Passive Monitoring

What is Passive Monitoring?

Passive Monitoring is an observation method in which network traffic is analysed without actively intervening or sending packets. It provides visibility into what is happening on the network without disrupting the production process.

In OT environments, passive Monitoring is crucial because many systems are legacy, fragile or Real-time and therefore cannot tolerate active scans.

🧠 Why is passive monitoring important in OT?

OT challenge	What passive monitoring offers
Vulnerable PLCs and HMIs	Safe visibility without risk of disruption
Lack of documentation	Automatic detection of devices, protocols and communication
Shadow OT	Unknown assets or connections become visible
Malware or undetected anomalies	Real-time behavioural analysis without endpoint installation

🔍 What is monitored?

Element	Examples
Devices	PLCs, HMIs, sensors, engineering stations
Communication protocols	Modbus, S7 Comm, OPC UA, ProfiNET, Ethernet IP
Network behaviour	Frequency, timing, retransmissions, unusual commands
Asset information	Serial numbers, firmware versions, vendor information
Connection patterns	Which devices communicate with whom and how often

🛠️ How is passive monitoring carried out?

Method	Description
SPAN port (switch mirror)	Traffic is copied to a monitoring device
TAP (Test Access Point)	Physical ‘split’ of network traffic with no latency
Inline sniffer	Equipment such as Nozomi, Claroty, Tenable.ot, ForeScout
Sensor in the OT zone	Sensor in the L2 zone that only observes (no IP interaction)

🔐 Security insights from passive monitoring

Behaviour	Possible interpretation
New device on the network	Shadow IT or unauthorised access
Unusual protocol traffic	Malware, misconfiguration or attacker activity
Irregular polling or write actions	Potential manipulation of PLCs or spoofing
External communication	Unauthorised remote access or data exfiltration
Changes in firmware version	Undocumented updates or supply chain incidents

✅ Best practices

Use SPAN/TAP in segments containing critical Assets
Combine with Asset Inventory and anomaly detection
Integrate with SIEM or SOC for alerting and Logging
Also monitor during maintenance windows (temporary vulnerabilities)
Define clear roles and responsibilities in the monitoring policy

📌 In summary

Passive Monitoring is the way to gain safe insight into OT networks, without endangering the stability of production or processes. IT is an essential pillar within Defense in Depth and IEC 62443 architectures.

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