In enterprise network security systems, both intrusion detection systems (IDS) and intrusion prevention systems (IPS) are crucial. Deploying only IDS/IPS without monitoring and analyzing the massive amounts of logs they generate renders the security investment meaningless. These logs are direct evidence of abnormal behavior in network traffic, the only clues to connecting fragments of attack events and reconstructing the attacker's path, and the starting point for shifting from passive defense to proactive hunting. For modern enterprises, monitoring these logs is no longer optional, but the cornerstone of building effective defense-in-depth, meeting compliance requirements, and achieving real-time response and post-incident traceability.

IDS and IPS differ in their functional positioning, which directly affects the value dimension of the logs they generate. Intrusion detection systems, deployed in a bypass manner, deeply analyze network traffic or host behavior, logging and generating alerts for any activity that matches known attack characteristics or deviates from the normal baseline. The core value of IDS logs lies in their breadth and insight; they strive to leave no suspicious signs unchecked, providing the security team with a complete, unvarnished "threat observation report." A typical network-based Intrusion Detection System (IDS) log contains key information such as timestamps, source and destination IP addresses, ports, triggering attack signatures, and raw packet fragments. These records constitute the raw material for threat analysis.

Intrusion Prevention Systems (IPS) can drop malicious packets or terminate connections in real time when an attack is detected. Therefore, IPS logs not only record threat discovery but, more importantly, the actions taken. An IPS log clearly tells a security analyst that at a certain point in time, a specific attack from a specific IP was successfully blocked. This provides direct evidence for measuring the effectiveness of security policies and confirming the integrity of protection boundaries. However, before being systematically collected, correlated, and analyzed, both IDS alert logs and IPS intercept logs are merely discrete data points and cannot be transformed into actionable security intelligence.

The primary importance of continuous monitoring of IDS/IPS logs lies in their ability to detect successful intrusions that have bypassed other layers of protection. No single defense is perfect; advanced persistent threat (APS) attacks or well-crafted zero-day attacks can bypass firewalls and evade malware detection. However, these attacks inevitably leave abnormal patterns in network traffic as they move laterally, communicate with command and control servers, or carry out data infiltration. A mature attack will eventually trigger a characteristic rule or behavioral anomaly alert in the IDS. By monitoring logs, security teams may be able to capture these clues early or mid-stages of an attack, thus having the opportunity to minimize losses.

Secondly, logs are the absolute core of building a complete chain of evidence. After a security incident occurs, questions such as "what happened," "how it happened," "how significant the impact," and "who was the source of the attack" must be answered by logs. IDS/IPS logs provide objective records at the network layer. They can be correlated chronologically with endpoint detection response logs, firewall logs, and authentication logs to accurately reconstruct the attacker's tactics, techniques, and steps. Without these logs, incident response is like the blind men and the elephant—it's impossible to accurately assess the damage, implement effective containment and cleanup measures, or produce rigorous reports for management or regulatory agencies. From a legal and compliance perspective, complete and tamper-proof security logs are themselves indispensable evidence.

Furthermore, stringent industry compliance requirements almost invariably mandate the collection, retention, and analysis of security logs. Whether it's regulatory requirements in the financial industry or data protection laws like the General Data Protection Regulation (GDPR), they all explicitly require companies to demonstrate that they have implemented appropriate technical measures to protect data security and can promptly detect and report breaches. Detailed IDS/IPS log recordings and monitoring reports are the most direct evidence to meet these compliance audit requirements. It demonstrates the company's proactive threat monitoring and fulfillment of its security responsibilities.

In addition, continuous log monitoring is the foundation for proactive threat hunting. Threat hunting is not about passively waiting for alerts, but about proactively searching for hidden threat indications in log data based on hypotheses. For example, security analysts can write queries to search past IDS logs for communication characteristics related to the latest disclosed exploit kits, thereby discovering potentially lurking exploit activities. This proactive discovery capability significantly reduces the time threats remain "hidden" in the environment. A simple example of log retrieval using a command-line tool (such as `grep`) can quickly filter out all records communicating with suspicious domains, which is often the starting point for threat hunting:

# Search the Snort (a popular open-source IDS) alert logs for all DNS query alerts containing known malicious domains
grep -i "alert.*dns" /var/log/snort/alert | grep -E "(evil-domain|malware-c2)\.com"

Finally, log monitoring forms a feedback loop for optimizing security policies and device performance. By analyzing IPS intercept logs, it's clear which attack rules are triggered most frequently and which source IP addresses are persistent threat sources. These analyses can guide security teams to adjust their strategies: for example, permanently blocking IP addresses from specific geographic regions that are continuously scanning maliciously at the firewall level; or adjusting rules that are frequently triggered but verified as false alarms to reduce the workload of analysts, allowing them to focus more on high-risk alerts. This process enables the security system to learn and continuously improve.

To achieve effective log monitoring, simply collecting logs is far from sufficient. Best practice is to establish a centralized security information and event management platform (SIEM). A SIEM platform can collect logs in real time from distributed IDS, IPS, and other security devices, performing standardization, correlation analysis, and visualization. It can automatically correlate abnormal connection alerts from the network side (IDS/IPS) with a suspicious process creation event from the host side, generating a higher-confidence security event, significantly improving detection accuracy and response speed. Simultaneously, a clear log retention policy should be established to ensure logs are searchable and traceable for a sufficiently long period (typically at least 6 months to 1 year) to meet investigation and compliance needs.

In short, the answer to the question "Why must we keep an eye on IDS and IPS logs?" is simple: because they record the most authentic truth about all your adversaries and the weaknesses in your own defenses.