I noticed this odd message “Networking interface unmanaged” in the Network icon on the system tray, which is supposed to display a list of network interfaces (at least it does that in KUbuntu). It’s impossible to manually connect or disconnect to LAN when this occurs, though an automatic DHCP connection may still be made.
sudo kate /etc/NetworkManager/NetworkManager.conf
The file probably contains:
Change false to true and restart Network Management/etc/init.d/network-manager restart[/code]