By Christian Kreibich, Senior Engineer at Corelight

If you’ve ever used Bro, you’ve likely noticed that it’s rather more flexible than other network monitoring solutions. This is not coincidence — it reflects a core principle that has underpinned the evolution of the Bro platform since its beginnings two decades ago. This principle has afforded users a wealth of benefits that continue to shape today’s product vision here at Corelight.

Let’s start with Bro’s basic design. It uses a traffic-parsing core to feed protocol events into its built-in script interpreter, which separates mechanism (the nitty-gritty parsing of traffic) from policy (what to do in response to observed activity). These events (450+ different types) cover a vast semantic range and span the entire protocol stack. Combined with the Bro scripting language — designed specifically to simplify network-typical compute and state-keeping tasks (a key difference to other popular languages such as Lua) — extensibility is not just a feature: it defines the system. Let that sink in for a moment: while Bro is a fantastic network flight recorder (consider the breadth and depth of the traffic logs the Corelight Sensor produces), this log production is merely one configuration of the system. Behavioral profiling of your end-hosts? Check. Arbitrary cross-flow/protocol state-keeping? Check. Lateral movement? Check. The system provides the building blocks, you provide the analysis — either in real-time on the appliance or in the form of actionable data in your broader analytics pipeline.

Extensibility doesn’t stop at Bro’s core design. It’s designed from the ground up to support clustering at the process and machine levels and features a powerful communication and data-persistence infrastructure to scale your deployment as your network grows.

Your needs go beyond those 450 event types? Bro has you covered. Its plugin architecture supports adding compiled code that allows you to add new functionality — for example a new protocol analyzer or packet source — on your own and without ever needing to patch the Bro source tree. No more worrying about release cycles, licensing, or development workflow. The core is designed to support extensibility. At the script level, you can derive new event types as needed. Concerned about managing those extensions? The Bro package manager makes it just as easy to maintain your in-house Bro feature set as it is to manage other distributions in your infrastructure.

What about integrating Bro with the rest of your infrastructure? BroControl provides a handy remote control for your Bro installation and supports plugins. On the network side, the NetControl framework provides a wide range of connectors to mesh Bro into your enforcement infrastructure. Naturally, the framework is designed for extensibility so you can add additional connectors with ease. Thinking of leveraging your existing threat intel feeds in Bro? The input framework makes it easy. Your file processing pipeline to check whether those PDFs are malicious? Bro’s file analysis framework feeds right into it. Finally, Bro’s forthcoming osquery integration allows it to include host-based events.

As a Corelight customer, you benefit from all of these features: we’re committed to running open-source Bro on our Sensors. We’ve streamlined custom script installation via our APIs, taken care of data export to Splunk, Kafka, or your favorite SIEM, and taken the hassle out of managing and tuning fast packet analysis pipelines for you. To ensure stability, the Sensor doesn’t yet expose all of Bro’s functionality — for example, customers cannot currently deploy their own plugins — but we’re aiming for feature convergence over time.

To us, extensibility is not an afterthought that we try to tuck on in a few release cycles. It permeates the way we think about network monitoring and has enabled scalability, visibility, profiling, learning, and detections battle-tested over two decades of real-world use in some of the world’s most demanding network environments.