This commit fixes three issues with Zeek's Modbus message logging:
1 - Some exception responses (e.g., READ_COILS_EXCEPTION) are logged
twice: once without and once with the exception message.
2 - Some exception responses (e.g., PROGRAM_484_EXCEPTION) are not
logged.
3 - Some known but reserved function codes (e.g., PROGRAM_UNITY) are
logged as unk-xxx (e.g., unk-90), while it would be possible to
log their known name.
To address these inconsistencies, the modbus parser has been updated
to parse all exception responses (i.e., all responses where the MSB
of the function code is set) using the already defined Exception
message.
Also, the Modbus main.zeek script has been updated to consistently
demand logging exception responses to the specialized
modbus_exception event, rather than logging some exception responses
in the modbus_message event and others in the modbus_exception event.
Finally, the main.zeek script has been updated to make sure that
for every known function code, the corresponding exception code was
also present, and the enumeration of known function codes in
consts.zeek has been expanded.
Closes#3984
Add the `pdu_type` field to Modbus over TCP logs to indicate whether the Modbus
message was a request or a response. Due to the client/server nature of Modbus
over TCP/IP, all messages from the TCP session originator are requests, while
all messages from the TCP session responder are responses.
Adding this information to the default log surfaces protocol metadata in a way
that doesn't require users to understand the Modbus over TCP protocol.
