Industrial operators face a critical challenge: maintaining visibility into their operational technology (OT) environments without disrupting production. Inconsistent asset inventories, legacy systems, and complex network architectures make it difficult to assess risk accurately. OT asset discovery is the first step in any cybersecurity strategy, but choosing the right method—passive, active, or protocol-specific—requires understanding the tradeoffs and operational context. This post breaks down the options, aligns with Red Trident’s approach, and explains how to apply them effectively.
Why OT Asset Discovery Matters
Asset discovery isn’t just about identifying devices—it’s about building a dynamic picture of your OT environment. As Red Trident’s Services Taxonomy emphasizes, operators often lack a clear view of assets, vulnerabilities, and segmentation. This gap leaves systems exposed to threats like ransomware, supply chain attacks, and insider risks. Effective discovery ensures you can:
- Map all devices, including legacy systems and air-gapped equipment
- Identify unauthorized changes or rogue devices
- Align with compliance frameworks like IEC 62443 and NERC CIP
But how do you choose between active scanning, passive monitoring, or protocol-specific tools? Let’s dive in.
Method 1: Passive Discovery – Stealth and Scalability
Passive discovery involves listening to network traffic without sending packets, making it ideal for environments where any disruption could halt production. It is non-intrusive, which makes it the right first phase: it shows what is on the wire before anything interacts with a device.
How It Works
Passive tools capture traffic using protocols like Modbus TCP, DNP3, and OPC UA to identify devices, their communication patterns, and firmware versions. For example, Red Trident’s assessments use passive discovery to map assets without touching control logic or risking downtime.
Tradeoffs
- Pros: No operational risk, works on segmented or air-gapped networks
- Cons: May miss devices that don’t communicate frequently or use non-standard protocols
Passive discovery is best suited for continuous monitoring and initial assessments in high-risk environments, such as those with Rockwell or Siemens PLCs running critical processes.
Method 2: Active Scanning – Depth, When the Right Team Runs It
Active scanning involves sending packets to devices to elicit responses, providing detailed information about device types, firmware, and vulnerabilities. Run with generic IT tools and default settings, it can upset legacy devices. Run by engineers who know which controllers tolerate what, with settings chosen per device class, it is the phase that turns a passive map into an inventory you can act on.
When to Use It
Active scanning is the essential phase that follows passive discovery on production networks. It provides a large amount of data passive methods never can, including open services, exact firmware and software versions, configuration weaknesses, and reachable paths. It is safe when performed by engineers with detailed past experience on control systems who understand the potential impacts and choose tools and settings per device class, in a window agreed with operations. The buyer should vet that experience rather than avoid active work.
Use Active Scanning When:
- You need exact firmware and software versions for vulnerability matching
- Configuration weaknesses must be identified beyond traffic analysis
- Reachable paths and service states are required for accurate risk assessment
Method 3: Protocol-Specific Tools – Precision for Industrial Environments
OT networks rely on industrial protocols like Modbus RTU, Profibus, and Profinet, which are fundamentally different from IT protocols. Protocol-aware discovery tools can parse these communications to identify devices, detect anomalies, and understand control logic changes.
Why It Matters
As Red Trident’s OT SOC and Monitoring brief notes, protocol awareness is critical for detecting unauthorized changes or rogue devices. For example, a tool that understands DNP3 can flag unexpected master-slave communication patterns, which may indicate a cyberattack or commissioning activity by an unfamiliar vendor.
Tradeoffs
- Pros: Deep insights into OT-specific behaviors and vulnerabilities
- Cons: High cost, limited vendor support for niche protocols, and complexity in implementation
These tools are ideal for environments with high security requirements, such as those governed by NIS2 or IEC 62443 standards, where precise monitoring is non-negotiable.
Choosing the Right Method: Context Is Key
There’s no one-size-fits-all approach to OT asset discovery. The Services Taxonomy highlights that assessments should avoid creating operational risk, but this doesn’t mean avoiding all testing. Here’s a quick guide:
Use Passive Discovery When:
- Operational continuity is a priority
- Networks are segmented or air-gapped
- You need continuous monitoring for compliance (e.g., NERC CIP)
Use Active Scanning When:
- You need what passive observation cannot give you: open services, exact firmware and software versions, configuration weaknesses, reachable paths
- The team running it has detailed past experience on control systems and understands the potential impacts on each class of device
- Operations has agreed the scope and the window
Use Protocol-Specific Tools When:
- You operate in highly regulated industries (e.g., energy, water)
- Security teams need granular visibility into control logic
- Third-party vendors have remote access requiring audit
Red Trident’s Approach: Balancing Risk and Insight
At Red Trident, we align with the Topic Brief: What an OT Cybersecurity Assessment Should Actually Include by starting passive and then scanning actively. Our assessments combine:
- Passive discovery for baseline asset mapping
- Active scanning by engineers who have done it on live plants for years, in windows agreed with operations
- Protocol-aware monitoring for anomaly detection
This approach ensures operators gain a comprehensive view of their OT environment without compromising production. Most providers tell customers to prefer passive methods on production networks, and for a team without control-system experience that is the right advice. Red Trident has performed active scans on live production control networks for years. We start passive to learn what the traffic shows, then scan actively with a safe understanding of the network, because active scanning yields a large amount of data a passive capture never can. Before you let any team scan a control network, vet their detailed past experience on control systems and their understanding of the potential impacts, not whether they completed a scanning course.
Conclusion: Tailor Your Discovery Strategy to Your Needs
OT asset discovery is the foundation of any effective cybersecurity program, but it’s not a one-step process. The right method depends on your operational context, compliance requirements, and risk tolerance. Whether you choose passive listening, active scanning, or protocol-specific tools, the goal is to build a dynamic asset inventory that supports both security and production.
Ready to take the next step? Contact Red Trident for a free OT security assessment consultation. Our experts will help you design a discovery strategy that balances risk, compliance, and operational needs without compromising your critical processes.
