Following our Twinkie analogy, the interior network is the creamy filling, the good stuff so to speak. And much like a Twinkie, the interior network is often completely unprotected . The real problem presents itself in a couple of different ways.
First, the majority of security incidents occur from the inside of the network. Now on the surface one might think, OK, so why harden the perimeter? The reason that so many incidents occur from the internal network is because we have hardened the perimeter. We have done such a good job of separating our internal network resources from the external world that it is difficult for an external attacker to exploit the network. This does not mean, however, that we can stop worrying about the perimeter, but it does mean that we need to start focusing more resources on hardening the internal network.
Second, there is a predominance of worms that wreak havoc on many corporate networks. The sad reality is that most of the people reading this book (including this author) have worked at a company that experienced a worm outbreak that had a negative impact on the network. In my case, I worked at a company that had its network effectively crippled for almost three days, with lingering effects for another two days, as a result of CodeRed. Worms such as this one are able to decimate our networks because in many cases we have designed the networks like the Internet: open access is the order of the day, which means that any system can communicate with any other system anywhere in the network.
This chapter is going to build upon the device-hardening methods that we have discussed in previous chapters to look at how to design a hardened interior network. Like Chapter 11, this chapter is going to follow and build upon the Cisco SAFE design methodology. To that end, we are going to look at the following topics:
Using Virtual LANs (VLANs) to segment the network
Designing the Enterprise Campus
Hardening Branch/Remote Offices