[nsd-users] Ignored IPv6 requests

[W.C.A. Wijngaards]

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Hi Stephane,

On 01/27/2014 03:29 PM, Stephane Bortzmeyer wrote:
> I switched a low-usage DNS server from NSD 3 to NSD 4 and I see
> now that, from time to time, it stops serving IPv6 requests
> (timeout). The problem seem to occur randomly, it lasts a few
> minutes, then cures itself. Nothing similar for IPv4. (By the way,
> the -6 option of -ex-Nagios monitoring plugins is broken, see 
> <https://github.com/monitoring-plugins/monitoring-plugins/issues/1228>.)
>
>  It is not a network/L3 issue: when the problem happens, even a dig
> on the name server to itself over the local interface fails. Also,
> with tcpdump on the name server, I see DNS requests coming in but
> nothing coming out.

I think this (could be) a bug in recvmmsg.  This is a (relatively) new
syscall that NSD4 uses to get more performance.  On Linux, new recent
kernels.  However, it shows to have issues.

You can compile --disable-recvmmsg.  Then it uses good-old recvfrom,
and that should work.

> strace on the daemon does not show a
> recvmmsg(sa_family=AF_INET6...) for my address when tcpdump sees
> the packet coming in.

Does the strace also show a result for that recvmmsg call, i.e. -1
errno=.. or so?  I need some sort of way to determine if this bug
happens, otherwise I'd have to switch of using recvmmsg completely ...
 At least no longer ship it by default, or perhaps perform a linux
kernel-version check in configure (but that gives binaries that then
later do not work when run on another machine).

> Could it be because of something else than NSD? At the same moment,
> I switched to Linux kernel 3.12.6 on a Linode VPS. Any similar
> problems with this setup? (If I have time, I will try with Knot, to
> compare and to see if it's the fault of the kernel.)

It'll be recvmmsg.  Not sure if Knot uses that.  The kernel version
may or may not have fixes for it, I guess, so the kernel version could
be important here.

Best regards,
   Wouter


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