By RUGERO Tesla (@404Saint).
We see a lot of clean "wins" on tech blogs. People post beautifully formatted walk-throughs where every exploit chain connects on the first try, every payload returns a root shell, and the terminal looks like a movie script.
This isn't one of those posts.
This is the raw, unedited chronology of what happens when you spend hours fighting industrial protocols, getting brick-walled by application layers, and chasing down silent network failures at 2:00 AM.
1. The Trap of Protocol Assumptions (DNP3 vs. Modbus)
The cycle started yesterday. Fresh off building a custom toolkit for Modbus TCP which is a beautifully simple, stateless, and completely predictable protocol, I figured I would easily pivot the framework to handle DNP3.
I expected a similar flow. I was entirely wrong.
Five hours of continuous, grueling socket programming later, I was staring at a wall. DNP3 isn’t just a protocol; it’s an intricate architectural ecosystem. It requires dealing with complex outstation internal states, multi-layered packet fragmentations, and strict link-layer confirmations. Treating DNP3 like Modbus isn't just a minor mistake; it’s a fundamental misunderstanding of the wire.
Realizing I was fighting a losing battle without deep-diving into the complete specification first, I made a tactical decision: I parked DNP3 for a dedicated project later, stepped back, and redirected my raw Python socket architecture toward EtherNet/IP (ENIP) and the Common Industrial Protocol (CIP).
2. The Illusions of the Network Layer (The TCP ACK Trap)
By late tonight, the initial EtherNet/IP encapsulation code was ready. I fired off the initial Phase 1 registration packets (0x0065) against my local lab target running an OpenPLC instance. The handshakes were flawless, and the session handles were rotating cleanly in hexadecimal. The transport layer felt bulletproof.
0x0065) yielding an active handle.
Then came Phase 2: Attempting a Symbolic Segment Tag Read (0x4C) over an unconnected data path (SendRRData, 0x006F). I hit enter on the execution script and watched tcpdump on my secondary monitor.
On the network level, it looked like a total success:
- The script pushed the raw custom ENIP payload to port 44818.
- The target container immediately shot back a TCP
. ACK.
But on my main terminal, everything just froze.
[*] Phase 02: Building and delivering Vendor class 0x64 read requests...
[-] Wire execution crashed. Context: timed out
Five seconds of total silence, followed by a socket timeout exception. This is the ultimate illusion of network testing. The host OS kernel swallowed the packet and acknowledged it at the transport layer, but the actual PLC application layer went completely dark. No TCP Reset (RST). No CIP error frames. Just an empty void.
3. Demolishing the Target (Inside the OpenPLC Logs)
I spent the next hour questioning my own code. When you're crafting raw binary payloads via Python’s struct.pack(), a single misaligned byte, a broken word size, or an incorrect item count in the Common Packet Format (CPF) wrapper will cause an industrial parser to drop the ball.
I tore the script apart, rebuilt the hexadecimal mapping from scratch, and even stripped out the proprietary Rockwell tag-read service down to a completely generic, standard CIP Identity Object discovery path (Class 0x01, Instance 0x01, Service 0x0E).
# Re-aligning the CPF wrapper and Encapsulation Header
rr_data_packet = struct.pack(
'<HHII8sI IHHHHHH',
0x006F, 24, handle, 0, b'SAINT404', 0, # Encap Header
0, 0, 2, 0x0000, 0, 0x00B2, 8 # CPF Data
) + cip_data
I ran it again. Same result. Another 5-second hang.
At this point, you have to stop assuming your tool is broken and start looking at the target's internal state. I dropped out of my code editor and pulled the live stdout logs from the target runtime container:
docker logs openplc-lab
There, buried in the container timestamps, was the exact smoking gun:
2026-06-23T14:05:56.165168622Z ENIP: Received unsupported EtherNet/IP Type
2026-06-23T14:05:56.165170130Z Server: Error writing response: -1
The realization was incredibly clarifying. The script wasn't broken. The packet wasn't misaligned. OpenPLC’s lightweight, modular EtherNet/IP listener simply does not support Rockwell-style proprietary symbolic tag reading or standard unconnected explicit message routing (UCMM).
Instead of cleanly replying over the wire with a standard CIP protocol error frame (like a normal PLC would), OpenPLC’s backend code hit an unhandled exception path, threw an internal error (-1), and dropped the socket completely without writing a single byte back to my network interface.
4. Upgrading the Arsenal
In offensive security and asset fingerprinting, a silent application-layer drop is incredibly valuable telemetry. It tells you exactly who and what is on the other side of the wire.
- A real, production-grade Allen-Bradley controller or a high-fidelity simulator will return either tag data or a structured CIP error frame (
Path Segment Error). - An OpenPLC instance will choke internally, log an unhandled error, and leave the client script hanging until a timeout.
The ultimate lesson? Hobbyist software teaches you about hobbyist bugs. Fighting with a stripped-down implementation only maps out its internal software limitations rather than showing you how real-world OT systems respond to packet manipulation.
To truly reverse-engineer industrial connection managers, Forward_Open loops, and complex CIP state machines, you need a target that acts like the real thing.
Tomorrow, the lightweight containers are getting parked. We are upgrading the lab environment to high-fidelity target harnesses.
