TCP length header included device ID?

[muhlemmer]

I wanted to learn more about the MODBUS TCP protocol and found this library and started reading the sources to get a better understanding. I came across this in the tcpTransporter.Send() method:

modbus/tcpclient.go

Lines 173 to 178 in 606c02f

	var data [tcpMaxLength]byte
	if _, err = io.ReadFull(mb.conn, data[:tcpHeaderSize]); err != nil {
	return
	}
	// Read length, ignore transaction & protocol id (4 bytes)
	length := int(binary.BigEndian.Uint16(data[4:]))

Where:

modbus/tcpclient.go

Lines 22 to 23 in 606c02f

	tcpHeaderSize = 7
	tcpMaxLength = 260

Unless I'm not seeing it right: After io.ReadFull, data has 7 bytes populated, remaining 253 bytes are 0x00. Decoding it with data[4:] practically passes 3 populated bytes.

This will result in length to contain a larger value if the device ID is non zero. This could then lead to a false positive on the length error check further down the code.

Also, the aduResponse might end up being longer. This wouldn't be a problem if the PDU unpacking takes proper precautions on lengths, haven't checked.

modbus/tcpclient.go

Line 194 in 606c02f

aduResponse = data[:length]

I'm not actually using this project, so I'm not able to confirm if this an actual bug and I might be completely wrong about my assumption. Just trying to learn new things :D.