[dns-operations] question for DNS being attacked

Paul Vixie paul at redbarn.org
Thu Jun 28 18:55:17 UTC 2012

On 6/28/2012 5:13 PM, Michael Graff wrote:
> On Jun 28, 2012, at 9:06 AM, Vernon Schryver wrote:
>> That conclusion does not hold, because it does not define the narrow
>> window alternative.  11 times as wide as what?
> With a slip factor of 2, every other packet will be dropped, and the
> other packets returned will have the truncated bit set.  If this is
> incorrect, please explain what it does do.
> This translates to a "normal" truncated response 50% of the time, and
> a timeout the other 50%.  Ignoring the penalty BIND 9 and other
> servers are likely to assign to this misbehaving server, the timeout
> keeps the "Waiting for a response" window open much, much longer.
>  This timeout is largely server-dependent, and some may wait multiple
> seconds.  This leaves the window open for a spoofing flood attack to
> sneak in.

as we discussed 1x1 when you brought this up the first time, longer
request windows are the equivilent of no birthday protection in terms of
their kaminsky danger-factor. at this time the remaining
kaminsky-vulnerable recursives are dominated by "does not do source port
randomization at all" followed by "lives behind a port-derandomizing
NAT". so while i am in favour of birthday protection, the risk posed by
RRL whereby this protection can be turned off on a query by query basis
is among my half dozen top concerns. (the risks of reflected amplified
DDoS would also be higher, so even if it were a direct tradeoff, the
decision would be obvious to me in favour of RRL.)

> While I commend you and Paul on the RLL work you've made, I think it
> is improper to not mention this in the documents you write.

that's a fair point. the technical note currently has the following text:

   5 - Attacker Behaviour

   5.1. A forged-source reflective amplifying attacker who wants to be
   successful will either have to select authority servers who do not
   practice rate limiting yet, or will have to select a large number of
   authority servers and use round robin to distribute the attack flows.
   Each authority server will have to be asked a question within one of
   that server's zones chosen at random in order to get an amplification
   effect. An attacker would do well to select DNSSEC-signed zones and to
   use DNSSEC signalling in their forged queries to maximize response size.
   This will be more effective than QTYPE ANY queries which are often
   blocked altogether due to their diagnostic rather than operational

i will add text to the effect that a kaminsky attacker could use forged-source packet bursts
to quietize a DNS server that uses RRL in order to make their attack against a distant 
recursive DNS server more effective.

the math isn't obvious to me, i'm not sure how much easier this makes a kaminsky attack, but
it's a tradeoff that every DNS operator should make explicitly and with complete knowledge.

>  It may be "not a big deal" to the administrator of the zone, but it
> is up to them to decide that.  Some may prefer to be a flooding source
> rather than make their zone more prone to spoofing, even if the actual
> odds are low.  The biggest problem here is that the zone publisher's
> goals of not being spoofable are entirely dependent on the resolver
> asking the questions, without DNSSEC in the mix.

we are now in the post-apocalyptic road-warrior phase of non-DNSSEC's
history. it's difficult for me to imagine anyone choosing to remain an
attack amplifier when they could instead sign their zones. but you're
entirely right about the tradeoff transparency; vernon and i do not
intend to slip this decision into an operator's life without their
knowledge and consent.

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.dns-oarc.net/pipermail/dns-operations/attachments/20120628/1b28c6f2/attachment.html>

More information about the dns-operations mailing list