TLS: modbus client doesn't seem to support the last revision of modbus/tcp security specification (MB-TCP-Security-v36_2021-07-30)

[tintigx]

Versions

• Python: 3.11.7
• OS: W10
• Pymodbus: 3.6.5
• Modbus Hardware (if used):

Pymodbus Specific

• Client: tls

Description

I was trying to connect a pymodbus client with to a modbus server supporting TLS1.2/1.3 TLS connection between the client and the server is OK.

But it seems there is an issue with the data encapsulated in TLS by pymodbus.

The last modbus/tcp security protocol specification (https://www.modbus.org/docs/MB-TCP-Security-v36_2021-07-30.pdf) indicates that mbap ADU must be encapsulated (mbap ADU = mbap header + function code + data) but pymodbus only encapsulate "function + data".

tls_framer.py

class ModbusTlsFramer(ModbusFramer): """Modbus TLS Frame controller.

No prefix MBAP header before decrypted PDU is used as a message frame for
Modbus Security Application Protocol.  It allows us to easily separate
decrypted messages which is PDU as follows:

    [ Function Code] [ Data ]
      1b               Nb
"""

[janiversen]

that correct it is currently not supported in the TLS_framer...but I suppose if you use the socket_framer it would work (have not tried it)

[tintigx]

Indeed, it works well using socket_framer instead of the default one tls_framer