[dns-operations] rate-limiting state

Colm MacCárthaigh colm at stdlib.net
Thu Feb 6 23:59:40 UTC 2014

On Thu, Feb 6, 2014 at 3:28 PM, Paul Vixie <paul at redbarn.org> wrote:
> first, i think we need smaller numbers, since at those volumes, opendns's
> pipes are full, and nobody would get any answers to any questions. so let's
> pick some number of requests and SYNs per second that is enough to use all
> the head room opendns had, but without affecting response flows to queries
> not related to your attack.

I chose a fairly typical number, which is actually below average. Arbor's
data on DDOS puts 10M somewhere between the 40th and 50th percentile.  I'd
be really surprised if OpenDNS's pipes fill up with that kind of small

second, RRL does not see SYNs. the kernel probably has SYN flood
> protection, which like a stateful firewall might penalize a host or
> netblock's real SYNs, but that has nothing to do with RRL's logic.

So the reflection attack is completed? If you're going to respond to all of
the SYNs, you may as well respond to the UDP queries. It'll be the same
reflected PPS. The byte count will be a little higher, but most networks
are bottlenecked on PPS at DNS payload sizes.

> so, third, let's look squarely at "large enough UDP flow to activate RRL".

10M requests/sec for www.example.com, type=A. Would that be large enough?

in that steady state situation, opendns's legitimate queries whose response
> matches an RRL flow are mixed with an avalanche of forged questions
> soliciting the same answer. opendns will retry three times over ten to 90
> seconds. if opendns ever gets an answer, it will fill its cache and stop
> asking that question. the possibility of opendns receiving a TC=1 and
> retrying with TCP, or receiving one of our periodic normal answers, and
> either way filling its cache is high, on the order of unity. of course,
> opendns might ask other authorities in between retries to any one
> authority, so you'll need to spoof all of the potential authorities who
> could help with the terminal cache-fill operation that ends the race.

I agree with all of this, but I don't think that the numbers work out. If
you're getting an attack of 10M PPS, which is very realistic, you'll end up
denying service to real users.

Important to consider here, is that if you did nothing, and let the
responses go answered (if you can), there's no impact on the real users.
The reflection target does get hit of course though.  So in effect, at
realistic DDOS scales, RRL can be used to deny service to your real users
to protect victims of reflection attacks. That's a form of asymmetric
altruism. I'm not against it, doing the internet a favour is worthwhile, we
all benefit ; but it's worth calling out.

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