Modbus Keyword
==============

The modbus keyword can be used for matching on various properties of
Modbus requests.

There are two ways of using this keyword:

* matching on functions properties with the setting "function";
* matching on directly on data access with the setting "access".

With the setting **function**, you can match on:

* an action based on a function code field and a sub-function code when applicable;
* one of three categories of Modbus functions;
* public functions that are publicly defined (setting "public")
* user-defined functions (setting "user")
* reserved functions that are dedicated to proprietary extensions of Modbus (keyword "reserved")
* one of the two sub-groups of public functions:

  * assigned functions whose definition is already given in the Modbus specification (keyword "assigned");
  * unassigned functions, which are reserved for future use (keyword "unassigned").

Syntax::

  modbus: function <value>
  modbus: function <value>, subfunction <value>
  modbus: function [!] <assigned | unassigned | public | user | reserved | all>

Sign '!' is negation

Examples::

  modbus: function 21                # Write File record function
  modbus: function 4, subfunction 4  # Force Listen Only Mode (Diagnostics) function
  modbus: function assigned          # defined by Modbus Application Protocol Specification V1.1b3
  modbus: function public            # validated by the Modbus.org community
  modbus: function user              # internal use and not supported by the specification
  modbus: function reserved          # used by some companies for legacy products and not available for public use
  modbus: function !reserved         # every function but reserved function

With the **access** setting, you can match on:

* a type of data access (read or write);
* one of primary tables access (Discretes Input, Coils, Input Registers and Holding Registers);
* a range of addresses access;
* a written value.

Syntax::

  modbus: access <read | write>
  modbus: access <read | write> <discretes | coils | input | holding>
  modbus: access <read | write> <discretes | coils | input | holding>, address <value>
  modbus: access <read | write> <discretes | coils | input | holding>, address <value>, value <value>

With _<value>_ setting matches on the address or value as it is being
accessed or written as follows::

  address 100      # exactly address 100
  address 100<>200 # greater than address 100 and smaller than address 200
  address >100     # greater than address 100
  address <100     # smaller than address 100

Examples::

  modbus: access read                                    # Read access
  modbus: access write                                   # Write access
  modbus: access read input                              # Read access to Discretes Input table
  modbus: access write coils                             # Write access to Coils table
  modbus: access read discretes, address <100            # Read access at address smaller than 100 of Discretes Input table
  modbus: access write holding, address 500, value >200  # Write value greather than 200 at address 500 of Holding Registers table

(cf. http://www.modbus.org/docs/Modbus_Application_Protocol_V1_1b3.pdf)

**Note:** Address of read and write are starting at 1. So if your system
is using a start at 0, you need to add 1 the address values.

**Note:** According to MODBUS Messaging on TCP/IP Implementation Guide
V1.0b, it is recommended to keep the TCP connection opened with a
remote device and not to open and close it for each MODBUS/TCP
transaction. In that case, it is important to set the depth of the
stream reassembling as unlimited (stream.reassembly.depth: 0)

(cf. http://www.modbus.org/docs/Modbus_Messaging_Implementation_Guide_V1_0b.pdf)

Paper and presentation (in french) on Modbus support are available :
http://www.ssi.gouv.fr/agence/publication/detection-dintrusion-dans-les-systemes-industriels-suricata-et-le-cas-modbus/