[dns-operations] Interesting messages in our logs

Paul Vixie paul at redbarn.org
Sun Nov 2 03:45:27 UTC 2014

> Lyle Giese <mailto:lyle at lcrcomputer.net>
> Saturday, November 01, 2014 5:37 PM
> Now on the TCP side, I am seeing 'no more TCP clients: quota
> reached'.  I am still not clear on what the other side is told or what
> it thinks is going on at this point.  My thought process/logic is that
> a TCP Reset should be handled above the application layer and that an
> application could not create/generate that.  Of course, I can be wrong
> on this point and would welcome corrections.
> Now that I have written this, I suspose it's within reason that an
> application can tell the upper layers, I am to busy to handle that
> request, drop the TCP session and those layers use the RST function to
> accomplish that.

yes. in general, when you receive a tcp connection that puts you over
your application layer quota, you close() it, possibly shutdown()'ing it
first. this will send a FIN immediately, and send an RST if non-meta
data keeps coming.
> But it sounds more and more like an DoS attempt against my
> authoritative servers by exhausting TCP resources.  And they were
> utilizing Google's servers to assist in that process.  Not at all sure
> what the heck they would be trying to accomplish hitting up my servers.

my intuition is that this is not a TCP attack per se. if you look at the
QNAME's google is sending you, they will probably be pseudo-random
subdomains of the zones you are authoritative for. and if your zone is
signed with DNSSEC, and has no wildcard, then you may find that google's
EDNS buffer size is not large enough to contain the responses you are
sending, so, you're sending TC=1 for other reasons, and google is coming
back with TCP a lot more often than RRL could possibly cause them to do.

in other words TCP is a side show, the real thrust of this attack is
likely elsewhere in your mix.

> And I would assume that it would take a lot more to accomplish a DoS
> attack against Google's servers this way!  But it's an excellent way
> for the attackers to hide their identity from me as I see only
> Google's IP addresses in the requests.

google has been doing their own version of RRL for a long time, longer
than DNS RRL has been specified, and they do a pretty good job. however,
pseudo-random subdomains are the attack du jour, and google's rate
limiting machinery might not be coping with it well. even so, it would
take a botnet-sized client population hammering google public dns with
pseudo-random subdomains of your zones to get google to actually launch
that number of simultaneous queries toward your authority servers.

i recommend analyzing your QNAMEs so that you can isolate the target.

google's dns people are certainly monitoring this thread and it's my bet
that they'll be in touch with you privately.

Paul Vixie
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