[Dnsmasq-discuss] Finding actual DNS server used

[Albert ARIBAUD]

Hi again Chris,

Le Sat, 14 Jan 2017 16:06:39 +0000
Chris Green <cl at isbd.net> a écrit:

> On Sat, Jan 14, 2017 at 03:40:52PM +0100, Albert ARIBAUD wrote:
> > > I've not spotted anything in the manual page that stands out for
> > > that purpose.  
> > 
> > There is not much point for it, is there? I mean, if dnsmasq has
> > upstream servers (possibly per request domain) and acts as a local
> > server, it is so that DNS clients on the LAN do *not* have to know
> > these upstream servers.
> > 
> > Or maybe I am missing something. What is your use case?
> >   
> Well for one it's useful to be able to check whether dnsmasq is using
> a sensible DNS server.  

Whatever server dnsmasq uses, it does so because its configuration
tells it to. The servers in this configuration are there either
because they were put in there by the dnamasq host admin (e.g., for
handling unqualified names as LAN names), or because the host has one
or more interfaces on which it is a DHCP client, not server, and the
actual DHCP server announces a DNS server which e.g. the Network
Manager added to the dnsmasq config. Do you see another case?

> On my home LAN I have a full dnsmasq running on a Raspberry Pi and
> point all the other systems at that for DNS.  The other systems
> include a number of xubuntu Linux systems which run the 'local only'
> dnsmasq which is run automatically by Network Manager.

Pretty much the same here on the very machine I am typing on right
now except my LAN's dnsmasq does not run on a RPi. :)

> Thus in my case, to prove that everything is working as intended, I'd
> like to be able to see that all systems (except the Raspberry Pi) are
> using the Raspberry Pi as their DNS server.  In addition I might also
> want to check what upstream servers the Pi is using.

I don't understand your problem... The local dnsmasq on my Xubuntu
machine takes its servers from its only source of DNS server IPs: the
DHCP (and RA) info it receives from my LAN's dnsmasq, and I control that
(as you control that on your RPi's dnsmasq), therefore I *know* which
DNS servers my Xubuntu machine can use.

Why do you think any of your Xubuntu machine would use a DNS server it
was not provided through DHCP or RA ?

Amicalement,
-- 
Albert.