Q:

How to use multicast on a multi-homed system (Java, Linux)

This is in Java, but I can always revert to C via JNI if needed.
I have a system with two NICs, each connected to a distinct subnet. I want to use multicast (in particular, SDP) to discover other hosts on both networks.
One network is easy: create a MulticastSocket on the specified port, joinGroup it, and I get packets. Simplicity.
Two networks: so far impossible. I've tried:
1) creating two sockets, binding to the same port and using setInterface() or setNetworkInterface() to "connect" to the right interface. No luck, even after various permutations of setReuseAddress().
2) create a single socket, and then attempt to join twice, with two calls to joinGroup(SocketAddress mcastaddr, NetworkInterface netIf). The second join call fails.
Solutions outside of Java would be great. In particular, if I could set up multicast routes that would effectively 'combine' the two interfaces (I could then look at each packet to determine which network) that would be fine. As I mentioned before, any amount of native code is usable in this environment (Linux, with the Apache "luni" java infrastructure).
Thanks!

A:

Coincidentally, I was working on a similar problem recently.
Here's some Java code which does what you want -- it picks up SDP packets on several interfaces.  joinGroup is used to "attach" to the specified interfaces.
/**
 * Demonstrate multi-homed multicast listening
 *
 * usage: java Multihome eth0 eth1 lo <etc>
 */

import java.io.IOException;
import java.net.DatagramPacket;
import java.net.InetAddress;
import java.net.InetSocketAddress;
import java.net.MulticastSocket;
import java.net.NetworkInterface;
import java.net.SocketException;
import java.util.Enumeration;
import java.util.HashSet;
import java.util.Set;

public class Multihome {
    // SDP constants
    public static final String MULTICAST_ADDRESS = "239.255.255.250";
    public static final int MULTICAST_PORT = 1900;

    // args: each arg is the name of an interface.
    public void doMain(Set<String> args)
            throws Exception
    {
        InetSocketAddress socketAddress =
                new InetSocketAddress(MULTICAST_ADDRESS, MULTICAST_PORT);
        MulticastSocket socket = new MulticastSocket(MULTICAST_PORT);
        Enumeration<NetworkInterface> ifs =
                NetworkInterface.getNetworkInterfaces();

        while (ifs.hasMoreElements()) {
            NetworkInterface xface = ifs.nextElement();
            Enumeration<InetAddress> addrs = xface.getInetAddresses();
            String name = xface.getName();

            while (addrs.hasMoreElements()) {
                InetAddress addr = addrs.nextElement();

                System.out.println(name + " ... has addr " + addr);
            }

            if (args.contains(name)) {
                System.out.println("Adding " + name + " to our interface set");
                socket.joinGroup(socketAddress, xface);
            }
        }

        byte[] buffer = new byte[1500];
        DatagramPacket packet = new DatagramPacket(buffer, buffer.length);

        while (true) {
            try {
                packet.setData(buffer, 0, buffer.length);
                socket.receive(packet);
                System.out.println("Received pkt from " + packet.getAddress() +
                                   " of length " + packet.getLength());
            } catch (IOException ex) {
                ex.printStackTrace();
            }
        }
    }

    public static void main(String[] args)
            throws Exception
    {
        Set<String> argSet = new HashSet<String>();
        Multihome multi = new Multihome();

        for (String arg : args) {
            argSet.add(arg);
        }

        multi.doMain(argSet);
    }
}

