Chapter 9. Proxy Systems

9.1. Why Proxying?

There's no point in connecting to the Internet if your users can't access it. On the other hand, there's no safety in connecting to the Internet if there's free access between it and every host at your site. Some compromise has to be applied.

The most obvious compromise is to provide a single host with Internet access for all your users. However, this isn't a satisfactory solution because these hosts aren't transparent to users. Users who want to access network services can't do so directly. They have to log in to the dual-homed host, do all their work from there, and then somehow transfer the results of their work back to their own workstations. At best, this multiple-step process annoys users by forcing them to do multiple transfers and work without the customizations they're accustomed to.

The problem is worse at sites with multiple operating systems; if your native system is a Macintosh, and the dual-homed host is a Unix system, the Unix system will probably be completely foreign to you. You'll be limited to using whatever tools are available on the dual-homed host, and these tools may be completely unlike (and may seem inferior to) the tools you use on your own system.

Dual-homed hosts configured without proxies therefore tend to annoy their users and significantly reduce the benefit people get from the Internet connection. Worse, they usually don't provide adequate security; it's almost impossible to adequately secure a machine with many users, particularly when those users are explicitly trying to get to the external universe. You can't effectively limit the available tools because your users can always transfer tools from internal machines that are the same type. For example, on a dual-homed host, you can't guarantee that all file transfers will be logged because people can use their own file transfer agents that don't do logging.

Proxy systems avoid user frustration and the insecurities of a dual-homed host. They deal with user frustration by automating the interaction with the dual-homed host. Instead of requiring users to deal directly with the dual-homed host, proxy systems allow all interaction to take place behind the scenes. The user has the illusion of dealing directly (or almost directly) with the server on the Internet, with a minimum of direct interaction with the dual-homed host. Figure 9-1 illustrates the difference between reality and illusion with proxy systems.

Figure 9-1. Proxies -- reality and illusion

Proxy systems deal with the insecurity problems by avoiding user logins on the dual-homed host and by forcing connections through controlled software. Because the proxy software works without requiring user logins, the host it runs on is safe from the randomness of having multiple logins. It's also impossible for anybody to install uncontrolled software to reach the Internet; the proxy acts as a control point.