Cluster Architecture

Zeek is not multithreaded, so once the limitations of a single processor core are reached the only option currently is to spread the workload across many cores, or even many physical computers. The cluster deployment scenario for Zeek is the current solution to build these larger systems. The tools and scripts that accompany Zeek provide the structure to easily manage many Zeek processes examining packets and doing correlation activities but acting as a singular, cohesive entity. This document describes the Zeek cluster architecture. For information on how to configure a Zeek cluster, see the documentation for ZeekControl.


The figure below illustrates the main components of a Zeek cluster.


For more specific information on the way Zeek processes are connected, how they function, and how they communicate with each other, see the Broker Framework Documentation.


The tap is a mechanism that splits the packet stream in order to make a copy available for inspection. Examples include the monitoring port on a switch and an optical splitter on fiber networks.


The frontend is a discrete hardware device or on-host technique that splits traffic into many streams or flows. The Zeek binary does not do this job. There are numerous ways to accomplish this task, some of which are described below in Frontend Options.


The manager is a Zeek process that has two primary jobs. It receives log messages and notices from the rest of the nodes in the cluster using the Zeek communications protocol (note that if you use a separate logger node, then the logger receives all logs instead of the manager). The result is a single log instead of many discrete logs that you have to combine in some manner with post-processing. The manager also supports other functionality and analysis which requires a centralized, global view of events or data.


A logger is an optional Zeek process that receives log messages from the rest of the nodes in the cluster using the Zeek communications protocol. The purpose of having a logger receive logs instead of the manager is to reduce the load on the manager. If no logger is needed, then the manager will receive logs instead.


A proxy is a Zeek process that may be used to offload data storage or any arbitrary workload. A cluster may contain multiple proxy nodes. The default scripts that come with Zeek make minimal use of proxies, so a single one may be sufficient, but customized use of them to partition data or workloads provides greater cluster scalability potential than just doing similar tasks on a single, centralized Manager node.

Zeek processes acting as proxies don’t tend to be extremely hard on CPU or memory and users frequently run proxy processes on the same physical host as the manager.


The worker is the Zeek process that sniffs network traffic and does protocol analysis on the reassembled traffic streams. Most of the work of an active cluster takes place on the workers and as such, the workers typically represent the bulk of the Zeek processes that are running in a cluster. The fastest memory and CPU core speed you can afford is recommended since all of the protocol parsing and most analysis will take place here. There are no particular requirements for the disks in workers since almost all logging is done remotely to the manager, and normally very little is written to disk.

Frontend Options

There are many options for setting up a frontend flow distributor. In many cases it is beneficial to do multiple stages of flow distribution on the network and on the host.

Discrete hardware flow balancers


If you are monitoring one or more 10G physical interfaces, the recommended solution is to use either a cFlow or cVu device from cPacket because they are used successfully at a number of sites. These devices will perform layer-2 load balancing by rewriting the destination Ethernet MAC address to cause each packet associated with a particular flow to have the same destination MAC. The packets can then be passed directly to a monitoring host where each worker has a BPF filter to limit its visibility to only that stream of flows, or onward to a commodity switch to split the traffic out to multiple 1G interfaces for the workers. This greatly reduces costs since workers can use relatively inexpensive 1G interfaces.

OpenFlow Switches

We are currently exploring the use of OpenFlow based switches to do flow-based load balancing directly on the switch, which greatly reduces frontend costs for many users. This document will be updated when we have more information.

On host flow balancing


The PF_RING software for Linux has a “clustering” feature which will do flow-based load balancing across a number of processes that are sniffing the same interface. This allows you to easily take advantage of multiple cores in a single physical host because Zeek’s main event loop is single threaded and can’t natively utilize all of the cores. If you want to use PF_RING, see the documentation on how to configure Zeek with PF_RING.


FreeBSD has an in-progress project named Netmap which will enable flow-based load balancing as well. When it becomes viable for real world use, this document will be updated.

Click! Software Router

Click! can be used for flow based load balancing with a simple configuration. This solution is not recommended on Linux due to Zeek’s PF_RING support and only as a last resort on other operating systems since it causes a lot of overhead due to context switching back and forth between kernel and userland several times per packet.