Inside DNP3: Building an Outstation from the Ground Up

This story was originally published on HackerNoon at: https://hackernoon.com/inside-dnp3-building-an-outstation-from-the-ground-up. Hands-on DNP3 protocol analysis in Python: state machines, transport reassembly anomalies, administrative functions, and Secure Authentication (SA v5). Check more stories related to cybersecurity at: https://hackernoon.com/c/cybersecurity. You can also check exclusive content about #cybersecurity, #scada, #ot-security, #ics-ot-security, #dnp3-protocol, #python, #reverse-engineering, #inside-dnp3, and more. This story was written by: @404saint. Learn more about this writer by checking @404saint's about page, and for more stories, please visit hackernoon.com. We built a custom Python-based DNP3 lab to analyze protocol mechanics from the raw socket layer up across 5 phases: 1. Reconnaissance: Parsed Class 0 Integrity Reads to extract Internal Indication (IIN) status bytes (e.g: IIN1.7 Device Restart). 2. Control Execution: Evaluated Select-Before-Operate (SBO) workflows, verifying that un-armed `OPERATE` (`FC 0x04`) requests are rejected while `DIRECT OPERATE` (`FC 0x05`) waives arming. 3. Administrative Functions: Tested system-level functions (FC 0x0D, FC 0x0E, FC 0x12, FC 0x18), showing how `STOP APPLICATION` halts application-layer processing while the network socket remains reachable. 4. Transport Layer: Probed single-byte transport header reassembly (FIR/FIN/SEQ) using orphaned fragments to trigger IIN2.2 errors, and verified unsolicited response forgery (FC 0x82). 5. Secure Authentication (SA v5): Implemented IEEE 1815-2012 Group 120 challenge-response HMAC exchanges to secure critical function codes without encrypting underlying payloads.