Using UDP broadcast for pods/peers discovery in Kubernetes

Question

I need to use UDP broadcast for peer discovery.

Environment:

• docker-desktop with a single node Kubernetes cluster

My code looks as follows:

import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class MainApp {

    public static void main(String[] args) throws ExecutionException, InterruptedException {
        int inPort = Integer.parseInt(System.getenv("IN_PORT"));
        int outPort = Integer.parseInt(System.getenv("OUT_PORT"));
        String name = System.getenv("NAME");
        Client client = new Client(name, outPort);
        Server server = new Server(name, inPort);

        ExecutorService service = Executors.newFixedThreadPool(2);
        service.submit(client);
        service.submit(server).get();
    }


    static class Client implements Runnable {
        final String name;
        final int port;

        Client(String name, int port) {
            this.name = name;
            this.port = port;
        }

        @Override
        public void run() {
            System.out.println(name + " client started, port = " + port);
            try (DatagramSocket socket = new DatagramSocket()) {
                socket.setBroadcast(true);
                while (!Thread.currentThread().isInterrupted()) {
                    byte[] buffer = (name + ": hi").getBytes();

                    DatagramPacket packet = new DatagramPacket(buffer, buffer.length,
                            InetAddress.getByName("255.255.255.255"), port);
                    socket.send(packet);
                    Thread.sleep(1000);
                    System.out.println("packet sent");
                }
            } catch (Exception e) {
                throw new RuntimeException(e);
            }
        }
    }


    static class Server implements Runnable {
        final String name;
        final int port;

        Server(String name, int port) {
            this.name = name;
            this.port = port;
        }

        @Override
        public void run() {

            System.out.println(name + " server started, port = " + port);

            try (DatagramSocket socket = new DatagramSocket(port)) {

                byte[] buf = new byte[256];
                while (!Thread.currentThread().isInterrupted()) {
                    DatagramPacket packet = new DatagramPacket(buf, buf.length);
                    socket.receive(packet);
                    String received = new String(packet.getData(), 0, packet.getLength());

                    System.out.println(String.format(name + " received '%s' from %s:%d", received,
                            packet.getAddress().toString(),
                            packet.getPort()));
                }
            } catch (Exception e) {
                throw new RuntimeException(e);
            }
        }

    }

}

Kubernetes pod settings:

For peer-1:

    spec:
      containers:
        - name: p2p
          image: p2p:1.0-SNAPSHOT
          env:
          - name: NAME
            value: "peer-1"
          - name: IN_PORT
            value: "9996"
          - name: OUT_PORT
            value: "9997"

For peer-2 :

    spec:
      containers:
        - name: p2p-2
          image: p2p:1.0-SNAPSHOT
          env:
          - name: NAME
            value: "peer-2"
          - name: IN_PORT
            value: "9997"
          - name: OUT_PORT
            value: "9996"

I used a different in/out ports for simplicity's sake. In reality, it should be the same port, e.g.: 9999

I see that each pod has a unique IP address

kubectl get pods -o wide

NAME                                READY   STATUS    RESTARTS   AGE     IP          NODE             NOMINATED NODE   READINESS GATES
p2p-deployment-2-59bb89f9d6-ghclv   1/1     Running   0          2m26s   10.1.0.38   docker-desktop   <none>           <none>
p2p-deployment-567bb5bd77-5cnsl     1/1     Running   0          2m29s   10.1.0.37   docker-desktop   <none>           <none>

Logs from peer-1:

peer-1 received 'peer-2: hi' from /10.1.0.1:57565

Logs from peer-2:

peer-2 received 'peer-1: hi' from /10.1.0.1:44777

Question: why peer-1 receives UDP packets from 10.1.0.1 instead of 10.1.0.37 ?

If I log into peer-2 container: kubectl exec -it p2p-deployment-2-59bb89f9d6-ghclv -- /bin/bash

Then

socat - UDP-DATAGRAM:255.255.255.255:9996,broadcast
test
test
...

in peer-1 logs I see peer-1 received 'test' from /10.1.0.1:43144. Again why network address is 10.1.0.1 instead of 10.1.0.37.

Could you please tell me what I'm doing wrong?

Note: when using the same port to send/receive UDP packets, some peer can receive a packet from its own IP address. In other words, a peer can only discover its own IP address but always gets 10.1.0.1 for packets received from other peers/pods

Answer 1

For some reason, UDP broadcast doesn't work as expected in Kubernetes infrastructure, however multicast works fine.

Thanks Ron Maupin for suggesting multicast.

Here you can find java code + kube config