What Should You Actually Customize in Zeek?

You got Zeek running, logs are flowing, and the default configuration is doing its job. And then you hit the wall that almost every Zeek user hits eventually: you know Zeek is extensible, you know you’re supposed to customize it, but nobody has given you a clear answer for how to decide what to touch and what to leave alone.

It’s a reasonable thing to be stuck on. The surface area for Zeek customization is large: configuration tweaks, custom Zeek scripts, packages, log modifications, performance tuning. The documentation tells you how to do most of it without telling you whether you should.

How to Decide What to Customize in Zeek

What a recent community discussion made clear is that the people with the most intentional deployments tend to run potential changes through a similar set of questions before they make them:

• Does this help me aggregate and view logs more easily?
• Does this help me identify unknown traffic or enrich my data?
• Does this help me detect malicious activity or troubleshoot problems?

If a change doesn’t serve at least one of those, it’s worth asking whether it’s necessary. It sounds almost too simple, but it’s a surprisingly effective filter, especially when you’re starting out and the list of things you could do feels overwhelming.

What Zeek Users Actually Change

The practical effect of that filter is that it leads different people to very different places, depending on what they’re actually trying to accomplish.

The most common area of customization was configuration tweaks: local.zeek adjustments, environment variable-driven behavior, turning things on and off based on deployment context. Close behind that were detection and analysis scripts, and installing Zeek packages via zkg. Log modifications and performance tuning came up too, though less frequently.

What’s interesting isn’t the distribution. It’s that people with the same underlying goal often reached different implementations. Take subnet enrichment: two practitioners independently solved the problem of associating subnet names with Zeek logs and landed on opposite approaches. One added the subnet name directly to each log at collection time, letting Zeek propagate it automatically as connection objects move through the pipeline. The other kept a separate lookup table entirely and enriched at analysis instead, on the grounds that storing the same metadata repeatedly in every log event is redundant when you can join it downstream.

Both are correct. They reflect different downstream tooling, different team preferences, different answers to the question of where work should happen in the pipeline. The same three questions got both practitioners to the same destination: richer, more useful logs, by completely different roads.

Building a Zeek Deployment That Improves Over Time

That’s what Zeek customization actually looks like in practice. Not a canonical set of scripts every serious user runs, not a prescribed package list, but a set of decisions that reflect your environment, your threat model, and what you’ve decided is worth the maintenance cost.

The deployments that improve deliberately tend to have that in common: a clear sense of what problem they’re solving before they start writing Zeek scripts or installing packages. The ones that drift tend to accumulate customizations without a clear reason for each one, which makes them harder to maintain and harder to hand off.

If you’re staring at Zeek wondering where to start, the three questions are a reasonable place. Run your first idea through them. If it passes, build it. If it doesn’t, set it aside and revisit when your priorities shift, because they will.

Your Zeek won’t look like anyone else’s. That’s not a problem to solve. It’s the whole point.

Thank you to the community members who contributed to February’s Topic of the Month discussion on scripts and customization: David, Sebastian, Yacin, Dop, Mark, Seth, Aashish, Chris, Pedro, James, Carlos, Jakob, Rastislav, Van, Arne, Aaron, and Jan.

This month we’re talking about: I Didn’t Know Zeek Could Do That! If Zeek has ever surprised you, a capability you stumbled on or a behavior you didn’t expect, we want to hear about it.