In this chapter we'll explore essential aspects of properly installing, maintaining, and interacting with the iptables firewall on Linux systems. We'll cover iptables administration from the perspectives of both kernel and userland, as well as how to build and maintain an iptables firewall policy. A default policy will be constructed that will serve as a guide throughout several chapters in the book; a script that implements it and a network diagram are included for reference in this chapter. Many of the example attacks throughout this book will be launched from hosts shown in this network diagram. Finally, we'll cover testing the default iptables policy to ensure that it is functioning as designed.