Running a command when a specific interface is created or destroyed

[Rolf Nielsen]

Hi,

I use my phone's 3G/4G connection to connect my computer to the
internet. With the help of the urndis driver, it comes up as ue0.
However, if I reboot the computer, the phone automatically disables USB
tethering, and I can't re-enable it until the boot is complete.

Being somewhat paranoid, I use an OpenVPN service, and I want the cilent
to start automatically. I obviously can't set it to start from rc.conf,
as I can't log in to the server before I have an internet connection.

So, is there a way to start the OpenVPN client automatically upon
creation of the ue0 interface? E.g. can dhclient be set up to run the
openvpn start script after it has given ue0 an IP address? And is there
a way to automatically stop the openvpn client if ue0 is destroyed, e.g.
if I disconnect my phone from the computer?

Rolf Nielsen