Fake DNS Server

fakedns.py

#!/bin/env python

import sys
import getopt
import socket

def response(data, ancount):
    # id and flags
    packet = data[:2] + '\x81\x80'
    # counts
    packet = ''.join([data[4:6], chr(ancount>>8), chr(ancount&0xff), '\x00\x00\x00\x00'])
    # original question
    packet += data[12:]
    # answers
    for i in range(ancount):
        # pointer to domain name
        packet += '\xc0\x0c'
        # response type, ttl, resource length (4 bytes), resource
        packet += '\x00\x01\x00\x01\x00\x00\x00\x3c\x00\x04'
        packet += ''.join([chr(x) for x in [192, 168, 0, i%256]])
    # id and flags (optional truncation)
    return data[:2] + ('\x83\x80' if len(packet) > 508 else '\x81\x80') + packet

if __name__ == '__main__':
    try:
        opts, args = getopt.getopt(sys.argv[1:], 'h:p:a:')
    except getopt.GetoptError, err:
        print(str(err))
        print('usage: fakedns.py [-h HOST] [-p PORT] [-a ANCOUNT]')
        sys.exit(2)
    host = 'localhost'
    port = 50053
    ancount = 42
    for o, a in opts:
        if o == '-h':
            host = a
        elif o == '-p':
            port = int(a)
        elif o == '-a':
            ancount = int(a)
    print('listen on {0}:{1} (ancount: {2})'.format(host, port, ancount))
    udp_sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
    udp_sock.bind((host, port))
    tcp_sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
    tcp_sock.bind((host, port))
    tcp_sock.listen(1)
    try:
        while True:
            data, addr = udp_sock.recvfrom(1024)
            print('[UDP] received {0} bytes from {1}'.format(addr, len(data)))
            packet = response(data, ancount)
            print('[UDP] send {0} bytes to {1}'.format(len(packet), addr))
            udp_sock.sendto(packet, addr)
            if len(packet) > 512:
                conn, addr = tcp_sock.accept()
                data = conn.recv(1024)[2:]
                print('[TCP] received {0} bytes from {1}'.format(addr, len(data)))
                packet = response(data, ancount)
                l = len(packet)
                print('[TCP] send {0} bytes to {1}'.format(l, addr))
                conn.send(''.join([chr(l>>8), chr(l&0xff), packet]))
                conn.close()
    except KeyboardInterrupt:
        print('exiting')
    udp_sock.close()
    tcp_sock.close()