By Kirk R. Halyk

Editor’s note: As part of a recent IT
blog post I asked Linux users and
evangelists in the TechRepublic community to step up to the plate and take a
crack at producing some informative articles and downloads on the Linux
operating system. This document is just one of the submissions inspired by that
challenge. Just click the Linux challenge tag to track other published submissions
stemming from this grass roots project.

1: Your purpose

Linux, like
Microsoft Windows, is simply a computer operating system. When I talk to
friends or co-workers who are embarking on the Linux experience for their
initial time, this is the first point I stress. Linux in itself is not a magic
wand that can be waved and make all sorts of computing problems disappear.
While Windows has its own set of problems, so too does Linux. There is no such
thing as a perfect or completely secure computer operating system. Will the
machine be a desktop computer or a server; purpose is a key to understanding
how to initially install and configure your Linux PC.

2: Installation

Windows, Linux does not present itself as a “server” version or as a “desktop”
version. During a typical installation of Linux the choice is yours as to
exactly what software you wish to install and therefore exactly what type of a
system you are constructing. Because of this, you need to be aware of the
packages that the installation program is installing for you. For example, some
distributions will configure and start a Samba server or a mail server as part
of the base install. Depending upon the purpose of your Linux PC and the
security level you are prepared to accept, these services may not be needed or
desired at all. Taking the time to familiarize yourself with your distributions’
installer can prevent many headaches and/or reinstalls down the road.

3: Install and configure a software firewall

A local
software firewall can provide a “just in case” layer of security to
any type of network. These types of firewalls allow you to filter the network
traffic that reaches your PC and are quite similar to the Windows Firewall. The
package called Shorewall along with a component of the Linux kernel
called Netfilterprovides a software
firewall. By installing and configuring Shorewall
during the installation process, you can restrict or block certain types of
network traffic, be it coming to or going out from your PC.

To access
and configure your firewall for Mandriva simply run
the mcc (or Mandriva
Control Center) command from a command prompt or, depending upon your graphical
environment, you may be able to access the Mandriva
Control Center from your base system menu. In the security options, select the
firewall icon and you will be presented with a list of common applications that
may need access through your firewall. For example, checking the box for “SSH
” will open port 22 needed by the Secure Shell server for secure
remote access. There is also an advanced section which will allow you to enter
some less commonly used ports. For example, entering “8000/tcp” will
open port 8000 on your PC to TCP-based network traffic.

Blocking or
allowing network traffic is one layer of security, but how do you secure a
service that you do allow the Internet or your intranet to connect to? Host
based security is yet another layer.

4: Configuring the /etc/hosts.deny and /etc/hosts.allow files

In the
preceding section we looked at the example of opening the Secure Shell service
to network traffic by opening port 22 on our firewall. To further secure this
server from unwanted traffic or potentially hackers, we may wish to limit the
hosts or computers that can connect to this server application. The /etc/hosts.deny and /etc/hosts.allow files allow us to do just that.

When a
computer attempts to access a service such as a secure shell server on your new
Linux PC the /etc/hosts.deny and /etc/hosts.allow files will be processed and access
will be granted or refused based on some easily configurable rules. Quite often
for desktop Linux PC’s it is very useful to place the following line in the /etc/hosts.deny file:


This will
deny access to all services from all hosts. It seems pretty restrictive at
first glance, but we then add hosts to the /etc/hosts.allow file that will allow us to access
services. The following are examples that allow some hosts remote secure shell

sshd:             #allow to access ssh
sshd:  #allow to access ssh

These two
files provide powerful host based filtering methods for your Linux PC.

5: Shutoff or remove non-essential services

Just like
Windows there can be services running in the background that you either don’t
want or don’t have a purpose for. By using the Linux command chkconfig you can see what services are
running and turn them on and off as needed. Services that are not running don’t
provide security holes for potential hackers and don’t take up those precious
CPU cycles.

6: Secure your required services

If your new
Linux PC has some services that will receive connections from the Internet make
sure you understand their configurations and tune them as necessary. For
example, if your Linux PC will receive secure shell connections make sure you
check the sshconfig file (for Mandriva
it is /etc/ssh/sshd_config) and disable options like root
login. Every Linux PC has a root user so you should disable root login via ssh in order to dissuade brute force
password crack attempts against your super-user account.

7: Tune kernel networking security options

The Linux
kernel itself can provide some additional networking security. Familiarize
yourself with the options in the /etc/sysctl.conf
file and tune them as needed. Options in this file control, for example, what
type of network information is logged in your system logs.

8: Connect the PC to a router

A hardware
router is a pretty common piece of household computer hardware these days. This
is the front line security to any home or business network and provides
multiple PC’s to share one visible or external Internet address. This is
generally bad news for any hacker or otherwise malicious program that may take
a look at your new Linux PC as it blocks any and all network traffic that you
don’t specifically allow. Home networking routers are just smaller versions of
what the big companies use to separate their corporate infrastructure from the

9: Update

Always keep
the software on your computer up to date with the latest security patches
should you be running Linux, Windows, BSD or WhoKnowsWhat.
Your distribution will release regular security patches that should be applied
and are available off the Internet. As with Windows, this should always be your
first Internet destination.

10: Other software

Your second
Internet stop may be to install some other hardening or system monitoring

Bastille-Linux is a
program that can be used to “harden” or secure certain aspects of
your new Linux PC. It interactively develops a security policy that is applied
to the system and can produce reports on potential security shortcomings. On
top of that it is a great tool to use for learning the in and out of securing
your Linux PC.

Tripwire is
a software package that monitors your system binaries for unauthorized
modifications. Often a hacker may modify system binaries that may be useful in
detecting a system intrusion. The modified programs would then report false
information to you allowing the hacker to maintain his control over your system.