10.8.3 Triggering network configuration – ifplugd

The ifplugd daemon brings an interface up or down according to whether or not its underlying hardware is plugged in to a network. The program can detect a live cable connected to an Ethernet interface or an access point associated to a Wi-Fi interface (although waproamd is probably what you want to use in the latter case). When ifplugd sees that the state of the link has changed it runs a proxy script which by default calls ifup or ifdown.