Chapter 1. Perimeter Security Fundamentals

The security of your network is evaluated daily. A rich question to ask is, "Are you the one doing it?" The answer, hopefully, is that someone on your side is involved in assessing the effectiveness of your defenses; however, overwhelming evidence reports that you are not the only party probing your network's perimeter. Internet-facing systemscomputers with IP addresses that can be reached from the Internetreceive between several and hundreds or even thousands of attack attempts every day. Many of these are simple scans that we know how to defend against, but others catch us by surprise, unexpectedly shifting us into incident investigation and cleanup mode.

Does your organization have access to expertise in all aspects of perimeter security, including networking, firewalls, intrusion detection systems (IDSs), intrusion prevention systems (IPSs), Virtual Private Networks (VPNs), UNIX security, and Windows security? In the pages ahead, we will show you how all these protective measures work together. Can you definitively say how secure or insecure your network is? Does everyone in your organization understand the policies related to information security and their implications? One hint that they do not is the famous expression, "But we have a firewall!" If you work in information security, you probably hear this phrase more often than you would like to, because it seems to express the opinion of many people, both technical and nontechnical.

One of the most challenging aspects of securing modern networks, even those that already have firewalls, is that they exhibit porous properties. Wireless connections, portable storage devices, mobile systems, and links to partner sites offer a multitude of ways in which data can get in and out of our networks, bypassing our border defenses. This is one of the reasons why a single security component cannot properly defend a network. However, many components working together can. Defense in depth, a major theme of this chapter and this book, is the process of layering these components to capitalize on their respective strengths. It is flexible, in that it allows us to select components based on technical, budgetary, and organizational constraints and combine them in a way that doesn't compromise the overall security or usability of the network.

We will begin this chapter by defining some common terms of the trade to ensure that we're all on the same page. Then we'll discuss core components of defense in depth, to illustrate how various aspects of the security perimeter can complement each other to form a balanced whole. We will close with a discussion of the Nimda worm and show how defense in depth can help protect your network against such an attack.