Passive Discovery
Identifying and profiling assets by analyzing existing network traffic without sending probes. Zero-disruption approach that is the default for clinical environments and active OT networks.
What is Passive Discovery?
Identifying and profiling assets by analyzing existing network traffic without sending probes. Zero-disruption approach that is the default for clinical environments and active OT networks.
Passive discovery identifies devices on a network by analyzing traffic they naturally generate — DHCP requests, mDNS/DNS-SD announcements, ARP broadcasts, protocol handshakes — without sending any probes or queries to the devices themselves. It creates an inventory of what exists on the network purely through observation.
The contrast with active discovery is critical in sensitive environments. Active scanners (Nmap, Nessus, network sweepers) send packets to devices and analyze responses. In IT environments, this is generally safe. In OT and IoMT environments, it can be catastrophic: PLCs, RTUs, and medical devices may respond to unexpected packets by crashing, entering a fault state, or executing unintended commands. A vulnerability scan that would take seconds on a Windows workstation can halt a production line or silence a patient monitor.
Passive discovery also captures device behavior over time, not just a point-in-time snapshot. This has two advantages: it catches devices that are only intermittently connected (shift-based equipment, portable medical devices), and it builds the communication patterns needed for behavioral baseline and anomaly detection. The same traffic that reveals device identity also reveals how the device normally communicates — creating a foundation for everything from vulnerability correlation to segmentation policy generation.
Key Facts
- Active scanning can crash or destabilize over 40% of common OT device types when exposed to unexpected traffic
- Passive discovery finds an average of 30% more devices than active scanning alone in mixed IT/OT environments
- Passive observation over 30 days captures intermittently connected devices missed in point-in-time scans
- CISA guidance for ICS networks explicitly recommends passive-first discovery approaches
How ORDR Addresses Passive Discovery
ORDR's discovery engine is built on passive analysis as the primary method, supplemented by optional active queries only to safe targets and at safe rates. Passive traffic monitoring, integration with DHCP/DNS logs, SPAN ports, and network taps gives ORDR complete visibility into connected assets without sending a single probe to OT or IoMT devices.
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ORDR gives security teams complete visibility into every connected asset—and the intelligence to act on what matters most.