It should go without saying that you need to stay up-to-date on patches. We won't focus too much on that, aside from saying: automate! Securing a machine is an entirely different topic all together, but here are a few points to consider:

Enabling SELinux (Security-Enhanced Linux) is your first line of defense against unknown attacks. SELinux can prevent buffer overflows, as opposed to simply taking the "updates" path, which requires that a publicly known hole be fixed before some tries to exploit it. SELinux provides a significantly improved access system to limit programs from accessing things they don't require to be operational. That, combined with overflow prevention makes it quite difficult to compromise a Linux system.

Further, on the topic of securing a multi-user machine, there is a much-debated precept: that users shouldn't be able to see what processes are running, unless they own them. This restriction is simple to enable in Linux and the BSD's, but does it really buy you anything? The answer is "maybe," and at the same time, "not really."

To satisfy the "maybe" camp, consider a process's arguments. When you run a command with a given set of arguments, the command as well as the arguments will show up in a 'ps' listing. If you have provided a password on the command line, it will be visible to anyone running a 'ps' while your process is still running. Many people think that allowing users to see running daemon processes on a server will allow them to know what to try attacking. This information is trivial to obtain via other means anyway, so "not really."

Every time this discussion starts, someone quickly suggests a chroot jail. "chroot" stands for "change root," which does just that.

If you run the command: 'chroot /home/charlie /bin/bash,' chroot will execute the shell /home/charlie/bin/bash, and then proceed to lock you into /home/charlie. The new root of the file system, for the lifetime of the bash shell, is /home/charlie. You now have zero access to any other part of the actual file system. Any available command, and its required libraries, needs to be copied into the chroot jail. Providing a usable environment is a ton of work. It's actually easier to give each user their own Linux Xen or Solaris Zone instance.