How Does an Intrusion Detection System (IDS) Work?

An Intrusion Detection System (IDS) monitors network or host activity to detect threats and policy violations. Unlike firewalls that block traffic, IDS tools alert analysts to suspicious behavior—supporting threat detection, compliance, and investigation. IDS tools remain a foundational element of layered network defense, even though they may not directly stop attacks alone.

Operating passively at the network or host level, IDS monitors traffic and system activity to identify known attack signatures or behavioral anomalies. Unlike firewalls or intrusion prevention systems (IPS), which sit inline and actively enforce policy, IDS operates out-of-band, providing critical detection without introducing latency or disruption.

Modern IDS platforms are evolving to meet the complexity of distributed and hybrid architectures. Key capabilities may include:

• Deep packet inspection
• Machine learning-assisted anomaly detection
• Integration with Security Information and Event Management (SIEM), Security Orchestration, Automation, and Response (SOAR), and Extended Detection and Response (XDR) platforms
  

Why IDS Still Matters in 2025

Despite the rise of Endpoint Detection and Response (EDR), Zero Trust, and threat intelligence platforms, IDS fills a unique gap in layered defense:

• Monitoring east-west traffic (lateral traffic flows within the network)
• Alerting on known signatures and behavioral anomalies
• Contributing to compliance audits through detailed logging of security events
• Enriching SIEM visibility when integrated with third-party intelligence

Types of IDS and Where They Fit

Different types of IDS serve specific needs:

• NIDS (Network IDS): Used at strategic points in the network to monitor traffic flowing to and from devices.
• HIDS (Host IDS): Observes logs and activity on individual devices via installation on devices in the network.
• Signature-Based IDS: Matches threats to known attack signatures. Best for known exploits.
• Anomaly-Based IDS: Flags deviations from normal behavior. Can detect unknown or zero-day attacks if normal baselines are established.
• Hybrid IDS: Combines methods

How Does an IDS Work?

Understanding the types of IDS is crucial, but equally important is how these systems detect threats. Let’s explore the detection methods they employ:

Traffic Capture:
Taps into network traffic using span ports, sensors, or agents. Inspects packet headers and payloads.

Analysis Engine:

• Signature detection: Compares traffic to a database of known attack patterns.
• Anomaly detection: Identifies unusual behavior, including lateral movement.

Alerting:
Triggers alerts for suspicious activity like brute-force attempts or unauthorized port access.

Response Integration:
While IDS doesn’t block threats, it can forward alerts to SOAR, firewalls, or human workflows that may trigger containment actions.

What Can IDS Detect?

A well-tuned IDS can identify many common intrusion patterns, including:

• • Network reconnaissance and port scanning activity
  • Some attacker attempts to evade detection—but not all 
  • Known exploit attempts, such as SQL injection

• Brute-force authentication attempts across services

• Suspicious lateral movement across internal systems
• Unusual outbound traffic patterns that may indicate data exfiltration

IDS can generally detect a whole host of known threats and suspicious behaviors, but they are not foolproof—advanced attackers often use evasion techniques such as encryption or obfuscation to bypass detection.

IDS vs. IPS: What’s the Difference?

Understanding the difference between Intrusion Detection Systems (IDS) and Intrusion Prevention Systems (IPS) is essential for building a layered security architecture that balances visibility with control. Choosing the right mix of IDS, IPS, and other tools depends on your environment’s tolerance for disruption, need for real-time enforcement, and capacity for forensic investigation.

IDS and IPS serve distinct but complementary roles. An IDS operates passively,  monitoring traffic, analyzing patterns, and generating alerts when it detects suspicious activity. It does not take direct action to block or stop the traffic, making it valuable for post-incident forensics, compliance auditing, and Security Operations Center (SOC) triage. It poses minimal risk of interrupting legitimate traffic.

In contrast, an IPS operates inline and can actively block threats. It’s designed to stop malicious activity before it reaches its target, making it effective for enforcing security policies and preventing exploit attempts. However, this enforcement role also brings operational risk: Overly aggressive rules or false positives can interrupt legitimate business traffic.

IDS Deployment Best Practices

Effective IDS deployment requires more than just placing sensors. Thoughtful integration, continuous tuning, and alignment with your environment’s threat landscape are crucial to successful IDS use. The following best practices help maximize detection coverage: 

Strategic Sensor Placement:

• At entry or exit points, network borders, and internet gateways
• Near crown-jewel assets and critical apps

Regular Updates:

• Regular signature updates for pattern-based detection
• Periodically retrain machine learning models depending on false positive rate

Tune for Context:

• Suppress false positives from legitimate activity
• Customize detection thresholds for each environment

Integrate with SIEM and SOAR:

• Use your SIEM to enrich and correlate alerts from IDS
• Automate triage and escalation using SOAR playbooks

Monitor and Review:

• Prioritize by severity and known indicators of compromise
• Track missed detections and refine models regularly
• Regularly review incidents and logs

Final Takeaway

Intrusion Detection Systems remain a foundational component of layered security. While IDS does not block threats, it can enhance visibility across east-west traffic, support incident investigation, and contribute to compliance mandates, especially when baselines are established and updated on a regular basis. When integrated with SOAR or SIEM, IDS enables faster triage and informed responses.

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