ModBus TCP proxy

Many modbus devices support only one or very few clients. This proxy acts as a bridge between the client and the modbus device. It can be seen as a layer 7 reverse proxy. This allows multiple clients to communicate with the same modbus device.

When multiple clients are connected, cross messages are avoided by serializing communication on a first come first served REQ/REP basis.

Installation

From within your favorite python 3 environment type:

$ pip install modbus-proxy

Note: On some systems pip points to a python 2 installation. You might need to use pip3 command instead.

Additionally, if you want logging configuration:

• YAML: pip install modbus-proxy[yaml] (see below)
• TOML: pip install modbus-proxy[toml] (see below)

Running the server

First, you will need write a configuration file where you specify for each modbus device you which to control:

• modbus connection (the modbus device url)
• listen interface (to which url your clients should connect)

Configuration files can be written in YAML (.yml or .yaml) or TOML (.toml).

Suppose you have a PLC modbus device listening on plc1.acme.org:502 and you want your clients to connect to your machine on port 9000. A YAML configuration would look like this:

devices:
- modbus:
    url: plc1.acme.org:502     # device url (mandatory)
    timeout: 10                # communication timeout (s) (optional, default: 10)
    connection_time: 0.1       # delay after connection (s) (optional, default: 0)
  listen:
    bind: 0:9000               # listening address (mandatory)

Assuming you saved this file as modbus-config.yml, start the server with:

$ modbus-proxy -c ./modbus-config.yml

Now, instead of connecting your client(s) to plc1.acme.org:502 you just need to tell them to connect to *machine*:9000 (where machine is the host where modbus-proxy is running).

Note that the server is capable of handling multiple modbus devices. Here is a configuration example for 2 devices:

devices:
- modbus:
    url: plc1.acme.org:502
  listen:
    bind: 0:9000
- modbus:
    url: plc2.acme.org:502
  listen:
    bind: 0:9001

If you have a single modbus device, you can avoid writting a configuration file by providing all arguments in the command line:

modbus-proxy -b tcp://0:9000 --modbus tcp://plc1.acme.org:502

(hint: run modbus-proxy --help to see all available options)

Running the examples

To run the examples you will need to have umodbus installed (do it with pip install umodbus).

Start the simple_tcp_server.py (this will simulate an actual modbus hardware):

$ python examples/simple_tcp_server.py -b :5020

You can run the example client just to be sure direct communication works:

$ python examples/simple_tcp_client.py -a 0:5020
holding registers: [1, 2, 3, 4]

Now for the real test:

Start a modbus-proxy bridge server with:

$ modbus-proxy -b tcp://:9000 --modbus tcp://:5020

Finally run a the example client but now address the proxy instead of the server (notice we are now using port 9000 and not 5020):

$ python examples/simple_tcp_client.py -a 0:9000
holding registers: [1, 2, 3, 4]

Docker

This project ships with a basic Dockerfile which you can use as a base to launch modbus-proxy inside a docker container.

First, build the docker image with:

$ docker build -t modbus-proxy .

To bridge a single modbus device without needing a configuration file is straight forward:

$ docker run -d -p 5020:502 modbus-proxy -b tcp://0:502 --modbus tcp://plc1.acme.org:502

Now you should be able to access your modbus device through the modbus-proxy by connecting your client(s) to <your-hostname/ip>:5020.

If, instead, you want to use a configuration file, you must mount the file so it is visible by the container.

Assuming you have prepared a conf.yml in the current directory:

devices:
- modbus:
    url: plc1.acme.org:502
  listen:
    bind: 0:502

Here is an example of how to run the container:

docker run -p 5020:502 -v $PWD/conf.yml:/src/conf.yml modbus-proxy -c /src/conf.yml

Note that for each modbus device you add in the configuration file you need to publish the corresponding bind port on the host (-p <host port>:<container port> argument).

Logging configuration

Logging configuration can be added to the configuration file by adding a new logging keyword.

The logging configuration will be passed to logging.config.dictConfig() so the file contents must obey the Configuration dictionary schema.

Here is a YAML example:

devices:
- modbus:
    url: plc1.acme.org:502
  listen:
    bind: 0:9000
logging:
  version: 1
  formatters:
    standard:
      format: "%(asctime)s %(levelname)8s %(name)s: %(message)s"
  handlers:
    console:
      class: logging.StreamHandler
      formatter: standard
  root:
    handlers: ['console']
    level: DEBUG

--log-config-file (deprecated)

Logging configuration file.

If a relative path is given, it is relative to the current working directory.

If a .conf or .ini file is given, it is passed directly to logging.config.fileConfig() so the file contents must obey the Configuration file format.

A simple logging configuration (also available at log.conf) which mimics the default configuration looks like this:

[formatters]
keys=standard

[handlers]
keys=console

[loggers]
keys=root

[formatter_standard]
format=%(asctime)s %(levelname)8s %(name)s: %(message)s

[handler_console]
class=StreamHandler
formatter=standard

[logger_root]
level=INFO
handlers=console

A more verbose example logging with a rotating file handler: log-verbose.conf

The same example above (also available at log.yml) can be achieved in YAML with:

version: 1
formatters:
  standard:
    format: "%(asctime)s %(levelname)8s %(name)s: %(message)s"
handlers:
  console:
    class: logging.StreamHandler
    formatter: standard
root:
  handlers: ['console']
  level: DEBUG

Credits

Development Lead

• Tiago Coutinho coutinhotiago@gmail.com

Contributors

None yet. Why not be the first?

History

0.6.1 (2021-09-29)

• Change default command line --modbus-connection-time from 0.1 to 0
• Add basic unit tests
• Github actions
• Repository cleanup

0.5.0 (2021-09-28)

• Add support for multiple devices
• Adapt docker to changes
• Deprecate --log-config-file command line parameter

0.4.2 (2021-09-23)

• Add connection time delay (fixes #4)

0.4.1 (2021-01-26)

• Logging improvements

0.4.0 (2021-01-26)

• Logging improvements

0.3.0 (2021-01-25)

• More robust server (fixes #2)

0.2.0 (2021-01-23)

• Document (README)
• Add docker intructions (fixes #1)
• Fix setup dependencies and meta data

0.1.1 (2020-12-02)

• Fix project package

0.1.0 (2020-11-11)

• First release on PyPI.