A: Listening to network traffic and C: Monitoring the audit trails on a server are considered passive activities in regard to the functioning of an Intrusion Detection System.
An Intrusion Detection System (IDS) refers to a system that monitors network traffic for suspicious activities and issues alert when such activities are discovered. An intrusion Detection System is a software application that scans a network or server for harmful activities or policy breaches. Any malicious venture or violation activity is reported either to an administrator or collected centrally using a SIEM or security information and event management system. A SIEM integrates outputs from various sources and uses alarm filtering mechanisms to differentiate malicious activities from false alarms.
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