Oh, I see it’s something Debian specific. On an Ubuntu Jammy host, networking is working in an Ubuntu Jammy container, but doesn’t work in a Debian Bullseye container. Is the reason known?
Apparently systemd-networkd doesn’t start:
Oct 21 08:03:57 c systemd[1]: Starting Network Service...
Oct 21 08:03:57 c systemd[151]: systemd-networkd.service: Failed to set up mount namespacing: Permission denied
Oct 21 08:03:57 c systemd[151]: systemd-networkd.service: Failed at step NAMESPACE spawning /lib/systemd/systemd-networkd: Permission denied
Oct 21 08:03:57 c systemd[1]: systemd-networkd.service: Main process exited, code=exited, status=226/NAMESPACE
Oct 21 08:03:57 c systemd[1]: systemd-networkd.service: Failed with result 'exit-code'.
Oct 21 08:03:57 c systemd[1]: Failed to start Network Service.
Oct 21 08:03:57 c systemd[1]: systemd-networkd.service: Scheduled restart job, restart counter is at 1.
Oct 21 08:03:57 c systemd[1]: Stopped Network Service.
The following topic looks a bit similar. Debian Bullseye runs systemd-247.
Submitted an issue, because it looks like a bug.