An Intrusion Detection System (IDS) is used to detect unauthorized attempts at accessing and manipulating computer systems locally, through the Internet or through an intranet. It can detect several types of attacks and malicious behaviors that can compromise the security of a network and its computers. This includes network attacks against vulnerable services, unauthorized logins and access to sensitive data, and malware (e.g. viruses, worms, etc.). An IDS also detects attacks that originate from within a system. In most cases, an IDS has three main components: Sensors, Console, and Engine. Sensors generate security events. A console is used to alert and control sensors and to monitor events. An engine is used to record events and to generate security alerts based on received security events. In many IDS implementations, these three components are combined into a single device. Basically, the two following types of IDS are used :

  • Network-based IDS
  • Host-based IDS

Network-based IDS: A Network-based Detection System (NIDS) analyzes data packets flowing through a network. It can detect malicious packets that are designed to be overlooked by a firewall’s simplistic filtering rules. It is responsible for detecting anomalous or inappropriate data that may be considered ‘unauthorized’ on a network. An NIDS captures and inspects all data traffic, regardless of whether it is permitted for checking or not.

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Host-Based IDS: Host-based IDS (HIDS) is an Intrusion Detection System that runs on the system to be monitored. HIDS monitors only the data that is directed to or originating from that particular system on which HIDS is installed. Besides network traffic for detecting attacks, it can also monitor other parameters of the system such as running processes, file system access and integrity, and user logins for identifying malicious activities. BlackIce Defender and Tripwire are good examples of HIDS. Tripwire is an HIDS tool that automatically calculates the cryptographic hashes of all system files as well as any other files that a Network Administrator wants to monitor for modifications. It then periodically scans all monitored files and recalculates the information to see whether the files have been modified or not. It raises an alarm if changes are detected.

IDS Responses

The following are types of responses generated by an IDS:

  1. True Positive: A valid anomaly is detected, and an alarm is generated.
  2. True Negative: No anomaly is present, and no alarm is generated.
  3. False Positive: No anomaly is present, but an alarm is generated. This is the worst case. If any IDS response is a false positive high rate, IDS is ignored and not used.
  4. False Negative: A valid anomaly is present, but no alarm has been generated.

IDS Detection Methods

  • Statistical Anomaly Detection: The Statistical Anomaly Detection method, also known as behavior-based detection, compares the current system operating characteristics on many base-line factors such as CPU utilization, file access activity and disk usages, etc. In this method, the Intrusion Detection System provides the facility for either a Network Administrator to make the profiles of authorized activities or place the IDS in learning mode so that it can learn what is to be added as normal activity. A large amount of time needs to be dedicated to ascertain whether the IDS is producing few false negatives or not. Hence, the main drawback of IDS is that if an attacker slowly changes his activities over time, the IDS might be fooled into accepting the new behavior.
  • Pattern Matching Detection: The Pattern Matching IDS, also known as knowledge-based or signature-based IDS, is mainly based on a database of known attacks. These known attacks are loaded into the IDS as signatures. When this happens, the IDS begins to guard the network. These signatures are usually given a number or name so that the Network Administrator can easily identify the occurring attack. Alerts from this IDS can be triggered for fragmented IP packets, streams of SYN packets (DoS), or any malformed Internet Control Message Protocol (ICMP) packets. The main disadvantage of the Pattern Matching System is that such an IDS can only trigger on signatures that are stored in the database of the IDS. However, any new or any obfusticated attack performed by an attacker will be undetected.
  • Protocol Detection Method: In the Protocol Detection Method, IDS keeps state information and can detect abnormal activities of protocols such as IP, TCP, and UDP protocols. If there is any violation in an incoming protocol rule, the IDS sends an alert message to the Network Administrator. Such an IDS is usually installed on the Web server and monitors the communication between a user and the system on which it is installed.


Anti-x is a component of Cisco Adaptive Security Appliance (ASA). Anti-x provides an in-depth security design that prevents various types of problems such as viruses. The security provided by the tool includes the following:

  • Anti-virus: It scans network traffic and prevents the transmission of known viruses. It detects viruses through their virus signatures.
  • Anti-spyware: It scans network traffic and prevents the transmission of spyware programs. As spyware can damage the system, this tool becomes very critical for any organization. Spyware eats into a lot of precious bandwidth too.
  • Anti-spam: It deletes and segregates all junk e-mails before forwarding them to users. It examines all e-mails that arrive in the network.
  • Anti-phishing: It prevents any phishing attacks from reaching network users.
  • URL filtering: It filters Web traffic based on URLs to prevent users from connecting to inappropriate sites.
  • E-mail filtering: Apart from providing the anti-spamming feature, it also filters e-mails containing offensive materials, thus potentially protecting an organization from lawsuits.

The Cisco ASA appliance can be configured for a network-based role for all functions of Anti-x.