After an upgrade today, resolved appears to have stopped resolving mDNS queries,
despite previously having done so and still being configured to do so. This
appears to be the result of a new override file that appeared in the
package. Unlike multiple other changes that require action to maintain an
existing configuration, I did not get any sort of alert in the upgrade process.

I am aware that I can alter my configuration to override the override, but I do
have to wonder if it isn't more appropriate for the package to alter the default
configuration file than to override whatever is in the configuration file. At
the very least, it should be possible to alert users to the need for a
configuration change, rather than assuming that users will know why things have
broken.

Hi,

This is correct. The Debian Technical Committee asked the systemd
maintainers to disable mDNS support in the default configuration. The
chosen implementation was to place an override file.

The alternative of changing the default at compile time was the method
preferred by the Debian Technical Committee, but that was not acceptable
by the systemd maintainers.

I agree that the disablement deserves a NEWS fragment. Do you use
apt-listchanges to display such NEWS? Would that have made you aware of
the problem? Would adding a note to Debian's release notes help here?

Helmut

Huh. Wouldn't that cause anyone relying on the default setting to run into the
same problem? Or does that cause the default config file to change significantly
enough to show up during an upgrade? The only reason I explicitly enabled mDNS
is because I figured I might as well remove the comment character when disabling
LLMNR.

It's maybe worth noting that it appears the comments in the default
resolved.conf are out of sync with reality. Most relevant here, they explicitly
say that deleting the conf file and drop-ins under /etc/ will restore the
defaults (which appear in the file), which is flatly incorrect when there are
drop-ins present in /usr. Should I suggest changing that to upstream?
I believe so. That's installed by default, isn't it? I have made configuration
changes due to those messages before (and ignored a lot more) over the years, so
I suspect that would have helped.

With greetings,
  Herbert Snorrason

The method of doing this is overriding the setting of
'MulticastDNS=yes' in the /etc/systemd/resolved.conf file. The only
way to get past this is to remove the override file, or create a
overriding override file in /etc/systemd/resolved.conf.d/