Diagnose DNS problem

[Ralf Mardorf]

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.

Hi,

I run Debian Edu 11 as a Virtual Machine Manager QUEM/KVM guest. In the
beginning the guest run without any issues. Since a while the guest
suffer from often occurring DNS issues, from one to the other moment the
guest can't resolve domain names anymore, hence either an upgrade of the
Arch Linux host or an upgrade of the Debian Edu 11 guest is the culprit.

For testing purpose I usually run
  
  dig a.domain.org

by the guest. Note, the host is always able to resolve domain names.

I can't rule out that it's a host related issue, maybe a QEMU/KVM bug,
but way more likely it's a Debian Edu 11 guest related issue. Running

  systemctl --type=service

by the guest shows a lot of services I don't use for my host install,
such as

  NetworkManager.service

or

  resolveconf.service

Actually restarting resolveconf.service by the guest sometimes solves
the issue, but sometimes I need to reboot the guest. I never do anything
with the host to get rid of the problem, since the host is always able
to resolve domain names.

I'm interested in suggestions, too.

Regards,
Ralf