Multicast behavior on different platforms

This is a result table of MulticastSocket behavior investigation.

The first column in the table says if the address used was a loopback one. Next 3 columns shows parameters:

• was loopbackMode requested?
• was setInterface() called?
• was a host used in MultiSocket.bind()?

The last 4 columns show when the multicast socket was able to receive a datagram sent from the same host.

address	setLoopbackMode to true	setInterface called	bind to explicit InetAddress		Solaris	Mac OS	Linux	Windows
non‑loopback	FALSE	FALSE	FALSE		FALSE	FALSE	FALSE	FALSE
non‑loopback	FALSE	FALSE	TRUE		FALSE	FALSE	FALSE	FALSE
non‑loopback	FALSE	TRUE	FALSE		FALSE	TRUE	FALSE	FALSE
non‑loopback	FALSE	TRUE	TRUE		FALSE	FALSE	FALSE	FALSE
non‑loopback	TRUE	FALSE	FALSE		TRUE	TRUE	TRUE	TRUE
non‑loopback	TRUE	FALSE	TRUE		FALSE	FALSE	FALSE	TRUE
non‑loopback	TRUE	TRUE	FALSE		TRUE	TRUE	TRUE	TRUE
non‑loopback	TRUE	TRUE	TRUE		FALSE	FALSE	FALSE	FALSE
loopback	FALSE	FALSE	FALSE		FALSE	FALSE	FALSE	FALSE
loopback	FALSE	FALSE	TRUE		FALSE	FALSE	FALSE	FALSE
loopback	FALSE	TRUE	FALSE		FALSE	FALSE	FALSE	FALSE
loopback	FALSE	TRUE	TRUE		FALSE	FALSE	FALSE	FALSE
loopback	TRUE	FALSE	FALSE		TRUE	TRUE	TRUE	TRUE
loopback	TRUE	FALSE	TRUE		FALSE	FALSE	FALSE	FALSE
loopback	TRUE	TRUE	FALSE		FALSE	TRUE	FALSE	TRUE
loopback	TRUE	TRUE	TRUE		FALSE	FALSE	FALSE	TRUE

So, in tests, Solaris and Linux behaved in the same way. Mac OS and Windows required different parameters to be configured. Don't forget to use -Djava.net.preferIPv4Stack=true vm option with Mac OS.

I used just 1 machine for each tested OS, so there may be other environmental effects I didn't notice.

Sources for the testing:

import java.net.DatagramPacket;
import java.net.Inet4Address;
import java.net.InetAddress;
import java.net.InetSocketAddress;
import java.net.MulticastSocket;
import java.net.NetworkInterface;
import java.net.SocketTimeoutException;
import java.net.UnknownHostException;
import java.time.LocalDateTime;
import java.util.Arrays;
import java.util.Collections;
import java.util.UUID;
import java.util.concurrent.TimeUnit;

public class App {

    private static final int DATAGRAM_BUFFER_SIZE = 64 * 1024;
    private static final int PORT = 54327;
    private static final InetAddress GROUP_ADDRESS;

    static {
        try {
            GROUP_ADDRESS = InetAddress.getByName("224.2.2.3");
        } catch (UnknownHostException e) {
            throw new RuntimeException(e);
        }
    }

    final InetAddress bindAddress;
    final boolean isLoopbackMode;
    final boolean isSetInterface;
    final boolean isSetBindAddress;

    final String uuid;

    private volatile boolean received;

    public App(InetAddress bindAddress, boolean isLoopbackMode, boolean isSetInterface, boolean isSetBindAddress) {
        this.bindAddress = bindAddress;
        this.isLoopbackMode = isLoopbackMode;
        this.isSetInterface = isSetInterface;
        this.isSetBindAddress = isSetBindAddress;
        this.uuid = UUID.randomUUID().toString();
    }

    public static void main(String[] args) throws Exception {
        // if (args == null || args.length != 4) {
        // System.err.println("Unexpected number of arguments");
        // System.err.println("Usage:");
        // System.err.println("\tjava -jar app.jar [bindAddress] [loopbackModeEnabled] [setInterface] [setBindAddress]");
        // System.exit(2);
        // }
        // InetAddress bindAddress = InetAddress.getByName(args[0]);
        // boolean isLoopbackMode = Boolean.parseBoolean(args[1]);
        // boolean isSetInterface = Boolean.parseBoolean(args[2]);
        // boolean isSetBindAddress = Boolean.parseBoolean(args[3]);
        // App app = new App(bindAddress, isLoopbackMode, isSetInterface, isSetBindAddress);
        for (NetworkInterface ni : Collections.list(NetworkInterface.getNetworkInterfaces())) {
            boolean skip = ni.isVirtual() || !ni.isUp();
            if (skip) continue;
            for (InetAddress ia : Collections.list(ni.getInetAddresses())) {
                if (ia instanceof Inet4Address) {
                    for (boolean loopbackMode : Arrays.asList(false, true)) {
                        for (boolean setInterface : Arrays.asList(false, true)) {
                            for (boolean setBindAddr : Arrays.asList(false, true)) {
                                System.out.println("========================================");
                                App app = new App(ia, loopbackMode, setInterface, setBindAddr);
                                Thread sender = app.sender();
                                System.out.println(app.received()
                                        + "\t" + app.bindAddress.getHostAddress()
                                        + "\t" + app.isLoopbackMode
                                        + "\t" + app.isSetInterface
                                        + "\t" + app.isSetBindAddress
                                        + "\t" + app.uuid
                                );
                                Thread.sleep(2000);
                                sender.interrupt();
                                Thread.sleep(2000);
                                System.out.println("========================================");
                            }
                        }
                    }
                }
            }
        }
    }

    private MulticastSocket configureSocket() throws Exception {
        MulticastSocket multicastSocket = new MulticastSocket(null);
        multicastSocket.setReuseAddress(true);
        multicastSocket.bind(isSetBindAddress ? new InetSocketAddress(bindAddress, PORT) : new InetSocketAddress(PORT));

        multicastSocket.setLoopbackMode(!isLoopbackMode);
        if (isSetInterface) {
            multicastSocket.setInterface(bindAddress);
        }
        multicastSocket.joinGroup(GROUP_ADDRESS);
        return multicastSocket;
    }

    public Thread sender() {
        Thread sender = new Thread(() -> {
            try (MulticastSocket multicastSocket = configureSocket()) {
                byte[] packetData = uuid.getBytes();
                DatagramPacket packet = new DatagramPacket(packetData, packetData.length, GROUP_ADDRESS, PORT);
                while (!received) {
//                    System.out.println("Sending");
                    multicastSocket.send(packet);
                    TimeUnit.SECONDS.sleep(1);
                }
            } catch (InterruptedException ie) {
                System.out.println("Failed sending: " + uuid);
            } catch (Exception e) {
                e.printStackTrace();
            }
        });
        sender.start();
        return sender;
    }

    public boolean received() {
        try (MulticastSocket multicastSocket = configureSocket()) {
            DatagramPacket datagramPacketReceive = new DatagramPacket(new byte[DATAGRAM_BUFFER_SIZE], DATAGRAM_BUFFER_SIZE);
            multicastSocket.setSoTimeout(1000);
            LocalDateTime end = LocalDateTime.now().plusSeconds(5);
            while (LocalDateTime.now().isBefore(end)) {
                try {
                    multicastSocket.receive(datagramPacketReceive);
                    String receivedStr = new String(datagramPacketReceive.getData(), datagramPacketReceive.getOffset(),
                            datagramPacketReceive.getLength());
                    System.out.println("Received: '" + receivedStr + "' from " + datagramPacketReceive.getSocketAddress() + "("
                            + (uuid.equals(receivedStr) ? "self" : "other") + ")");
                    if (!received) {
                        received = uuid.equals(receivedStr);
                    }
                    if (received) {
                        return true;
                    }
                } catch (SocketTimeoutException ignored) {
                }
            }
            multicastSocket.leaveGroup(GROUP_ADDRESS);
        } catch (Exception e) {
            e.printStackTrace();
        }
        return false;
    }
}

This page was originally a GH issue comment: https://github.com/hazelcast/hazelcast/pull/19251#issuecomment-891375270