The technology behind content filtering is fairly simple. If the device is set up to be a monitor, technicians can attach it to the network by using a network tap, span port or similar replication technology, ensuring that the network has a copy of all traffic. If it is designed to serve as a filter, it can be placed at a choke point in the network.
The important criteria to evaluate when deciding if a content filter meets your business requirements is how the filter decides which traffic is allowed and which is denied. Most of the current generation of content filters use whitelist/blacklist technology to build lists of acceptable and unacceptable content. Depending upon the organization's security requirements, either a default "allow" or "deny" rule is applied. This approach is often seen in Web content filtering, where users are blocked from accessing inappropriate Web sites. While maintaining these lists can be quite a chore, filter manufacturers often provide a subscription service that offers access to a centrally maintained site categorization scheme.
Some companies are experimenting with newer content-filtering technologies. Using document signatures, traffic profiles and other techniques, these approaches seek to identify leaks of confidential information and other inappropriate content. While they hold promise, they're probably only useful if you have extremely high security requirements or a desire to be on the cutting edge of security technology. Otherwise, I'd recommend waiting a couple of years until these technologies mature.
This was first published in February 2007