Inside the Linux Packet Filter, Part II
Following a packet's journey through the kernel can be an interesting experience. In our trip we encountered typical kernel data structures (such as skbuffs), discovered idiomatic programming techniques (such as the use of structures with function pointers as an efficient alternative to C++ objects) and met some new 2.4 mechanisms (softirqs).
If you are eager to learn more on the subject, arm yourself with kernel sources and a comfortable editor, swallow a good cup of coffee and start peeking here and there. The price is cheap, and fun is guaranteed!