Symptoms: after doing a big release upgrade on Ubuntu, the name resolution stops working. You can’t use domain names to reach websites or servers. The file /etc/resolv.conf shows the nameserver is set to 127.0.0.53
What’s happened here is that the traditional DNS resolution system has been replaced by the systemd name resolution system.
The quick fix is to edit the file /etc/systemd/resolved.conf, and set the value of DNS thus:
DNS = 184.108.40.206 220.127.116.11 2001:4860:4860::8888 2001:4860:4860::8844
That sets the name resolver to Google’s. The config file lists a few other services.
Then restart the systemd-resolved service:
systemctl restart systemd-resolved
I don’t know when name resolution changed in Ubuntu, but I was running a very old LTS, and upgraded to the latest non-LTS version.
The old style of name resolution was only DNS. The new style is DNS and a lot more – you can set different name resolvers for different network interfaces.
See also: systemd-resolved man page.