Operational technology (OT) and IoT devices are now among the most attractive targets in any network — and the hardest to monitor. You cannot install an endpoint agent on a PLC, a building management controller, a medical imaging device, or a factory sensor. This guide explains how to gain security visibility into these environments using a SIEM with agentless network monitoring.
Traditional IT security assumes you can deploy an agent on every endpoint — a piece of software that reports detailed telemetry back to your SIEM. OT and IoT devices break this assumption completely:
The visibility gap
If your security monitoring depends entirely on endpoint agents, your OT and IoT estate is a blind spot. Attackers know this. The Stuxnet, Triton, and Industroyer attacks all targeted OT precisely because it is so poorly monitored.
Since you cannot put software on the devices, you monitor the network they communicate over. Every OT and IoT device talks to something — a controller, a historian, a cloud endpoint, each other. That traffic is observable from the network layer without touching the devices themselves. This is where nPro's NRTG (network real-time monitoring) component complements the SIEM.
Agentless network monitoring works through three mechanisms:
A network tap or SPAN port mirrors traffic to nPro, which analyses it without injecting any packets. This is completely non-intrusive — the OT devices are never touched, scanned, or probed. nPro parses industrial protocols (Modbus, DNP3, OPC-UA, BACnet, S7) to understand what the devices are actually doing.
OT networks are highly predictable — a sensor reports every few seconds, a controller polls its devices on a fixed cycle. nPro learns this baseline, then alerts on deviations: a device suddenly talking to a new destination, an unexpected protocol appearing, or command traffic outside normal operating patterns.
Managed switches, gateways, and newer IoT devices often support SNMP or syslog. nPro polls these for device health, interface status, and event logs, adding another layer of visibility on top of passive traffic analysis.
OT security is built around the Purdue Model, which organises industrial networks into hierarchical zones from physical processes (Level 0) up to enterprise IT (Level 5). The most important security control is enforcing — and monitoring — the boundaries between these zones. A SIEM with network monitoring lets you verify that traffic crossing zone boundaries follows policy, and alerts immediately when something crosses a boundary it should not.
For organisations in manufacturing, utilities, oil and gas, and healthcare across Southeast Asia, this is increasingly a regulatory expectation as well as a security best practice.
OT environments frequently operate in air-gapped or tightly network-restricted settings where sending telemetry to a cloud SIEM is either impossible or prohibited. A self-hosted SIEM like nPro runs entirely within the OT network boundary, requires no outbound internet connectivity, and keeps sensitive operational data — which can reveal production processes and physical infrastructure details — fully under your control. See our guide on deploying a self-hosted SIEM for the setup walkthrough.
To start monitoring an OT or IoT environment: identify the network choke points where you can tap or mirror traffic (typically the boundary between OT zones), deploy nPro with a network tap at those points, let it baseline normal traffic for two to four weeks, then enable alerting on cross-zone communication and new-device detection as your highest-priority rules. Layer in SNMP polling for any managed devices that support it.
nPro combines SIEM and agentless network monitoring in one self-hosted platform.
Related: What is a SIEM? · Detecting Ransomware · SIEM for Healthcare · nPro SIEM