Designing Data Diode Protection Strategies for Industrial Network Isolation​

​

​​I. Core Principles for Protection Strategy Design​​

1. ​​Define Security Objectives​​

   • ​​Primary Goal​​:
     • Ensure one-way data flow from OT to IT networks (blocking IT→OT traffic).
     • Eliminate all reverse access attempts (including protocol-level exploits).
   • ​​Secondary Goals​​:
     • Data integrity (anti-tampering), transmission reliability (low packet loss), and auditability.

2. ​​Layered Defense Architecture​​

   [OT Network] → [Data Diode] → [DMZ Buffer] → [Firewall] → [IT Network]
   

   • ​​Data Diode​​: Physical unidirectional isolation (hardware-enforced).
   • ​​DMZ​​: Secondary data sanitization (e.g., protocol stripping, content filtering).
   • ​​Firewall​​: Restrict IT network access to the DMZ.

​​II. Key Protection Strategies​​

1. ​​Data Flow Control Policies​​

2. ​​Security Enhancements​​

• ​​Data Encryption​​: AES-256 for transmitted data (even if unidirectional).
• ​​Integrity Checks​​: Compare SHA-256 hashes of received data (DMZ) with OT-side originals.
• ​​Anti-Replay Attacks​​: Add sequence numbers; discard duplicate/out-of-order packets.

​​III. Deployment Topology​​

1. ​​Typical Deployment Locations​​

                         [OT Network]  
                                 │  
                                 ▼  
                 [Industrial Firewall] ←→ [Data Diode]  
                                 │  
                                 ▼  
                       [DMZ (Data Sanitization)]  
                                 │  
                                 ▼  
                         [IT Network]  

• ​​Data Diode Placement​​:
  • Directly connected to OT boundary switches (e.g., industrial ring core switches).
  • Avoid serial connections to critical devices (e.g., PLCs) to prevent single points of failure.

2. ​​High-Availability (Dual Diodes)​​

[OT Network] → [Primary Diode] → [DMZ]  
             ↘ [Backup Diode] ↗  

• ​​Failover​​: Backup diode activates if the primary fails (requires synchronized configurations).
• ​​Load Balancing​​: Split traffic (e.g., alarms via primary, sensor data via backup).

​​IV. Technical Implementation​​

1. ​​Hardware Selection Guidelines​​

2. ​​Protocol Handling​​

• ​​Allowed Protocols​​:

  OPC UA (Pub/Sub mode)  
  Modbus RTU (read-only function codes)  
  MQTT (unidirectional publish)  
  

• ​​Blocked Protocols​​:

  SSH/Telnet (management protocols)  
  HTTP/S (prevent web exploits)  
  ICMP (block probes)  
  

3. ​​Logging & Auditing​​

• ​​Diode Logs​​: Record transmission metrics (packet loss, data volume).
• ​​DMZ Auditing​​:
  • Use SIEM tools (e.g., Splunk) to detect anomalies.
  • Example alert rules:

    IF packet contains "write" command THEN alert and drop  
    IF throughput spikes >200% THEN alert  
    

​​V. Operations & Incident Response​​

1. ​​Maintenance​​

• ​​Regular Testing​​:
  • Simulate reverse attacks (e.g., TCP SYN packets) monthly to verify diode blocking.
• ​​Configuration Backups​​:
  • Store diode rules offline to prevent loss during failures.

2. ​​Incident Handling​​

​​VI. Compliance​​

1. ​​China’s Classified Protection 2.0​​:
   • Mandates diode-based access control (Clause 9.1.3).
2. ​​IEC 62443​​:
   • Meets SL-T (Security Level 3) for "unidirectional communication channels".

​​Conclusion​​

By combining ​​physical unidirectionality + protocol stripping + DMZ buffering​​, data diodes become a robust OT/IT isolation solution. Key takeaways:

1. ​​Absolute Unidirectionality​​: Hardware-enforced reverse traffic blocking.
2. ​​Data Minimization​​: Transmit only essential OT data.
3. ​​Defense-in-Depth​​: Complement with firewalls and DMZs.

For production deployment, validate strategies in a test environment and document a detailed "Data Diode Operations Manual".