Why are there no upstream resolver entries in /etc/resolv.conf ?
Basically, there should be upstream resolvers that your server uses to resolve domain names, which typically are in the form
nameserver xxx.xxx.xxx.xxx
(IPv6 will also work.)
You would also be running BIND, which is a DNS server running on your server. You can name the primary nameserver whatever you like as long as it’s resolvable, which basically means that you went to your registrar and registered the nameserver.
It’s pretty common to use “ns1.example.tld” as a nameserver name, but that’s just common, not mandatory. Virtualmin uses the server’s full hostname by default, which is also fine. So would “barney.example.tld” or “matilda.example.tld” as long as the names are properly registered with the registrar for the domain.
I suggest you add entries for the three nameservers you provided to /etc/resolv.conf in format
nameserver 79.143.182.242
nameserver 178.238.234.231
nameserver 5.189.191.29
I also suggest that you create you own BIND nameserver as “ns.yourdomain.tld” to distinguish it from the machine’s hostname (just to make things clearer for you when diagosing DNS issues: it makes no practical difference what the nameserver is named), and register the nameserver name you created with your registrar to make it resolvable. Then use that in the “Primary nameserver” field.
By the way, I’m no Ubuntu expert; so if someone smarter than me says “NO, don’t listen to that idiot!” by all means defer to the smarter person. I think this sort of thing is pretty uniform across distros, but I don’t know that for sure.
Richard
EDIT: Also, obviously, make sure that any sites hosted on the server have their authoritative nameservers set to whatever nameserver name you choose in their registrars.