General Steps to Increase System Security

Preparation: The first step is to understand security by doing research and then use that knowledge to plan your defense.

Planning: Security is an ongoing process, not just something you do once. Planning should be continual. Any security plan is only as good as its weakest point. After an attacker breaches [...]

Firestarter is an Open Source visual firewall program. The software aims to combine ease of use with powerful features, therefore serving both Linux desktop users and system administrators.
Firestarter features

Open Source software, available free of charge
User friendly, easy to use, graphical interface
A wizard walks you through setting up your firewall on your first time
Suitable for use [...]

Crackers use many methods to gain access to a system, here is a list of most common methods to block unwanted access :

Every Servers that you run is a potential doorway for both legitimate users and cracker to have access to your system. So if you don’t absolutely need a server to run, shut it [...]

PDFCrack is a GNU/Linux (other POSIX-compatible systems should work too) tool for recovering passwords and content from PDF-files. It is small, command line driven without external dependencies.

Supports the standard security handler (revision 2 and 3) on all known PDF-versions
Supports cracking both owner and userpasswords
Both wordlists and bruteforcing the password is supported
Simple permutations (currently only trying [...]