There are different ways to make a device managed/unmanaged in NetworkManager.

by configuration:
  (1) NM_CONTROLLED=yes|no in ifcfg-rh file
  (2) keyfile.unmanaged-devices=spec in NetworkManager.conf
  (3) ENV{NM_UNMANAGED}="0|1" in udev

by explicit user decision via D-Bus
  - nmcli device set $DEV managed yes|no




Previously, a configuration from file/udev could not be overturned and the device was always unmanaged.

Since https://cgit.freedesktop.org/NetworkManager/NetworkManager/commit/?id=87a3df2e572ed47b5f76f6d1cad63ce622296e21 , the user decision overwrites configuration.


Which means, for example you can have a device unmanaged in NetworkManager.conf, but if you choose, you can still explicitly activate a connection.
Once a connection is explicitly managed by the user, the other configuration means are ignored: https://cgit.freedesktop.org/NetworkManager/NetworkManager/tree/src/devices/nm-device.c?id=89d6fa6aa93cdc982992c137f0b694cd339c315c#n9086


This is neat, but it's a change in behavior on which users might rely.

Let's revert that for ifcfg-rh (1) and keyfile/NM.conf (2) and make those settings authoritative again.




Eventually, we should add per-device configuration via NetworkManager.conf and thereby allow to specify which devices do manage/unmanage. This configuration should deprecate keyfile.default-unmanaged and NM_CONTROLLED as per-device configuration should not be handled by settings-plugins.