[Dnsmasq-discuss] Occasional "communications error", how to diagnose?

[Geert Stappers]

On Thu, Dec 14, 2023 at 10:04:02AM +0000, Chris Green wrote:
> On Wed, Dec 13, 2023 at 08:59:05PM +0000, Simon Kelley wrote:
> > On 13/12/2023 15:25, Chris Green wrote:
> > > I run dnsmasq version 2.89 on my laptop
> > > which is running [x]ubuntu 23.04.
> > > 
> > > I have systemd.resolvd disabled.
> > > 
> > > I'm occasionally seeing the following error when getting a host's IP:-
> > > 
> > >      chris$ host homepi
> > >      ;; communications error to 127.0.0.1#53: timed out
> > >      homepi has address 192.168.1.113
> > >      chris$ ps -ef | grep dnsmasq
> > > dnsmasq 933 1 0 Dec06 ? 00:00:22 /usr/sbin/dnsmasq -x /run/dnsmasq/dnsmasq.pid 
> > -u dnsmasq -7 /etc/dnsmasq.d,.dpkg-dist,.dpkg-old,.dpkg-new --local-service 
> > --trust-anchor=.,20326,8,2,e06d44b80b8f1d39a95c0b0d7c65d08458e880409bbc683457104237c7f8ec8d 
> > 
> > >      chris      86541    3774  0 15:05 pts/1    00:00:00 grep --color=auto dnsmasq
> > >      chris$
> > > 
> > > As can be seen dnsmasq is running and subsequent queries work without any
> > > error (or delay).  The above timeout is a few seconds, maybe five or a bit
> > > less.
> > > 
> > > There's no dnsmasq related error message in syslog (nothing for today at
> > > all).  The system homepi is a Raspberry Pi on the same LAN as the laptop
> > > running dnsmasq, The error isn't only for one particular host, I've seen
> > > it for other systems on my LAN.
> > > 
> > > Can anyone suggest what might be causing the error and/or how to diagnose
> > > what's wrong?
> > > 
> > 
> > It looks like the first query (or its reply) was dropped, host retried, 
> > and it worked second time around.
> > 
> > Since DNS transport is normally across UDP, which is defined as 
> > unreliable, this is completely normal. Except that the UDP packets are 
> > not actually traversing a network, they're going via the lo interface 
> > within one machine. I'm sure there are circumstances where UDP packets 
> > can get dropped in the kernel when going via the lo interface, but it 
> > shouldn't happen very often. Is the machine under heavy load or memory 
> > pressure? Maybe a network reconfiguration event could drop packets?
> > 
> No, it's not a heavily loaded system by any means.

Acknowledge.


> It's a Thinkpad T470 laptop with an I7 processor and is virtually
> never worked hard at all.  Just randomly running top now shows:-
> 
>     top - 09:59:28 up 12:04,  3 users,  load average: 0.20, 0.12, 0.10
>     Tasks: 254 total,   1 running, 253 sleeping,   0 stopped,   0 zombie
>     %Cpu(s):  1.5 us,  0.2 sy,  0.0 ni, 97.9 id,  0.3 wa,  0.0 hi,  0.0 si,  0.0 st
>     MiB Mem :   7790.8 total,    296.7 free,   1032.4 used,   6461.8 buff/cache
>     MiB Swap:  15258.0 total,  15255.5 free,      2.5 used.   6370.8 avail Mem 
> 
> That's about the way it always is (three users are all me).
> 
> What I don't understand is that there's nothing at all in the logs about the 
> failure/timeout.

Imagination is more important as knowledge    --Albert Einstein

The sympthoms are that client request doesn't reach the server,
hence the report of "time out".


> Can I increase dnsmasq's logging to see if anything shows
> up?  It's just 'my' laptop so there isn't a lot of DNS.

Add another DNS client for collecting more datapoints.
So try to reproduce the issue with `dig` and/or `nslookup`
whenever you encounter it with `host`.


Groeten
Geert Stappers

P.S.
Thanks for making it possible that we can read in the discussion order.
-- 
Silence is hard to parse