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88 lines
3.9 KiB
ReStructuredText
88 lines
3.9 KiB
ReStructuredText
Modbus Keyword
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==============
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The modbus keyword can be used for matching on various properties of
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Modbus requests.
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There are two ways of using this keyword:
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* matching on functions properties with the setting "function";
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* matching on directly on data access with the setting "access".
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With the setting **function**, you can match on:
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* an action based on a function code field and a sub-function code when applicable;
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* one of three categories of Modbus functions;
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* public functions that are publicly defined (setting "public")
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* user-defined functions (setting "user")
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* reserved functions that are dedicated to proprietary extensions of Modbus (keyword "reserved")
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* one of the two sub-groups of public functions:
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* assigned functions whose definition is already given in the Modbus specification (keyword "assigned");
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* unassigned functions, which are reserved for future use (keyword "unassigned").
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Syntax::
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modbus: function <value>
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modbus: function <value>, subfunction <value>
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modbus: function [!] <assigned | unassigned | public | user | reserved | all>
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Sign '!' is negation
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Examples::
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modbus: function 21 # Write File record function
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modbus: function 4, subfunction 4 # Force Listen Only Mode (Diagnostics) function
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modbus: function assigned # defined by Modbus Application Protocol Specification V1.1b3
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modbus: function public # validated by the Modbus.org community
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modbus: function user # internal use and not supported by the specification
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modbus: function reserved # used by some companies for legacy products and not available for public use
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modbus: function !reserved # every function but reserved function
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With the **access** setting, you can match on:
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* a type of data access (read or write);
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* one of primary tables access (Discretes Input, Coils, Input Registers and Holding Registers);
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* a range of addresses access;
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* a written value.
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Syntax::
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modbus: access <read | write>
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modbus: access <read | write> <discretes | coils | input | holding>
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modbus: access <read | write> <discretes | coils | input | holding>, address <value>
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modbus: access <read | write> <discretes | coils | input | holding>, address <value>, value <value>
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With _<value>_ setting matches on the address or value as it is being
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accessed or written as follows::
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address 100 # exactly address 100
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address 100<>200 # greater than address 100 and smaller than address 200
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address >100 # greater than address 100
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address <100 # smaller than address 100
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Examples::
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modbus: access read # Read access
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modbus: access write # Write access
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modbus: access read input # Read access to Discretes Input table
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modbus: access write coils # Write access to Coils table
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modbus: access read discretes, address <100 # Read access at address smaller than 100 of Discretes Input table
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modbus: access write holding, address 500, value >200 # Write value greather than 200 at address 500 of Holding Registers table
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(cf. http://www.modbus.org/docs/Modbus_Application_Protocol_V1_1b3.pdf)
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**Note:** Address of read and write are starting at 1. So if your system
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is using a start at 0, you need to add 1 the address values.
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**Note:** According to MODBUS Messaging on TCP/IP Implementation Guide
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V1.0b, it is recommended to keep the TCP connection opened with a
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remote device and not to open and close it for each MODBUS/TCP
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transaction. In that case, it is important to set the depth of the
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stream reassembling as unlimited (stream.reassembly.depth: 0)
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(cf. http://www.modbus.org/docs/Modbus_Messaging_Implementation_Guide_V1_0b.pdf)
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Paper and presentation (in french) on Modbus support are available :
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http://www.ssi.gouv.fr/agence/publication/detection-dintrusion-dans-les-systemes-industriels-suricata-et-le-cas-modbus/
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