Tomas Krizek <tomas.krizek(a)nic.cz>cz>, 2019-03-18 11:35:17 (+0100):
  `systemctl mask kresd.socket` is the correct approach.
I tested this
 with a clean installation of Knot Resolver 3.2.1 and the service works
 as expected (it is in 'active' state, not 'degraded').
 I'd suggest to remove any drop-ins to restore the service and sockets to
 its pristine state, do `systemctl daemon-reload` and try masking the
 kresd.socket again. If you're still able to reproduce the degraded
 state, please tell us which OS distribution you use and its systemd
 version to debug this further. 
Thanks, and I would have found out about masking had I read kresd.systemd
more carefully than I did.
I had to do `systemctl reset-failed kresd.socket` to get back to `running`
state.