Silently drop queries for AAAA records

Kevin Darcy kcd at
Mon Dec 13 23:08:15 UTC 2010

On 12/7/2010 5:31 PM, David A. Evans wrote:
>         I'm in the mood to prove a point.   I have a very poorly 
> written application that is generating a few hundred queries per 
> second of completely bogus AAAA records before attempting a lookup of 
> the correct A records.  This is because the application was compiled 
> with a IPv6 interface enabled on the severs so it assumes that v6 is 
> available.  It is not.  The application owner does not see an issue as 
> they get the handful NXDOMAIN responses back in ~2 ms for each valid 
> response and don't see any performance hit.
>         I would like to silently drop the AAAA record lookups instead 
> of responding back with NXDOMAIN.
NXDOMAIN? Is the application looking up a different *name* for its AAAA 
queries than for its A queries? If a single name owned A records but no 
AAAA records then the correct response from an AAAA-capable resolver to 
an AAAA query of the name, would be the so-called "NODATA" response 
(NOERROR with 0 answers and an SOA RR in Authority Section for negative 
caching purposes, see RFC 2308 for details). NXDOMAIN, as another poster 
pointed out, could inhibit even A-record queries of the name, and would 
be the wrong response in that situation.

> Thusly generating a performance hit as the application waits 2 seconds 
> for the reply.
>         I have found the filter-aaaa-on-v4  but it doesn't quiet do 
> what I want.  From the description and my testing it appears to still 
> reply with NXDOMAIN to these queries, it simply filters out the 
> 'valid' AAAA records from IPV4 based replies. (which is a really cool 
> solution to other issues, but not what I need.)
How nasty do you want to be? You could always add an AAAA record for 
that name. Point it anywhere you want <evil laugh>

If you point it to something simply non-existent, this solution seems to 
me only slightly ruder than silently dropping the queries.

>         Besides spinning up a bind 4.x box which google tells me did 
> this by default, is there any way of doing this?

I think it would be a really *bad* idea to spin up a BIND 4.x instance. 
Do you really want a big ugly security hole on your network? What about 
the person that inherits this setup from you? Would they be conversant 
in BIND 4.x setup and maintenance? I wouldn't wish BIND 4.x on anyone...

If you really want to go in the direction of dropping packets, I'd look 
at some sort of software-firewall intervention (iptables or whatever) to 
do the packet-dropping.

On the other hand, if the app really is looking up a different name for 
AAAA than for A (see above), that opens up all sorts of options for you. 
You could set up that name as a zone by itself and simply return REFUSED 
for all of those queries (the response packet count, and potentially the 
application delay, would be the same, but the response packets would be 
smaller and your intent crystal clear). Or set up a forwarder and play 
some games that way.

- Kevin

> *David A. Evans*
> *Enterprise IP/DNS Management*
> *Network Infrastructure Tools and Services*
> *_Evans_David_A at cat.com_* <mailto:Evans_David_A at>
> **
> /Eschew Obfuscation/
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