无法使用Java DatagramSocket捕获UDP广播数据包

Question

I am creating a client-server communication application and would like the client to be able to detect and connect to the server automatically, given that they are on the same network.

Previously, my code was working across a Linux and Windows machine. I would broadcast a simple message and it could be read. I could also see the message while watching network traffic with Wireshark.

The approach I am taking is to

1. Get the broadcast address(es) on the network on the server.
2. For a given duration, broadcast a message (soon to be the server IP)
3. On the client side, wait until a message is received.

I am quite new to networking, so any obvious errors may not be immediately obvious to me.

Server broadcast code:

public class Broadcaster {
    /* ... */
    public void pulse() throws InterruptedException, IOException, SocketException {
        Long elapsed = new Date().getTime();
        Long timeout = elapsed + this.duration;
        DatagramPacket packet = new DatagramPacket(this.message.getBytes(), this.message.length());
        HashSet<InetAddress> channels = Broadcaster.getBroadcastChannels();

        while(elapsed <= timeout) {
            for(InetAddress channel : channels) {
                DatagramSocket socket = new DatagramSocket(this.port);      
                socket.setBroadcast(true);
                socket.connect(channel, this.port);
                socket.send(packet);        
                System.out.println("Broadcast sent to " + channel.getHostAddress() + " (" + socket.getPort() + "): " + this.message);       
                socket.close();
            }   
            Thread.sleep(this.frequency);
            elapsed = new Date().getTime();
        }
    }

    private static HashSet<InetAddress> getBroadcastChannels() throws SocketException {
        /* Returns 192.168.0.255 */
    }

    public static void main(String[] args) {
        Broadcaster heart = new Broadcaster("Hello from the Raspberry Pi!", 120000, 5000, 8027);
        try {
            heart.pulse();
        } catch(SocketException e) {
            /* ...etc... */
        } finally {
            System.out.println("Broadcasting completed.");
        }
    }
}

Client code:

public class BroadcastListener {
    private int port;
    private int length;

    public BroadcastListener(int length, int port) {
        this.port = port;
        this.length = length;
    }

    public String getNext() throws IOException {
        byte buffer[] = new byte[this.length];  
        DatagramSocket socket = new DatagramSocket(this.port);
        DatagramPacket packet = new DatagramPacket(buffer, buffer.length);  
        System.out.println("Waiting on " + socket.getLocalSocketAddress()); 
        socket.receive(packet);
        socket.close();

        return new String(buffer);
    }

    public static void main(String[] args) {
        System.out.println("Listening for network broadcasts...");
        BroadcastListener broadcast = new BroadcastListener(128, 8027);

        try {
            System.out.println("Received broadcast: " + broadcast.getNext());
        } catch(IOException e) {
            System.out.println("Could not receive broadcasts:");
            System.out.println(e.getMessage());
        }
    }
}

The broadcast/netmask address as seen on both devices ifconfig output is netmask 255.255.255.0 broadcast 192.168.0.255

What confuses me the most is that Wireshark is still seeing the broadcast but when I run the client Java program, it just sits at socket.receive(packet);

Wireshark screenshot on Imgur

Both client & server are on port 8027. It is clear that the broadcaster is working, but the client broadcast listener is not. Does anyone have any idea what could be happening? Thanks!

Answer 1

As mentioned in the comments: Check your firewall :-)

Another thing i recognized was that if I sniff with wireshark, no other processes could receive that datagrams. After realizing that, I wrote a nodejs script, which exclusive=false by default but even that did not help. Maybe there is a kernel flag or something, that UDP datagrams cannot be 'consumed' by one process.

Answer 2

It looks like your server may be brodcasting on a different address than your client. Your client is not being assigned an InetAddress, try using this constructor for your socket in BroadcastListener

DatagramSocket socket = new DatagramSocket(new InetSocketAddress("192.168.0.255", this.port));

If that doesnt work you might try binding both your server and client to 127.0.0.1

Answer 3

You keep creating and destroying DatagramSockets . If the packet arrives at your host at a moment when you don't have a DatagramSocket bound to the port, it will be thrown away.

Create one DatagramSocket and leave it open for this life of this code.