for unmanaged devices, provide details about what software is managing the device

We had a person using Debian bookworm XFCE on an RPi enter the #debian-raspberrypi IRC channel and ask a question about getting their RPi Ethernet working with NM. The NM version was likely 1.36.2-1.

The reason it didn't work with NM is that on the Debian RPi images, eth0 is managed with ifupdown, so it is unmanaged in NM.

The device isn't present in the NM Applet in XFCE so they tried nmtui, which told them Could not activate connection: Connection 'Ethernet connection' is not available on device eth0 because device is strictly unmanaged.

That message lead them to modify the managed parameter in the ifupdown section of /etc/NetworkManager/NetworkManager.conf, which fixed the issue for them, but could lead to weirdness if both ifupdown and NM manage the same device.

The correct fix though, would have been to shut down the network with ifupdown, remove the ifupdown eth0 config in /etc/network/interfaces.d/eth0 and then restart/reload NM so that it picks up the change.

I think it would be helpful if error messages about unmanaged devices in both nmtui and any other UIs written by NM developers be changed to say things like "this device is managed by ifupdown instead of NM" rather than the current messages, seem to lead some people to suboptimal solutions.

I think it would also be good if the GUI tools written by NM developers would expose unmanaged devices, but still refuse to manage them, with a "this device is managed by ifupdown instead of NM" message alongside them in the interface. This would make it more likely that people could help themselves in situations where someone else like distro devs made a device managed with ifupdown/networkd or another too.

PS: while Debian might disable the ifupdown eth0 config by default or switch to NM or something, I think there are other images and situations out there where the above change is likely to be useful.