For Allen Gwinn, senior IT director at Southern Methodist University in
Dallas, the question is not if his 15,000-user network will be exposed,
but how to control the damages when it happens.
Gwinn knows that as part of an educational institution, his network is a
prime target for hackers. At the same time, there is a lot of information
on his network that he must allow the public to access. To that end,
Gwinn relies on an old military technique to layer his network defenses
— he’s created a demilitarized zone, or DMZ.
DMZs are no-man’s lands created to ferret out the enemy. Anything in the
DMZ is negligible. In the enterprise, a DMZ is an area where the public
can access certain elements of the network, but not the back-end
While firewalls, intrusion detection and intrusion prevention all are
good strategies for detecting unwanted traffic on a network, Gwinn says
the DMZ analogy provides a blueprint for bringing all these security
tools together for maximum protection.
”A DMZ is a frame of mind on how you’re going to treat things exposed to
the Internet,” Gwinn says. ”Everyone needs a plan for how you approach
having devices in an untrusted world.”
Rick Fleming, CTO at Digital Defense, a computer security firm in San
Antonio, Texas, says the trick is to put things in the DMZ that would not
lead to catastrophic results if they were jeopardized.
He recommends Web servers, e-mail servers, and other information stores
such as PDF silos. ”You have to ask, if someone were to gain root level
access to this Web server, what would be the net effect? The answer
should be that there is no net effect,” Fleming says.
He adds that a lot of companies want to be able to put information out to
the public, such as financial institutions. But they often make the
mistake of linking those public machines with back-end private networks.
He points to cases where banks offer public access to Web servers by
giving customers digital forms to fill out. ”Suddenly, a hacker can
access customer information in the back-end SQL database with simple
queries,” he points out.
”You don’t want someone to compromise a machine in your DMZ and then
come inside and compromise a machine within,” he says. ”Machines
classified as DMZ machines should have as little connectivity or as
limited a connection as humanly possible with machines on your trusted
As an example, Gwinn says he puts his e-mail server in the private
network, but his e-mail smart host in the DMZ to take in and send out
messages. He says putting a Microsoft Exchange Server in the DMZ is
foolish. ”The first thing that happens is someone comes along and finds
an exploit in the code and they exploit your server. They can then own
IT managers should create a DMZ that takes these possibilities into
consideration. ”If the DMZ is set up correctly, it should limit outbound
connections in some way, as well,” Fleming says. ”If someone does
compromise a box, they are limited to where they can go and what they can
do. The server should not initiate outband connections, but people don’t
often think that way.”
Winn Schwartau, president of security consultancy The Security Awareness
Company, says the biggest thing to have in a DMZ is deception.
”Whoever is coming in there should not know whether it’s real or not,”
he says. ”The good guys will be fine and dandy but the bad guys will
not. You want to create a hall of mirrors.”
He recommends adding a honeypot to the DMZ where you can get information
on anyone trying to access your network. ”It allows you to spoof the bad
guys and collect data on the tools they are using,” he adds.
A DMZ does have its drawbacks, including the overhead on traffic. While
Schwartau recommends examining all inbound and outbound traffic, he
recommends using a variety of appliances to do this. ”Perform side tasks
— have your e-mail and virus checking on one box and your content
filtering on another. Make it as distributed as possible so you’re not
overloading the CPU,” he says.
Other considerations for a DMZ are the computation power required and
temporary data storage, which for large enterprises could become costly.
For this reason, some companies reject the DMZ blueprint. ”They would
rather deal with an occasional attack,” Fleming says.
Not Gwinn. He says the costs associated with a DMZ are well worth it.
”Sure, it raises your costs until you prevent your first network
security disaster. If you don’t know what a costly network is, just wait
until you have a major security compromise.”