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Single-firewall DMZ or dual-firewall DMZ? What's it gonna be?

Is a dual-firewall DMZ worth its expense? This article examines the tradeoffs of a single-firewall vs. a dual-firewall DMZ.

So you've latched onto the idea of using a DMZ to offer more secure and robust protection for some of your machines, as opposed to simply using a traditional firewall in front of your entire network, passing everything inside. Good for you. But one question remains: Will you go the simple route and have a DMZ residing off your single firewall? Or, is the extra expense worth it to have the ultimate in protection -- two firewalls and the DMZ residing between both of them?

You would be reasonably secure with a traditional DMZ to protect your public-facing servers, which, at the same time will guard your more sensitive internal network from nefarious external users. In this scenario, the firewall looks at all incoming traffic to figure out if it should be shunted over to the DMZ network (where one or more machines providing outward-facing services reside) or passed onto the protected internal network. The traditional DMZ examines all outgoing traffic from the internal network to determine if it should be passed (a) from the protected network to the DMZ network for those internal packets asking for Web and mail services, (b) to the protected network from the DMZ network as the response to requests from within, or (c) to the Internet. You might know this architecture as the dual-homed gateway architecture, as the firewall will have two interfaces -- one to the DMZ and one to the internal network.

Taking it one step further, the dual-firewall DMZ architecture (sometimes known as a sub-network firewall) adds another layer of defense and isolation between the internal network and the big, bad world outside. You also gain further protection for your public-facing hosts by having a firewall in front of them, as well as another firewall in front of your internal network. Using this architecture, traffic between the protected network and the Internet must traverse two firewalls, which gives you initial first-line defense against nefarious traffic targeted at your outward-facing machines.

So, now you have to make up your mind, and the following traffic questions can help you with that decision:

  • From a performance standpoint, can you afford to route external traffic through two firewall systems instead of just one?
  • How can you monitor traffic passing through either leg of the network?
  • From where might you monitor that traffic?
  • Do you need instant ability to recover from compromises, such as disabling one firewall system while keeping the other running and passing traffic?
  • Do you have the requisite number of network ports available?
  • Does your budget allow for two firewalls or is the expense prohibitive?

Here's the bottom line: A traditional DMZ architecture affords you an additional level of protection for machines offering public services, but it requires additional effort for ongoing operations and maintenance. The dual-firewall DMZ option is the most secure, but (as with any double-edged sword), it is also the most expensive to deploy and run.

About the author: Jonathan Hassell is author of Hardening Windows (Apress LP), and is a site expert. Hassell is a systems administrator and IT consultant residing in Raleigh, N.C., who has extensive experience in networking technologies and Internet connectivity. He runs his own Web-hosting business, Enable Hosting. His previous book RADIUS (O'Reilly & Associates), is a guide to the RADIUS authentication protocol and offers suggestions for implementing RADIUS and overall network security. Ask Hassell a hardening Windows question today.

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