Sending UDP from Java to Python not working

Question

Goal

I am trying to send data over UDP from a Java server to a Python client on the same machine.

Environment

• Device/OS: Jetson nano with Ubuntu 18.04.3 LTS (Bionic Beaver)
• JavaC version: javac 11.0.4
• Java version:
  • openjdk version "11.0.4" 2019-07-16
  • OpenJDK Runtime Environment (build 11.0.4+11-post-Ubuntu-1ubuntu218.04.3)
  • OpenJDK 64-Bit Server VM (build 11.0.4+11-post-Ubuntu-1ubuntu218.04.3, mixed mode)

Problem

I can send data from a Python test server, see code below, to the Python test client just fine. However, If I try to send data from the Java test server to the Python test client, nothing seems to arrive. The Java server doesn't throw an exception.

Test client in Python (working)

import socket

UDP_IP = "localhost"
UDP_PORT = 9999

sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
sock.bind((UDP_IP, UDP_PORT))

print("listening...")

while True:
    data, addr = sock.recvfrom(1024)
    print("received message from: ", addr)
    print("payload: ", data)

Test server in Python (working)

import socket

sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
sock.sendto("Hello World", ("localhost", 9999))

Test server in Java (not working or doesn't reach the python client)

import java.io.*;
import java.net.*;

public class TestSender {
    public static void main(String[] args) {
        try {
            byte[]         data    = "Hello world".getBytes();
            int            port    = 9999;
            InetAddress    address = InetAddress.getLocalHost();
            DatagramPacket packet  = new DatagramPacket(data, data.length, address, port);
            DatagramSocket socket  = new DatagramSocket();

            socket.send(packet);
            System.out.println("Data sent");

            socket.close();

        } catch (Exception e) {
            System.out.println("Something went wrong");
        }
    }
}

Answer 1

Issue

InetAddress address = InetAddress.getLocalHost();

returns the address of one of the interfaces of the machine.

Solution

As @Gomiero suggested, using either

InetAddress address = InetAddress.getByName("127.0.0.1");
// or
InetAddress address = InetAddress.getByName("localhost");

solves the problem because in this case the client bound to 127.0.0.1 and does not accept packets from any other IP address.

Note

As @user207421 suggested in most cases the proper solution to the problem would be to bind the client to 0.0.0.0 like this:

UDP_IP = "0.0.0.0"
UDP_PORT = 9999
sock.bind((UDP_IP, UDP_PORT))

Binding the client to 0.0.0.0 resolves the issue with using InetAddress.getLocalHost() on the server-side because it makes the client accept data to any of its IP addresses.

However, I finally decided to bind the client to 127.0.0.1 now with the proper understanding of what that does, because I only want data from the local machine.