pylresolv - DNS querying through libc libresolv.so using ctypes

Provides access to res_query (resolver(3)) and friends for very basic DNS querying (beyond gethostbyname).

Right now, it only supports MX record lookups.

This is not a replacement for fully featured DNS libraries like dnspython or pycares, but rather a small wrapper to provide a bare minimal lookup capability. Additionally, it serves as an example of how to use -lresolv routines.

Warning

BEWARE!

This library uses (a) not-so-well documented C library calls which (b) may differ in their ABI across different libc versions and operating systems. Proceed with caution. The only thing this has going for it, is its small size.

Most common usage

from pylresolv import ns_parse, ns_type, res_query

answer = res_query('gmail.com', rr_type=ns_type.ns_t_mx)
ret = ns_parse(answer, handler=ns_type.handle_t_mx)
print(ret)

Will produce a list of RR objects:

[RR<ns_type.ns_t_mx: 15>((40, u'alt4.gmail-smtp-in.l.google.com')),
 RR<ns_type.ns_t_mx: 15>((30, u'alt3.gmail-smtp-in.l.google.com')),
 RR<ns_type.ns_t_mx: 15>((10, u'alt1.gmail-smtp-in.l.google.com')),
 RR<ns_type.ns_t_mx: 15>((20, u'alt2.gmail-smtp-in.l.google.com')),
 RR<ns_type.ns_t_mx: 15>((5, u'gmail-smtp-in.l.google.com'))]

Example from main:

from pprint import pprint
queries = (
    ('gmail.com', ns_type.ns_t_ns),
    ('gmail.com', ns_type.ns_t_a),
    ('gmail.com', ns_type.ns_t_mx),
    ('gmail.com', ns_type.ns_t_txt),
    ('www.gmail.com', ns_type.ns_t_cname),
)
for qhost, qtype in queries:
    print('Querying host {!r} type {!r}:'.format(qhost, qtype))
    answer = res_query(qhost, rr_type=qtype)
    ret = ns_parse(answer)
    # ret = ns_parse(answer, handler=ns_type.handle_t_mx)
    # ret = ns_parse(answer, handler=ns_type.handle_bin)
    pprint([rr.contents for rr in ret])
    print()

Produces:

Querying host 'gmail.com' type <ns_type.ns_t_ns: 2>:
['ns2.google.com', 'ns4.google.com', 'ns3.google.com', 'ns1.google.com']

Querying host 'gmail.com' type <ns_type.ns_t_a: 1>:
[bytearray(b'\xac\xd9\x11e')]

Querying host 'gmail.com' type <ns_type.ns_t_mx: 15>:
[(40, 'alt4.gmail-smtp-in.l.google.com'),
 (30, 'alt3.gmail-smtp-in.l.google.com'),
 (10, 'alt1.gmail-smtp-in.l.google.com'),
 (20, 'alt2.gmail-smtp-in.l.google.com'),
 (5, 'gmail-smtp-in.l.google.com')]

Querying host 'gmail.com' type <ns_type.ns_t_txt: 16>:
['globalsign-smime-dv=CDYX+XFHUw2wml6/Gb8+59BsH31KzUr6c1l2BPvqKX8=',
 'v=spf1 redirect=_spf.google.com']

Querying host 'www.gmail.com' type <ns_type.ns_t_cname: 5>:
['mail.google.com']

2019-07-03: 0.2

• Renamed ns_type.handle_mx to ns_type.handle_t_mx.
• Added: - handle_bin - handle_compressed - handle_text - handle_t_cname - handle_t_ns` - handle_t_txt`
• ns_parse now wraps the returned values into RR objects, so the type can be retrieved afterwards. The value can be fetched from its contents property.
• You can use handle_bin to fetch A or AAAA records.
• res_query now raises LookupError on failure.

2019-03-17: 0.1

• Initial release.

Copyright

Copyright 2019, Walter Doekes (OSSO B.V.)

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program. If not, see http://www.gnu.org/licenses/.