This intermediate tutorial covers different mechanisms for capturing and manipulating packets. Security applications -- such as VPNs, firewalls, and sniffers, and network apps such as routers -- rely on methods like those described here to do their work. Once you have the hang of them, you will rely on them too. You won't want to miss this tutorial all about packet interception, covering kernels from 2.2.x to the present and techniques from divert socket and netfilter to interrupt handling and messing with the kernel source code itself.