4.14. Modbus KeywordΒΆ

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”).


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

Sign ‘!’ is negation


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.


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


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/