Python port forwarding/multiplexing server

Question

I would like to make server that listen on UDP port 162 (SNMP trap) and then forwards this traffic to multiple clients. Also important is that the source port & address stays same (address spoofing).

I guess that best tool for this would be Twisted or Scapy or maybe vanilla sockets, only I can't find anything in the documentation for Twisted about source address spoofing/forging.

Any solution for this?

Edit:added bounty, mybe any solution with iptables?

Answer 1

I am not comfortable with twisted or scapy, but it's quite straightforward to do this with vanilla python sockets. An extra advantage of that is that it will be even more portable. This code works in my limited tests:

#!/usr/bin/python
from socket import *
bufsize = 1024 # Modify to suit your needs
targetHost = "somehost.yourdomain.com"
listenPort = 1123

def forward(data, port):
    print "Forwarding: '%s' from port %s" % (data, port)
    sock = socket(AF_INET, SOCK_DGRAM)
    sock.bind(("localhost", port)) # Bind to the port data came in on
    sock.sendto(data, (targetHost, listenPort))

def listen(host, port):
    listenSocket = socket(AF_INET, SOCK_DGRAM)
    listenSocket.bind((host, port))
    while True:
        data, addr = listenSocket.recvfrom(bufsize)
        forward(data, addr[1]) # data and port

listen("localhost", listenPort)

Answer 2

The accepted answer didn't work for me and I end up using A simple TCP redirector in python :

#!/usr/bin/env python

import socket
import threading
import select
import sys

terminateAll = False

class ClientThread(threading.Thread):
    def __init__(self, clientSocket, targetHost, targetPort):
        threading.Thread.__init__(self)
        self.__clientSocket = clientSocket
        self.__targetHost = targetHost
        self.__targetPort = targetPort

    def run(self):
        print "Client Thread started"

        self.__clientSocket.setblocking(0)

        targetHostSocket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        targetHostSocket.connect((self.__targetHost, self.__targetPort))
        targetHostSocket.setblocking(0)

        clientData = ""
        targetHostData = ""
        terminate = False
        while not terminate and not terminateAll:
            inputs = [self.__clientSocket, targetHostSocket]
            outputs = []

            if len(clientData) > 0:
                outputs.append(self.__clientSocket)

            if len(targetHostData) > 0:
                outputs.append(targetHostSocket)

            try:
                inputsReady, outputsReady, errorsReady = select.select(inputs, outputs, [], 1.0)
            except Exception, e:
                print e
                break

            for inp in inputsReady:
                if inp == self.__clientSocket:
                    try:
                        data = self.__clientSocket.recv(4096)
                    except Exception, e:
                        print e

                    if data != None:
                        if len(data) > 0:
                            targetHostData += data
                        else:
                            terminate = True
                elif inp == targetHostSocket:
                    try:
                        data = targetHostSocket.recv(4096)
                    except Exception, e:
                        print e

                    if data != None:
                        if len(data) > 0:
                            clientData += data
                        else:
                            terminate = True

            for out in outputsReady:
                if out == self.__clientSocket and len(clientData) > 0:
                    bytesWritten = self.__clientSocket.send(clientData)
                    if bytesWritten > 0:
                        clientData = clientData[bytesWritten:]
                elif out == targetHostSocket and len(targetHostData) > 0:
                    bytesWritten = targetHostSocket.send(targetHostData)
                    if bytesWritten > 0:
                        targetHostData = targetHostData[bytesWritten:]

        self.__clientSocket.close()
        targetHostSocket.close()
        print "ClienThread terminating"

if __name__ == '__main__':
    if len(sys.argv) != 5:
        print 'Usage:\n\tpython SimpleTCPRedirector <host> <port> <remote host> <remote port>'
        print 'Example:\n\tpython SimpleTCPRedirector localhost 8080 www.google.com 80'
        sys.exit(0)     

    localHost = sys.argv[1]
    localPort = int(sys.argv[2])
    targetHost = sys.argv[3]
    targetPort = int(sys.argv[4])

    serverSocket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    serverSocket.bind((localHost, localPort))
    serverSocket.listen(5)
    print "Waiting for client..."
    while True:
        try:
            clientSocket, address = serverSocket.accept()
        except KeyboardInterrupt:
            print "\nTerminating..."
            terminateAll = True
            break
        ClientThread(clientSocket, targetHost, targetPort).start()

    serverSocket.close()