I am playing around with networking component of latest Ubuntu. I faced the above issue when I tried to manage networking devices with NM.
The command line output is
root@sankar:~# nmcli d s DEVICE TYPE STATE CONNECTION eth0 ethernet unmanaged -- eth1 ethernet unmanaged -- eth2 ethernet unmanaged -- lo loopback unmanaged --
I tried couple of solutions(comment #36 and #30) given in https://bugs.launchpad.net/ubuntu/+source/network-manager/+bug/1638842.
Any help is highly appreciated