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This chapter provides a brief introduction to securing Linux on a network. It covers some of the basics, such as the OSI model, that you need in order to get started on this process. The chapter describes how you can set iptables firewall rules manually and use the iptables service directly, without the firewalled service. A wonderful tool to help you review your network services from a network standpoint is the nmap security scanner. The nmap utility is available in most Linux distribution repositories. Completely disabling an unused service is fine, but for needed network services, you must set up access control. This control can be accomplished, via the /etc/ hosts.allow and /etc/hosts.deny files, for selected services on Linux systems that incorporate TCP Wrapper support. Firewalls can be placed into different categories, depending upon their function. Each category has an important place in securing your server and network.