tcptee.py is a tee for TCP. It acts as a simple TCP proxy and dumps all traffic onto stdout.

tcptee.py

# A tee for TCP, similar to `socal -v`.
#
#           | server
# client ---|
#           | stdout

import socket
from select import select
import sys
import logging

class TcpTee:

    def __init__(self, source_port, destination):
        self.destination = destination

        self.teesock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        self.teesock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
        self.teesock.bind(('127.0.0.1', source_port))
        self.teesock.listen(200)

        # Linked client/server sockets in both directions
        self.channel = {}

    def run(self):
        while 1:
            inputready, outputready, exceptready = select([self.teesock] + self.channel.keys(), [], [])
            for s in inputready:
                if s == self.teesock:
                    self.on_accept()
                    break

                data = s.recv(4096)
                if not data:
                    self.on_close(s)
                    break

                self.on_recv(s, data)

    def on_accept(self):
        clientsock, clientaddr = self.teesock.accept()
        serversock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        try:
            serversock.connect(self.destination)
        except Exception:
            logging.exception('Could not connect to server %s. Closing connection to client %s' % (self.destination, clientaddr))
            clientsock.close()
        else:
            logging.info("%r has connected", clientaddr)
            self.channel[clientsock] = serversock
            self.channel[serversock] = clientsock

    def on_close(self, sock):
        logging.info("%s has disconnected", s.getpeername())
        othersock = self.channel[sock]

        sock.close()
        othersock.close()

        del self.channel[sock]
        del self.channel[othersock]

    def on_recv(self, sock, data):
        print data
        self.channel[sock].send(data)

if __name__ == '__main__':
    import argparse
    parser = argparse.ArgumentParser()
    parser.add_argument("listen_port", help="The port this process will listen on.", type=int)
    parser.add_argument("server_host", help="The remote host to connect to.")
    parser.add_argument("server_port", help="The remote port to connect to.", type=int)
    args = parser.parse_args()

    logging.basicConfig(stream=sys.stderr, level=logging.DEBUG)

    tee = TcpTee(int(args.listen_port), (args.server_host, int(args.server_port)))
    try:
        tee.run()
    except KeyboardInterrupt:
        logging.info("Ctrl C - Good Bye")
        sys.exit(1)

Based on http://voorloopnul.com/blog/a-python-proxy-in-less-than-100-lines-of-code/

Getting this error when I try to run your code
python3 testing.py 2001 4chan.org 80
Traceback (most recent call last):
File "testing.py", line 73, in
tee.run()
File "testing.py", line 21, in run
inputready, outputready, exceptready = select([self.teesock] + self.channel.keys(), [], [])
TypeError: can only concatenate list (not "dict_keys") to list

Fyi, I changed line 58 to print (data)

this is great. However i do have an odd question. Is it possible to combine this with scapy, as for a test i would like to print (as an example) for each received packet, not only the data but also the tcp headers (and after that some SSL data).

i've got it partly working:
at def on_recv(self): i've added
layer = IP(data)
layer.show()
but the output is totally incorrect

Thanks for writing this, very useful.

When I tried to run it in Python 3.4 I saw a couple of errors so I fixed those in a fork https://gist.github.com/shearichard/14c78eb4a8f1aea4791787ab1d14c8f9 - if you want to you can update yours.

Thanks again.