What is an Intrusion Detection System?

Keeping your network safe from intrusion is one of the most vital parts of system and network administration and security. If your network is penetrated by a malicious attacker, it can lead to massive losses for your company, including potential downtime, data breaches, and loss of customer trust.

An intrusion detection system (IDS) is a tool or software that works with your network to keep it secure and flag when somebody is trying to break into your system. There are several different types of IDS and numerous tools on the market and figuring out which one to use can be daunting.

An intrusion detection system (IDS) can also be seen as a device or software application that monitors a network for malicious activity or policy violations. Any malicious activity or violation is typically reported or collected centrally using a security information and event management system. Some IDS’s are capable of responding to detected intrusion upon discovery. These are classified as intrusion prevention systems (IPS).

IDS Detection Types

There is a wide array of IDS, ranging from antivirus software to tiered monitoring systems that follow the traffic of an entire network. The most common classifications are:

  • Network intrusion detection systems (NIDS): A system that analyzes incoming network traffic.
  • Host-based intrusion detection systems (HIDS): A system that monitors important operating system files.

There is also subset of IDS types. The most common variants are based on signature detection and anomaly detection.

  • Signature-based: Signature-based IDS detects possible threats by looking for specific patterns, such as byte sequences in network traffic, or known malicious instruction sequences used by malware. This terminology originates from antivirus software, which refers to these detected patterns as signatures. Although signature-based IDS can easily detect known attacks, it is impossible to detect new attacks, for which no pattern is available.
  • Anomaly-based: a newer technology designed to detect and adapt to unknown attacks, primarily due to the explosion of malware. This detection method uses machine learning to create a defined model of trustworthy activity, and then compare new behavior against this trust model. While this approach enables the detection of previously unknown attacks, it can suffer from false positives: previously unknown legitimate activity can accidentally be classified as malicious.

Other Types of Intrusion Detection System

You might be wondering: what are the different ways to classify an IDS? There are three main types of intrusion detection software, or three main “parts,” depending on if you view these all as part of one system:

  1. Network Intrusion Detection System
  2. Network Node Intrusion Detection System
  3. Host Intrusion Detection System

At the most basic level, Network Intrusion Detection Systems and Network Node Intrusion Detection Systems look at network traffic, while Host Intrusion Detection Systems look at actions and files on the host devices.

What Is a Network Intrusion Detection System?

A Network Intrusion Detection System (NIDS) is generally deployed or placed at strategic points throughout the network, intended to cover those places where traffic is most likely to be vulnerable to attack. Generally, it’s applied to entire subnets, and it attempts to match any traffic passing by to a library of known attacks. It passively looks at network traffic coming through the points on the network on which it’s deployed. They can be relatively easy to secure and can be made difficult for intruders to detect. This means an intruder may not realize their potential attack is being detected by the NIDS.

Network-based intrusion detection system software analyzes a large amount of network traffic, which means they sometimes have low specificity. This means sometimes they might miss an attack or might not detect something happening in encrypted traffic. In some cases, they might need more manual involvement from an administrator to ensure they’re configured correctly.

What Is a Network Node Intrusion Detection System?

A Network Node Intrusion Detection System (NNIDS) is like a NIDS, but it’s only applied to one host at a time, not an entire subnet.

What Is a Host Intrusion Detection System?

The Host intrusion Detection system HIDs runs on all the devices in the network with access to the internet and other parts of the enterprise network. HIDS have some advantages over NIDS, due to their ability to look more closely at internal traffic, as well as working as a second line of defense against malicious packets a NIDS has failed to detect.

It looks at the entire system’s file set and compares it to its previous “snapshots” of the file set. It then looks at whether there are significant differences outside normal business use and alerts the administrator as to whether there are any missing or significantly altered files or settings. It primarily uses host-based actions such as application use and files, file access across the system, and kernel logs.

Network and host-based intrusion detection systems are the most common ways of expressing this classification, and you won’t find NNIDS mentioned very often in this space. Just think of it as a type of NIDS.

There are also two main approaches to detecting intrusion: signature-based IDS and anomaly-based IDS.

IDS Usage in Networks

When placed at a strategic point or points within a network to monitor traffic to and from all devices on the network, an IDS will perform an analysis of passing traffic, and match the traffic that is passed on the subnets to the library of known attacks. Once an attack is identified, or abnormal behavior is sensed, the alert can be sent to the administrator.

Evasion Techniques

Being aware of the techniques available to cyber criminals who are trying to breach a secure network can help IT departments understand how IDS systems can be tricked into not missing actionable threats:

  • Fragmentation: Sending fragmented packets allow the attacker to stay under the radar, bypassing the detection system’s ability to detect the attack signature.
  • Avoiding defaults: A port utilized by a protocol does not always provide an indication to the protocol that’s being transported. If an attacker had reconfigured it to use a different port, the IDS may not be able to detect the presence of a trojan.
  • Coordinated, low-bandwidth attacks: coordinating a scan among numerous attackers, or even allocating various ports or hosts to different attackers. This makes it difficult for the IDS to correlate the captured packets and deduce that a network scan is in progress.
  • Address spoofing/proxying: attackers can obscure the source of the attack by using poorly secured or incorrectly configured proxy servers to bounce an attack. If the source is spoofed and bounced by a server, it makes it very difficult to detect.
  • Pattern change evasion: IDS rely on pattern matching to detect attacks. By making slight adjust to the attack architecture, detection can be avoided.


No firewall is foolproof, and no network is impenetrable. Attackers continuously develop new exploits and attack techniques designed to circumvent your defenses. Many attacks leverage other malware or social engineering to obtain user credentials that grant them access to your network and data. A network intrusion detection system (NIDS) is crucial for network security because it enables you to detect and respond to malicious traffic.

The primary benefit of an intrusion detection system is to ensure IT personnel is notified when an attack or network intrusion might be taking place. A network intrusion detection system (NIDS) monitors both inbound and outbound traffic on the network, as well as data traversing between systems within the network. The network IDS monitors network traffic and triggers alerts when suspicious activity or known threats are detected, so IT personnel can examine more closely and take the appropriate steps to block or stop an attack.

Modern networked business environments require a high level of security to ensure safe and trusted communication of information between various organizations. An intrusion detection system acts as an adaptable safeguard technology for system security after traditional technologies fail. Cyber attacks will only become more sophisticated, so it is important that protection technologies adapt along with their threats.

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Author: refuge_2020

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