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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