Definition Monday: Network Access Control

Welcome to Definition Monday, where we define and explain a common technology or security concept for the benefit of our less experienced readers. This week: Network Access Control.

In an extraordinarily high security environment, it’s possible that only devices personally vetted by the IT team could be connected to a network. Using strict procedures as well as technologies like 802.1x, multifactor authentication, and MAC address port locking, it would be possible to ensure that only a specific set of network devices would be able to pass data.


However, very, very few networks are run with that sort of tight security. In a typical enterprise environment, previously unknown and unvetted clients need to connect all the time. Salesmen or consultants visiting the office will need network resources to work. Student interns might bring their own laptops or palmtops, since few companies will actually issue computers to the unpaid. Employees might want to use an iPad or some other personally owned device in addition to their corporate computer.

So the question becomes this: how do we ensure the security of the network as a whole, including all of the information assets on it, while still being flexible enough to accommodate any random piece of hardware that someone brings in that happens to speak TCP/IP?

Network Access Control.

As the name implies, a Network Access Control (or NAC) system acts as a gatekeeper, controlling client access to network resources. Whether you’re looking at an Open Source system like Packetfence or a commercial product like Cisco Clean Access or Bradford Campus Manager, the methodology is more or less the same.

When a device is connected to the network, a message is sent to a central database server with the hardware address of the device; this is to determine whether this is something that has been used on the network in the past or if it is some entirely new visitor. If it is a new device, the system will generally ask for some user credentials to ensure that the person plugging in this item is an authorized user of the network. This is especially important in wireless environments, where clients cannot be assumed to be in a particular geographic area but may be out in a parking lot or on another floor of a shared building.

Once the credentials are authenticated, generally via a RADIUS or LDAP central directory server, the NAC system will evaluate the security posture of the device. This is usually done via a piece of software called an “agent”, which is downloaded to the client machine and executed to gather data. This agent will retrieve information like the patch level of the operating system, the presence or absence of items like antivirus software, the networking settings, and so on. Information retrieved from the agent is then relayed back to the NAC, which will use it to define network connection parameters for the new client.

For example, imagine a student intern brings in his home netbook to use on the company network. When he connects, he is prompted for his username and password; this establishes that the item is owned by an intern, not a full time employee, so he may be placed on a VLAN for end users who don’t need access to database servers and other critical infrastructure. The agent then relays that the netbook has antivirus software installed, but the definition file is out of date; this information could be used to put the netbook into a “guest” VLAN with only Internet access, sealed off from company resources. It could even be used to put the device into a “remediation” VLAN that only has access to Windows Update, Symantec, and other web sites that would be useful for getting the machine up to snuff. Once it has been brought up to date, the agent will run again, realize that it is fixed, and reallocate network resources accordingly.

Obviously, the initial deployment of a NAC requires a lot of thought and planning. But with more and more employees wanting to just use their own equipment in the office, a Network Access Control system can save tremendous amounts of time for your IT staff by relieving them of the need to personally evaluate and update each new machine that someone wants to use at work.

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