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IDFAQ: What is Intrusion Detection?

Intrusion Detection can be defined as "...the act of detecting actions that attempt to compromise the confidentiality, integrity or availability of a resource." More specifically, the goal of intrusion detection is to identify entities attempting to subvert in-place security controls.

Common types of Intrusion Detection:

Network Based (Network IDS)

Network based intrusion detection attempts to identify unauthorized, illicit, and anomalous behavior based solely on network traffic. A network IDS, using either a network tap, span port, or hub collects packets that traverse a given network. Using the captured data, the IDS system processes and flags any suspicious traffic. Unlike an intrusion prevention system, an intrusion detection system does not actively block network traffic. The role of a network IDS is passive, only gathering, identifying, logging and alerting. Examples of Network IDS:

Host Based (HIDS)

Often referred to as HIDS, host based intrusion detection attempts to identify unauthorized, illicit, and anomalous behavior on a specific device. HIDS generally involves an agent installed on each system, monitoring and alerting on local OS and application activity. The installed agent uses a combination of signatures, rules, and heuristics to identify unauthorized activity. The role of a host IDS is passive, only gathering, identifying, logging, and alerting. Examples of HIDS:

Physical (Physical IDS)

Physical intrusion detection is the act of identifying threats to physical systems. Physical intrusion detection is most often seen as physical controls put in place to ensure CIA. In many cases physical intrusion detection systems act as prevention systems as well. Examples of Physical intrusion detections are:

  • Security Guards
  • Security Cameras
  • Access Control Systems (Card, Biometric)
  • Firewalls
  • Man Traps
  • Motion Sensors

Intrusion Prevention

Intrusion prevention follows the same process of gathering and identifying data and behavior, with the added ability to block (prevent) the activity. This can be done with Network, Host, and Physical intrusion detection systems.


Matthew Berge
Ernst & Young