IDS can be defined as the tools, methods, and resources to help identify, assess, and report unauthorized or unapproved network activity. An IDS detects activity in traffic that may or may not be an intrusion. IDSes can detect and deal with insider attacks, as well as, external attacks, and are often very useful in detecting violations of corporate security policy and other internal threats.

Host Based Intrusion Detection

Host based IDS are usually installed on servers and are more focused on analyzing the specific operating systems and applications, resource utilization and other system activity residing on the Host-based IDS host. It will log any activities it discovers to a secure database and check to see whether the events match any malicious event record listed in the knowledge base. Host-based IDS are often critical in detecting internal attacks directed towards an organization’s servers such as DNS, Mail, and Web Servers.

Network Based Intrusion Detection

Network based IDS are dedicated network devices distributed within networks that monitor and inspect network traffic flowing through the device. Instead of analyzing information that originates and resides on a host, Network-based IDS uses packet sniffing techniques to pull data from TCP/IP packets or other protocols that are traveling along the network. }Most Network-based IDS log their activities and report or alarm on questionable events. Network-based IDS work best when located on the DMZ, on any subnets containing mission critical servers and just inside the firewall.

Comparison of Host based IDS and Network based IDS

Host based IDSNetwork based IDS
Narrow in scope (watches only specific host activities)Broad in scope (watches all network activities)
More complex setupEasier setup
Better for detecting attacks from the insideBetter for detecting attacks from the outside
More expensive to implementLess expensive to implement
Detection is based on what any single host can recordDetection is based on what can be recorded on the entire network
Does not see packet headersExamines packet headers
Usually only responds after a suspicious log entry has been madeNear real-time response
Detects local attacks before they hit the networkDetects network attacks as payload is analyzed
Verifies success or failure of attacksDetects unsuccessful attack attempts

Hybrid Intrusion Detection

Hybrid IDS are systems that combine both Host-based IDS, which monitors events occurring on the host system and Network-based IDS, which monitors network traffic, functionality on the same security platform. A Hybrid IDS, can monitor system and application events and verify a file system’s integrity like a Host-based IDS, but only serves to analyze network traffic destined for the device itself. A Hybrid IDS is often deployed on an organization’s most critical servers.

Signature Based IDS

Monitor network or server traffic and match bytes or packet sequences against a set of predetermined attack lists or signatures. When a particular intrusion or attack session match a signature configured on the IDS, the system alerts administrators or takes other pre-configured action.  Signatures are easy to develop and understand if you know what network behavior you’re trying to identify. However, because they only detect known attacks, a signature must be created for every attack. New vulnerabilities and exploits will not be detected until administrators develop new signatures. Another drawback to signature-based IDS is that they are very large and it can be hard to keep up with the pace of fast moving network traffic.

Anomaly Based IDS

Use network traffic baselines to determine a “normal” state for the network and compare current traffic to that baseline. Use a type of statistical calculation to determine whether current traffic deviates from “normal” traffic, which is either learned and/or specified by administrators. If network anomalies occur, the IDS alerts administrators. A new attack for which a signature doesn’t exist can be detected if it falls out of the “normal” traffic patterns. High false alarm rates created by inaccurate profiles of “normal” network operations.

Why are IDS important?

The ability to know when an intruder or attacker is engaged in reconnaissance or other malicious activity can mean the difference between being compromised and not being compromised. An IDS can alert the administrator of a successful compromise, allowing them the opportunity to implement mitigating actions before further damage is caused. As Corporations and other Institutions are being legally compelled to disclose data breaches and compromises to their affected customers, this can have profound effects upon a compromised company, in the way of bad press, loss of customer trust, and the effects on their stock.

Pros of IDS

  • Can detect external hackers, as well as, internal network-based attacks
  • Scales easily to provide protection for the entire network
  • Offers centralized management for correlation of distributed attacks
  • Provides defense in depth
  • Gives administrators the ability to quantify attacks
  • Provides an additional layer of protection

Cons of IDS

  • Generates false positives and negatives
  • Reacts to attacks rather than preventing them
  • Requires full-time monitoring and  highly skilled staff dedicated to interpreting the data
  • Requires a complex incident response process
  • Cannot monitor traffic at higher network traffic rates
  • Generates an enormous amount of data to be analyzed
  • Cannot deal with encrypted network traffic
  • It is expensive

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