The next step toward better security is to use secure passwords on your network and ensure that users use them as well. For somewhat more physical security, you can force the use of a password with the LILO or GRUB bootloaders, remove bootable devices such as floppy and CD-ROM drives, or configure a network-booting server for Ubuntu. This approach is not well supported or documented at the time of this writing, but you can read about one way to do this in Brieuc Jeunhomme's Network Boot and Exotic Root HOWTO, available at http://www.tldp.org/HOWTO/Network-boot-HOWTO/. You can also read more about GRUB and LILO in Chapter 35, "Kernel and Module Management."
Also, keep in mind that some studies show that as much as 90% of network break-ins are by current or former employees. If a person no longer requires access to your network, lock out access or, even better, remove the account immediately. A good security policy also dictates that any data associated with the account first be backed up and retained for a set period of time to ensure against loss of important data. If you are able, remove the terminated employee from the system before he leaves the building.
Finally, be aware of physical security. If a potential attacker can get physical access to your system, getting full access becomes trivial. Keep all servers in a locked room, and ensure that only authorized personnel are given access to clients.
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