What you will achieve

Restore NetworkManager connectivity when Wi‑Fi or Ethernet shows "connecting" forever — stale profiles, wrong MAC randomisation, or conflicting netplan.

1) Restart and logs

sudo systemctl restart NetworkManager
journalctl -u NetworkManager -b --no-pager | tail -50

2) Delete and recreate profile

nmcli connection show
nmcli connection delete "Broken Profile"
nmcli device wifi connect "SSID" password "secret"

3) Conflicts with netplan (Ubuntu server)

ls /etc/netplan/
sudo netplan try

Mixing static netplan with NM on the same interface causes fights — pick one renderer per interface.

4) Wi‑Fi power save and randomisation

nmcli connection modify "SSID" wifi.powersave 2
nmcli connection modify "SSID" wifi.cloned-mac-address permanent

Verify

nmcli general status
nmcli device status
ping -c3 1.1.1.1

5) Reset NM state completely

sudo systemctl stop NetworkManager
sudo rm -rf /var/lib/NetworkManager/*
sudo systemctl start NetworkManager

Back up /etc/NetworkManager/system-connections/ first if profiles are precious.

6) Managed devices

grep -r managed /etc/NetworkManager/
# /etc/NetworkManager/NetworkManager.conf
[ifupdown]
managed=true

7) Ethernet carrier detect

ethtool eth0 | grep Link
dmesg | grep eth0

Bad cable shows "Link detected: no" — no amount of NM restarts fixes physics.

Corporate VPN interference

Stale routes from OpenVPN may block NM reconnect. Flush routes or restart VPN after NM establishes base connectivity.

8) DNS after connect

resolvectl status
nmcli dev show wlan0 | grep DNS

Connected with no DNS — set ipv4.dns manually or fix DHCP option 6 from router.

Prerequisites

Root/sudo, interface name from nmcli dev, backup of /etc/NetworkManager/system-connections/. Physical link verified (cable seated, Wi‑Fi switch on). Router DHCP pool not exhausted.

Journal follow during reconnect

journalctl -u NetworkManager -f

Watch real-time errors while toggling Wi‑Fi — EAP failures and DHCP NAK show here clearly.

ModemManager conflict

Some USB LTE modems confuse NM — systemctl mask ModemManager on servers without cellular modems.

802.1x certificate expiry

Corporate Wi‑Fi stops working when RADIUS client cert expires — NM logs show TLS handshake failure not wrong password. Renew cert, update profile, delete old connection UUID. For DHCP starvation, nmcli dev show shows IP4.ADDRESS empty while associated — tcpdump on wlan0 for DHCP discover/offer ratio.

Bluetooth tethering

Phone USB tether creates rndis interface — NM sometimes needs nmcli connection add type ethernet ifname rndis0 con-name phone manually first pairing. IPv6-only networks break legacy apps — set NM ipv6.method ignore on problematic captive portals.

Kernel downgrade test

If Wi‑Fi broke immediately after HWE kernel update, boot previous kernel from GRUB Advanced Options — confirms driver regression while you file bug or hold linux-image until fix ships.