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This paper describes the design and implementation of protocol scrubbers. Protocol scrubbers are transparent, interposed mechanisms for explicitly removing network scans and attacks at various protocol layers. The transport scrubber supports downstream passive network-based intrusion detection systems by converting ambiguous network flows into well-behaved flows that are unequivocally interpreted by all downstream endpoints. The fingerprint scrubber restricts an attacker's ability to determine the operating system of a protected host. As an example, this paper presents the implementation of a TCP scrubber that eliminates insertion and evasion attacks-attacks that use ambiguities to subvert detection-on passive network-based intrusion detection systems, while preserving high performance. The TCP scrubber is based on a novel, simplified state machine that performs in a fast and scalable manner. The fingerprint scrubber is built upon the TCP scrubber and removes additional ambiguities from flows that can reveal implementation-specific details about a host's operating system.
Date of Publication: April 2004