ifplugd is a Linux daemon that can monitor network interfaces
for a change in state (i.e. up or down) then trigger custom
actions or scripts based on the change.

This could be useful in a number of situations.
For example where the network is configured manually
(no NetworkManager etc), and you wish to reconfigure your
network settings when an ethernet cable is connected 
and then deconfigure those same settings when the cable is
disconnected.

To change which interfaces are monitored by default, and
the general start up options, edit "/etc/ifplugd/ifplugd.conf".
Actions can be added/changed by editing "/etc/ifplugd/ifplugd.action".

To make ifplugd auto start on boot.
First run "chmod 644 /etc/rc.d/rc.ifplugd"
Then add the following to the end of your "/etc/rc.local" file:

if [ -x /etc/rc.d/rc.ifplugd ]; then
   sh /etc/rc.d/rc.ifplugd start || /bin/true
fi