[Dnsmasq-discuss] Still investigating delay on first dns query - more info

Chris Green cl at isbd.net
Thu Mar 26 12:55:21 GMT 2020


On Thu, Mar 26, 2020 at 12:02:45PM +0000, Simon Kelley wrote:
> On 23/03/2020 13:25, Chris Green wrote:
> > 
> > I'm running dnsmasq version 2.76 on a Raspberry Pi.  The systems
> > seeing the delay when they send a query are (mostly) running xubuntu
> > 19.10.
> > 
> > The delay only occurs when querying names on the LAN, requests for
> > external names run normally.  It makes no difference whether I give a fully
> > qualified name or just the machine name (the domain gets added by the
> > 'search' option in /etc/resolv.con anyway).
> > 
> > It appears to be something to do with IPV6 and AAAA records (or lack
> > of them) that causes the issue but I'm still stumped as to how to fix
> > it.  
> > 
> > Having set 'log-queries=extra' in /etc/dnsmasq.conf I see the
> > following in /var/log/syslog when I query (using 'host') the name
> > 'esprimo' twice from my laptop after booting (booting the laptop that
> > is).
> > 
[snip log]
> > 
> > So for some reason first time round the querying system asks
> > repeatedly for the AAAA record, then waits 5 seconds, asks again and
> > then gives up.  But it only does this the first time it sends the
> > query.  (I suspect that the delay re-occurs after a long idle time but
> > I can't reproduce the delay by clearing the systemd resolvctl cache)
> > 
> > I realise this probably isn't directly a dnsmasq problem but I'd
> > really appreciate any suggestions or workarounds that might help me
> > fix this issue.  I can easily add more debug flags or configuration if
> > they might help.
> > 
> 
> 
> Dnsmasq looks to be doing the right thing here, if one assumes that what
> it sends is the same as what it logs (and I have not reason to believe
> it isn't).
> 
> Given the MX query, it looks like the originator may be a mail transfer
> agent (sendmail, exim and friends). Can you determine what is making the
> queries and maybe configure it not to use IPv6?
> 
> 
Yes, thank you Simon.  It just dawned on me (thinking about it in bed
last night) that the log only shows what dnsmasq *sent* and not what
was received or acted on at the other end.  I think my next job is
probably to set up wireshark or something like it and actually
observer what is going in and out of the client system.

-- 
Chris Green



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