NIDS (Network Intrusion Detection System) and NIPS (Network Intrusion Prevention System)

NIDS and NIPS (Behavior based, signature based, anomaly based, heuristic)

An intrusion detection system (IDS) is software that runs on a server or network device to monitor and track network activity. By using an IDS, a network administrator can configure the system to monitor network activity for suspicious behavior that can indicate unauthorized access attempts. IDSs can be configured to evaluate system logs, look at suspicious network activity, and disconnect sessions that appear to violate security settings.

IDSs can be sold with firewalls. Firewalls by themselves will prevent many common attacks, but they don’t usually have the intelligence or the reporting capabilities to monitor the entire network. An IDS, in conjunction with a firewall, allows both a reactive posture with the firewall and a preventive posture with the IDS.

In response to an event, the IDS can react by disabling systems, shutting down ports, ending sessions, deception (redirect to honeypot), and even potentially shutting down your network. A network-based IDS that takes active steps to halt or prevent an intrusion is called a network intrusion prevention system (NIPS). When operating in this mode, they are considered active systems.

Passive detection systems log the event and rely on notifications to alert administrators of an intrusion. Shunning or ignoring an attack is an example of a passive response, where an invalid attack can be safely ignored. A disadvantage of passive systems is the lag between intrusion detection and any remediation steps taken by the administrator.

Intrusion prevention systems (IPS) like IDSs follows the same process of gathering and identifying data and behavior, with the added ability to block (prevent) the activity.

A network-based IDS examines network patters, such as an unusual number or requests destined for a particular server or service, such as an FTP server. Network IDS systems should be located as upfront as possible, e.g. on the firewall, a network tap, span port, or hub, to monitor external traffic. Host IDS systems on the other hand, are placed on individual hosts where they can more efficiently monitor internally generated events.

Using both network and host IDS enhances the security of the environment.

Snort is an example of a network intrusion detection and prevention system. It conducts traffic analysis and packet logging on IP networks. Snort uses a flexible rule-based language to describe traffic that it should collect or pass, and a modular detection engine.

Network based intrusion detection attempts to identify unauthorized, illicit, and anomalous behavior based solely on network traffic. Using the captured data, the Network IDS 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.

Host based intrusion detection system (HIDS) 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. Tripwire is an example of a HIDS.

There are no fully mature open standards for ID at present. The Internet Engineering Task Force (IETF) is the body which develops new Internet standards. They have a working group to develop a common format for IDS alerts.

The following types of monitoring methodologies can be used to detect intrusions and malicious behavior: signature, anomaly, heuristic and rule-based monitoring.

A signature based IDS will monitor packets on the network and compare them against a database of signatures or attributes from known malicious threats. This is similar to the way most antivirus software detects malware. The issue is that there will be a lag between a new threat being discovered in the wild and the signature for detecting that threat being applied to your IDS.

A network IDS signature is a pattern that we want to look for in traffic. Signatures range from very simple – checking the value of a header field – to highly complex signatures that may actually track the state of a connection or perform extensive protocol analysis.

An anomaly-based IDS examines ongoing traffic, activity, transactions, or behavior for anomalies (things outside the norm) on networks or systems that may indicate attack. An IDS which is anomaly based will monitor network traffic and compare it against an established baseline. The baseline will identify what is “normal” for that network, what sort of bandwidth is generally used, what protocols are used, what ports and devices generally connect to each other, and alert the administrator when traffic is detected which is anomalous to the baseline.

A heuristic-based security monitoring uses an initial database of known attack types but dynamically alters their signatures base on learned behavior of network traffic. A heuristic system uses algorithms to analyze the traffic passing through the network. Heuristic systems require more fine-tuning to prevent false positives in your network.

A behavior-based system looks for variations in behavior such as unusually high traffic, policy violations, and so on. By looking for deviations in behavior, it is able to recognize potential threats and respond quickly.
Similar to firewall access control rules, a rule-based security monitoring system relies on the administrator to create rules and determine the actions to take when those rules are transgressed.

• CompTIA Security+ Study Guide: Exam SY0-301, Fifth Edition by Emmett Dulaney
• Mike Meyers’ CompTIA Security+ Certification Passport, Second Edition by T. J. Samuelle


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