Problems with TCP hole punching

Question

I've tried to write a basic TCP hole puncher for a firewall in Python 3 using the principles outlined in this article. I'm having trouble getting anything to connect, though. Here is the code:

#!/usr/bin/python3

import sys
import socket
import _thread as thread

def client():
    c = socket.socket()

    c.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
    c.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEPORT, 1)

    c.bind((socket.gethostbyname(socket.gethostname()), int(sys.argv[3])))
    while(c.connect_ex((sys.argv[1], int(sys.argv[2])))):
        pass
    print("connected!")
    thread.interrupt_main()

def server():
    c = socket.socket()

    c.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
    c.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEPORT, 1)

    c.bind((socket.gethostbyname(socket.gethostname()), int(sys.argv[3])))
    c.listen(5)
    c.accept()
    print("connected!")
    thread.interrupt_main()

def main():
    thread.start_new_thread(client, ())
    thread.start_new_thread(server, ())

    while True:
        pass

if __name__ == '__main__':
    main()

I decided to try the puncher on my local machine, so that I could capture all the traffic sent by both instances. I first set up a loopback firewall:

iptables -A INPUT -i lo -m state --state RELATED,ESTABLISHED -j ACCEPT
iptables -A INPUT -i lo -j DROP

Then I launched two copies of the script:

left$ ./tcphole.py localhost 20012 20011

and

right$ ./tcphole.py localhost 20011 20012

I can see according to Wireshark that the SYN packets are being transmitted both ways:

But nothing ever prints "connected!" What am I doing wrong?

Answer 1

The answer turned out to be quite simple: packets aren't considered RELATED if they aren't coming to the same IP address!

Changing the bind lines to

c.bind('', int(sys.argv[3])))

(the '' binds to the loopback address) fixes the problem entirely.