Snort 3 Inspector Reference

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Updated:
December 13, 2023

Chapter: Modbus Inspector

Modbus Inspector

Modbus Inspector Overview

Type

Inspector (service)

Usage

Inspect

Instance Type

Multiton

Other Inspectors Required

stream_tcp

Enabled

false

The Modbus protocol defines a communication standard to exchange messages between a Supervisory Control and Data Acquisition (SCADA) system and a Programmable Automation Controller (PLC). The Modbus protocol uses TCP port 502.

The modbus inspector detects and analyzes Modbus messages in network traffic. When enabled, the Modbus intrusion rule options provide access to certain Modbus protocol fields.

Best Practices for Configuring the Modbus Inspector

If your network does not contain an enabled Modbus device, you should not enable the modbus inspector in a network analysis policy that you apply to traffic.

Modbus Inspector Parameters

Modbus TCP port configuration

The binder inspector defines the Modbus TCP port configuration. For more information, see the Binder Inspector Overview.

Example: 
[
    {
        "when": {
            "role": "server",
            "proto": "tcp",
            "ports": "502"
        },
        "use": {
            "type": "modbus"
        }
    },
    {
        "when": {
            "role": "any",
            "service:" "modbus"
        },
        "use": {
            "type":"modbus"
        }
    }
]

Note

The modbus inspector does not provide any parameters.

Modbus Inspector Rules

Enable the modbus inspector rules to generate events and, in an inline deployment, drop offending packets.

Table 1. Modbus Inspector Rules

GID:SID

Rule Message
144:1

length in Modbus MBAP header does not match the length needed for the given function

144:2

Modbus protocol ID is non-zero

144:3

reserved Modbus function code in use

Modbus Inspector Intrusion Rule Options

You can use a modbus option alone or in combination with the content and byte_jump intrusion rule options.

modbus_data

Sets the data cursor to the beginning of the Modbus Data field.

Syntax: modbus_data;

Examples: modbus_data;

modbus_func

Verifies that the Modbus Function field matches the specified Modbus function code. You can set a positive integer or string literal to represent a Modbus function code.

Type: string

Syntax: modbus_func: <function>;

Valid values:

Table 2. Modbus Function Code Values

Code

String

1

read_coils

2

read_discrete_inputs

3

read_holding_registers

4

read_input_registers

5

write_single_coil

6

write_single_register

7

read_exception_status

8

diagnostics

11

get_comm_event_counter

12

get_comm_event_log

15

write_multiple_coils

16

write_multiple_registers

17

report_slave_id

20

read_file_record

21

write_file_record

22

mask_write_register

23

read_write_multiple_registers

24

read_fifo_queue

43

encapsulated_interface_transport

Examples: 
modbus_func: read_coils;
modbus_func: 8;

modbus_unit

Verifies that the Modbus Unit ID in the message matches the specified unit ID. You can set a number to represent the Modbus Unit ID.

Type: integer

Syntax: modbus_unit: <unit_id>;

Valid range: 0 to 255

Examples: 
modbus_unit: 1;

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