Strange failure with recursive zen.spamhaus.org NS query

5 msg	view for a zone?
1 msg	Re: Recursion ceases for 5-10 minutes at random...
6 msg	Bind and possible redundancy flaw.
3 msg	BIND 9.5.0b2 named-checkzone gives false diagno...
2 msg	debugging
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3 msg	Slave DNS Server: what the significance of sh m...
2 msg	Calculating QPS
2 msg	dig segment fault
3 msg	named executables not found in my box ;(Copied ...
3 msg	Bind listening on IPs it shouldn't
2 msg	linux blog for sys administrators

Strange failure with recursive zen.spamhaus.org...
\ Simeon Miteff (15 Feb 2008)
. \ Ronan Flood (15 Feb 2008)

3 msg	allow-update to localhost and (but not or) TSIG...
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6 msg	Delegation
3 msg	Understanding concepts

Subject:	Strange failure with recursive zen.spamhaus.org NS query
Group:	Bind-users
From:	Simeon Miteff
Date:	15 Feb 2008

Hi All

Some of our users are reporting that our bind9 caching nameservers are
failing to resolve certain DNS records (especially NS records for
zen.spamhaus.org).

I found a similar problem reported on this list from August last year:
http://marc.info/?l=bind-users&m=118714610726126&w=4

When I followed Mark's advice and did some packet dumps, I discovered
that bind is receiving a response from one of the spamhaus.org name
servers, but not sending that to the client.

I reproduced the behavior on my Debian workstation with bind 9.4.2, from
where I am able to resolve the NS records for zen.spamhaus.org using
dig, directly.

A debug log is available at:
http://filterline.its.uct.ac.za/~smiteff/bind_issues/debug_log.txt

Bind seems to receive the response at "15-Feb-2008 12:54:51.552", but
I'm unable to figure out exactly where it decides to discard it (perhaps
the 12th line from the end of the log?). I've copied the bind config
files into the same directory above.

Does anyone know what is causing this or what else I should explore?

Regards,
Simeon Miteff.