4.14. 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”).
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
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)
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/