In this chapter, we describe two sample configurations for basic firewalls. Almost any real firewall is going to be more complex than those described in this chapter, but this presentation should give you some idea of the tasks involved in building a firewall and how the various pieces fit together.
We want to emphasize that these examples are just that: examples. You shouldn't blindly implement one of these examples without first taking the time to understand your own needs, your environment, and the implications and complications of the services you want your firewall to provide.
The services that we're going to provide through these sample firewalls are just the basics: the World Wide Web, terminal access, file transfer, electronic mail, Usenet news, and DNS.
The screened subnet architecture, described in Chapter 6, and shown in Figure 24.1, is probably the most common do-it-yourself firewall architecture. This architecture provides good security (including multiple layers of redundancy) at what most sites feel is a reasonable cost.
Figure 24.1. Screened subnet architecture
There are two-router and single-router variations of the screened subnet architecture. Basically, you can use either a pair of two-interface routers or a single three-interface router. The single-router screened subnet architecture works about as well ...