[SOLVED] 18.04 - how to force usage of DNS server assigned by DHCP?

Hi all

How can I force 18.04 to always and only use the DNS server assigned to it by DHCP?

We have some 20 laptops running Slackware. We're testing Ubuntu, and so far this is a right proper showstopper. I can't for the life of me figure out why 18.04 insists on using the WAN addresses of all our servers, even when the laptop is on the LAN and its network is configured to use DHCP and a DNS that resolves hosts to their proper LAN addresses. The WAN address is perfectly fine when the laptops are outside of the LAN, but on the LAN we need the LAN addresses.

Note that this is our first attempt at Ubuntu. This has never been an issue with Slackware (nor the few OpenBSD machines we have). Those just do what we tell them to do, no more no less. Ubuntu 18.04 seems to have a will of its own when it comes to DNS.

We have these 4 scenarios:

Slackware machines on LAN, network configured by DHCP
$ host foo.mydomain
foo.mydomain has address [the hosts LAN address]
YAY! All is well

Slackware machines roaming, network configured by DHCP
$ host foo.mydomain
foo.mydomain has address [the hosts WAN address]
YAY! All is well

Ubuntu 18.04 machine on LAN, network configured by DHCP - TROUBLE AHEAD!
$ host foo.mydomain
foo.mydomain has address [the hosts WAN address]
WAN!! ARGH!!! What the heck??

Ubuntu 18.04 machine roaming, network configured by DHCP
$ host foo.mydomain
foo.mydomain has address [the hosts WAN address]
YAY! All is well

I hope (and suspect) this is all very simple, and that it is just us being stuck in our Slackware ways. Any and all help would be greatly appreciated.

It seems I'm getting hit by the same problem as this individual:

https://github.com/systemd/systemd/issues/8851

The plot thickens!

After reboot:

$ systemd-resolve foo.mydomain # returns proper LAN address
$ ping foo.mydomain # pings LAN address
$ systemd-resolve foo.mydomain # now returns WAN address
$ ping foo.mydomain # pings WAN

What. The. Heck.

Solved by doing the following:

1. Add domains to /etc/systemd/resolved.conf
2. rm /etc/resolv.conf
3. ln -s /run/systemd/resolve/resolv.conf /etc/resolv.conf
4. reboot

This seems to have fixed the issue, for now at least.