NetworkManager fails at boot with "Could not connect to D-Bus" on Linux Mint, leaving interfaces down

Direct answer

The most common reason for the boot‑time error “Could not connect to D‑Bus” is that the network-manager.service unit starts before dbus.service is ready. Adding an After=dbus.service ordering (via a drop‑in file) makes NetworkManager wait for the system bus, allowing it to bind and bring interfaces up.

Confirmed facts

• The unit file /lib/systemd/system/network-manager.service does not list dbus.service in its After= dependency.
• When the daemon cannot bind to D‑Bus, it exits and no network interfaces are configured, leaving them disconnected.
• UFW retains its previous state; if it was enabled it stays enabled, but without active interfaces the system has no traffic to filter.

Steps to fix and verify

1. Check whether the D‑Bus daemon is active:

   systemctl is-active dbus.service

   . If it is not active or is masked, resolve that first.
2. Create a drop‑in to add the ordering:

   sudo mkdir -p /etc/systemd/system/network-manager.service.d
   sudo tee /etc/systemd/system/network-manager.service.d/dbus-order.conf >/dev/null <<'EOF'
   [Unit]
   After=dbus.service
   EOF
   

3. Reload systemd and restart the service:

   sudo systemctl daemon-reload
   sudo systemctl restart network-manager.service

4. Verify the change:

   systemctl show --property=After network-manager.service | grep dbus

   Should show After=...dbus.service....
5. Reboot and check the journal for persistence:

   journalctl -u network-manager -b | grep "Could not connect to D‑Bus"

   If no lines appear, the failure is resolved.
6. Confirm interfaces are up:

   nmcli device status

   Devices should show connected or unmanaged only if intentionally left unmanaged.

Missing diagnostic detail

If systemctl is-active dbus.service returns anything other than “active”, the ordering fix alone will not help; please report the exact status of dbus.service so we can address a possible D‑Bus failure instead.