Ubuntu 21.10: Cinnamong network manager applet shows "No connection" while Internet works fine over ethernet


I don't have any problem with Internet access in Ubuntu 21.10 over ethernet, but the network manager applet in Cinnamon shows "No connection".

My settings

 cat /etc/network/interfaces
 # interfaces(5) file used by ifup(8) and ifdown(8)
 auto lo
 iface lo inet loopback
 allow-hotplug enp3s0
 iface enp3s0 inet dhcp

and

cat /etc/NetworkManager/NetworkManager.conf
[main]
plugins=ifupdown,keyfile

[ifupdown]
managed=false

[device]
wifi.scan-rand-mac-address=no

If I set managed=true and restart the network manager

sudo systemctl restart NetworkManager 

then the applet shows "connected to the wired network". But this also allows leads to non working Internet connection after rebooting.

---------------Answer---------------

If the interface exists in /etc/network/interfaces NM won't manage it. So, just comment out the lines pertaining to enp3s0 if you want NM to manage the connection.


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