The first layer of security you need to take into account is the physical security of your computer systems. Who has direct physical access to your machine? Should they? Can you protect your machine from their tampering? Should you?
How much physical security you need on your system is very dependent on your situation, and/or budget.
If you are a home user, you probably don't need a lot (although you might need to protect your machine from tampering by children or annoying relatives). If you are in a lab, you need considerably more, but users will still need to be able to get work done on the machines. Many of the following sections will help out. If you are in an office, you may or may not need to secure your machine off-hours or while you are away. At some companies, leaving your console unsecured is a termination offense.
Obvious physical security methods such as locks on doors, cables, locked cabinets, and video surveillance are all good ideas, but beyond the scope of this document. :)
Many modern PC cases include a "locking" feature. Usually this will be a socket on the front of the case that allows you to turn an included key to a locked or unlocked position. Case locks can help prevent someone from stealing your PC, or opening up the case and directly manipulating/stealing your hardware. They can also sometimes prevent someone from rebooting your computer from their own floppy or other hardware.
These case locks do different things according to the support in the motherboard and how the case is constructed. On many PC's they make it so you have to break the case to get the case open. On some others, they will not let you plug in new keyboards or mice. Check your motherboard or case instructions for more information. This can sometimes be a very useful feature, even though the locks are usually very low-quality and can easily be defeated by attackers with locksmithing.
Some machines (most notably SPARCs and macs) have a dongle on the back that, if you put a cable through, attackers would have to cut the cable or break the case to get into it. Just putting a padlock or combo lock through these can be a good deterrent to someone stealing your machine.
The BIOS is the lowest level of software that configures or manipulates your x86-based hardware. LILO and other Linux boot methods access the BIOS to determine how to boot up your Linux machine. Other hardware that Linux runs on has similar software (OpenFirmware on Macs and new Suns, Sun boot PROM, etc...). You can use your BIOS to prevent attackers from rebooting your machine and manipulating your Linux system.
Many PC BIOSs let you set a boot password. This doesn't provide all that much security (the BIOS can be reset, or removed if someone can get into the case), but might be a good deterrent (i.e. it will take time and leave traces of tampering). Similarly, on S/Linux (Linux for SPARC(tm) processor machines), your EEPROM can be set to require a boot-up password. This might slow attackers down.
Many x86 BIOSs also allow you to specify various other good security settings. Check your BIOS manual or look at it the next time you boot up. For example, some BIOSs disallow booting from floppy drives and some require passwords to access some BIOS features.
Note: If you have a server machine, and you set up a boot password, your machine will not boot up unattended. Keep in mind that you will need to come in and supply the password in the event of a power failure. ;(
The various Linux boot loaders also can have a boot password set.
LILO, for example, has password and restricted
settings; password requires password at boot time,
whereas restricted requires a boot-time password only if you
specify options (such as single) at the LILO prompt.
From the lilo.conf man page:
password=password
The per-image option `password=...' (see below) applies to all images.
restricted
The per-image option `restricted' (see below) applies to all images.
password=password
Protect the image by a password.
restricted
A password is only required to boot the image if
parameters are specified on the command line
(e.g. single).
Keep in mind when setting all these passwords that you need to remember them. :) Also remember that these passwords will merely slow the determined attacker. They won't prevent someone from booting from a floppy, and mounting your root partition. If you are using security in conjunction with a boot loader, you might as well disable booting from a floppy in your computer's BIOS, and password-protect the BIOS.
If anyone has security-related information from a different boot
loader, we would love to hear it. (grub, silo, milo, linload, etc).
Note: If you have a server machine, and you set up a boot password, your machine will not boot up unattended. Keep in mind that you will need to come in and supply the password in the event of a power failure. ;(
If you wander away from your machine from time to time, it is nice to
be able to "lock" your console so that no one can tamper with, or look at,
your work. Two programs that do this are: xlock and vlock.
xlock is a X display locker. It should be included in any Linux
distributions that support X. Check out the man page for it for more
options, but in general you can run xlock from any xterm on your
console and it will lock the display and require your password to
unlock.
vlock is a simple little program that allows you to lock some or all
of the virtual consoles on your Linux box. You can lock just the one
you are working in or all of them. If you just lock one, others can
come in and use the console; they will just not be able to use your
virtual console until you unlock it. vlock ships with redhat
Linux, but your mileage may vary.
Of course locking your console will prevent someone from tampering with your work, but won't prevent them from rebooting your machine or otherwise disrupting your work. It also does not prevent them from accessing your machine from another machine on the network and causing problems.
More importantly, it does not prevent someone from switching out of the X Window System entirely, and going to a normal virtual console login prompt, or to the VC that X11 was started from, and suspending it, thus obtaining your privileges. For this reason, you might consider only using it while under control of xdm.
The first thing to always note is when your machine was rebooted. Since Linux is a robust and stable OS, the only times your machine should reboot is when you take it down for OS upgrades, hardware swapping, or the like. If your machine has rebooted without you doing it, that may be a sign that an intruder has compromised it. Many of the ways that your machine can be compromised require the intruder to reboot or power off your machine.
Check for signs of tampering on the case and computer area. Although many intruders clean traces of their presence out of logs, it's a good idea to check through them all and note any discrepancy.
It is also a good idea to store log data at a secure location, such as a dedicated log server within your well-protected network. Once a machine has been compromised, log data becomes of little use as it most likely has also been modified by the intruder.
The syslog daemon can be configured to automatically send log data to a central syslog server, but this is typically sent unencrypted, allowing an intruder to view data as it is being transferred. This may reveal information about your network that is not intended to be public. There are syslog daemons available that encrypt the data as it is being sent.
Also be aware that faking syslog messages is easy -- with an exploit program having been published. Syslog even accepts net log entries claiming to come from the local host without indicating their true origin.
Some things to check for in your logs:
su entries or logins from strange places. We will discuss system log data later in the HOWTO.