Lua Function for Snort 3 Rule Creation

This document outlines a Lua function that generates rules for Snort 3 Intrusion Detection System based on provided criteria. The function will handle the construction of the rule string and ensure that the necessary components are included.

Function Overview

Purpose

The function, create_snort_rule, will create a Snort 3 rule based on the specified parameters such as action, protocol, source and destination IPs and ports, and any additional options.

Parameters

• action (string): The action to take ("alert", "drop", "pass").
• protocol (string): The protocol type (e.g., "tcp", "udp", "icmp").
• src_ip (string): The source IP address (e.g., "192.168.1.1").
• src_port (string): The source port (e.g., "any", "80").
• dst_ip (string): The destination IP address.
• dst_port (string): The destination port.
• options (string or nil): Additional options for the rule (optional).

Returns

• (string): A correctly formatted Snort 3 rule string.

Exceptions

• Raises an error if invalid parameters are provided.

Code Implementation

-- Function to create a Snort 3 rule
function create_snort_rule(action, protocol, src_ip, src_port, dst_ip, dst_port, options)
    -- Validate inputs
    local valid_actions = {alert=true, drop=true, pass=true}
    local valid_protocols = {tcp=true, udp=true, icmp=true}
    
    if not valid_actions[action] then
        error("Invalid action: " .. tostring(action))
    end
    
    if not valid_protocols[protocol] then
        error("Invalid protocol: " .. tostring(protocol))
    end
    
    -- Construct the rule
    local rule = string.format("%s %s ", action, protocol)
    rule = rule .. string.format("src %s:%s, dst %s:%s; ", src_ip, src_port, dst_ip, dst_port)
    
    -- Append options if provided
    if options then
        rule = rule .. options .. " "
    end
    
    rule = rule .. "sid:1000001;"
    return rule
end

Code Explanation

• Input Validation: The function first checks if the action and protocol parameters are valid by comparing against predefined lists of valid actions and protocols.
• Rule Construction: It uses string.format to build the rule string with the specified parameters, ensuring proper formatting.
• Option Handling: If additional options are provided, they are appended to the rule.
• Return Value: The function concludes by returning the complete Snort 3 rule as a string.

Example Usage

Here is how you can utilize the create_snort_rule function to generate a Snort 3 rule:

-- Example to create a Snort rule
local rule = create_snort_rule("alert", "tcp", "192.168.1.0/24", "any", "10.0.0.1", "80", "msg:\"Possible attack!\";")
print(rule)

Output

This would generate a rule similar to:

alert tcp src 192.168.1.0/24:any, dst 10.0.0.1:80; msg:"Possible attack!"; sid:1000001;

Conclusion

This Lua function provides a robust solution for creating Snort 3 rules dynamically based on user-defined input. It adheres to best practices by including input validation and structured code, making it suitable for scalable applications in network security monitoring.

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