DNS keyserver (was Re: gnupg-1.0.7: keyserver subdir?)

[Michael Graff]

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Simon Josefsson <jas@extundo.com> writes:

> > Cool.  A key server DDOS attack is in your future.  :)
> 
> How so?

One of the popular DDOS attacks is to use a multiplier of some sort.
For example, AOL used to have about 26 or so MX records, which just
fit in a 512 byte response packet.  Attackers would forge a source
address on a packet to be some poor victim machine, and send a request
for "aol.com. mx" -- a 63 byte (or so) packet would multiply into a
512 byte packet.  So, the attacker needed but 1/8th the bandwidth of
the target to get this trick to work.

What you're proposing (and I still like the idea, mind you :) is even
worse -- nearly a 1000x increase.  I send you a small packet as a
request, and you send me back up to 64k of data.

> > Remember that the max payload (and max rdata size) is still 64k -
> > headers, so you'll have to handle broken up data even over TCP.
> 
> Hm.  That is a problem. Perhaps it is easier to state that it won't
> work with keys larger than 64kb though.

Then your server won't hold the most popular keys.  :)

I'd say reserve a flag somewhere in the returned data that means "more
data" and they can just tack on .0 on the end.  And then .1, .2, .3,
etc.  That way, while less efficient, it will work for even large
keys.

BTW, remember also that a 64k UDP packet will be fragmented over
nearly every type of wire, and if even one of those fragments are
dropped, the entire packet is lost.

It may be better to limit the EDNS0-sized responses to perhaps 1400
bytes of payload, and use some sort of chaining method to get the full
key data.  Of course, this is not atomic.

You can see why I never actually wrote the thing.  :)

What I was considering was to split the signatures up, and return each
signature in a different packet.  One packet would be the self-signed
key (if it is self-signed, the bare key otherwise) and each following
request would return one signature.  It would stop when it was told to
- -- that is, requesting

        keyid-here.sig.21

would return "END OF SIGNATURES"

It got messy fast.

- --Michael
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