DNS delay on first lookup, Ubuntu 19.10 but not earlier versions

[Colin Law]

Are the failing systems ones that have been updated over a period?  I
think there may have been an issue with upgrades not cleanly managing
upgrades to the DNS system.
What does
ls -l /etc/resolv.conf
show? it should be a link to /run/resolveconf/resolv.conf, at least
that is what it is on mine.

If it isn't then move it and create the link


Colin

On Wed, 1 Apr 2020 at 12:19, Chris Green <cl at isbd.net> wrote:
>
> On Wed, Apr 01, 2020 at 09:43:13PM +1100, Karl Auer wrote:
> > On Wed, 2020-04-01 at 11:16 +0100, Chris Green wrote:
> > > No difference (but 'dig' doesn't show a delay), here's a 'dig'
> > > followed by a 'host' immediately after reboot on one of the systems
> > > that shows the delay:-
> >
> > If dig doesn't show a delay, then it is NOT the DNS itself that is
> > causing the delay, but the querier (browser, whatever).
> >
> Yes, I know that really because it's only two systems (running xubuntu
> 19.10) that show the delay.  I've always been pretty sure that it's a
> client (or something on the client) problem.
>
>
> > BTW those two queries were for different names, so not a useful
> > comparison.
> >
> Oops, sorry, I keep putting 'esprimo.co.uk' by mistake.  In reality I
> nearly always use short names (i.e. just 'esprimo') because there is a
> 'search zbmc.eu' in /etc/resolv.conf.  I have just been putting the
> FQDN here to make things clearer.
>
>
> > You could also bypass dnsmasq by doing this:
> >
> >    dig esprimo.zbmc.eu @ip-address-of-nameserver
> >
> No local dnsmasq on these systems, the 'dnsmasq run by Network Manager
> for local DNS cache' was dropped a few Ubuntu versions ago I think.
> Now it's systemd that is supposed to do local DNS caching and I
> suspect it's something in that which is causing this delay.
>
> Just to prove the problem is local/client here's another sequence of
> 'host' commands directly after reboot:-
>
>     chris$ time host esprimo 192.168.1.4
>     Using domain server:
>     Name: 192.168.1.4
>     Address: 192.168.1.4#53
>     Aliases:
>
>     esprimo.zbmc.eu has address 192.168.1.3
>
>     real        0m0.023s
>     user        0m0.005s
>     sys 0m0.009s
>     chris$ time host esprimo
>     esprimo.zbmc.eu has address 192.168.1.3
>
>     real        0m10.024s
>     user        0m0.001s
>     sys 0m0.013s
>     chris$ time host esprimo
>     esprimo.zbmc.eu has address 192.168.1.3
>
>     real        0m0.016s
>     user        0m0.006s
>     sys 0m0.005s
>     chris$
>
> Going direct to my DHCP/DNS server 192.168.1.4 the lookup is fast,
> *then* using the local system it's slow once, then subsequent lookup
> is fast.  If I could simply turn off the local cache/lookup it would
> probably speed things up for me, as the DNS server is on the LAN
> and I'm really the only user it's fast enough without a local cache.
>
>
> > ... but from the sound of it there will be no difference anyway.
> >
> > What are you doing that causes the lookups? A browser, something else?
> >
> Most things in the 'real world', e.g. 'ssh esprimo.zbmc.eu', 'host
> esprimo.zbmc.eu', 'nslookup esprimo.zbmc.eu' all show the delay when
> run immediately after boot (and after a long idle time).  I've not
> tried browsing a page on esprimo.co.uk (though there are web pages I
> could try immediately after boot).
>
>
> It would be interesting to know what the idle time is before the delay
> happens again.  I've tried clearing the systemd-resolvd cache but that
> doesn't make the delay happen again.  As it is the only guaranteed way
> I have of seeing the delay (other than leaving the system idle for a
> * long* indeterminate time) is to reboot it.  Fortunately one of the
> * 19.10 systems reboots very quickly so it's not that painful doing
> * lots of reboots.
>
> --
> Chris Green
>
> --
> ubuntu-users mailing list
> ubuntu-users at lists.ubuntu.com
> Modify settings or unsubscribe at: https://lists.ubuntu.com/mailman/listinfo/ubuntu-users