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A critical problem faced by a Network Intrusion Detection System (NIDS) is that of ambiguity. The NIDS cannot always determine what traffic reaches a given host nor how that host will interpret the traffic, and attackers may exploit this ambiguity to avoid detection or cause misleading alarms. We present a novel, lightweight solution, Active Mapping, which eliminates TCP/IP-based ambiguity in a NIDS' analysis with minimal runtime cost. Active Mapping efficiently builds profiles of the network topology and the TCP/IP policies of hosts on the network; a NIDS may then use the host profiles to disambiguate the interpretation of the network traffic on a per-host basis. Active Mapping avoids the semantic and performance problems of traffic normalization, in which traffic streams are modified to remove ambiguities.

We have developed a prototype implementation of Active Mapping and modified a NIDS to use the Active Mapping-generated profile database in our tests. We found wide variation across operating systems' TCP/IP stack policies in real-world tests (about 6,700 hosts), underscoring the need for this sort of disambiguation.

We discuss the capabilities and limitations of Active Mapping in detail, including real-world challenges. We also present results on the performance impact of using Active Mapping in terms of time and memory.

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