How to implement a UDP proxy for debugging network packet loss?

Question

I want to create a debugging solution that acts as a UDP proxy between a client and a server. The purpose of this proxy is to inspect and possibly log network traffic to diagnose issues like packet loss, without modifying the code on the client or server.

What I'm looking for:

How to implement such a UDP proxy using sockets in a programming language like Python, C++, or any other suitable language. An explanation of how the proxy would receive packets from the client and forward them to the server, and vice versa. Any potential challenges or considerations when implementing this, such as handling bidirectional communication, maintaining performance, or ensuring reliability.

What I've tried:

I have a basic understanding of how to use UDP sockets for sending and receiving packets, but I'm not sure how to handle the bidirectional nature of the proxy and how to ensure packets are properly relayed without corruption or unnecessary delay.

Example scenario:

Client sends UDP packets to Proxy (on some IP:Port). Proxy forwards these packets to the Server (on another IP:Port). Server's response is intercepted by Proxy and forwarded back to the Client. Any code snippets, tools, or resources to get started would be greatly appreciated!

Answer 1

Here is Python code written for this purpose:

import socket
from threading import Thread

class Proxy(Thread):
    """ used to proxy single udp connection 
    """
    BUFFER_SIZE = 4096 
    def __init__(self, listening_address, forward_address):
        print " Server started on", listening_address
        Thread.__init__(self)
        self.bind = listening_address
        self.target = forward_address

    def run(self):
        # listen for incoming connections:
        target = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
        target.connect(self.target)

        s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
        try:
            s.bind(self.bind)
        except socket.error, err:
            print "Couldn't bind server on %r" % (self.bind, )
            raise SystemExit
        while 1:
            datagram = s.recv(self.BUFFER_SIZE)
            if not datagram:
                break
            length = len(datagram)
            sent = target.send(datagram)
            if length != sent:
                print 'cannot send to %r, %r !+ %r' % (self.target, length, sent)
        s.close()


if __name__ == "__main__":
    LISTEN = ("0.0.0.0", 8008)
    TARGET = ("localhost", 5084)
    while 1:
        proxy = Proxy(LISTEN, TARGET)
        proxy.start()
        proxy.join()
        print ' [restarting] '

I used this two scripts to test it.

import socket

target = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
target.connect(("localhost", 8008))
print 'sending:', target.send("test data: 123456789")

and

import socket

s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
s.bind(("localhost", 5084))
while 1:
    datagram = s.recv(1024)
    if not datagram:
        break
    print repr(datagram)

s.close()