This article is excerpted from The Official Ubuntu Book by Benjamin Mako Hill, Jono Bacon, Corey Burger, Jonathan Jesse and Ivan Krstic, copyright Prentice Hall. Reprinted with permission of Prentice Hall, all rights reserved.

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.

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