Host based & Network based IDS

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Host-Based IDS (HIDS)

Host-based systems were the first type of IDS to be developed and implemented. These systems collect and analyze data that originate on a computer that hosts a service, such as a Web server. Once this data is aggregated for a given computer, it can either be analyzed locally or sent to a separate/central analysis machine. One example of a host-based system is programs that operate on a system and receive application or operating system audit logs. These programs are highly effective for detecting insider abuses. Residing on the trusted network systems themselves, they are close to the network’s authenticated users. If one of these users attempts unauthorized activity, host-based systems usually detect and collect the most pertinent information in the quickest possible manner. In addition to detecting unauthorized insider activity, host-based systems are also effective at detecting unauthorized file modification.

On the down side, host-based systems can get unwieldy. With several thousand possible endpoints on a large network, collecting and aggregating separate specific computer information for each individual machine may prove inefficient and ineffective. In addition, if an intruder disables the data collection on any given computer, the IDS on that machine will be rendered useless because there is no backup.

Possible host-based IDS implementations include Windows NT/2000 Security Event Logs, RDMS audit sources, Enterprise Management systems audit data (such as Tivoli), and UNIX Syslog in their raw forms or in their secure forms such as Solaris' BSM; host-based commercial products include RealSecure, ITA, Squire, and Entercept, to name a few.

Network-Based IDS (NIDS)

As opposed to monitoring the activities that take place on a particular network, Network-based intrusion detection analyzes data packets that travel over the actual network. These packets are examined and sometimes compared with empirical data to verify their nature: malicious or benign. Because they are responsible for monitoring a network, rather than a single host, Network-based intrusion detection systems (NIDS) tend to be more distributed than host-based IDS. Software, or appliance hardware in some cases, resides in one or more systems connected to a network, and is used to analyze data such as network packets. Instead of analyzing information that originates and resides on a computer, network-based IDS uses techniques like “packet-sniffing” to pull data from TCP/IP or other protocol packets traveling along the network. This surveillance of the connections between computers makes network-based IDS great at detecting access attempts from outside the trusted network. In general, network-based systems are best at detecting the following activities:

   * Unauthorized outsider access: When an unauthorized user logs in successfully, or attempts to log in, they are best tracked with host-based IDS. However, detecting the unauthorized user before their log on attempt is best accomplished with network-based IDS.
   * Bandwidth theft/denial of service: These attacks from outside the network single out network resources for abuse or overload. The packets that initiate/carry these attacks can best be noticed with use of network-based IDS.

Some possible downsides to network-based IDS include encrypted packet payloads and high-speed networks, both of which inhibit the effectiveness of packet interception and deter packet interpretation. Examples of network-based IDS include Shadow, Snort!, Dragon, NFR, RealSecure, and NetProwler.

HIDS and NIDS Used in Combination

The two types of intrusion detection systems differ significantly from each other, but complement one another well. The network architecture of host-based is agent-based, which means that a software agent resides on each of the hosts that will be governed by the system. In addition, more efficient host-based intrusion detection systems are capable of monitoring and collecting system audit trails in real time as well as on a scheduled basis, thus distributing both CPU utilization and network overhead and providing for a flexible means of security administration.

In a proper IDS implementation, it would be advantageous to fully integrate the network intrusion detection system, such that it would filter alerts and notifications in an identical manner to the host-based portion of the system, controlled from the same central location. In doing so, this provides a convenient means of managing and reacting to misuse using both types of intrusion detection.

That said, as an organization introduces an IDS into its network to augment its current information security strategy, the primary focus of the intrusion detection system should be host-based. Although network intrusion detection has its merits and certainly must be incorporated into a proper IDS solution, it has historically been incapable of evolving to comply with the growing technology of data communications. Most NIDS perform miserably, if at all, on switched networks, fast networks of speeds over 100 Mbps, and encrypted networks. Furthermore, somewhere in the range of 80 - 85 percent of security incidents originate from within an organization. Consequently, intrusion detection systems should rely predominantly on host-based components, but should always make use of NIDS to complete the defense. In short, a truly secure environment requires both a network and host-based intrusion detection implementation to provide for a robust system that is the basis for all of the monitoring, response, and detection of computer misuse.