There are plenty of online guides covering iptables topics such as how to construct rules, how to build a firewall in Linux, etc. What is often lacking are clear explanations of how networking architecture is structured in Linux and how various firewall and networking modules are interrelated. This category primarily covers iptables, ipchains, and ip rules. It also touches on ufw (Uncomplicated FireWall), and nftables ("NetFilter Tables"), the successor to iptables.
The following topics are covered:
My paper on Linux Networking Concepts is broken down into parts to make it easier to find bits and pieces of information. Or, if you'd prefer you may download the entire document: The Whole Enchilada.
Minor content differences may exist between the online version on this website and the document referenced above.