server.py

#!/usr/bin/env python
"""UDP hole punching server."""
from twisted.internet.protocol import DatagramProtocol
from twisted.internet import reactor

import sys


class ServerProtocol(DatagramProtocol):
    """
    Server protocol implementation.

    Server listens for UDP messages. Once it receives a message it registers
    this client for a peer link in the waiting list.

    As soon as a second client connects, information about one client (public
    IP address and port) is sent to the other and vice versa.

    Those clients are now considered linked and removed from the waiting list.
    """

    def __init__(self):
        """Initialize with empy address list."""
        self.addresses = []

    def addressString(self, address):
        """Return a string representation of an address."""
        ip, port = address
        return ':'.join([ip, str(port)])

    def datagramReceived(self, datagram, address):
        """Handle incoming datagram messages."""
        if datagram == '0':
            print 'Registration from %s:%d' % address
            self.transport.write('ok', address)
            self.addresses.append(address)

            if len(self.addresses) >= 2:
                msg_0 = self.addressString(self.addresses[1])
                msg_1 = self.addressString(self.addresses[0])

                self.transport.write(msg_0, self.addresses[0])
                self.transport.write(msg_1, self.addresses[1])

                self.addresses.pop(0)
                self.addresses.pop(0)

                print 'Linked peers'

if __name__ == '__main__':
    if len(sys.argv) < 2:
        print "Usage: ./server.py PORT"
        sys.exit(1)

    port = int(sys.argv[1])
    reactor.listenUDP(port, ServerProtocol())
    print 'Listening on *:%d' % (port)
    reactor.run()