mbproxy

A lightweight Modbus TCP proxy with in-memory caching. Designed to reduce load on Modbus devices by serving cached responses to multiple clients.

Features

• Caching: In-memory cache with configurable TTL
• Request coalescing: Identical concurrent requests share a single upstream fetch
• Read-only mode: Optionally block or ignore write requests
• Auto-reconnect: Automatic upstream reconnection on failure
• Stale data fallback: Optionally serve stale cache on upstream errors
• Graceful shutdown: Complete in-flight requests before terminating
• Minimal footprint: ~6MB Docker image (scratch base)

Quick Start

docker run --rm \
  -e MODBUS_UPSTREAM=192.168.1.100:502 \
  -p 5502:5502 \
  ghcr.io/tma/mbproxy

Configuration

All configuration is via environment variables:

Variable	Description	Default
MODBUS_LISTEN	TCP address to listen on	:5502
MODBUS_UPSTREAM	Upstream Modbus device address	(required)
MODBUS_SLAVE_ID	Default slave ID	1
MODBUS_CACHE_TTL	Cache time-to-live	10s
MODBUS_CACHE_SERVE_STALE	Serve stale data on upstream error	false
MODBUS_READONLY	Read-only mode: false, true, deny	true
MODBUS_TIMEOUT	Upstream connection timeout	10s
MODBUS_REQUEST_DELAY	Delay after each upstream request	0 (disabled)
MODBUS_CONNECT_DELAY	Silent period after connecting to upstream	0 (disabled)
MODBUS_SHUTDOWN_TIMEOUT	Graceful shutdown timeout	30s
LOG_LEVEL	Log level: INFO, DEBUG	INFO

Read-Only Modes

• false: Full read/write passthrough to upstream device
• true: Silently ignore write requests, return success response
• deny: Reject write requests with Modbus illegal function exception

Docker Compose Examples

Basic Setup

services:
  mbproxy:
    image: ghcr.io/tma/mbproxy
    ports:
      - "5502:5502"
    environment:
      MODBUS_UPSTREAM: "192.168.1.100:502"
    restart: unless-stopped

All Configuration Options

services:
  mbproxy:
    image: ghcr.io/tma/mbproxy
    ports:
      - "5502:5502"
    environment:
      MODBUS_LISTEN: ":5502"
      MODBUS_UPSTREAM: "192.168.1.100:502"
      MODBUS_SLAVE_ID: "1"
      MODBUS_CACHE_TTL: "10s"
      MODBUS_CACHE_SERVE_STALE: "false"
      MODBUS_READONLY: "true"
      MODBUS_TIMEOUT: "10s"
      MODBUS_REQUEST_DELAY: "0"
      MODBUS_CONNECT_DELAY: "0"
      MODBUS_SHUTDOWN_TIMEOUT: "30s"
      LOG_LEVEL: "INFO"
    restart: unless-stopped

Multiple Devices (Multiple Proxies)

services:
  inverter-proxy:
    image: ghcr.io/tma/mbproxy
    ports:
      - "5502:5502"
    environment:
      MODBUS_UPSTREAM: "192.168.1.100:502"
      MODBUS_CACHE_TTL: "10s"

  meter-proxy:
    image: ghcr.io/tma/mbproxy
    ports:
      - "5503:5502"
    environment:
      MODBUS_UPSTREAM: "192.168.1.101:502"
      MODBUS_CACHE_TTL: "2s"

Building from Source

# Build Docker image
docker build -t mbproxy .

# Run tests
docker build --target test .

# Or run tests directly
docker run --rm -v $(pwd):/app -w /app golang:1.24 go test ./...

Supported Modbus Functions

Code	Function
0x01	Read Coils
0x02	Read Discrete Inputs
0x03	Read Holding Registers
0x04	Read Input Registers
0x05	Write Single Coil
0x06	Write Single Register
0x0F	Write Multiple Coils
0x10	Write Multiple Registers

Cache Behavior

• Key format: {slave_id}:{function_code}:{start_address}:{quantity}
• Read requests: Served from cache if available and not expired
• Write requests: Forwarded to upstream (if allowed), exact matching cache entries invalidated
• Request coalescing: Multiple identical requests during a cache miss share a single upstream fetch

License

MIT