dnsserver.py

# -*- coding: utf-8 -*-
# vim: set expandtab tabstop=4 shiftwidth=4 :

## {{{ http://code.activestate.com/recipes/491264/ (r4)
import socket

class DNSQuery:
    def __init__(self, data):
        self.data=data
        self.domain=''

        tipo = (ord(data[2]) >> 3) & 15   # Opcode bits
        if tipo == 0:                     # Standard query
            ini=12
            lon=ord(data[ini])
            while lon != 0:
                self.domain+=data[ini+1:ini+lon+1]+'.'
                ini+=lon+1
                lon=ord(data[ini])

    def response(self, ip):
        packet=''
        if self.domain:
            packet+=self.data[:2] + "\x81\x80"
            packet+=self.data[4:6] + self.data[4:6] + '\x00\x00\x00\x00'   # Questions and Answers Counts
            packet+=self.data[12:]                                         # Original Domain Name Question
            packet+='\xc0\x0c'                                             # Pointer to domain name
            packet+='\x00\x01\x00\x01\x00\x00\x00\x3c\x00\x04'             # Response type, ttl and resource data length -> 4 bytes
            packet+=str.join('',map(lambda x: chr(int(x)), ip.split('.'))) # 4bytes of IP
        return packet

if __name__ == '__main__':
    ip='192.168.1.1'
    print 'DistDNS:: dom.query. 60 IN A %s' % ip
  
    udps = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
    udps.bind(('',53))
  
    try:
        while 1:
            data, addr = udps.recvfrom(1024)
            p=DNSQuery(data)
            udps.sendto(p.response(ip), addr)
            print 'Response: %s -> %s' % (p.domain, ip)
    except KeyboardInterrupt:
        print 'Finished'
        udps.close()
## end of http://code.activestate.com/recipes/491264/ }}}