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Passive network monitoring is the standard for OT asset inventory as well as threat and anomaly detection, but today industrial environments can safely rely on a combination of passive network and wireless monitoring as well as active polling, remote collection and endpoint security techniques.
The reliance on passive network monitoring is namely because of the sensitive nature and criticality of industrial environments, which is compounded by heavy reliance on legacy systems with insecure, proprietary protocols that can be destabilized or knocked offline by active scanning. OT systems often support critical infrastructure, controlling processes that can impact safety, security and delivery of essential services. Passive monitoring allows for visibility without any interference, whereas active scanning and probing techniques, common in IT environments, can be disruptive, potentially leading to operational failures or safety hazards.
Passive industrial network monitoring tools work by being connected to a SPAN or mirror port of a switch or router and observing network communications. For example, Nozomi Guardian sensors apply this approach to deliver comprehensive OT and IoT asset visibility and monitoring. They identify devices without any agents or active interrogation. Guardian sensors can continuously monitor activity to discover new assets communicating on the network, provide network visualizations for troubleshooting and research, identify critical vulnerabilities, and detect cybersecurity threats and operational issues.
Specifically, Guardian sensors use passive deep-packet inspection to automatically discover, in real time, the industrial network including its components, connections and topology. Its advanced learning capability then builds profiles specific to each ICS and uses behavioral analytics to monitor for anomalous, suspicious and malicious activity. The result is the rapid detection of cyberattacks and critical process anomalies.
Remote collectors are another form of passive monitoring designed to cover hard-to-reach and unmanned locations. These small, low-resource sensors that work in conjunction with network sensors to capture data from remote field sites, offshore platforms, electrical substations, and smaller plans within a larger campus, where a full Guardian sensor is not viable. The Nozomi Networks Remote Collector captures, deduplicates, compresses and encrypts this traffic before sending it to an associated Guardian sensor for processing.
As wireless technology has become more prevalent in OT environments, it has also become an attack vector that has not traditionally been monitored. Wireless sensors provide another form of passive monitoring that should be leveraged in industrial environments. Some common wireless use cases include vendors connecting laptops to guest wi-fi, or creating rogue wireless access points. It’s difficult to detect wireless-specific threats such as bruteforce attacks, spoofing and bluejacking, and even more difficult to determine the location of the devices performing the attacks.
In addition to Wi-Fi and Bluetooth, process control networks rely on specialized wireless protocols designed to facilitate reliable communication between sensors and controllers with low power consumption. These protocols play a crucial role in collecting, concatenating and transmitting data that enables both system operation and surveillance. For example, the IEEE 802.15.4 standard is used in smart buildings for real-time monitoring of HVAC and lighting systems and in farming for wireless irrigation system control to optimize crop yields.
Nozomi Guardian Air is the industry’s first wireless security sensor purpose-built for OT and IoT environments, providing visibility into wireless assets — everything from thermostats to robots to drones — which until now were only detected once connected to a wired network. It continuously monitors wireless spectrum technologies operating between 800 MHz to 5895 MHz (WiFi, Bluetooth, IEEE802.15.4, LoRaWAN, Zwave, cellular and drones) and detecting and locating threats through triangulation to enable faster response.
Like any approach, passive monitoring comes with tradeoffs that areworth weighing against an industrial environment's specific needs.
Network sensors are cyber workhorses. But there will always be dozens of scenarios where adding a network sensor isn’t feasible or additional details are needed to understand and troubleshoot your environment. Today’s industrial environments can safely rely on a combination of passive network, remote collection and wireless monitoring as well as active polling and endpoint security techniques. The Nozomi Networks platform offers the full spectrum of monitoring methods to provide continuous visibility into all your assets and their risk levels, even when they aren’t actively communicating.
Active querying is an agentless, non-invasive technique that can be used to gather detailed information not available from passive sensors for specific assets of interest, included embedded OT/IoT devices. Referred to as Smart Polling in the Nozomi Networks platform, it interrogates devices based on knowledge of their protocols, leveraging special messages and instructions to return useful information without affecting device stability.
Smart Polling can be used to:
In IT security, endpoint agents are ubiquitous for anti-virus protection and patching. Unfortunately, negative experiences deploying IT-focused agents on OT devices have led to scarce adoption of much-needed endpoint monitoring. Traditional ICS network monitoring solutions monitor North-South traffic between Purdue levels or firewalls, but East-West communications between devices within a zone, especially at lower Purdue levels have long been a blind spot. Moreover, endpoint monitoring is the only way to correlate user activity and events as they happen.
In OT, malicious attacks involving credential theft and other malicious behavior certainly occur, but many threats involve inadvertent mistakes by employees or authorized third-party technicians who come and go, often remotely. Without endpoint security, there’s no way to know who’s plugging in when and what they’re doing until their commands have been executed on the network. That’s too late.
Released in 2023, Nozomi Arc is a safe, non-disruptive security agent purpose-built to protect the unique high availability requirements for OT endpoints. Unlike traditional EPP and EDR tools, Arc operates primarily in user space, using kernel space modules only when absolutely necessary, mainly in read‑only mode. This design ensures precise, controlled interaction with system objects while minimizing write operations and preserving system stability.
Passive monitoring is the foundation of a sound OT and IoT security strategy because it delivers continuous, non-disruptive visibility into assets, communications, and behavior across even the most sensitive industrial environments. But passive monitoring alone cannot answer every question. Remote collectors and wireless sensors extend that same passive monitoring approach into hard-to-reach and wireless-connected corners of the network, while active querying and endpoint security agents fill in the details that passive monitoring, by design, cannot reach on its own. Together, these methods give SOC and operations teams a complete, layered view of their OT and IoT systems without ever putting fragile legacy systems at risk.
Building that kind of layered visibility from scratch is a significant undertaking for any internal team, especially in environments running specialized OT and IoT protocols. The Nozomi platform is purpose-built for complex industrial, commercial and critical infrastructure environments, combining visibility from the endpoint to the air with continuous monitoring and AI-powered analysis. It helps organizations minimize cyber risk, maximize operational resilience, and streamline compliance with the toughest standards and regulations, all built on the same passive monitoring foundation described throughout this article.
Request a demo to see the platform in action, or learn how we use AI to strengthen passive monitoring with predictive, real-time analysis.
In IT, security teams focus on protecting data, whereas in OT, availability is the priority. OT networks can be put at risk with traditional IT security scanning methodologies. Therefore, passive network monitoring is one of the primary methods for security detection in OT environments to minimize unplanned downtime that can cost millions or create safety risks. Many OT devices are also old and sensitive, so even a simple network scan can cause a controller to stop working. Passive monitoring avoids this problem because it never sends traffic into the network and simply listens to mirrored traffic from the network. Safe active methods, such as Nozomi’s Smart Polling, can then add detail on top of this passive foundation.
Passive monitoring is a detection capability, not a prevention control. It identifies threats and anomalies in real time, but it doesn’t act on them. Containment and remediation are handled by your SOC team’s response process or by endpoint-level controls such as Arc.
In short, passive monitoring tells you what is happening on the network, and other tools and processes determine what you do about it.
No. Since passive monitoring works off a SPAN or mirror port copy of traffic rather than sitting inline, deployment doesn’t require rerouting production traffic or taking systems offline. The main upfront work is configuring the switch to mirror the right traffic, which is a one-time change rather than an ongoing risk.
Passive monitoring listens to a copy of network traffic, usually from a SPAN port or tap, and never sends anything to your devices. Active monitoring works the opposite way: it sends requests directly to devices and uses their responses to gather information. In short, passive monitoring observes what devices are already saying, while active monitoring asks them questions.
However not all active polling is the same, especially in OT. Traditional IT active tools sweep IP ranges, probe ports, and query every device they find. While that can be accepted in corporate networks, it could cause risk in OT environments, which usually contain legacy devices.
Active polling can be made safe for OT by being far more targeted, leveraging the native protocols of OT devices. It only contacts devices that are already known, uses protocol-specific messages those devices expect, and keeps request volume low. Nozomi’s Smart Polling is built this way. It works from the devices that passive monitoring has already detected and retrieves firmware versions, configurations, and other details that might not have been gathered passively. You also control which nodes are polled, when, and by which method. This gives users the added detail of active methods without the risk of a traditional scan.
No. Passive monitoring only sees devices that communicate on the traffic it can observe. A device that is powered on but silent, or one on a segment you aren’t mirroring, won’t show up. Good sensor placement closes most of the gap, and pairing passive monitoring with other methods, such as targeted queries or endpoint data, covers the rest.
Yes, though it doesn’t make you compliant on its own. It gives you an up-to-date asset inventory, continuous monitoring, and a record of network activity, all of which support frameworks like IEC 62443, NERC CIP, NIS2, and NIST 800-82. Auditors still expect policies, processes, and response procedures alongside the tooling.
No. Smart Polling sends only the specific, protocol-native messages a device already expects rather than generic scans, which is why it can fill gaps passive monitoring can’t reach without the instability risk of conventional active scanning. You choose which nodes are polled, when, and by which method.
They’re additive layers, not competing options: Guardian typically goes in first for wired asset visibility, Guardian Air and Remote Collector extend that to wireless and hard-to-reach assets, and Arc adds endpoint-level detail once network visibility is established. Which combination makes sense depends on where an environment’s specific blind spots are.
