Strategies for Enabling Logging in Legacy ICS Devices: Technical Guidance for Industrial Networks

Introduction

Industrial Control Systems (ICS) and Operational Technology (OT) environments are receiving unprecedented focus from the cybersecurity and industrial networking community. Logging is foundational for forensics, intrusion detection, compliance, and operational resilience, but a persistent challenge remains: many legacy ICS devices lack native logging capabilities or standardized event reporting interfaces. Modernizing these environments requires precise strategies for enabling, extending, or extracting logging from these critical yet often constrained assets.

Historical Context: Why Legacy ICS Devices Lack Logging

Unlike IT devices, most ICS equipment historically prioritized availability, deterministic response, and longevity over extensive diagnostics or logging. Many PLCs, RTUs, and field devices deployed in the 1980s, 1990s, and early 2000s predated widespread networked operations.

• Resource Limitations: Limited memory and processor capabilities left little room for persistent logs.

• Protocol Simplicity: Protocols like Modbus (1979), PROFIBUS (1989), and serial-based DNP3 were designed for real-time control rather than auditability.

• Physical Isolation: The "air gap" mentality meant security through obscurity was assumed, with the implicit belief that threats could not penetrate isolated control networks.

Classification of Legacy Device Logging Capabilities

Before deploying augmentation strategies, categorize equipment:

• No Native Event Logging: Devices may not retain or emit system or communication logs (e.g., early-gen PLCs, many field sensors).

• Limited Logging (Local Storage/Indicators): Minimal status/error codes accessible via proprietary engineering tools or local display.

• Externalized Logging via SCADA/MES: Logging occurs only when events are polled or pushed to supervisory systems, often via custom drivers or legacy interfaces.

Strategy 1: Protocol and Traffic Monitoring ("Passive Logging")

Overview

When devices themselves cannot produce logs, operators can extract valuable information by monitoring network traffic at aggregation points.

Technical Considerations

• Traffic Mirroring: Use SPAN (Switched Port Analyzer) or TAPs (Test Access Points) on critical network segments to capture ICS protocol exchanges.

• Protocol Decoders (DPI): Tools like Wireshark, Zeek, and dedicated ICS DPI engines (e.g., Claroty, Nozomi, Dragos) can parse Modbus, DNP3, IEC 60870-5-104, etc., to reconstruct commands, state changes, and data reads.

• Syslog Export: Develop connectors or use network appliances to convert relevant protocol transactions into syslog or structured log entries for SIEM or data lake ingestion.

Limitations & Risks

• No Local Device State: Passive monitoring observes only what traverses the wire; direct device faults may go undetected.

• Encryption Barriers: Increasing deployment of secure protocols (e.g., IEC 62351-protected DNP3) impedes traffic inspection.

Strategy 2: Collecting Logs from Adjacent Systems

SCADA, HMI, and Engineering Workstations as Sources

While individual field devices aren't log-capable, their master controllers, HMIs, and engineering stations often capture alarms, operator actions, firmware updates, and communication failures.

• Centralized Logging via SIEM: Integrate these higher-level logs with IT log aggregation for cross-domain analysis.

• Event Mapping: Correlate HMI events (such as tag changes, unauthorized configuration attempts) to underlying device behavior.

Additional Benefits

• User Attribution: Enrich event context with authenticated user actions or workstation identities.

• Legacy Protocol Bridge Logging: Consider historian platforms as a secondary log repository, which often keep change or error records using proprietary formats.

Strategy 3: Device Augmentation – Serial Gateways and I/O Adapters

Where permitted by infrastructure constraints, deploy protocol-aware gateways or I/O devices that interpose on legacy serial or simple Ethernet/routed links.

Best Practices

• Non-Invasive Serial Splitters: Devices such as serial-to-Ethernet gateways or logging bridges can monitor RS-232/485 communication—recording commands, values, and error frames without affecting control logic.

• Replay & Parsing Considerations: Be wary of "man-in-the-middle" approaches—ensure gateways are certified for ICS safety, and guarantee that they do not introduce latency or jitter.

Historical Example

Bristol Babcock Network Interface Units from the 1990s could dual-home serial data to maintenance terminals and control stations, showcasing early design patterns for monitoring without disrupting legacy bus communications.

Strategy 4: Custom Instrumentation and Firmware Upgrades

In rare but business-critical scenarios, retrofitting devices with updated firmware or custom modules to emit logs can be considered:

• Vendor Collaboration: Work with original vendors for supported firmware patches to enable syslog/SNMP trap support or add event buffers.

• Open Source/Community Projects: For hardware with open interfaces (rare in ICS), custom firmware (e.g., IEC 61131-3 PLCs) may add remote logging or cloud telemetry. Extreme caution required; safety and process validation are paramount.

Strategy 5: IT/OT Collaboration for Logging Architecture

Integrating IT Security Practices

• Log Normalization: Define parsing and normalization standards for diverse industrial logs to enable SIEM correlation (e.g., CEF, LEEF, JSON mappings).

• Secure Log Transfer: Use hardened jump servers, unidirectional diodes, or data brokers to move logs from OT networks to IT analytics infrastructure, enforcing strict segmentation.

OT Operator Involvement

Ensure operators can tune what is logged based on process priorities (alarms, overrides, setpoint changes), balancing bandwidth and storage constraints with forensic and compliance needs.

Conclusion: Navigating Constraints with Layered Logging Strategies

While legacy ICS devices pose inherent challenges for direct logging, a layered strategy that combines protocol monitoring, adjacent system event harvesting, hardware augmentation, and careful IT/OT collaboration can drastically improve visibility and event traceability in critical industrial settings. Long-term roadmaps should prioritize equipment refreshes with native, standards-based logging and remote monitoring—but even in legacy-dense environments, substantial advances remain within reach through well-considered technical and architectural interventions.