Книга: Iptables Tutorial 1.2.2
In this chapter we have discussed several of the chains and tables and how they are traversed, including the standard built-in chains and userspecified chains. This is a very important area to understand. It may be simple, but unless fully understood, fatal mistakes can be equally easily.
The next chapter will deal in depth with the state machine of netfilter, and how states are traversed and set on packets in a connection tracking machine. The next chapter is in other words just as important as this chapter has been.