Re: recursive DNS servers DDoS as a growing DDoS problem

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Chris Thompson wrote:

Michael Sierchio <kudzu@xxxxxxxxxxxx> writes:

Robert Story wrote:

VG> In the scenario you describe, I cannot see any actual amplification...

The amplification isn't in the number of hosts responding, but in
packet size.
A very small DNS request packet results in a huge response packet.

Are you talking about rogue authoritative servers? Otherwise, responses
will be limited to 512 bytes, possibly with the truncation bit set.


Unless it supports EDNS, in which case it may be persuaded to send
larger replies. BIND does currently have "you cannot be serious"
cutoff at 4096 bytes.

The reason that it is more awkward to use the method against
authoritative-only nameservers is that you have to find a large
RRset in the wild (or one that will come with large authority and/or
additional sections in the reply) and then use the authoritative
nameservers for that RRset, not any old open recursive nameserver
(or many of them). You cannot craft your own RRset for the purpose.

That is not a problem. As usually MCI at your service. They have
switched from RFC 3258 DNS design to having a very long list of name
servers each of which is separate. That is at least 345 bytes of
extra/authority section instead of the usual 70-100. All you need is
to find a domain hosted with them. If you are happy with a 5x
amplification you can simply use MCI.com

They are not the only ones.

It is a general trend in large ISPs/Telcos to exterminate with extreme
prejudice any DNS design that requires some networking competence.
Once again - transitions from RFC3258 to long lists are only one
example. Plenty of others.

But you can still get amplification, certainly.

The real solution to this problem is people finally starting to
enforce antispoofing on access networks. It is the same story as with
smurf and broadcast amplification 7 years ago. It is time to put up a
name and shame list out there.

- --

A. R. Ivanov
E-mail: aivanov@xxxxxxxxxx
WWW: http://www.sigsegv.cx/
pub 1024D/DDE5E715 2002-03-03 Anton R. Ivanov <ai1-n@xxxxxxxxxx>
Fingerprint: C824 CBD7 EE4B D7F8 5331 89D5 FCDA 572E DDE5 E715


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