I want to run a script or program immediately after my ppp interface is up. How do I do that?

The network control scripts in /etc/sysconfig/network-scripts allow you to add a script that will run after the ppp device is up.

For example, looking at the file /etc/sysconfig/network-scripts/ifup-ppp, which controls ppp interfaces, the first few lines are:

#!/bin/bash

. /etc/init.d/functions

cd /etc/sysconfig/network-scripts
. network-functi...