dnslib 0.3

Simple library to encode/decode DNS wire-format packets

dnslib
------

A simple library to encode/decode DNS wire-format packets. This was originally
written for a custom nameserver.

The key classes are:

* DNSRecord (contains a DNSHeader and one or more DNSQuestion/DNSRR records)
* DNSHeader
* DNSQuestion
* RR (resource records)
* RD (resource data - superclass for TXT,A,MX,CNAME,PRT,SOA)

To decode a DNS packet:

>>> packet = 'd5ad818000010005000000000377777706676f6f676c6503636f6d0000010001c00c0005000100000005000803777777016cc010c02c0001000100000005000442f95b68c02c0001000100000005000442f95b63c02c0001000100000005000442f95b67c02c0001000100000005000442f95b93'.decode('hex')
>>> d = DNSRecord.parse(packet)
>>> print d
<DNS Header: id=0xd5ad type=RESPONSE opcode=QUERY flags=RD,RA rcode=None q=1 a=5 ns=0 ar=0>
<DNS Question: 'www.google.com' qtype=A qclass=IN>
<DNS RR: 'www.google.com' rtype=CNAME rclass=IN ttl=5 rdata='www.l.google.com'>
<DNS RR: 'www.l.google.com' rtype=A rclass=IN ttl=5 rdata='66.249.91.104'>
<DNS RR: 'www.l.google.com' rtype=A rclass=IN ttl=5 rdata='66.249.91.99'>
<DNS RR: 'www.l.google.com' rtype=A rclass=IN ttl=5 rdata='66.249.91.103'>
<DNS RR: 'www.l.google.com' rtype=A rclass=IN ttl=5 rdata='66.249.91.147'>

To create a DNS Request Packet:

>>> d = DNSRecord(q=DNSQuestion("google.com"))
>>> print d
<DNS Header: id=... type=QUERY opcode=QUERY flags=RD rcode=None q=1 a=0 ns=0 ar=0>
<DNS Question: 'google.com' qtype=A qclass=IN>
>>> d.pack()
'...'

>>> d = DNSRecord(q=DNSQuestion("google.com",QTYPE.MX))
>>> print d
<DNS Header: id=... type=QUERY opcode=QUERY flags=RD rcode=None q=1 a=0 ns=0 ar=0>
<DNS Question: 'google.com' qtype=MX qclass=IN>
>>> d.pack()
'...'

To create a DNS Response Packet:

>>> d = DNSRecord(DNSHeader(qr=1,aa=1,ra=1),
... q=DNSQuestion("abc.com"),
... a=RR("abc.com",rdata=A("1.2.3.4")))
>>> print d
<DNS Header: id=... type=RESPONSE opcode=QUERY flags=AA,RD,RA rcode=None q=1 a=1 ns=0 ar=0>
<DNS Question: 'abc.com' qtype=A qclass=IN>
<DNS RR: 'abc.com' rtype=A rclass=IN ttl=0 rdata='1.2.3.4'>
>>> d.pack()
'...'

To create a skeleton reply to a DNS query:

>>> q = DNSRecord(q=DNSQuestion("abc.com",QTYPE.CNAME))
>>> a = q.reply(data="xxx.abc.com")
>>> print a
<DNS Header: id=... type=RESPONSE opcode=QUERY flags=AA,RD,RA rcode=None q=1 a=1 ns=0 ar=0>
<DNS Question: 'abc.com' qtype=CNAME qclass=IN>
<DNS RR: 'abc.com' rtype=CNAME rclass=IN ttl=0 rdata='xxx.abc.com'>
>>> a.pack()
'...'

Add additional RRs:

>>> a.add_answer(RR('xxx.abc.com',QTYPE.A,rdata=A("1.2.3.4")))
>>> print a
<DNS Header: id=... type=RESPONSE opcode=QUERY flags=AA,RD,RA rcode=None q=1 a=2 ns=0 ar=0>
<DNS Question: 'abc.com' qtype=CNAME qclass=IN>
<DNS RR: 'abc.com' rtype=CNAME rclass=IN ttl=0 rdata='xxx.abc.com'>
<DNS RR: 'xxx.abc.com' rtype=A rclass=IN ttl=0 rdata='1.2.3.4'>
>>> a.pack()
'...'

Changelog:

0.1 2010-09-19 Initial Release
0.2 2010-09-22 Minor fixes
0.3 2010-10-02 Add DNSLabel class to supportt arbitrary labels (embedded '.')