On upgrade tries to replace /etc/systemd/resolved.conf

[Léa GRIS]

When upgrading from Ubuntu 20.04.1 to 20.10, the new package tries to replace resolved.conf which would break custom DNS configuration

[Dan Streetman (ddstreet)]

when you say "tries to replace" do you mean this:

Configuration file '/etc/systemd/resolved.conf'
 ==> Modified (by you or by a script) since installation.
 ==> Package distributor has shipped an updated version.
   What would you like to do about it ? Your options are:
    Y or I : install the package maintainer's version
    N or O : keep your currently-installed version
      D : show the differences between the versions
      Z : start a shell to examine the situation
 The default action is to keep your current version.
*** resolved.conf (Y/I/N/O/D/Z) [default=N] ?

that's what I see when upgrading with a modified conf file, and it's expected behavior. If that's what you are seeing, then there is no bug, just say "N" to keep your modified version, or say "Y" and then modify it again after upgrade is complete.

[Léa GRIS (lea-gris)]

When upgrading a system with this file altered, it is important to keep it unchanged or the system will not resolve DNS on reboot.

No need to ask. Just don't overwrite the file if it has been changed.

Because if it has been changed, there is a reason, and pushing the package's default config is surely going to break that intended change. This is common-sense isn't it?