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Five steps to controlling network access-2

Wes Noonan describes five ways to harden the network infrastructure and protect data in the event that the network perimeter fails.

Wes Noonan, author of "Hardening Network Infrastructures," reviews steps you can take from both a Windows and network perspective to protect your data regardless of what is occurring at the network perimeter.

One common security mistake is to treat the network and applications as separate entities that never interact. You may have separate people maintaining them, separate security policies, separate procedures and so on. Hardening Windows servers will go a long way toward protecting the integrity of the data on those servers, but you must also harden the network infrastructure itself. Start by taking the following five steps.

1. Implement access control lists (ACLs)

If someone can get inside your network, they can gain access to your Windows systems. You need to implement strict ACLs on your network equipment and grant access only to those users that require it. For example, do users in Houston ever need access to systems in New York? If not, chances are the traffic passing between those systems isn't essential to the business.

2. Implement network-based access control (NBAC)

Connecting systems to the network used to be a hassle: You had to build the network drivers, assign addresses and physically connect systems to get them to talk. Although this made it difficult for unauthorized systems to easily connect to the network, it created excessive administrative overhead. Then technologies like star-wired networks and Dynamic Host Configuration Protocol (DHCP) made it exceedingly simple to connect systems to the network. At first I rejoiced! But now I realize anyone can connect to the network. In fact, approximately 90% of the customers I visit have live network jacks that I can easily plug into to gain network access even if they have some written policy that states unauthorized connections are not permitted.

NBAC seeks to provide an enforcement mechanism to support those written policies. With NBAC, you want to define what is an authorized user and ensure connected systems are running the appropriate patches and software versions. If they aren't, they are placed in quarantine until the system is patched or updated.

3. Restrict remote connections

Implementing a VPN can be a risky endeavor. It permits network access for both users and viruses. Instead of allowing VPN access to your entire network, implement network ACLs that restrict remote users only to the servers and resources they need. For instance, using a VPN to connect Citrix or Terminal Server farms ensures that the only traffic allowed through the VPN is the Citrix traffic to the Citrix servers; if a remote client's system is infected, it will not infect your network.

4. Restrict and secure wireless connections

If implemented behind your firewall, wireless LAN connections create a particularly large, gaping hole in your network perimeter. As a result, your wireless LAN connections should be treated like any other remote connection: Terminate them outside your firewall and require a VPN connection to gain access to internal and protected resources.

5. Implement IPsec

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Implementing IPsec on your network is a great way to protect data in transit from being compromised. But it's no panacea. For example, if a machine is infected with Slammer, IPsec will only ensure the Slammer traffic is encrypted before it is transmitted. When used in conjunction with the other hardening methods, however, IPsec can serve as an effective method for protecting your internal traffic from prying eyes.


Due to network de-perimeterization, you can no longer rely exclusively on the network perimeter to protect systems and data. Removing the perimeter entirely is not the solution, nor is hardening the perimeter alone. You must also harden your Windows systems and network infrastructures to protect data in the event that the network perimeter fails or is circumvented.

About the Author
Wesley J. Noonan has been working in the computer industry for over 12 years, specializing in Windows-based networks and network infrastructure security design and implementation. He is a senior network consultant for Collective Technologies, LLC ( Wes recently authored the book "Hardening Network Infrastructures" for Osborne/McGraw-Hill and previously authored a chapter on network security and design for "The CISSP training guide" by QUE Publishing.


Note: This article originally appeared on

This was last published in November 2004

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