Simple Packet Authentication client for xt_pknock

knockmac.py

#!/usr/bin/env python

import hashlib
import hmac
import re
import struct
import socket
from socket import AF_INET, SOCK_DGRAM
import sys
import urllib2
from time import time

_IPRE = re.compile(r'Current IP Address: (?P<addr>(?:(?:(?:2(?:5[0-5]|[0-4][0-9]))|[0-1]?[0-9]?[0-9])\.){3}(?:(?:(?:2(?:5[0-5]|[0-4][0-9]))|[0-1]?[0-9]?[0-9])))')

def _get_external_address():
    ufd = urllib2.urlopen('http://checkip.dyndns.org/')
    udata = ufd.readlines()
    ufd.close()
    assert len(udata) >= 1, "Empty reply from DynDNS"
    m = _IPRE.search(udata[0])
    assert m is not None, "Invalid reply from DynDNS: %r" % udata[0]
    return m.group('addr')

class Spa(object):
    def __init__(self, target, secret, myaddr=None):
        self.target = target
        self.secret = secret
        self.myaddr = myaddr if myaddr is not None else _get_external_address()

    def __gen_spa(self):
        epoch_mins = int(time() / 60)
        s = hmac.HMAC(self.secret, digestmod=hashlib.sha256)
        s.update(socket.inet_aton(self.myaddr))
        s.update(struct.pack("i", epoch_mins))
        return s.hexdigest() + '\0'

    def send(self):
        spa = self.__gen_spa()
        s = socket.socket(family=AF_INET, type=SOCK_DGRAM)
        s.sendto(spa, self.target)
        s.close()

def main(target, secret, addr=None):
    spa = Spa(target, secret, addr)
    print "Knocking %s:%d" % target
    spa.send()
    print "Done"

if __name__ == '__main__':
    if len(sys.argv) < 4:
        print >>sys.stderr, "Usage: %s target port secret [local_addr]" % sys.argv[0]
    else:
        taddr = socket.gethostbyname(sys.argv[1])
        tport = int(sys.argv[2])
        secret = sys.argv[3]
        addr = socket.gethostbyname(sys.argv[4]) if len(sys.argv) > 4 else None
        main((taddr, tport), secret, addr)