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Decoding Modbus TCP on different port number

Hi there,

great work developers and community for this great website. This has helped me numerous times in trouble shooting and learning the protocols etc.

I have a Question regarding Modbus/TCP. The default port is 502, but I have seen that some controllers can send Modbus TCP messages through a different portnumer, such as 503 for example. I have captured the these packets, but Wireshark does not decode these messages as Modbus. How can I tell/configure Wireshark to decode these messages as Modbus/TCP?

Many thanks

Ramon de Vries's avatar
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Ramon de Vries
asked 2020-05-04 11:29:34 +0000
grahamb's avatar
23.8k
grahamb
updated 2020-05-04 12:04:08 +0000
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Add an entry to the "Decode As..." table. This can be done by right-clicking a packet in the packet list that isn't being dissected correctly due to not being on port 502, choosing the option "Decode As..." and then adjusting the "Value" and "Current" fields as required. You can have as many entries as needed and can go directly to the table from the Analyze menu.

The table should look like this, I've added an entry for port 503.

image description

grahamb's avatar
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grahamb
answered 2020-05-04 12:09:58 +0000
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Thanks for the information!

Ramon de Vries's avatar Ramon de Vries (2020-05-13 06:06:43 +0000) edit
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