After replacing DNS resolver daemons a bunch of weeks ago in couple places, found the hard way that nothing is quite as reliable as (t)rusty dnscache from djbdns, which is sadly too venerable and lacks some essential features at this point.
Complex things like systemd-resolved and unbound either crash, hang or just start dropping all requests silently for no clear reason (happens outside of conditions described in that email as well, to this day).
But whatever, such basic service as name resolver needs some kind of watchdog anyway, and seem to be easy to test too - delegate some subdomain to a script (NS entry + glue record) which would give predictable responses to arbitrary queries and make/check those.
Implemented both sides of that testing process in dns-test-daemon script, which can be run with some hash-key for BLAKE2S HMAC:
% ./dns-test-daemon -k hash-key -b 127.0.0.1:5533 & % dig -p5533 @127.0.0.1 aaaa test.com ... test.com. 300 IN AAAA eb5:7823:f2d2:2ed2:ba27:dd79:a33e:f762 ...
And then query it like above, getting back first bytes of keyed name hash after inet_ntop conversion as a response address.
Good thing about it is that name can be something random like "o3rrgbs4vxrs.test.mydomain.com", to force DNS resolver to actually do its job and not just keep returning same "google.com" from the cache or something. And do it correctly too, as otherwise resulting hash won't match expected value.
So same script has client mode to use same key and do the checking, as well as randomizing queried names:
% dns-test-daemon -k hash-key --debug @.test.mydomain.com
(optionally in a loop too, with interval/retries/timeout opts, and checking general network availability via fping to avoid any false alarms due to that)
Ended up running this tester/hack to restart unbound occasionally when it craps itself, which restored reliable DNS operation for me, essential thing for any outbound network access, pretty big deal.