Linux Firewalls, Third Edition

Chapter 4. Building and Installing a Standalone Firewall

Chapter 2, “Packet-Filtering Concepts,” covered the background ideas and concepts behind a packet-filtering firewall. Each firewall rule chain has its own default policy. Each rule not only applies to an individual INPUT or OUTPUT chain, but also can apply to a specific network interface, message protocol type (such as TCP, UDP, or ICMP), and service port number. Individual acceptance, denial, and rejection rules are defined for the INPUT chain and the OUTPUT chain, as well as for the FORWARD chain, which you'll learn about at the end of this chapter and in Chapter 6, “Packet Forwarding.” This chapter pulls together those ideas to demonstrate how to build a simple, single-system firewall ...