Installing and Using a Firewall Program



Larry Rogers

This library item is related to the following area(s) of work:

Security and Survivability

This article was originally published in News at SEI on: December 1, 2002

Your home computer is a popular target for intruders. They look for credit card numbers and bank account information so they can use your money to buy themselves goods and services. Intruders also want your computer’s resources—your hard disk space, your fast processor, and your Internet connection. They use these resources to attack other computers on the Internet. In fact, the more computers an intruder uses, the harder it is for law enforcement to figure out where the attack is really coming from. If intruders can’t be found, they can’t be stopped, and they can’t be prosecuted.

Why are intruders paying attention to home computers? Home computers are typically not very secure and are easy to break into. When combined with high-speed Internet connections that are always turned on, intruders can quickly find and then attack home computers. While intruders also attack home computers connected to the Internet through dial-in connections, high-speed connections (cable modems and DSL modems) are a favorite target.

One tool you can use to help defend your home computer against intruder attack is a firewall. This column describes a firewall, its importance to your home computer strategy, and some tips about creating firewall rules. You can read about other tools and guidelines in Home Computer Security.

The Security Guard

Have you ever visited a business where you first stopped at the reception desk to interact with a security guard? That guard’s job is to assess everybody who wishes to enter or leave the building to decide if they should continue on or be stopped. The guard keeps the unwanted out and permits only appropriate people and objects to enter and leave the business’s premises.

Let’s dig deeper into this analogy. When someone enters a building, the security guard usually greets them. If they have an appropriate identification badge, they show it to the guard or swipe it through a reader. If all is okay, they pass through the guard’s checkpoint. However, if something’s wrong or if they are a visitor, they must first stop at the guard desk.

The guard asks whom they wish to see. The guard may also ask for identification such as a driver’s license or their company ID. The guard reviews the list of expected guests to see if this person is approved to visit the party in question. If the guard decides everything is all right, the visitor may pass. The visitor usually signs a logbook with their name, the company they represent, whom they are seeing, and the time of day.

On a computer, the firewall acts much like a guard when it looks at network traffic destined for or received from another computer. The firewall determines if that traffic should continue on to its destination or be stopped. The firewall “guard” is important because it keeps the unwanted out and permits only appropriate traffic to enter and leave the computer.

To do this job, the firewall has to look at every piece of information—every packet—that tries to enter or leave a computer. Each packet is labeled with where it came from and where it wants to go. Some packets are allowed to go anywhere (the employee with the ID badge) while others can only go to specific places (visitors for a specific person). If the firewall allows the packet to proceed (being acceptable according to the rules), it moves the packet on its way to the destination. In most cases, the firewall records where the packet came from, where it’s going, and when it was seen. For people entering a building, this is similar to the ID card system keeping track of who enters or the visitor signing the visitor’s log.

The building’s guard may do a few more tasks before deciding that the person can pass. If the person is a visitor and is not on the visitors list, the guard calls the employee being visited to announce the visitor’s arrival and to ask if they may pass. If the employee accepts the visitor, they may proceed. The guard may also give the visitor a badge that identifies them as a visitor. That badge may limit where in the building they can go and indicate if they need to be escorted. Finally, no matter whether the person is a visitor or an employee, the guard may inspect their briefcase or computer case before they pass.

The firewall can also check whether a given packet should pass, allowing the computer’s user to respond to unanticipated network traffic (just as the guard does with the unexpected visitor). Individual packets can be allowed to pass, or the firewall can be changed to allow all future packets of the same type to pass. Some firewalls have advanced capabilities that make it possible to direct packets to a different destination and perhaps even have their contents concealed inside other packets (similar to the visitor being escorted). Finally, firewalls can filter packets based not only on their point of origin or destination, but also on their content (inspecting the briefcase or computer case before being allowed to pass).

Back to the office building, when employees leave the building, they may also have to swipe their ID card to show that they’ve left. A visitor signs out and returns their temporary badge. Both may be subject to having their possessions inspected before being allowed to leave.

Firewalls can also recognize and record when a computer-to-computer connection ends. If the connection was temporary (like a visitor), the firewall rules can change to deny future similar connections until the system’s user authorizes them (just as visitors must re-identify themselves and be re-approved by an employee). Finally, outgoing connections can also be filtered according to content (again, similar to inspecting possessions at the exit).

What does this all mean? It means that with a firewall, you can control which packets are allowed to enter your home computer and which are allowed to leave. That’s the easy part.

The hard part is deciding the details about the packets that are allowed to enter and exit your home computer. If your firewall supports content filtering, you also need to learn which content to allow and which not to allow. To help you get a handle on this harder task, let’s return to our security guard analogy.

Imagine that you are that security guard and it’s your first day on the job. You have to decide who’s allowed in, who’s allowed out, and what people can bring into and take out of the building. How do you do this?

One strategy is to be very conservative: let no one in or out and let no possessions in or out. This is very simple, very easy to achieve, but not particularly helpful to the business if none of its employees or visitors can get in or out. Nor is it helpful if they can’t bring anything with them. With this type of strategy, your tenure as a security guard may be short-lived.

If you try this, you quickly learn that you need to change your strategy to allow people in and out only if they have acceptable identification and possessions using some agreed-to criteria. Add the requirement that if you don’t meet the precise criteria for admittance, you don’t get in.

With most firewalls, you can do the same thing. You can program your firewall to let nothing in and nothing out. Period. This is a deny-all firewall strategy and it does work, though it effectively disconnects you from the Internet. It is impractical for most home computers.

You can do what the security guard did: review each packet (employee or visitor) to see where it’s coming from and where it’s going. Some firewall products let you easily review each packet so that you can decide what to do with it. When you are shopping for a firewall, look for this review feature because it can be quite helpful. Practically speaking, it isn’t easy to decide which traffic is all right and which is not all right. Any feature that makes this job easier helps you achieve your goal of securing your home computer.

Just like the security guard who learns that anybody with a company photo ID is allowed to pass, you too can create firewall rules that allow traffic to pass without reviewing each packet each time. For example, you may choose to allow your Internet browsers to visit any web site. This rule would define the source of that traffic to be your browsers (Netscape Navigator and Microsoft Internet Explorer, for example) and the destination location to be any web server. This means that anybody using your home computer could visit any Internet web site, as long as that web server used the well-known standard locations.

Building Firewall Rules

Now that you have an idea of what your firewall security guard is trying to do, you need a method for gathering information and programming your firewall. Here is a set of steps to use to do just that:

  1. Program test: What’s the program that wants to make a connection to the Internet? Although many programs may need to make the same type of connection to the same Internet destination, you need to know the name of each. Avoid general rules that allow all programs to make a connection. This often results in unwanted and unchecked behavior.
  2. The Location test: What’s the Internet location of the computer system to which your computer wants to connect? Locations consist of an address and a port number. Sometimes a program is allowed to connect to any Internet location, such as a web browser connecting to any web server. Again, you want to limit programs so that they only connect to specific locations where possible.
  3. The Allowed test: Is this connection allowed or denied? Your firewall rules will contain some of each.
  4. The Temporary test: Is this connection temporary or permanent? For example, if you’re going to connect to this specific location more than five times each time you use the computer, you probably want to make the connection permanent. This means that you ought to add a rule to your firewall rules. If you aren’t going to make this connection often, you should define it as temporary.

With each connection, apply the PLAT tests to get the information you need to build a firewall rule. (A worksheet in PDF format is available to help you with this.) The answer to the PLAT tests tells you if you need to include a new firewall rule for this new connection. For most firewall programs, you can temporarily allow a connection but avoid making it permanent by not including it in your rules. Where possible, allow only temporary connections.

As you run each program on your home computer, you’ll learn how it uses the Internet. Slowly you’ll begin to build the set of rules that define what traffic is allowed into and out of your computer. By only letting in and out what you approve and denying all else, you will strike a practical balance between allowing everything and allowing nothing in or out.

Along the way, you may come across exceptions to your rules. For example, you might decide that anybody who uses your home computer can visit any web site except a chosen few web sites. This is analogous to the security guard letting every employee pass except a few who need more attention first.

To do this with firewall rules, the exception rules must be listed before the general rules. For example, this means that the web sites whose connections are not allowed must be listed before the rules that allow all connections to any web site.

Why? Most firewall programs search their rules starting from the first through the last. When the firewall finds a rule that matches the packet being examined, the firewall honors it, does what the rule says, and looks no further. For example, if the firewall finds the general rule allowing any web site connections first, it honors this rule and doesn’t look further for rules that might deny such a connection. So, the order of firewall rules is important.

Many firewalls can be programmed to require a password before changing the rules. This extra level of protection safeguards against unwanted changes no matter their source, that is, you, an intruder, or another user. Follow the guidance in “Use Strong Passwords” in Home Computer Security when assigning a password to your firewall.

Finally, make a backup of your firewall rules. You’ve probably taken a lot of time to build and tune them to match how your home computer is used. These rules are important to your computer’s security, so back them up using the guidance in “Make Backups of Important Files and Folders” in Home Computer Security.

Types of Firewalls

Firewalls come in two general types: hardware and software (programs). The software versions also come in two types: free versions and commercial versions (ones that you purchase). At a minimum, you should use one of the free versions on your home computer. This is especially important if you have a laptop that you connect to your home network as well as a network at a hotel, a conference, or your office.

If you can afford a hardware firewall, you should install one of these too. The same issues apply to the hardware versions that apply to the software versions. Many can also be password protected against unwanted changes. Search the Internet with your browser to see what’s available and what they cost. The price of hardware firewalls is coming down as the demand grows.


A firewall is your security guard that stands between your home computer and the Internet. It lets you control which traffic your computer accepts. It also controls which of your programs can connect to the Internet. With a firewall, you define which connections between your computer and other computers on the Internet are allowed and which are denied. There are free firewall products that provide the capabilities you need to secure your home computer. Commercial versions have even more features that can further protect your computer.

Firewalls are an important part of your home computer’s security defenses.

Example: Operating a Firewall Program (contains several graphics).

About the Author

Lawrence R. Rogers is a senior member of the technical staff in the Networked Systems Survivability Program at the Software Engineering Institute (SEI). The CERT Coordination Center® is a part of this program. Rogers’s primary focus is analyzing system and network vulnerabilities and helping to transition security technology into production use. His professional interests are in the areas of the administering systems in a secure fashion and software tools and techniques for creating new systems being deployed on the Internet. Rogers also works as a trainer of system administrators, authoring and delivering courseware. Before joining the SEI, Rogers worked for 10 years at Princeton University. Rogers co-authored the Advanced Programmer’s Guide to UNIX Systems V with Rebecca Thomas and Jean Yates. He received a BS in systems analysis from Miami University in 1976 and an MA in computer engineering in 1978 from Case Western Reserve University.

This and other columns by Larry Rogers, along with extensive information about computer and network security, can be found at

The views expressed in this article are the author's only and do not represent directly or imply any official position or view of the Software Engineering Institute or Carnegie Mellon University. This article is intended to stimulate further discussion about this topic.

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