Feature Request: Modbus RTU over TCP (RTU Encapsulation)

[cybcon]

Summary

Extend the mock server's protocol support to handle Modbus RTU frames encapsulated over TCP connections, in addition to the existing native Modbus TCP implementation.

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Motivation

While Modbus TCP is the standard for network-based communication, a significant number of real-world devices — particularly serial-to-Ethernet converters, legacy PLCs, and embedded systems — tunnel Modbus RTU frames directly over TCP without converting them to the native Modbus TCP framing (i.e. without the MBAP header). Testing against such devices requires a mock server that can speak RTU-over-TCP, which is currently not supported. Without this, teams are forced to use real hardware or build custom shims just to run integration tests against these device types.

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Technical Background

There are two distinct Modbus-over-TCP variants that are often confused:

Variant	MBAP Header	CRC	Typical Use Case
Modbus TCP	✅ Yes	❌ No	Native Ethernet devices
Modbus RTU over TCP	❌ No	✅ Yes	Serial-to-Ethernet converters, legacy devices

RTU over TCP uses the raw RTU frame format — including the CRC16 checksum — but transports it over a TCP socket instead of a serial line. No MBAP header is present.

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Proposed Implementation

The existing protocol configuration parameter is the natural extension point for this feature:

# Current native Modbus TCP
protocol: tcp

# Proposed new option
protocol: rtu-over-tcp

The server should then expect and respond with raw RTU-framed PDUs over the TCP connection, including correct CRC16 validation and generation.

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Use Cases

• Testing against serial-to-Ethernet gateways or converters that pass through RTU frames unmodified
• Simulating legacy PLCs and embedded controllers in CI/CD pipelines without physical hardware
• Validating that a Modbus master correctly handles RTU framing over TCP
• Supporting industrial IoT scenarios where RTU-over-TCP is the de facto standard for brownfield integrations

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Expected Behavior

• The server accepts incoming TCP connections and parses raw RTU frames (no MBAP header)
• CRC16 validation is performed on all incoming frames; invalid frames are silently dropped or logged, consistent with RTU behavior
• Responses are sent as valid RTU frames including a correctly computed CRC16
• The rtu-over-tcp mode shares the same register space and configuration as the existing tcp mode, requiring minimal migration effort
• Error handling and logging clearly distinguishes between the two protocol modes

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Why This Matters

RTU over TCP is not an edge case — it is a daily reality in industrial environments wherever serial devices have been retrofitted with Ethernet connectivity. Adding this mode with a single protocol parameter change would make the mock server meaningfully more useful for brownfield testing scenarios, with relatively low implementation overhead given that RTU framing logic is well-defined and self-contained.