How can I write a udev rule to be run/matched when a device is unplugged? I have a 3G modem that I wrote a rule for to run "ifup ppp0" when the modem is plugged in. I would like to have "ifdown ppp0" be automatically run when the modem is unplugged. Without a rule to run "ifdown ppp0", the network state of ppp0 is remembered as up, so if I plug in the modem again, "ifup ppp0" then fails because it says the interface is already configured.