This document will attempt to explain some procedures and commonly used software to help your linux system be more secure.
The first thing to keep in mind is that there is never any such thing as a "completely" secure computer system. All you can do is make it increasingly difficult for someone to compromise your system. For the average home linux user, not much is required to keep the causal cracker at bay. For high profile linux users (banks, telecommunications companies, etc) much more work is required.
Another factor to take into account is that the more you increase your system security, the more intrusive your security becomes. You need to decide where in this balancing act your system is still usable and yet secure for your purposes. For instance, you could require everyone dialing into your system to use a call back modem to call them back at their home number. This is more secure, but if someone is not at home, it makes it difficult for them to login. You could also setup your linux system with no network or connection to the Internet, but this makes it harder to surf the web. If you are a large to medium sized site, you should establish a "Security Policy" stating how much security is required by your site and what auditing is in place to check it.
This document has been segregated into several sections. They cover several broad kinds of security issues. The first, physical security, covers how you need to protect your physical machine from tampering. The second describes how to protect your system from tampering by local users. The third, network security, describes how to better secure your linux system from network attacks. The next discusses what to do when you detect a system compromise in progress or detect one that has recently happened. Then links to other security resources are enumerated, and finally a few closing words.
The two main points to realize when reading this document are: