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 SearchWindowsSecurity.com 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.


More information from SearchWindowsSecurity.com

  • Tip: Windows Firewall: Love it or hate it
  • Ask the Expert: How do I set up a DMZ?
  • ITKnowledgeExchanage: Using Windows authentication to access a Linux DMZ


  • This was last published in September 2005

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    I do not believe that there truly is a value add benefit in todays climate for small to medium enterprise organizations.  I believe it solely depends on the circumstances.  Especially when one considers that a router on a stick model can serve as the aggregated edge.  This article doesn't deep dive enough into possible outcomes.  

    When one considers the added expense it comes down to the simple philosophy of how well do you know your architecture and what is your end stated goals.  How is one going to proxy and filter and ultimately where does this leave my customer base - both internal lines of business and external consumers of said product from a security standpoint.  

    The next critical juncture is simplicity of design.  Even in a two tier DMZ model - aggregated edge and internal edge one still has to consider that more often than not, there are holes that are poked through the firewall.  The value add of a single tier versus two tier design cannot be overlooked.  At the end of the day, I believe it comes down to how technical is your staff?  Do they understand the concepts of VRF's?  What does your IDS/IPS schema lookalike?  Are you planning on using SDN for multisite connections?  What technology will one use for SDN?  Is it Cisco IWAN? Viptella?  Is one using interconnected sites?  Is one using colocation?  

    Having worked in the past at both major enterprises and on small to mid tier enterprises I believe that one may be truly surprised about the security benefits of single tier if it is designed correctly.  All that being said one cannot deny that the traditional sandwich model is still in place and ultimately if not designed correctly can leave your organization with the same vulnerabilities as a single tier.  

    I would say that one also has to look at compliancy efforts - and outbreak mitigation, as well as traditional DDOS.  Are you looking towards PCI and SOX to be your guide?  What about HIPPA?  

    That is to say that it todays world of information technology, its all about the end state architecture.  I believe that if one has a colocation facility and multi end points then yes one can utilize a single tier firewall model.  One has to be familiar with subnetting, Firewalling, VRF, BGP, SDN, SBC's, IDS/IPS, Managed CPE, MPLS, Internet Edge, DMVPN, ACVPN, Business Partner Connections, Sandboxing, Site to Site VPN, Aggregated Edge and true deep dive planning to achieve the goals of your organization.  Every VAR that is out there will try and sell you a bill of goods if one does not understand the tradeoffs.  I would strongly recommend that one reads about the internet of things design.  I would also recommend that one understands conceptual delivery for Mobile and deep certificate chaining.  One must also understand what applications are they serving.  There are many effective ways to deliver complete walling off applications to keep your business and your customers secure.

    At the end of the day if one does not feel comfortable with the said technologies above - then one should interview at least four firms and have them each present a high level design.  Take those in aggregate and study the pros and cons.  Not trying to outshine the intent of the article but I don't think it delves enough into the philosophy of end state architecture for 2016 to 2024.  What's coming down the pipeline may allow one to conceptualize what your organization may look like.


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