Bug#1017713: systemd: upgrade breaks DNS resolution in some cases

[Luca Boccassi]

Control: severity -1 wishlist
Control: tags -1 wontfix
Control: close -1

On Fri, 19 Aug 2022 13:01:10 +0200 =?UTF-8?Q?Rapha=c3=abl_Halimi?=
<raphael.halimi at gmail.com> wrote:
> Package: systemd
> Version: 251.3-2~exp1
> Severity: critical
> 
> (filing the bug as critical since it "makes unrelated software on the
> system (or the whole system) break", feel free to downgrade)
> 
> Dear developers,
> 
> A recent update of systemd splits systemd-resolved in its own
package, 
> and the new systemd-resolved is not installed by default, thus,
during 
> the upgrade, the systemd-resolved service is stopped and removed
(which 
> seems to be the intended behavior).
> 
> In the (admittedly probably rare) case where systemd-resolved's stub 
> resolver was already in use beforehand (meaning, /etc/resolv.conf was
> already symlinked to /run/systemd/resolve/stub-resolv.conf), the
upgrade 
> completely breaks DNS resolution, since the file (which remains in 
> /run/systemd/resolve) lists 127.0.0.53 as the only nameserver, which 
> doesn't respond anymore since the systemd-resolved service was
stopped.
> 
> The breakage lasts until the user manually fixes it by installing 
> systemd-resolved, but this simple operation may be tricky, because 
> there's no DNS resolution anymore and apt will fail to download the
new 
> package, unless the user manually creates a temporary
/etc/resolv.conf 
> file listing a working name server, or symlinks /etc/resolv.conf to 
> /run/systemd/resolve/resolv.conf instead (which also remains in /run 
> after the service is stopped, and doesn't use the stub resolver since
> this file, unlike stub-resolv.conf, lists the upstream name servers).
> 
> One possible solution would be to check in the maintainer scripts if
the 
> stub resolver is already in use (in other terms, if /etc/resolv.conf
is 
> a symlink to /run/systemd/resolve/stub-resolv.conf), and, if it's the
> case, do what's described above (symlink /etc/resolv.conf to 
> /run/systemd/resolve/resolv.conf instead, thus bypassing the stub 
> resolver). This would keep DNS resolution working (until the next 
> reboot, that is), but the user will at least have the time to read
the 
> NEWS entry, and act accordingly.
> 
> Regards,
> 
> -- 
> Raphaël Halimi

Custom unsupported non-default setups are not supported. The NEWS entry
gives notice, and that's enough.

-- 
Kind regards,
Luca Boccassi
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