[dns-operations] Interesting messages in our logs

Paul Vixie paul at redbarn.org
Sat Nov 1 23:10:31 UTC 2014

> Lyle Giese <mailto:lyle at lcrcomputer.net>
> Saturday, November 01, 2014 1:41 PM
> On 11/01/14 12:21, Paul Vixie wrote:
>>> Stephane Bortzmeyer <mailto:bortzmeyer at nic.fr>
>>> Saturday, November 01, 2014 8:49 AM
>>> On Sat, Nov 01, 2014 at 10:10:07AM -0500,
>>>  Lyle Giese <lyle at lcrcomputer.net> wrote 
>>>  a message of 23 lines which said:
>>>> Oct 31 04:10:52 linux1 named[2899]: client
>>>> 2607:f8b0:4001:c07::151#61651: no more TCP clients: quota reached
>>> If you wish to handle this amount of requests, you can raise
>>> the tcp-clients parameter.
>>> options { tcp-clients 300; };
>> there is no number you can insert here, including the largest number
>> your OS can support, such as 2^16, which will make your tcp listener
>> robust in the face of attacks. even if both sides of a non-attack
>> flow (so, client and server) fully implemented the recommendations of
>> <https://tools.ietf.org/html/draft-dickinson-dnsop-5966-bis-00>,
>> intentional tcp state exhaustion will remain a viable attack vector.
> While interesting and I learn from discussions like this, it doesn't
> answer my original question.  When Named goes into SLIP via UDP
> queries, the other party should(and did) retry using TCP.  What
> happens when we throttle via TCP, like above?  Does NAMED just drop
> the connection? Or does it send back a meaningful error message or
> status of some sort?

first, SLIP isn't a mode, it's a ratio. "SLIP X;" means every X'th drop
will be turned into a TC=1 response. so, higher values for X result in
fewer TC=1 responses (since 1/N > 1/M for all M > N). from your syslogs,
you would benefit from more drops and fewer slips, which is why i
advised you to increase your slips to at least 5, and be willing to
consider 8 or 10 if the problem persists.

second, you're not throttling via TCP, you're doing two
related-but-not-that things. you are encouraging TCP by deliberately
mixing slip frames (TC=1) in with your rate-related intentional drops.
second, you're seeing more TCP traffic than you have state quota for.
these are probably related, but need not be related. if i were attacking
you i would make sure you had lots of TCP sessions open at the same time
that i blasted you with UDP in a way that forced you to start sending
TC=1 responses. all i have to do is ask google dns over and over for
answers it won't have in cache and that won't fit in UDP responses. (you
can only stop me by ensuring that IP fragmentation and therefore EDNS
are working to your authority servers, which is often not under your
control. BWA HA HA HA!) i can also just open a lot of TCP sessions from
other parts of the internet, since it's unlikely that you have
per-client-ip flow quotas, and i can just ask lots of questions,
returning ACK's very slowly, tar-pit style. i'll never be idle, and if
you RST or FIN me i'll just call back.

to precisely answer your question, in the log snippet you showed (no
more TCP clients: quota reached) there is no meaningful error message or
status to send. you could send SERVFAIL but it's more likely you're just
RST'ing. neither one is meaningful to the far end. you have no choices
available to you which will be meaningful to the far end. (as i said,

happy belated all-hallows eve, everybody.

Paul Vixie
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