Bro: a system for detecting network intruders in real-time

  • Authors:
  • Vern Paxson

  • Affiliations:
  • Network Research Group, Lawrence Berkeley National Laboratory, Berkeley, CA

  • Venue:
  • SSYM'98 Proceedings of the 7th conference on USENIX Security Symposium - Volume 7
  • Year:
  • 1998

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Abstract

We describe Bro, a stand-alone system for detecting network intruders in real-time by passively monitoring a network link over which the intruder's traffic transits. We give an overview of the system's design, which emphasizes high-speed (FDDI-rate) monitoring, real-time notification, clear separation between mechanism and policy, and extensibility. To achieve these ends, Bro is divided into an "event engine" that reduces a kernel-filtered network traffic stream into a series of higher-level events, and a "policy script interpreter" that interprets event handlers written in a specialized language used to express a site's security policy. Event handlers can update state information, synthesize new events, record information to disk, and generate real-time notifications via syslog. We also discuss a number of attacks that attempt to subvert passive monitoring systems and defenses against these, and give particulars of how Bro analyzes the four applications integrated into it so far: Finger, FTP, Portmapper and Telnet. The system is publicly available in source code form.