Silently drop queries for AAAA records

Mark Andrews marka at
Mon Dec 13 23:37:43 UTC 2010

In message <4D06A75F.7080400 at>, Kevin Darcy writes:
> 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.

It's likely to be applying the search list to AAAA queries and *not*
stopping on NODATA then applying the search list to A queries.  I've
argued that this is wrong behaviour and that searches should stop
on NODATA but some people are worried that this change in behaviour
will break something that is depending on the searches skipping
NODATA responses.

If you are worried about this then complain to the OS vendor to fix
the resolver library.

> > 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?

BIND 4 did not block AAAA queries.
> 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.

Or stop worrying about this and realise that it will self correct
as more sites start using IPv6 and AAAA records.  IPv4 really has
passed its "use by" date.

> - Kevin
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: marka at

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