Tips on keeping your Ubuntu Linux server secure
As a system administrator, one of your chief tasks is dealing with server security. If your server is connected to the Internet, for security purposes, it's in a war zone. If it's only an internal server, you still need to deal with (accidentally) malicious users, disgruntled employees and the guy in accounting who really wants to read the boss's secretary's e-mail.
In general, Ubuntu Server is a very secure platform. The Ubuntu Security Team, the team that produces all official security updates, has one of the best turnaround times in the industry. Ubuntu ships with a no open ports policy, meaning that after you install the machine - be it an Ubuntu desktop or a server - no applications will be accepting connections from the Internet by default. Like Ubuntu desktops, Ubuntu Server uses the sudo mechanism for system administration, eschewing the root account. And finally, security updates are guaranteed for at least 18 months after each release (five years for some releases, like Dapper), and are free.
In this section, we want to take a look at filesystem security, system resource limits, dealing with logs and finally some network security. But Linux security is a difficult and expansive topic; remember that we're giving you a crash course here, and leaving a lot of things out - to be a good administrator, you'll want to learn more.