Intrusion detection systems monitor the use of computers and the network over which they communicate, searching for unauthorized use, anomalous behavior, and attempts to deny users, machines or portions of the network access to services. Potential users of such systems need information that is rarely found in marketing literature, including how well a given system finds intruders and how much work is required to use and maintain that system in a fully functioning network with significant daily traffic. Researchers and developers can specify which prototypical attacks can be found by their systems, but without access to the normal traffic generated by day-to-day work, they can not describe how well their systems detect real attacks while passing background traffic and avoiding false alarms. This information is critical: every declared intrusion requires time to review, regardless of whether it is a correct detection for which a real intrusion occurred, or whether it is merely a false alarm. To meet the needs of researchers, developers and ultimately system administrators we have developed the first objective, repeatable, and realistic measurement of intrusion detection system performance. Network traffic on an Air Force base was measured, characterized and subsequently simulated on an isolated network on which a few computers were used to simulate thousands of different Unix systems and hundreds of different users during periods of normal network traffic. Simulated attackers mapped the network, issued denial of service attacks, illegally gained access to systems, and obtained super-user privileges. Attack types ranged from old, well-known attacks, to new, stealthy attacks. Seven weeks of training data and two weeks of testing data were generated, filling more than 30 CD-ROMs. Methods and results from the 1998 DARPA intrusion detection evaluation will be highlighted, and preliminary plans for the 1999 evaluation will be presented.