What Is Bro Scripting?


Heather Bennett

What Is Bro Scripting?

Bro scripting is a powerful tool for network traffic analysis and intrusion detection. It allows users to write custom scripts that can extend the functionality of the Bro Network Security Monitor (NSM) framework.

Why Use Bro Scripting?

Bro NSM is designed to analyze network traffic in real-time and generate high-level logs that provide deep insights into network activities. However, there are times when the default functionality of Bro is not sufficient to meet specific analysis requirements.

This is where Bro scripting comes into play. By writing custom scripts, users can extract additional information from network traffic and perform complex analysis tailored to their needs. Bro scripting offers flexibility and extensibility, making it a valuable tool for security analysts, researchers, and network administrators.

Getting Started with Bro Scripting

To start using Bro scripting, you need to have Bro NSM installed on your system. Once installed, you can create a new script by following these steps:

  1. Create a new file with a .bro extension (e.g., myscript.bro).
  2. Add necessary import statements at the beginning of your script to include predefined functionalities.
  3. Define event handlers that specify which network events you want your script to capture and process.
  4. Write your custom logic inside event handlers to extract valuable information from network traffic.

Your script can be as simple or as complex as needed. Bro provides a wide range of built-in functions and variables that you can use in your scripts to manipulate network data effectively.


@load base/protocols/http

event http_request(c: connection, method: string, uri: string, headers: HTTP::header_list)
    print fmt("HTTP request detected: %s %s", method, uri);
    print fmt("User-Agent: %s", headers?$?"User-Agent");

In the above example, we load the HTTP protocol module and define an event handler called http_request. This handler captures HTTP requests and prints the request method, URI, and User-Agent header if present. You can further enhance this script to perform more complex tasks like logging or alerting based on specific conditions.

Deploying Bro Scripts

Once you have written your Bro script, you can deploy it by running the Bro NSM framework with your script as a parameter:

$ bro -r myscript.bro

This command starts Bro NSM and instructs it to use your custom script for analysis. You can also specify input files or network interfaces to capture traffic.


Bro scripting empowers users to extend the capabilities of Bro NSM and perform advanced network traffic analysis. With its flexibility and extensibility, Bro scripting is an essential tool in the arsenal of network security professionals.

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