L2TP-over-IPSEC on Ubuntu

I expected it to be built-in but L2TP-over-IPSEC support in NetworkManager on Ubuntu 16.4 seems to be unavailable.  Turns out it’s available on Launchpad.

$ sudo add-apt-repository ppa:nm-l2tp/network-manager-l2tp
$ sudo apt-get update
$ sudo apt-get install network-manager-l2tp

The only hitch seems to be that xl2tpd is running by default but needs to be started and configured on-demand.  I disabled it.

$ sudo update-rc.d xl2tpd disable

Now I can start the VPN without issue.