The computer BIOS
This area is covered in depth in the "Practical Unix and Internet Security" book, but I'll give a brief overview of the basics. Someone turns your main accounting server off, turns it back on, boots it from a specially made floppy disk and transfers payroll.db to a foreign ftp site. Unless your accounting server is locked up what is to prevent a malicious user (or the cleaning staff of your building, the delivery guy, etc.) from doing just that? I have heard horror stories of cleaning staff unplugging servers so that they could plug their cleaning equipment in. I have seen people accidentally knock the little reset switch on power bars and reboot their servers (not that I have ever done that). It just makes sense to lock your servers up in a secure room (or even a closet). It is also a very good idea to put the servers on a raised surface to prevent damage in the event of flooding (be it a hole in the roof or a super gulp slurpee).
The computer's BIOS is on of the most low level components, it controls how the computer boots and a variety of other things. Older BIOS's are infamous for having universal passwords, make sure your bios is recent and does not contain such a backdoor. The bios can be used to lock the boot sequence of a computer to C: only, i.e. the first harddrive, this is a very good idea. You should also use the bios to disable the floppy drive (typically a server will not need to use it), and it can prevent users from copying data off of the machine onto floppy disks. You may also wish to disable the serial ports in users machines so that they cannot attach modems, most modern computers use PS/2 keyboard and mice, so there is very little reason for a serial port in any case (plus they eat up IRQ's). Same goes for the parallel port, allowing users to print in a fashion that bypasses your network, or giving them the chance to attach an external CDROM burner or harddrive can decrease security greatly. As you can see this is an extension of the policy of least privilege and can decrease risks considerably, as well as making network maintenance easier (less IRQ conflicts, etc.). There are of course programs to get the BIOS password from a computer, one is available from: http://www.esiea.fr/public_html/Christophe.GRENIER/, it is available for DOS and Linux.
Once the computer has decided to boot from C:, LILO (or whichever bootloader you use) takes over. Most bootloaders allow for some flexibility in how you boot the system, LILO especially so, but this is a two edged sword. You can pass LILO arguments at boot time, the most damaging (from a security point of view) being "image-name single" which boots Linux into single user mode, and by default in most distributions dumps you to a root prompt in a command shell with no prompting for passwords or other pesky security mechanisms. Several techniques exist to minimize this risk.
this controls how long (in tenths of seconds) LILO waits for user input before booting to the default selection. One of the requirements of C2 security is that this interval be set to 0 (obviously a dual boot machines blows most security out of the water). It is a good idea to set this to 0 unless the system dual boots something else.
forces the user to enter something, LILO will not boot the system automatically. This could be useful on servers as a way of disabling reboots without a human attendant, but typically if the hacker has the ability to reboot the system they could rewrite the MBR with new boot options. If you add a timeout option however the system will continue booting after the timeout is reached.
requires a password to be used if boot time options (such as "linux single") are passed to the boot loader. Make sure you use this one on each image (otherwise the server will need a password to boot, which is fine if youre never planning to remotely reboot it).
requires user to input a password, used in conjunction with restricted, also make sure lilo.conf is no longer world readable, or any user will be able to read the password.
Here is an example of lilo.conf from one of my servers.
This boots the system using the /boot/vmlinuz-2.2.5 kernel, stored on the first portion (right after the MBR) of the first IDE harddrive of the system, the prompt keyword would normally stop unattended rebooting, however it is set in the image, so it can boot linux no problem, but it would ask for a password if you entered linux single, so if you want to go into linux single you have 10 seconds to type it in, at which point you would be prompted for the password ("s0m3_pAsSw0rD_h3r3"). Combine this with a BIOS set to only boot from C: and password protected and you have a pretty secure system. One minor security measure you can take to secure the lilo.conf file is to set it immutable, using the chattr command. To set the file immutable simply:
chattr +i /sbin/lilo.conf
and this will prevent any changes (accidental or otherwise) to the lilo.conf file. If you wish to modify the lilo.conf file you will need to unset the immutable flag:
chattr -i /sbin/lilo.conf
only the root user has access to the immutable flag.