Just wanted to give an update in this thread as we now think we fully understand the problem.
The regression was indeed introduced through a bugfix in systemd 244.1 as linked in the post above.
The systemd developers refused to back this change to fix our users arguing that the new logic is correct and that the problem is that /sys is writable in our containers.
We can’t make /sys read-only as we specifically need it writable for a number of other network operations (bridges for libvirt and the like). We also care about having udev running in containers to handle our device hotplug logic for which we’ve done kernel work in the past few years.
So this gets us in a bit of a stuck situation as far as easy fixes are concerned. We have identified one kernel issue which prevents udev from behaving in the way networkd expects it and @brauner is working on fixing this upstream, though as with any kernel change, this will take time to roll out to all distros.
The issue can be worked around a few other ways in the mean time:
- Have individual distros revert the systemd change (we will push for Ubuntu to do that)
raw.lxc to force /sys to be read-only (as suggested above)
- Use a systemd override on the
systemd-networkd unit to give it a read-only /sys
It’s that last option we’re now investigating for our own images. The plan is to ship a very small systemd unit override in all affected images to make networkd behave as it did previously. Once our kernel change is widely available, this workaround can then be removed.