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Simple library to encode/decode DNS wire-format packets

Project description

From Version 0.9.12 the master repository for dnslib has been moved to GitHub (https://github.com/paulc/dnslib). Please update any links to the original BitBucket repository as this will no longer be maintained.

dnslib

A library to encode/decode DNS wire-format packets supporting both Python 2.7 and Python 3.2+.

The library provides:

  • Support for encoding/decoding DNS packets between wire format, python objects, and Zone/DiG textual representation (dnslib.dns)

  • A server framework allowing the simple creation of custom DNS resolvers (dnslib.server) and a number of example servers created using this framework

  • A number of utilities for testing (dnslib.client, dnslib.proxy, dnslib.intercept)

Python 3 support was added in Version 0.9.0 which represented a fairly major update to the library - the key changes include:

  • Python 2.7/3.2+ support (the last version supporting Python 2.6 or earlier was version 0.8.3)

  • The 'Bimap' interface was changed significantly to explicitly split forward (value->text) lookups via getitem and reverse (text->value) lookups via getattr. Applications using the old interface will need to be updated.

  • Hostnames are now returned with a trailing dot by default (in line with RFC)

  • Most object attributes are now typed in line with the record definitions to make it harder to generate invalid packets

  • Support for encoding/decoding resource records in 'Zone' (BIND) file format

  • Support for encoding/decoding packets in 'DiG' format

  • Server framework allowing (in most cases) custom resolvers to be created by just subclassing the DNSResolver class and overriding the 'resolve' method

  • A lot of fixes to error detection/handling which should make the library much more robust to invalid/unsupported data. The library should now either return a valid DNSRecord instance when parsing a packet or raise DNSError (tested via fuzzing)

  • Improved utilities (dnslib.client, dnslib.proxy, dnslib.intercept)

  • Improvements to encoding/decoding tests including the ability to generate test data automatically in test_decode.py (comparing outputs against DiG)

  • Ability to compare and diff DNSRecords

Classes

The key DNS packet handling classes are in dnslib.dns and map to the standard DNS packet sections:

  • DNSRecord - container for DNS packet. Contains:
    • DNSHeader
    • Question section containing zero or more DNSQuestion objects
    • Answer section containing zero or more RR objects
    • Authority section containing zero or more RR objects
    • Additional section containing zero or more RR objects
  • DNS RRs (resource records) contain an RR header and an RD object)
  • Specific RD types are implemented as subclasses of RD
  • DNS labels are represented by a DNSLabel class - in most cases this handles conversion to/from textual representation however does support arbitatry labels via a tuple of bytes objects

Usage

To decode a DNS packet:

>>> packet = binascii.unhexlify(b'd5ad818000010005000000000377777706676f6f676c6503636f6d0000010001c00c0005000100000005000803777777016cc010c02c0001000100000005000442f95b68c02c0001000100000005000442f95b63c02c0001000100000005000442f95b67c02c0001000100000005000442f95b93')
>>> d = DNSRecord.parse(packet)
>>> d
<DNS Header: id=0xd5ad type=RESPONSE opcode=QUERY flags=RD,RA rcode='NOERROR' 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'>

The default text representation of the DNSRecord is in zone file format:

>>> print(d)
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 54701
;; flags: qr rd ra; QUERY: 1, ANSWER: 5, AUTHORITY: 0, ADDITIONAL: 0
;; QUESTION SECTION:
;www.google.com.                IN      A
;; ANSWER SECTION:
www.google.com.         5       IN      CNAME   www.l.google.com.
www.l.google.com.       5       IN      A       66.249.91.104
www.l.google.com.       5       IN      A       66.249.91.99
www.l.google.com.       5       IN      A       66.249.91.103
www.l.google.com.       5       IN      A       66.249.91.147

To create a DNS Request Packet:

>>> d = DNSRecord.question("google.com")

(This is equivalent to: d = DNSRecord(q=DNSQuestion("google.com") )

>>> d
<DNS Header: id=... type=QUERY opcode=QUERY flags=RD rcode='NOERROR' q=1 a=0 ns=0 ar=0>
<DNS Question: 'google.com.' qtype=A qclass=IN>

>>> str(DNSRecord.parse(d.pack())) == str(d)
True

>>> print(d)
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: ...
;; flags: rd; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 0
;; QUESTION SECTION:
;google.com.                    IN      A

>>> d = DNSRecord.question("google.com","MX")

(This is equivalent to: d = DNSRecord(q=DNSQuestion("google.com",QTYPE.MX) )

>>> str(DNSRecord.parse(d.pack())) == str(d)
True

>>> print(d)
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: ...
;; flags: rd; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 0
;; QUESTION SECTION:
;google.com.                    IN      MX

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")))
>>> d
<DNS Header: id=... type=RESPONSE opcode=QUERY flags=AA,RD,RA rcode='NOERROR' 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'>
>>> str(DNSRecord.parse(d.pack())) == str(d)
True

>>> print(d)
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: ...
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0
;; QUESTION SECTION:
;abc.com.                       IN      A
;; ANSWER SECTION:
abc.com.                0       IN      A       1.2.3.4

It is also possible to create RRs from a string in zone file format

>>> RR.fromZone("abc.com IN A 1.2.3.4")
[<DNS RR: 'abc.com.' rtype=A rclass=IN ttl=0 rdata='1.2.3.4'>]

(Note: this produces a list of RRs which should be unpacked if being
passed to add_answer/add_auth/add_ar etc)

>>> q = DNSRecord.question("abc.com")
>>> a = q.reply()
>>> a.add_answer(*RR.fromZone("abc.com 60 A 1.2.3.4"))
>>> print(a)
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: ...
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0
;; QUESTION SECTION:
;abc.com.                       IN      A
;; ANSWER SECTION:
abc.com.                60      IN      A       1.2.3.4

The zone file can contain multiple entries and supports most of the normal format defined in RFC1035 (specifically not $INCLUDE)

>>> z = '''
...         $TTL 300
...         $ORIGIN abc.com
...
...         @       IN      MX      10  mail.abc.com.
...         www     IN      A       1.2.3.4
...                 IN      TXT     "Some Text"
...         mail    IN      CNAME   www.abc.com.
... '''
>>> for rr in RR.fromZone(textwrap.dedent(z)):
...     print(rr)
abc.com.                300     IN      MX      10 mail.abc.com.
www.abc.com.            300     IN      A       1.2.3.4
www.abc.com.            300     IN      TXT     "Some Text"
mail.abc.com.           300     IN      CNAME   www.abc.com.

To create a skeleton reply to a DNS query:

>>> q = DNSRecord(q=DNSQuestion("abc.com",QTYPE.ANY))
>>> a = q.reply()
>>> a.add_answer(RR("abc.com",QTYPE.A,rdata=A("1.2.3.4"),ttl=60))
>>> str(DNSRecord.parse(a.pack())) == str(a)
True
>>> print(a)
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: ...
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0
;; QUESTION SECTION:
;abc.com.                       IN      ANY
;; ANSWER SECTION:
abc.com.                60      IN      A       1.2.3.4

Add additional RRs:

>>> a.add_answer(RR("xxx.abc.com",QTYPE.A,rdata=A("1.2.3.4")))
>>> a.add_answer(RR("xxx.abc.com",QTYPE.AAAA,rdata=AAAA("1234:5678::1")))
>>> str(DNSRecord.parse(a.pack())) == str(a)
True
>>> print(a)
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: ...
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 3, AUTHORITY: 0, ADDITIONAL: 0
;; QUESTION SECTION:
;abc.com.                       IN      ANY
;; ANSWER SECTION:
abc.com.                60      IN      A       1.2.3.4
xxx.abc.com.            0       IN      A       1.2.3.4
xxx.abc.com.            0       IN      AAAA    1234:5678::1

It is also possible to create a reply from a string in zone file format:

>>> q = DNSRecord(q=DNSQuestion("abc.com",QTYPE.ANY))
>>> a = q.replyZone("abc.com 60 IN CNAME xxx.abc.com")
>>> print(a)
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: ...
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0
;; QUESTION SECTION:
;abc.com.                       IN      ANY
;; ANSWER SECTION:
abc.com.                60      IN      CNAME   xxx.abc.com.

>>> str(DNSRecord.parse(a.pack())) == str(a)
True

>>> q = DNSRecord(q=DNSQuestion("abc.com",QTYPE.ANY))
>>> a = q.replyZone(textwrap.dedent(z))
>>> print(a)
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: ...
;; flags: qr aa rd ra; QUERY: 1, ANSWER: 4, AUTHORITY: 0, ADDITIONAL: 0
;; QUESTION SECTION:
;abc.com.                       IN      ANY
;; ANSWER SECTION:
abc.com.                300     IN      MX      10 mail.abc.com.
www.abc.com.            300     IN      A       1.2.3.4
www.abc.com.            300     IN      TXT     "Some Text"
mail.abc.com.           300     IN      CNAME   www.abc.com.

To send a DNSSEC request (EDNS OPT record with DO flag & header AD flag):

>>> q = DNSRecord(q=DNSQuestion("abc.com",QTYPE.A))
>>> q.add_ar(EDNS0(flags="do",udp_len=4096))
>>> q.header.ad = 1
>>> print(q)
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: ...
;; flags: rd ad; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 1
;; QUESTION SECTION:
;abc.com.                       IN      A
;; ADDITIONAL SECTION:
;; OPT PSEUDOSECTION
; EDNS: version: 0, flags: do; udp: 4096

The library also includes a simple framework for generating custom DNS resolvers in dnslib.server (see module docs). In most cases this just requires implementing a custom 'resolve' method which receives a question object and returns a response.

A number of sample resolvers are provided as examples (see CLI --help):

  • dnslib.fixedresolver - Respond to all requests with fixed response
  • dnslib.zoneresolver - Respond from Zone file
  • dnslib.shellresolver - Call shell script to generate response

The library includes a number of client utilities:

  • DiG like client library

     # python -m dnslib.client --help
    
  • DNS Proxy Server

     # python -m dnslib.proxy --help
    
  • Intercepting DNS Proxy Server (replace proxy responses for specified domains)

     # python -m dnslib.intercept --help
    

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 Torbjorn Lonnemark (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
  • 0.9.0 2014-05-05 Major update including Py3 support (see docs)
  • 0.9.1 2014-05-05 Minor fixes
  • 0.9.2 2014-08-26 Fix Bimap handling of unknown mappings to avoid exception in printing Add typed attributes to classes Misc fixes from James Mills - thanks
  • 0.9.3 2014-08-26 Workaround for argparse bug which raises AssertionError if [] is present in option text (really?)
  • 0.9.4 2015-04-10 Fix to support multiple strings in TXT record Patch provided by James Cherry (https://bitbucket.org/james_cherry) - thanks NOTE: For consistency this patch changes the 'repr' output for TXT records to always be quoted
  • 0.9.5 2015-10-27 Add threading & timeout handling to DNSServer
  • 0.9.6 2015-10-28 Replace strftime in RRSIG formatting to avoid possible locale issues Identified by Bryan Everly - thanks
  • 0.9.7 2017-01-15 Sort out CAA/TYPE257 DiG parsing mismatch
  • 0.9.8 2019-02-25 Force DNSKEY key to be bytes object Catch Bimap wrapped attr (used by inspect module in 3.7)
  • 0.9.9 2019-03-19 Add support for DNSSEC flag getters/setters (from raul@dinosec.com - thanks) Added --dnssec flags to dnslib.client & dnslib.test_decode (sets EDNS0 DO flag) Added EDNS0 support to dnslib.digparser
  • 0.9.10 2019-03-24 Fixes to DNSSEC support Add NSEC RR support Add --dnssec flag to dnslib.client & dnslib.test_decode Quote/unquote non-printable characters in DNS labels Update test data (Thanks to raul@dinosec.com for help)
  • 0.9.11 2019-12-17 Encode NOTIFY Opcode (Issue #26)
  • 0.9.12 2019-12-17 Transition master repository to Github (Bitbucket shutting down hg)
  • 0.9.13 2020-06-01 Handle truncated requests in server.py (Issue #9) Replace thred.isAlive with thread.is_alive (Deprecated in Py3.9) Merged Pull Request #4 (Extra options for intercept.py) - thanks to @nolanl
  • 0.9.14 2020-06-09 Merged Pull Request #10 (Return doctest status via exit code) Thanks to @mgorny

License:

BSD

Author:

  • PaulC

Master Repository/Issues:

(Note: https://bitbucket.org/paulc/dnslib has been deprecated and will not be updated)

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