Run my own DNS on my network both 22.04 server and desktop for testing and understanding (have done for over 2 decades) and it keeps getting reset to the fallback DNS.
I configure my DNS using netplan (changed the values to protect the innocent)
network: version: 2 renderer: networkd ethernets: enp5s0: addresses: [10.10.1.70/24] gateway4: 10.10.1.99 nameservers: search: [mydomain.local] addresses: [10.10.1.1] dhcp4: yes
Originally I fixed this problem by changing /etc/systemd/resolved.conf
[Resolve] DNS=10.10.1.1 18.104.22.168 22.214.171.124 domains=mydomain.local
This seemed to work on 20.04 (I could be wrong). Really feel a failure fixing it by typing
sudo netplan apply
Happy to learn the correct way to configure a DNS now so it is now overwritten. (I am reasonably sure (sound like a politician) that it is not falling back because of network issues.
Love Ubuntu, know it is me, just would like some of that "teach a man to fish" karma