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,AAAA,MX,CNAME,PRT,SOA,NAPTR)
        * DNSLabel (envelope for a DNS label)

    The library has (in theory) very rudimentary support for EDNS0 options
    however this has not been tested due to a lack of data (anyone wanting
    to improve support or provide test data please raise an issue)

    Note: In version 0.3 the library was modified to use the DNSLabel class to
    support arbirary DNS labels (as specified in RFC2181) - and specifically
    to allow embedded '.'s. In most cases this is transparent (DNSLabel will
    automatically convert a domain label presented as a dot separated string &
    convert pack to this format when converted to a string) however to get the
    underlying label data (as a tuple) you need to access the DNSLabel.label
    attribute. To specifiy a label to the DNSRecord classes you can either pass
    a DNSLabel object or pass the elements as a list/tuple.

    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 support arbitrary labels (embedded '.')
        *   0.4     2012-02-26  Merge with dbslib-circuits
        *   0.5     2012-09-13  Add support for RFC2136 DDNS updates
                                Patch provided by Wesley Shields <wxs@FreeBSD.org> - thanks
        *   0.6     2012-10-20  Basic AAAA support
        *   0.7     2012-10-20  Add initial EDNS0 support (untested)
        *   0.8     2012-11-04  Add support for NAPTR, Authority RR and additional RR
                                Patch provided by Stefan Andersson (https://bitbucket.org/norox) - thanks
        *   0.8.1   2012-11-05  Added NAPTR test case and fixed logic error
                                Patch provided by Stefan Andersson (https://bitbucket.org/norox) - thanks
        *   0.8.2   2012-11-11  Patch to fix IPv6 formatting
                                Patch provided by Torbjörn Lönnemark (https://bitbucket.org/tobbezz) - thanks
        *   0.8.3   2013-04-27  Don't parse rdata if rdlength is 0
                                Patch provided by Wesley Shields <wxs@FreeBSD.org> - thanks

    License:

        *   BSD

    Author:

        *   Paul Chakravarti (paul.chakravarti@gmail.com)

    Master Repository/Issues:

        *   https://bitbucket.org/paulc/dnslib