For many enterprises, the cost of active scanning is so high -- downtime, aggravation, finger pointing -- that it's relegated to a semiannual event. Also, active scanning yields extremely sensitive security information that can be misused.
Alternatively, the idea behind passive scanning is that systems expose a lot of information about themselves in normal communications. Active scanning can discover more, but passive scanning may be enough to help target-based IDS.
For example, by watching TCP connection establishment and teardown and application-layer banners, a passive IDS scanner can make a fairly good guess as to the operating system running on the communicating systems, and application types and version information.
We ran NeVO, Tenable Network Security's passive scanner, and found the output to be very accurate. Operating systems, application versions, known bugs and protocols running on nonstandard ports were all in the logs.
While passive scanning will never replace active scanning for its depth of information, it's a powerful tool which we expect all IDS vendors will eventually incorporate in their products.
About the author:
Joel Snyder is a senior partner at Opus One, an IT consulting firm.