DNS A record lookups resolve to the host making the request.

echodns.py

#!/usr/bin/env python3

"""
Tiny Python DNS server that replies to all A record requests with the requester's IP.
"""

import socket
import sys

try:
    socket.SO_REUSEPORT
except AttributeError:
    socket.SO_REUSEPORT = 15

# DNSQuery class from http://code.activestate.com/recipes/491264-mini-fake-dns-server/
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 build_reply(self, ip):
        packet=''
        if ip == '': # Taken from crypt0s (https://github.com/Crypt0s/FakeDns/blob/master/fakedns.py)
            # Build the response packet         
            packet+=self.data[:2] + "\x81\x83"                         # Reply Code: No Such Name
                                                                       #0 answer rrs   0 additional, 0 auth
            packet+=self.data[4:6] + '\x00\x00' + '\x00\x00\x00\x00'   # Questions and Answers Counts
            packet+=self.data[12:]                                     # Original Domain Name Question
        
        if self.domain and packet == '':
            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('',[chr(int(x)) for x in ip.split('.')]) # 4bytes of IP
        return packet

def main():
    print('[+] PyEchoDNS')
    
    udps = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)

    #SO_REUSEPORT option allows multiple threads to bind to one port.
    # kernel >= 3.9 https://lwn.net/Articles/542629/
    try: 
        udps.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEPORT, 1)
    except socket.error:
        print('[!] SO_REUSEPORT not supported by your system.')
    
    udps.bind(('',53))
  
    try:
        while 1:
            data, src_addr = udps.recvfrom(1024)
            ip = src_addr[0]
            p=DNSQuery(data)
            
            udps.sendto(p.build_reply(ip), src_addr)
            print(('[+] Request from %s for %s' % (ip, p.domain)))
    except KeyboardInterrupt:
        print('[+] Closing')
        udps.close()

if __name__ == '__main__':
    main()