DNS server that always responds the same address

dnsd.py

import socket
import struct

respond_addr = '127.0.0.1'

def parse_qname(qname):
    labels = []
    while qname:
        length = ord(qname[0])
        label = qname[1:1+length]
        labels.append(label)
        qname = qname[1+length:]
    return '.'.join(labels)


s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
s.bind(('', 53))

while True:
    data, addr = s.recvfrom(8192)
    header = struct.unpack('>HHHHHH', data[:12])
    qname, data = data[12:].split('\x00', 1)
    qtype, qclass = struct.unpack('>HH', data[:4])

    # respond with respond_addr only if qtype is A
    if qtype == 1:
        print "[+] %s %d %d (from %s)" % (parse_qname(qname), qtype, qclass, addr[0])
        data = struct.pack('>HHHHHH', header[0], 0x8180, 1, 1, 0, 0)
        data += qname + '\x00'
        data += struct.pack('>HH', qtype, qclass)
        data += struct.pack('>HHHIH4s', 0xc00c, qtype, qclass, 86400, 4, socket.inet_aton(respond_addr))
    else:
        print "[!] %s %d %d (from %s)" % (parse_qname(qname), qtype, qclass, addr[0])
        data = struct.pack('>HHHHHH', header[0], 0x8180, 1, 0, 0, 0)
        data += qname + '\x00'
        data += struct.pack('>HH', qtype, qclass)

    s.sendto(data, addr)

result.txt

On server:
$ sudo python dnsd.py
[+] www.example.com 1 1 (from 127.0.0.1)
[+] www.example.net 1 1 (from 127.0.0.1)
[!] www.example.com 28 1 (from 127.0.0.1)

On client:
$ dig @localhost www.example.com A

; <<>> DiG 9.9.5-3ubuntu0.3-Ubuntu <<>> @localhost www.example.com A
; (2 servers found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 4904
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;www.example.com.               IN      A

;; ANSWER SECTION:
www.example.com.        86400   IN      A       127.0.0.1

;; Query time: 0 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Fri Nov 27 11:30:12 JST 2015
;; MSG SIZE  rcvd: 49


$ dig @localhost www.example.net A

; <<>> DiG 9.9.5-3ubuntu0.3-Ubuntu <<>> @localhost www.example.net A
; (2 servers found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 47312
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;www.example.net.               IN      A

;; ANSWER SECTION:
www.example.net.        86400   IN      A       127.0.0.1

;; Query time: 2 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Fri Nov 27 11:30:18 JST 2015
;; MSG SIZE  rcvd: 49


$ dig @localhost www.example.com AAAA

; <<>> DiG 9.9.5-3ubuntu0.3-Ubuntu <<>> @localhost www.example.com AAAA
; (2 servers found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 9375
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;www.example.com.               IN      AAAA

;; Query time: 3 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Fri Nov 27 11:30:24 JST 2015
;; MSG SIZE  rcvd: 33