3 Design and Implementation Issues
3.1 What are some of the basic design decisions in a firewall?There are a number of basic design issues that should be addressed by the lucky person who has been tasked with the responsibility of designing, specifying, and implementing or overseeing the installation of a firewall.
The first and most important decision reflects the policy of how your company or organization wants to operate the system: is the firewall in place explicitly to deny all services except those critical to the mission of connecting to the Net, or is the firewall in place to provide a metered and audited method of ``queuing'' access in a non-threatening manner? There are degrees of paranoia between these positions; the final stance of your firewall might be more the result of a political than an engineering decision.
The second is: what level of monitoring, redundancy, and control do you want? Having established the acceptable risk level (e.g., how paranoid you are) by resolving the first issue, you can form a checklist of what should be monitored, permitted, and denied. In other words, you start by figuring out your overall objectives, and then combine a needs analysis with a risk assessment, and sort the almost always conflicting requirements out into a laundry list that specifies what you plan to implement.
The third issue is financial. We can't address this one here in anything but vague terms, but it's important to try to quantify any proposed solutions in terms of how much it will cost either to buy or to implement. For example, a complete firewall product may cost between $100,000 at the high end, and free at the low end. The free option, of doing some fancy configuring on a Cisco or similar router will cost nothing but staff time and a few cups of coffee. Implementing a high end firewall from scratch might cost several man-months, which may equate to $30,000 worth of staff salary and benefits. The systems management overhead is also a consideration. Building a home-brew is fine, but it's important to build it so that it doesn't require constant (and expensive) attention. It's important, in other words, to evaluate firewalls not only in terms of what they cost now, but continuing costs such as support.
On the technical side, there are a couple of decisions to make, based on the fact that for all practical purposes what we are talking about is a static traffic routing service placed between the network service provider's router and your internal network. The traffic routing service may be implemented at an IP level via something like screening rules in a router, or at an application level via proxy gateways and services.
The decision to make is whether to place an exposed stripped-down machine on the outside network to run proxy services for telnet, FTP, news, etc., or whether to set up a screening router as a filter, permitting communication with one or more internal machines. There are pluses and minuses to both approaches, with the proxy machine providing a greater level of audit and potentially security in return for increased cost in configuration and a decrease in the level of service that may be provided (since a proxy needs to be developed for each desired service). The old trade-off between ease-of-use and security comes back to haunt us with a vengeance.
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