This chapter introduced netfilter/iptables and iproute2. A very important thing for anyone building firewalls is to know how and where packets are analyzed. For that, we introduced a diagram of how packets traverse the chains in the filter, nat, and mangle tables for netfilter.

For beginners, a first look the iptables syntax might seem a bit difficult. An iptables rule contains the table on which we make an operation (filter table being default), a command (append, insert, delete, list), some filtering specifications to match the packets we want, and a target (DROP, ACCEPT, REJECT, LOG) that specifies what we want to do with the packet.

The iproute2 package introduces two complex tools. One is ip, which can be used to set up Layer 3 communication like IP addresses and routing. tc stands for traffic control, and it is used to implement QoS.

Before digging into tc commands, we learned a bit of theory on classless and classful queuing disciplines. The best and most popular classful qdiscs...