External security can be split into two categories: intrusion prevention and intrusion detection. Intrusion prevention measures are taken to prevent unauthorized access to a system. If these measures fail, intrusion detection may prove useful in determining when unauthorized access has occurred, and what damage has been done.

A full Linux installation is a large and complex system. It's difficult to keep track of everything that's installed, and even harder to configure each package's security features. The problem becomes simpler when fewer packages are installed. A first step to intrusion prevention is to remove packages you don't need. Take a look back at Part 4 of the LPI 101 series for a review of packaging systems.