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.