Diagnose DNS problem

[Paul Smith]

On Sun, 2022-10-02 at 14:55 +0100, Colin Law wrote:
> I have an Ubuntu 18.04 system at a relative's home that has a DNS
> issue that I don't know how to diagnose.  When I run, for a
> particular
> domain
> 
> ping a.domain.com
> 
> I get
> ping: a.domain.com : Name or service not known
> but when I run ping with the actual IP address it is ok, and if I run
> it (using the name) from any other location it works fine.  I assume
> that means that I have a DNS problem with that machine or network.
> DNS seems to work ok on all other domains that I have tried.  I don't
> know how to diagnose this, so any suggestions will be gratefully
> received.

18.04 is pretty old and DNS configuration has gone through a number of
changes; even before it could be done a number of ways, so I'm not
exactly sure how it's configured on that system.

The thing to look at is the contents of the /etc/resolv.conf file on
the system.  That will tell you (or anyway, us :)) what type of DNS
setup the system uses.

There are two broad possibilities: either it will contain a specific
upstream DNS server IP address, either for the router on your local LAN
(e.g., 192.168.x.y) or for some upstream DNS server; or it will point
back to your local system (e.g., 127.0.0.1) which means you have a
local DNS server (maybe something like dnsmasq) running on your system.

Before we can advise you on next steps we need to know how the system
is configured.