Finished my investigation. Guess it is not a bug after all, rather — I
would say — a matter of extremely dispersed documentation plus very
unfriendly behavior from the ISP.
The situation is that even if you put mdns_minimal in front of other
entries in hosts in nsswitch, mdns /is not queried/ if the DNS declares
SOA for the "local" domain. Mine does. This is the reason why I was
getting the impression of systemd-resolved passing queries to DNS even
when it should not have. It was actually nss_mdns to let it do so. The
fact that for some time my isp has directed all dns queries to its own
nameserver even when another one was selected did not help in verifying
the behavior with a dns not declaring SOA for local.
Yet, this aspect of the linux mdns implementation should be documented
with much better emphasis, or even better, there should be a way to make
mdns_minimal report/log what it is doing. Currently, you find mention of
the mdns "handover" to dns for local only looking at the very end of
/usr/share/doc/libnss-mdns/README.md.gz where the heuristics is
explained.
Because heuristics /can go wrong/, the item should IMHO be given much
better emphasis. For instance, I suggest that ubuntu ships with a man
page for nss_mdns and nss_mdns_minimal.
** Changed in: systemd (Ubuntu)
Status: New => Invalid
** Package changed: systemd (Ubuntu) => nss-mdns (Ubuntu)
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https://bugs.launchpad.net/bugs/1883793
Title:
systemd-resolved leaks mDNS queries to DNS
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