There are several different types of firewalls on the market today. Choosing one for your organization can be a daunting task – especially in an industry filled with buzzwords and proprietary trademarks. Let's take a look

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at the basics of firewall technology and five questions you should ask when choosing a firewall for your organization.

  1. Why are you implementing a firewall? Sure, this sounds like a simple question. You're probably thinking to yourself, "Because we need one!" But it's important that you take the time to define the technical objectives that you have for firewall implementation. These objectives will drive the selection process. You don't want to choose an expensive, feature-rich firewall that's complicated to administer when your technical requirements could be met by a simpler product.

  2. How will the firewall fit into your network topology? Will this firewall sit at the perimeter of your corporate network and be directly connected to the Internet, or will it serve to segment a sensitive LAN from the remainder of the organization? How much traffic will it process? How many interfaces will it need to segment your traffic? Performance requirements such as these contribute a significant amount to the total cost of new firewall implementations, making it easy to under- or over-purchase.

  3. What type of traffic inspection do you need to perform? This is where the buzzwords start to come into play. Every vendor out there has a different trademark for their traffic-inspection technology, but there are essentially three different options (listed in order of increasing complexity and cost):
    • Packet-filtering firewalls use simple rules to evaluate each packet they encounter on its own merits. They maintain no history from packet to packet, and they perform basic packet header inspection. The simplicity of this inspection makes them speed demons. They're the most inexpensive option, but they are also the least flexible and vulnerable. There's a good chance you already own equipment capable of performing packet filtering – your routers!

    • Stateful-inspection firewalls go a step further. They track the three-way TCP handshake to ensure that packets claiming to belong to an established session (i.e., the SYN flag is not set) correspond to previous activity seen by the firewall. Requests to open the initial connection are subject to the stateful-inspection firewall rulebase.

    • Application-proxy firewalls contain the highest level of intelligence. In addition to stateful inspection, they broker the connection between client and server. The client connects to the firewall, which analyzes the request (including application-layer inspection of packet contents). If the firewall rules indicate that the communication should be allowed, the firewall then establishes a connection with the server and continues to act as an intermediary in the communication. When combined with Network Address Translation, both hosts may not even be aware that the other exists – they both believe they are communicating directly with the firewall.

  4. Is your organization better suited for an appliance or a software solution? Appliances are typically much easier to install. You normally just plug in the appropriate Ethernet cables, perform basic network configuration and you're ready to configure your firewall rules. Software firewalls, on the other hand, can be tricky to install and require tweaking. They also lack the security that's often built into the hardened operating systems of firewall appliances. What's the tradeoff? You guessed it! Appliances are more expensive.

  5. What operating system is best suited for your requirements? Even appliances run an OS and, chances are, you'll need to work with it at some point in your firewall administration career. If you're a Linux jockey, you probably don't want to choose a Windows-based firewall. On the other hand, if you don't know ⁄dev⁄null from ⁄var⁄log, you probably want to steer clear of Unix-based solutions.

While I can't recommend a specific type of firewall to you without knowing your needs, the process of answering these questions can help you solidify your thoughts and put you in the right direction. With these answers in hand, you should be able to intelligently evaluate the cost/benefit tradeoff for the various products available on the market today.


FIREWALL ARCHITECTURE TUTORIAL

  Introduction
  How to choose a firewall
  Choosing the right firewall topology
  Placing systems in a firewall topology
  Auditing firewall activity

ABOUT THE AUTHOR:
Mike Chapple, CISSP is an IT Security Professional with the University of Notre Dame. He previously served as an information security researcher with the National Security Agency and the U.S. Air Force. Mike is a frequent contributor to SearchSecurity, a technical editor for Information Security magazine and the author of several information security titles including the CISSP Prep Guide and Information Security Illuminated.


This was first published in October 2005

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