Chapter3.iptables: The Linux Firewall Administration Program

Chapter 3. iptables: The Linux Firewall Administration Program

Chapter 2, "Packet-Filtering Concepts," covers the background ideas and concepts behind a packet-filtering firewall. Each built-in rule chain has its own default policy. Each rule can apply not only to an individual chain, but also to a specific network interface, message protocol type (such as TCP, UDP, or ICMP), and service port or ICMP message type 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." The next chapter pulls those ideas together to demonstrate how to build a simple, single-system, custom-designed firewall for your site.

This chapter covers the iptables firewall administration program used to build a Netfilter firewall. For those of you who are familiar with or accustomed to the older ipfwadm and ipchains programs used with the IPFW technology, iptables will look very similar to those programs. However, it is much more feature-rich and flexible, and it is very different on subtle levels.

There is indeed a difference between iptables and Netfilter, though you'll often hear the terms used interchangeably. Netfilter is the Linux kernel-space program code to implement a firewall within the Linux kernel, either compiled directly into the kernel or included as a set of modules. On the other hand, iptables is the userland program used for administration of the Netfilter firewall. Throughout this text, I will refer to iptables as being inclusive of both Netfilter and iptables, unless otherwise noted.