[ldns-users] pyLDNS Problems

[Paul Wouters]

Forwarding on behalve of Chris,

---------- Forwarded message ----------
From: Christopher Olah <christopherolah.co at gmail.com>
To: ldns-users at open.nlnetlabs.nl
Date: Thu, 25 Aug 2011 15:15:52 -0400
Subject: pyLDNS Problems

Hey all,

Over the last few months I've made extensive use of pyLDNS and even
written a wrapper over it, ldnsx
(http://git.xelerance.com/cgi-bin/gitweb.cgi?p=ldnsx.git;a=summary).
I've ran a number of bugs, many of which you seem to have resolved in
the latest release. Still, there remain some that are deeply
problematic for me:

(1) ldns_resolver stops responding

Creating reproducible errors here is tricky, because things are so
dependent on the state of things outside my control. However, the
following code has been demonstrating it quite reliability on centos6
with pyLDNS 1.3.40

import ldns

ldns_resolver = ldns.ldns_resolver.new_frm_file("/etc/resolv.conf")
#Drop nameservers
while self._ldns_resolver.pop_nameserver():
   pass
#Set nameserver to xelerance's to aid repropducability
address = ldns.ldns_rdf_new_frm_str(ldns.LDNS_RDF_TYPE_A,"76.10.157.65")
ldns_resolver.push_nameserver(address)
#Make queries. The first will work OK, but the next two will break.
print ldns_resolver.query("google.com", ldns.LDNS_RR_TYPE_ANY,
ldns.LDNS_RR_CLASS_IN )
print ldns_resolver.query("xelerance.com", ldns.LDNS_RR_TYPE_ANY,
ldns.LDNS_RR_CLASS_IN )
print ldns_resolver.query("google.com", ldns.LDNS_RR_TYPE_ANY,
ldns.LDNS_RR_CLASS_IN )

It just returns Nones. Contrast this with dig ANY xelerance.com @76.10.157.65

Some observations: This bug seems to be associated with truncated packets. Using

ldns_resolver.set_usevc(True)

will make the above code run smoothly. On the other hand, it doesn't
seem to be the only thing going on here. I've had the exact same code
run fine on the past (though there was a server change since this
particular example started failing consistently) and I've had other
examples that weren't truncated fail in the past.

Another observation: If you run with ldns_resolver._debug = True, it will print:

;; ->>HEADER<<- opcode: QUERY, rcode: NOERROR, id: 0
;; flags: rd ; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 0
;; QUESTION SECTION:
;; xelerance.com.       IN      ANY

;; ANSWER SECTION:

;; AUTHORITY SECTION:

;; ADDITIONAL SECTION:

;; Query time: 0 msec
;; WHEN: Thu Aug 25 14:25:13 2011
;; MSG SIZE  rcvd: 0

My experience with this bug is that it will stop occurring seemingly
at random...

(2) Random segfaults

I don't have any reproducible instances of this bug in the latest
version of pyLDNS, though calls to ldnsx.get_rrs will consistently
cause it to segfault in older versions. Translating it into ldns code
would be somewhat tedious and long, so I'll leave it out.

This isn't an overly big issue, since it has become quite infrequent
in the latest release.

Christopher Olah