Dnslib3

A library to encode/decode DNS wire-format packets.

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:

• 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

Note that when using the library you should always validate the received TXID

q = DNSRecord.question("abc.com")
a_pkt = q.send(address,port,tcp=args.tcp)
a = DNSRecord.parse(a_pkt)
if q.header.id != a.header.id:
    raise DNSError('Response transaction id does not match query transaction id')

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
 *   0.9.15  2021-05-07  DNSServer fixes - support IPv6 (from Pull Request #21) - thanks to @mikma
                                         - deamon threads (Pull Request #19) - thanks to @wojons
                         Add unsupported RR types (Issue #27)
 *   0.9.16  2021-05-07  Merge pull request #23 from Tugzrida/patch-1
                            Add support for all RR types to NSEC type bitmap
                         Merge pull request #17 from sunds/issue_16
                            Issue 16: uncaught exceptions leak open sockets
 *   0.9.18  2022-01-09  Validate TXID in client.py (Issue #30 - thanks to @daniel4x)
 *   0.9.19  2022-01-09  Allow custom log function (logf) in  DNSLogger
                            (Issue #31 - thanks to @DmitryFrolovTri)
 *   0.9.20  2022-07-17  Fix DeprecationWarnings about invalid escape sequences
                            (Pull-Request #39 - thanks to @brianmaissy)
                         Make DNSLabel matchSuffix and stripSuffix case-insensitive
                            (Pull-Request #37 - thanks to @NiKiZe)
                         Add support for HTTPS RR
                            (Pull-Request #35 - thanks to @jkl-caliber)
                         Fix display of non-printable characters in TXT records
                            (Issue #32 - thanks to @sbv-csis)
                         Add --strip-aaaa option to dnslib.proxy
 *   0.9.21  2022-09-19  Minor clean-up / add wheels to distro
 *   0.9.22  2022-09027  Issue #43 (0.9.21 Raises TypeError instead of DNSError when failing to parse HTTPS records)
                         Note that we just fix the exception - there still seems to be a problem with parsing HTTPS records
                         (Thanks to @robinlandstrom)
 *   0.9.23  2022-10-28  Issue #43: HTTPS reads after RD end (thanks to @robinlandstrom for pull request)
                         Issue #45: Dnslib fails to handle unknown RR types in NSEC RD type bitmap
                            Bimap now supports a function to map unknown types which we use to
                            dynamically map from rtype <-> TYPExxxx for unknown record types
                            RR zone representation updated to match RFC3597
                         Pull Request #47: Add support for DS, SSHFP, and TLSA records (thanks to @rmbolger)
 *   0.9.24  2024-01-02  Merge multiple PRs
                            #49 - Generate README.md (via symlink)
                            #51 - Update Github CI checkout & setup-python actions (and remove Python 2.7 CI support)
                                  (thanks to @SpencerIsGiddy)
                            #54 - Support for RP records (thanks to @ryan-gang)
                            #57 - Support for LOC records (thanks to @valentinesd)
                         (Note that this will be the last release supporting Python 2.7 and Python <3.7)

License

BSD

Author

• PaulC

Master Repository/Issues

• https://github.com/paulc/dnslib

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