The issues of tuning the Linux OS for maximum performance and security are considered. Basic administration and access control, Firewall configuration, file sharing server, WEB, FTP and Proxy servers, email delivery programs, DNS services, as well as system monitoring policies and data archiving are described. Potential vulnerabilities are given, recommendations are given to prevent possible attacks, and it is shown how to act in the event of an attack or hacking of the system in order to restore its functionality as quickly as possible and prevent data loss. The sixth edition has updated the information to reflect the latest version of Ubuntu and added a description of programs for testing the security of the OS configuration.