[dns-operations] Stunning security discovery: AXFR may leak information
warren at kumari.net
Tue Apr 14 21:06:56 UTC 2015
On Tue, Apr 14, 2015 at 4:31 PM, Michael Sinatra <michael at brokendns.net> wrote:
> On 4/14/15 12:00 PM, Mike Hoskins (michoski) wrote:
>>> I disagree with this. There is no valid reason for exposing your
>>> network topology to the outside world. You are only making the job
>>> easier for potential attackers.
>> Yes agreed. The finding is nothing new, and it's not a weakness in AXFR
>> itself as others have rightly pointed out...so the timing and way in which
>> it was reported were less than ideal...but your point is spot on. Many
>> speak against "security by obscurity" but I think that is often taken too
>> far -- in some ways blocking AXFR is no different than DMZs and
>> firewalls...hey, why not have everything on public IP with all ports
>> exposed? Security is an onion, and as many layers as you can put between
>> you and the adversary are generally good assuming the "obscurity" is not
>> adding unnecessary complexity or other hidden cost (proper config of a DNS
>> server is quite easy and can be automated).
> The problem I have with the way that this is posed by the US-CERT
> advisory is that it neglects to point out that DNS is designed to be a
> public database. If you put information in the DNS that makes it easy
> to guess things about your network that you don't want people to guess,
> well, you have a problem then. Relying on AXFR restrictions to mask
> that problem is, at best, a weak control. (See Paul's post.) Because
> security is indeed an onion, AXFR restrictions really shouldn't be
> *that* important--just another layer in a set of good security practices.
> The real reason I see for restricting AXFR is to preserve resources on
> the server. This is less of an issue now than it was in the BIND 4 days
> (didn't BIND 4 used to fork() for outbound zone transfers?), but I still
> don't want any- and every-one to hammer my DNS servers with AXFR
Depends on who you are and who is interested in the contents of your
zone -- if you are operating a ccTLD (and depending on the number of
records, the distribution of records, phase of the moon, etc) it *may*
be cheaper to simply allow AXFR instead of having a bunch of spammers
do dictionary attacks trying to guess all the names. At least one
ccTLD (that I know of) became much happier after they just threw in
the towel and allowed AXFR...
> I am kind of surprised and disappointed that the US-CERT
> doesn't mention that component of the issue.
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I don't think the execution is relevant when it was obviously a bad
idea in the first place.
This is like putting rabid weasels in your pants, and later expressing
regret at having chosen those particular rabid weasels and that pair
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