = new UdpClient(8088);
        udpClient.JoinMulticastGroup(IPAddress.Parse("224.100.0.1"), 50);

        receiveThread = new Thread(Receive);
        receiveThread.Start();
    }

    public void Receive()
    {
        while (true)
        {
            var ipEndPoint = new IPEndPoint(IPAddress.Any, 0);
            var data = udpClient.Receive(ref ipEndPoint);

            Message = Encoding.Default.GetString(data);

            // Raise the AfterReceive event
            if (AfterReceive != null)
            {
                AfterReceive(this, new EventArgs());
            }
        }
    }

It works perfectly on my local machine but not across the network.

-Does not seem to be the firewall. I disabled it on both machines and it still did not work.

-It works if i do a direct send to the hard coded IP address of the client machine (ie not multicast).

Any help would be appreciated.

Answer 1

Does your local network hardware support IGMP?

It's possible that your switch is multicast aware, but if IGMP is disabled it won't notice if any attached hardware subscribes to a particular multicast group so it wouldn't forward those packets.

To test this, temporarily connect two machines directly together with a cross-over cable. That should (AFAICR) always work.

Also, it should be the server half of the code that has the TTL argument supplied to JoinMulticastGroup(), not the client half.

Answer 2

I've just spent 4 hours on something similar (I think), the solution for me was:

client.Client.Bind(new IPEndPoint(IPAddress.Any, SSDP_PORT));
client.JoinMulticastGroup(SSDP_IP,IP.ExternalIPAddresses.First());
client.MulticastLoopback = true;

Using a specific (first external) IP address on the multicast group.

Answer 3

I can't see a TTL specified anywhere in the code. Remember that TTL was originally meant to be in unit seconds, but is has become unit hops. This means that by using a clever TTL you could eliminate passing through the router. The default TTL on my machine is 32 - I think that should be more than adequate; but yours may actually be different (UdpClient.Ttl) if your system has been through any form of a security lockdown.

I can't recommend the TTL you need - as I personally need to do a lot of experimentation.

If that doesn't work, you could have a look at these articles:

• OSIX Article
• CodeProject Article

All-in-all it looks like there has been success with using Sockets and not UdpClients.

Your chosen multicast group could also be local-only. Try another one.

Your physical network layer could also be causing issues. I would venture to question switches and direct (x-over) connections. Hubs and all more intelligent should handle them fine. I don't have any literature to back that, however.