Comment 8 for bug 1449001

Just upgraded to 17.04 from 16.04 and found DNS resolution stopped randomly working.

As it turns out systemd-resolved decided for some obscure reason to switch to google DNS which it can't reach compared to the locally provided recursive resolver which continues to work just fine.

In summary in my case it isn't just a privacy concern but actually breaks DNS resolution.

Aug 05 11:29:07 dtank0 systemd-resolved[8051]: Switching to system DNS server 10.124.196.1.
Aug 05 11:29:07 dtank0 systemd[1]: Started Network Name Resolution.
Aug 05 11:33:58 dtank0 systemd-resolved[8051]: Switching to fallback DNS server 8.8.8.8.

After the switch to 8.8.8.8 DNS resolution on the host stopped working because 8.8.8.8 is not reachable from the host.

Interestingly stopping and disabling systemd-resolved followed by an "resolvconf -u" did not revert the config back to a working configuration.
It required removing /run/resolvconf/interface/systemd-resolved by hand (starting systemd-resolved will add that file but not remove on stop).