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Building Internet Firewalls

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7. Proxy Systems

Proxying provides Internet access to a single host, or a very small number of hosts, while appearing to provide access to all of your hosts. The hosts that have access act as proxies for the machines that don't, doing what these machines want done.

A proxy server for a particular protocol or set of protocols runs on a dual-homed host or a bastion host: some host that the user can talk to, which can, in turn, talk to the outside world. The user's client program talks to this proxy server instead of directly to the "real" server out on the Internet. The proxy server evaluates requests from the client and decides which to pass on and which to disregard. If a request is approved, the proxy server talks to the real server on behalf of the client (thus the term "proxy"), and proceeds to relay requests from the client to the real server, and to relay the real server's answers back to the client.

As far as the user is concerned, talking to the proxy server is just like talking directly to the real server. As far as the real server is concerned, it's talking to a user on the host that is running the proxy server; it doesn't know that the user is really somewhere else.

Proxying doesn't require any special hardware, although it does require special software for most services.

NOTE: Proxy systems are effective only when they are used in conjunction with some method of restricting IP -level traffic between the clients and the real servers, such as a screening router or a dual-homed host that doesn't route packets. If there is IP -level connectivity between the clients and the real servers, the clients can bypass the proxy system (and presumably so can someone from the outside).

7.1 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 he is dealing directly (or almost directly) with the server on the Internet that he really wants to access, with a minimum of direct interaction with the dual-homed host. Figure 7.1 illustrates the difference between reality and illusion with proxy systems.

Figure 7.1: Proxies - reality and illusion

Figure 7.1

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.

7.1.1 Advantages of Proxying

There are a number of advantages to using proxy services.

7.1.1.1 Proxy services allow users to access Internet services `directly'

With the dual-homed host approach, a user needs to log into the host before using any Internet services. This is often inconvenient, and some users become so frustrated that they look for ways around the firewall. With proxy services, users think they're interacting directly with Internet services.

Of course, there's more going on behind the scenes but it's usually transparent to users. While proxy services allow users to access Internet services from their own systems, they do so without allowing packets to pass directly between the user's system and the Internet. The path is indirect, either through a dual-homed host, or through a bastion host and screening router combination.

7.1.1.2 Proxy services are good at logging

Because proxy servers understand the underlying protocol, they allow logging to be performed in a particularly effective way. For example, instead of logging all of the data transferred, an FTP proxy server logs only the commands issued and the server responses received; this results in a much smaller and more useful log.

7.1.2 Disadvantages of Proxying

There are also some disadvantages to using proxy services.

7.1.2.1 Proxy services lag behind nonproxied services

Although proxy software is widely available for the older and simpler services like FTP and Telnet, proven software for newer or less widely used services is harder to find. There's usually a distinct lag between the introduction of a service and the availability of proxying servers for it; the length of the lag depends primarily on how well the service is designed for proxying. This makes it difficult for a site to offer new services immediately as they become available. Until suitable proxy software is available, a system that needs new services may have to be placed outside the firewall, opening up potential security holes.

7.1.2.2 Proxy services may require different servers for each service

You may need a different proxy server for each protocol, because the proxy server has to understand the protocol in order to determine what to allow and disallow, and in order to masquerade as a client to the real server and as the real server to the proxy client. Collecting, installing, and configuring all these various servers can be a lot of work.

Products and packages differ greatly in the ease with which they can be configured, but making things easier in one place can make it harder in others. For example, servers that are particularly easy to configure are usually limited in flexibility; they're easy to configure because they make certain assumptions about how they're going to be used, which may or may not be correct or appropriate for your site.

7.1.2.3 Proxy services usually require modifications to clients, procedures, or both

Except for a few services designed for proxying, proxy servers require modifications to clients and/or procedures. Either kind of modification has drawbacks; people can't always use the readily available tools with their normal instructions.

Because of these modifications, proxied applications don't work as well as nonproxied applications. They tend to bend protocol specifications, and some clients and servers are less flexible than others.

7.1.2.4 Proxy services aren't workable for some services

Proxying relies on the ability to insert the proxy server between the client and the real server; that requires relatively straightforward interaction between the two. A service like talk that has complicated and messy interactions may never be possible to proxy (see the discussion of talk in Chapter 8, Configuring Internet Services ).

7.1.2.5 Proxy services don't protect you from all protocol weaknesses

As a security solution, proxying relies on the ability to determine which operations in a protocol are safe. Not all protocols provide easy ways to do this. The X Window System protocol, for example, provides a large number of unsafe operations, and it's difficult to make it work while removing the unsafe operations. HTTP is designed to operate effectively with proxy servers, but it's also designed to be readily extensible, and it achieves that goal by passing data that's going to be executed. It's impossible for a proxy server to protect you from the data; it would have to understand the data being passed and determine whether it was dangerous or not.


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