fixing unmanaged network interfaces on ubuntu linux

The problem

I honestly get really stressed whenever rebooting my ubuntu crosses my mind. After a system reboot, network interfaces (specifically ethernet/eno1) showed as "unmanaged" in NetworkManager, preventing network connectivity. The system was running Ubuntu with NetworkManager as the network management service.

Symptoms

• Network interface eno1 showed status "unmanaged"
• No internet connectivity
• nmcli device status showed all devices (eno1, docker0, lo) as "unmanaged"
• NetworkManager service was running but not managing any interfaces

Failed attempts

Attempt 1: Basic NetworkManager Commands

sudo nmcli device connect eno1

Failed with error: "Error: Failed to add/activate new connection: Connection 'eno1' is not available on device eno1 because device is strictly unmanaged"

Attempt 2: Netplan Configuration

Created /etc/netplan/01-network-manager-all.yaml:

network:
  version: 2
  renderer: NetworkManager
  ethernets:
    eno1:
      dhcp4: true

Failed because netplan wasn't installed, and couldn't be installed due to no internet connection.

Attempt 3: Basic NetworkManager Configuration

Attempted to create a new connection:

sudo nmcli connection add type ethernet con-name "Wired" ifname eno1
sudo nmcli connection up "Wired"

Failed with error about device being strictly unmanaged.

Attempt 4: NetworkManager.conf Modification

Found that /etc/NetworkManager/NetworkManager.conf had managed=false in the [ifupdown] section, but simply changing it to managed=true didn't resolve the issue.

What worked

1. Remove existing NetworkManager configuration:

sudo rm /etc/NetworkManager/NetworkManager.conf
sudo rm /etc/NetworkManager/system-connections/*

2. Create new NetworkManager.conf:

sudo nano /etc/NetworkManager/NetworkManager.conf

With contents:

[main]
plugins=ifupdown,keyfile
dhcp=internal
no-auto-default=*

[ifupdown]
managed=true

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

3. Create global management file:

sudo touch /etc/NetworkManager/conf.d/10-globally-managed-devices.conf

4. Reset NetworkManager state:

sudo systemctl stop NetworkManager
sudo rm /var/lib/NetworkManager/NetworkManager.state
sudo systemctl start NetworkManager

5. Create new connection:

sudo nmcli connection add type ethernet con-name "Wired" ifname eno1 autoconnect yes

I figured that

1. Simply changing managed=false to managed=true in NetworkManager.conf is insufficient
2. NetworkManager needs (atleast seem so in my case) a complete reset (configurations and state) to properly recognize interfaces
3. The combination of removing state file and creating a fresh configuration was it
4. The presence of the 10-globally-managed-devices.conf file helps ensure NetworkManager manages all interfaces
5. I have a excuse to dig deeper into networking (now call me dr. cuz that's the cure to my anxiety)

Prevention

I don't like the idea of backup network config files so I got claude to write me script to fix this if it ever happens again.

• this is an offline solution for obvious reasons. We don't have connection in the first place.