ENOBUFS and DNS...
From: Barry Bouwsma (freebsd-misuser_at_remove-NOSPAM-to-reply.NOSPAM.dyndns.dk)
Date: 12/15/03
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Date: Mon, 15 Dec 2003 22:17:53 +0100 (CET) To: "%s" <freebsd-net@freebsd.org>
[Drop hostname part of IPv6-only address above to obtain IPv4-capable e-mail,
or just drop me from the recipients and I'll catch up from the archives]
Hello, "%s"!
I've read in this list from a couple years ago, several discussions about
ENOBUFS being returned to UDP-using applications. This is what I'm
experiencing with BIND when I get hit with lots of queries over a slow
link.
I'm serving DNS info for my subdomain, with an off-site secondary. I'm
on a dial-in now (no comments please); I don't ever remember seeing this
with a cable modem connection (about 2-4x upstream speed than now, with
downstream speed higher still).
When I send a mail to the FreeBSD lists, shortly after, I get hit with
lots of DNS queries to verify my address(es). My modem is saturated
both down- and upstream for some minutes. For a minute or two, `named'
spits out syslog messages about insufficient resources, as the replies
it tries to make return ENOBUFS.
If I were to tweak the sysctl net.inet.ip.intr_queue_maxlen from its
default of 50 up, would that possibly help named? Or might that cause
problems elsewhere? Or should I ignore this, or would the best possible
solution be for me simply not to send any more mail to the lists?
I can think of a few possibilties for this being made worse over my
thin pipe. Comments about my thoughts below are welcome, to help me
improve my understanding of things.
I'm usually filling the downstream pipe even without the queries
coming in (pay-per-minute so I'm trying to maximize use of pipe).
This alone may worsen things, as incoming queries see a high latency,
causing them to be repeated before a response is received, possibly
causing other nameservers to initiate queries to me, resulting in many
more queries coming in than if I returned answers promptly.
The size of the outgoing responses is larger than the queries, so it
takes more time to push out responses than it does for them to come in.
These factors combined with the timeouts/retries that resolvers and
nameservers have, mean that no matter what I do, things won't get a
lot better for me.
(As a note, when I sent mails over the cable modem, a different
mailing list software was used by FreeBSD. Still, I'd see heaps of
queries shortly after, just as now. This in the event the current
software makes the deliveries faster at the same time, causing more
simultaneous queries to me. Also, perhaps more sites are doing not
only sender validation but also validation of the from address due
to spam growth the last year.)
I suspect that not all sites are able to successfuly query me, as
after the initial couple minutes of ENOBUFS problems and as the
incoming queries taper off, some time later I'll see a repeat of
the ENOBUFS problem, as I'm assuming another round of attempts is
made to dispose of the queue built up at freebsd.org. If I'm still
online when that happens, to be queried, of course.
I haven't looked to see whether BIND does anything special when an
ENOBUFS pops up in order not to drop the response. Perhaps if it
were to do so, queueing responses, things would only get worse as
the backlog continues to increase, so by the time responses get
sent, the requester has already given up (after sending a few more
queries to increase the backlog further). Thus in such a case the
better thing is to drop random responses in order to get fewer of
them out in a more timely fashion.
Or perhaps I shouldn't worry, trusting that the sites which fail to
receive a response from me directly after a few tries might poke the
offsite secondary nameserver, and that the error-recovery is handled
by DNS, so I shouldn't do anything to UDP to try to help.
Anyway, just for fun, I'm going to double the above sysctl value for
this message and see how things change. Later I'll think about
suspending my downloads to speed up incoming queries. Also, I just
remembered that userland ppp allows me to prioritize certain traffic
so I should try that too, though normally the downloads I do only
snarf a few hundred bytes/sec from the outgoing pipe, so that might
help little....
As noted, comments about my ideas are welcome.
Thanks,
Barry Bouwsma
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