Due to the complexity of today's data breaches and intrusions, deploying and maintaining network security more frequently requires a promising system to defend against intruders and other security threats as well. Organizations securing their networks often use a combination of technologies to combat the countless cyber attack, intrusion, and compromise methods available to cyber criminals today.

Although a wide range of tools and methodologies exists, the two widespread fundamentals to all secure enterprise network configurations are the firewall and the intrusion detection/prevention system (IDS/IDPS). A firewall controls incoming and departing traffic based on rules and policies, and act as a wall between secure and un-trusted networks. Within the secure network, an IDS/IDPS discovers suspicious activities to/from hosts and within the traffic itself and can take proactive measures to log and block attacks.

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In intrusion detection system we have two common types of IDS, Network Based Intrusion Detection System (NIDS) and Host Based Intrusion Detection System (HIDS) that are widely used.

NIDS:

Network-based intrusion detection system (NIDS) attempts to identify unauthorized, illicit, and anomalous behavior based solely on network traffic. The role of a network IDS is passive, only gathering, identifying, logging and alerting. It monitors traffic on a network looking for suspicious activity, which could be an attack or unauthorized activity.

HIDS:

A host-based intrusion detection system (HIDS) examines all or parts of the dynamic behavior and the state of a computer system. It monitors and analyzes the internals of a computing system as well as (in some cases) the network packets on its network interfaces. An HIDS gives you deep visibility of what's happening on your critical systems.

Both types of Intrusion Detection System involve the gathering and analysis of information from a variety of areas contained in a computer or network to identify possible threats posed by hackers and crackers inside or outside the organization. However, the most effective fortification for a corporate network is provided by a combination of both technologies.

IDS is It's simply a security software which is termed to help user or system administrator by automatically alerting or notifying when a user attempts to compromise information system through any malicious activities or at the point where a violation of security policies is taken. These detections are operated by inspecting traffic that occurs between hosts. These mechanisms are prorated into two major forms.

1. Signature detection
2. Anomaly detection

Signature Detection:

This type of detection works well with the threads that are already determined or known. It implicates searching a series of bytes or sequence that are termed to be malicious. One of the most profitable points is that signatures are easy to apply and develop once you figure out the sort of network behavior to be found out.

The main drawback to signature based IDS is that it's easy to fool signature-based solutions by changing the ways in which an attack is made and the more advanced the IDS Signature database, the higher the CPU load for the system charged with analyzing each signature.

Anomaly Detection:

The anomaly detection technique is a centralized process that works on the concept of a baseline for network behavior. This baseline is a depiction of accepted network behavior, which is learned or specified by the network administrators, or both. It's like a guard personally interviewing everyone at the gate before they are let down the drive.

One of the major drawbacks of anomaly detection engines is the difficulty of defining rules. Each protocol being analyzed must be defined, implemented and tested for accuracy which is not always an easy task.

Whether you need to monitor your own network or Host by connecting them to identify any latest threats, there are various free Intrusion Detection Systems that offer outstanding functionalities and can be used at the enterprise level. Many, if not most, of these intrusion-detection systems (IDS), uses a combination of engines, to create solid, free intrusion-detection services.

Security Onion:

Security Onion is an Ubuntu-based Linux distribution used for network monitoring and intrusion detection. It can monitor multiple VLANs and subnets and works fine in VMware and other virtual environments. This configuration can be used as IDS only. At present, it isn't supported to be run as an IPS. However, there is the option to run as a network and host intrusion detection deployment that contains tools and services such as Squil, Bro IDS, and OSSEC to perform the IDS functionality. However, Security Onion needs more assistance with development, which will most likely happen in time. The easy-to-use Setup wizard allows building a group of numerous distributed sensors for enterprise in minutes.

OSSEC:

OSSEC is an open source host intrusion detection system (HIDS) which offers multiple additional modules that can be used with the core functionality of IDS. In addition to intrusion detection, the OSSEC can perform file integrity monitoring and rootkit detection with real-time alerts, all of which are centrally managed with the ability to create different policies, depending on a company's needs. The OSSEC can locally run on most operating systems, including Linux versions, Mac OSX and Windows.

OpenWIPS-NG:

OpenWIPS-NG is a wireless IDS/IPS that relies on a server, sensors, and interfaces and available freely. As developed by the author of Aircrack-NG, this system has many functionalities and services already built for scanning, detection and intrusion prevention. OpenWIPS-NG is modular and allows an administrator to download plug-ins for additional features. It also provides facility to perform WIPS on a tight budget.

Suricata:

Unlike other IDS/IPS systems, Suricata contends most directly with Snort. This system has an architecture that is similar to Snort that relies on signatures and can even use Snort rules. Being newer than Snort, Suricata has ways to catch up to in this area. If Snort isn't a preference in your organization, this is the contiguous free tool available to run on an enterprise network.

Bro IDS:

Bro IDS is almost like to Security Onion; however, it uses more than IDS rules to find out from where the attacks are coming. Bro IDS uses a combination of tools. Bro is considered a specification-based network IDS. It uses a wide range of protocol analysis modules to inspect traffic and make decisions regarding its conformance to various norms. It is a very powerful complement to Snort.

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However, the list free IDS is not limited to the IDS mentioned above only; there are many Intrusion Detection Systems as well. The power of IDS is that it demonstrates a positive degree of readiness, which may be critical for long-term success. If your business relies on networking, then implementing IDS is essential to prevent intruders from harming your business.