8.20. Modbus Keyword
The modbus keyword can be used for matching on various properties of Modbus requests.
There are three ways of using this keyword:
matching on functions properties with the setting "function";
matching on directly on data access with the setting "access";
matching on unit identifier with the setting "unit" only or with the previous setting "function" or "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 <discretes | coils | input | holding>
modbus: access read <discretes | coils | input | holding>, address <value>
modbus: access write < coils | holding>
modbus: access write < coils | holding>, address <value>
modbus: access write < coils | 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 greater than 200 at address 500 of Holding Registers table
With the setting unit, you can match on:
a MODBUS slave address of a remote device connected on the sub-network behind a bridge or a gateway. The destination IP address identifies the bridge itself and the bridge uses the MODBUS unit identifier to forward the request to the right slave device.
Syntax:
modbus: unit <value>
modbus: unit <value>, function <value>
modbus: unit <value>, function <value>, subfunction <value>
modbus: unit <value>, function [!] <assigned | unassigned | public | user | reserved | all>
modbus: unit <value>, access <read | write>
modbus: unit <value>, access read <discretes | coils | input | holding>
modbus: unit <value>, access read <discretes | coils | input | holding>, address <value>
modbus: unit <value>, access write < coils | holding>
modbus: unit <value>, access write < coils | holding>, address <value>
modbus: unit <value>, access write < coils | holding>, address <value>, value <value>
With _<value>_ setting matches on the address or value as it is being accessed or written as follows:
unit 10 # exactly unit identifier 10
unit 10<>20 # greater than unit identifier 10 and smaller than unit identifier 20
unit >10 # greater than unit identifier 10
unit <10 # smaller than unit identifier 10
Examples:
modbus: unit 10 # Unit identifier 10
modbus: unit 10, function 21 # Unit identifier 10 and write File record function
modbus: unit 10, function 4, subfunction 4 # Unit identifier 10 and force Listen Only Mode (Diagnostics) function
modbus: unit 10, function assigned # Unit identifier 10 and assigned function
modbus: unit 10, function !reserved # Unit identifier 10 and every function but reserved function
modbus: unit 10, access read # Unit identifier 10 and Read access
modbus: unit 10, access write coils # Unit identifier 10 and Write access to Coils table
modbus: unit >10, access read discretes, address <100 # Greater than unit identifier 10 and Read access at address smaller than 100 of Discretes Input table
modbus: unit 10<>20, access write holding, address 500, value >200 # Greater than unit identifier 10 and smaller than unit identifier 20 and Write value greater 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)
Note: According to MODBUS Messaging on TCP/IP Implementation Guide V1.0b, the MODBUS slave device addresses on serial line are assigned from 1 to 247 (decimal). Address 0 is used as broadcast address.
(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/