Firewalls: The Next Level of Understanding

Definition:

A firewall is a method for keeping a network secure. A firewall is a system or group of systems that enforces an access control policy between two networks. The actual means by which this is accomplished varies widely, but in principle, the firewall can be thought of as a pair of mechanisms: one which exists to block traffic, and the other which exists to permit traffic. Some firewalls place a greater emphasis on blocking traffic, while others emphasize permitting traffic. Probably the most important thing to recognize about a firewall is that it implements an access control policy. If you don't have a good idea what kind of access you want to permit or deny, or you simply permit someone or some product to configure a firewall based on what they or it think it should do, then they are making policy for your organization as a whole. It can be implemented in a single router that filters out unwanted packets, or it may use a combination of technologies in routers and hosts.

Firewalls are widely used to give users access to the Internet in a secure fashion as well as to separate a company's public Web server from its internal network. They are also used to keep internal network segments secure.

What can a firewall protect against?

Some firewalls permit only e-mail traffic through them, thereby protecting the network against any attacks other than attacks against the e-mail service. Other firewalls provide less strict protections, and block services that are known to be problems. Generally, firewalls are configured to protect against unauthenticated interactive logins from the "outside" world. This, more than anything, helps prevent vandals from logging into machines on your network. More elaborate firewalls block traffic from the outside to the inside, but permit users on the inside to communicate freely with the outside. The firewall can protect you against any type of network-borne attack if you unplug it. Firewalls are also important since they can provide a single "choke point" where security and audit can be imposed. Unlike in a situation where a computer system is being attacked by someone dialing in with a modem, the firewall can act as an effective "phone tap" and tracing tool. Firewalls provide an important logging and auditing function; often they provide summaries to the administrator about what kinds and amount of traffic passed through it, how many attempts there were to break into it, etc.

What can't a firewall protect against?

Firewalls can't protect against attacks that don't go through the firewall. Many corporations that connect to the Internet are very concerned about proprietary data leaking out of the company through that route. Unfortunately for those concerned, a magnetic tape can just as effectively be used to export data. Many organizations that are terrified (at a management level) of Internet connections have no coherent policy about how dial-in access via modems (and other back-doors to their systems) should be protected.

For a firewall to work, it must be a part of a consistent overall organizational security architecture. Firewall policies must be realistic, and reflect the level of security in the entire network. For example, a site with top secret or classified data doesn't need a firewall at all: they shouldn't be hooking up to the Internet in the first place, or the systems with the really secret data should be isolated from the rest of the corporate network.

Another thing a firewall can't really protect you against is traitors or idiots inside your network. While an industrial spy might export information through your firewall, he's just as likely to export it through a telephone, FAX machine, or floppy disk. Floppy disks are a far more likely means for information to leak from your organization than a firewall!

Firewalls also cannot protect you against stupidity. Users who reveal sensitive information over the telephone are good targets for social engineering; an attacker may be able to break into your network by completely bypassing your firewall, if he can find a "helpful" employee inside who can be fooled into giving access to a modem pool.

For an excellent FAQ on firewall technology, go to: http://www.faqs.org/faqs/firewalls-faq/

Copyright © 1995-1998 Marcus J. Ranum. Copyright © 1998 Matt Curtin. All rights reserved. This document may be used, reprinted, and redistributed as is providing this copyright notice and all attributions remain intact.

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