Networking is all about connecting computers together, so it follows that a computer network is no more secure than the machines that it connects. A single insecure host can make lots of trouble for your entire network, as it can act as a tool for reconnaissance or a strong base of attack if it is under the control of an adversary. Firewalls, intrusion detection, and other advanced security measures are useless if your servers offer easily compromised services. Before delving into the network part of network security, you should first make sure that the machines you are responsible for are as secure as possible.
This chapter offers many methods for reducing the risks involved in offering services on a Unix-based system. Even though each of these hacks can stand on its own, it is worth reading through this entire chapter. If you only implement one type of security measure, you run the risk of all your preparation being totally negated once an attacker figures out how to bypass it. Just as Fort Knox isn't protected by a regular door with an ordinary dead bolt, no single security feature can ultimately protect your servers. And the security measures you may need to take increase proportionally to the value of what you're protecting.
As the old saying goes, security isn't a noun, it's a verb. That is, security is an active process that must be constantly followed and renewed. Short of unplugging the machine, there is no single action you can take to secure your machine. With that in mind, consider these techniques as a starting point for building a secure server that meets your particular needs.