An Introduction to Using Linux as a Multipurpose Firewall

Feeling insecure? Here's a guide for getting the protection you need.

High-speed Internet connections are becoming more readily available and popular for home computer users. ADSL (Asymmetric Digital Subscriber Line), Nortel's 1MB modem and cable modems all offer connection speeds many times faster than that of a standard 56K POTS (plain old telephone service) modem that most of us know all too well. The other big advantage of these new services is that they are always connected. That is, you don't need to dial your service provider with your modem to start up your Internet connection. When you turn on your computer, the connection is already there, and your operating system will establish a link as it boots up.

Like the standard modem, these connections allow only one computer to connect to the Internet at a time. In some cases, additional IP addresses can be assigned to additional computers, but there is usually a monthly cost involved in providing this service.

By installing Linux on that old 486 you have sitting in the corner collecting dust, you can create a firewall so all the computers on your local LAN can see the Internet, and at the same time, transfer data back and forth between each other, (see Figure 1). You don't even need a dedicated PC. A faster PC can simultaneously be used for other purposes while acting as the firewall; however, there are two main drawbacks with this approach:

  • Users on your LAN may experience a slower connection to the Internet.

  • You could inadvertently open a security hole, allowing someone on the Internet to get in and play havoc with your system or files.

Figure 1. Generic Diagram of Small LAN Configuration

I will be discussing two different types of Linux firewalls. The first type consists of a 486 with 12MB of RAM, and a 200MB hard drive using either Red Hat 6.0 or Slackware 3.6. The second, called the Linux Router Project (LRP), uses a 486, 12MB of RAM, a 1.44MB floppy and no hard drive. Two Ethernet network interface cards (NICs) will be required, regardless of which firewall configuration you install.


Someone is always watching, and people are always on the lookout for computers on the Internet with poor security. Their motivation can be as simple as boredom, or more seriously, a need to find a system to penetrate so they can use it to hide behind while they continue breaking into other systems, leaving evidence that points to you.

If you are running a standard Windows installation, you probably don't have the means to see who is trying to check out your machine. As long as “File and Print Sharing” is turned off inside of Windows, for the most part, you are safe. However, it is possible someone may find a new security hole in your PC and exploit it.

If you have Linux running, you can check out your system logs. Upon doing an informal survey with friends who run Linux firewalls, I found on average five attempts by outsiders each day to use TELNET or FTP to break into their Linux boxes. In the case of a firewall, you can turn off or restrict most services. In general, the strength of your firewall security decreases for each service you open up to the Internet, since each service is an invitation for someone to try and sneak in to your system. For example, if you open TELNET, someone can use it to break in. A safer alternative is to restrict TELNET to certain incoming IP addresses, such as the IP addresses you might use to access your home system from work. If you have no plans to TELNET or FTP into your firewall from the Internet and all your traffic is originated inside your local LAN, you can lock your firewall fairly tightly. It is always a good idea to stay caught up on new security holes and the fixes for them. Check out for more information.

Theory of Operation

There are many reasons for having a firewall, some of which I have already mentioned. They include:

  • Ensuring that local traffic on your intranet does not spill out to the Internet.

  • Allowing the full use of file and print sharing in your LAN without having to worry about unwanted intrusions.

  • Providing security for your LAN.

  • Allowing yourself and authorized users access to your LAN to read e-mail, listen to MP3s or access file backups.