Writing Plugins

Zeek provides a plugin API that enables extending the system dynamically, without modifying the core code base. That way, custom code remains self-contained and can be maintained, compiled, and installed independently. Currently, plugins can add the following functionality to Zeek:

  • Zeek scripts.

  • Builtin functions/events/types for the scripting language.

  • Protocol analyzers.

  • File analyzers.

  • Packet sources and packet dumpers.

  • Logging framework backends.

  • Input framework readers.

A plugin’s functionality is available to the user just as if Zeek had the corresponding code built-in. Indeed, internally many of Zeek’s pieces are structured as plugins as well, they are just statically compiled into the binary rather than loaded dynamically at runtime.


Plugins and Zeek packages are related but separate concepts. Both extend Zeek’s functionality without modifying Zeek’s source code. A plugin achieves this via compiled, native code that Zeek links into its core at runtime. A Zeek package, on the other hand, is a modular addition to Zeek, managed via the zkg package manager, that may or may not include a plugin. More commonly, packages consist of script-layer additions to Zeek’s functionality. Packages also feature more elaborate metadata, enabling dependencies on other packages, Zeek versions, etc.

Quick Start

Writing a basic plugin is quite straight-forward as long as one follows a few conventions. In the following, we create a simple example plugin that adds a new Built-In Function (BIF) to Zeek: we’ll add rot13(s: string) : string, a function that rotates every letter in a string by 13 places.

Generally, a plugin comes in the form of a directory following a certain structure. To get started, Zeek’s distribution provides a helper script auxil/zeek-aux/plugin-support/init-plugin that creates a skeleton plugin that can then be customized. Let’s use that:

# init-plugin ./rot13-plugin Demo Rot13

As you can see, the script takes three arguments. The first is a directory inside which the plugin skeleton will be created. The second is the namespace the plugin will live in, and the third is a descriptive name for the plugin itself relative to the namespace. Zeek uses the combination of namespace and name to identify a plugin. The namespace serves to avoid naming conflicts between plugins written by independent developers; pick, e.g., the name of your organisation. The namespaces Bro (legacy) and Zeek are reserved for functionality distributed by the Zeek Project. In our example, the plugin will be called Demo::Rot13.

The init-plugin script puts a number of files in place. The full layout is described later. For now, all we need is src/rot13.bif. It’s initially empty, but we’ll add our new BIF there as follows:

# cat src/rot13.bif
#include <cstring>
#include <cctype>
#include "zeek/util.h"
#include "zeek/ZeekString.h"
#include "zeek/Val.h"

module Demo;

function rot13%(s: string%) : string
    char* rot13 = util::copy_string(s->CheckString());

    for ( char* p = rot13; *p; p++ )
        char b = islower(*p) ? 'a' : 'A';
        char d = *p - b + 13;

        if ( d >= 13 && d <= 38 )
            *p  = d % 26 + b;

    zeek::String* zs = new zeek::String(1, reinterpret_cast<byte_vec>(rot13),
    return make_intrusive<StringVal>(zs);

The syntax of this file is just like any other *.bif file; we won’t go into it here.

Now we are ready to compile our plugin. The configure script will just need to be able to find the location of either a Zeek installation-tree or a Zeek source-tree.

When building a plugin against a Zeek installation-tree, simply have the installation’s associated zeek-config in your PATH and the configure script will detect it and use it to obtain all the information it needs:

# which zeek-config
# cd rot13-plugin
# ./configure && make
[... cmake output ...]

When building a plugin against a Zeek source-tree (which itself needs to have first been built), the configure script has to explicitly be told its location:

# cd rot13-plugin
# ./configure --zeek-dist=/path/to/zeek/dist && make
[... cmake output ...]

This builds the plugin in a subdirectory build/. In fact, that subdirectory becomes the plugin: when make finishes, build/ has everything it needs for Zeek to recognize it as a dynamic plugin.

Let’s try that. Once we point Zeek to the build/ directory, it will pull in our new plugin automatically, as we can check with the -N option:

# export ZEEK_PLUGIN_PATH=/path/to/rot13-plugin/build
# zeek -N
Demo::Rot13 - <Insert description> (dynamic, version 0.1.0)

That looks quite good, except for the dummy description that we should replace with something nicer so that users will know what our plugin is about. We do this by editing the config.description line in src/Plugin.cc, like this:

plugin::Configuration Plugin::Configure()
    plugin::Configuration config;
    config.name = "Demo::Rot13";
    config.description = "Caesar cipher rotating a string's letters by 13 places.";
    config.version.major = 0;
    config.version.minor = 1;
    config.version.patch = 0;
    return config;

Now rebuild and verify that the description is visible:

# make
# zeek -N | grep Rot13
Demo::Rot13 - Caesar cipher rotating a string's letters by 13 places. (dynamic, version 0.1.0)

Zeek can also show us what exactly the plugin provides with the more verbose option -NN:

# zeek -NN
Demo::Rot13 - Caesar cipher rotating a string's letters by 13 places. (dynamic, version 0.1.0)
    [Function] Demo::rot13

There’s our function. Now let’s use it:

# zeek -e 'print Demo::rot13("Hello")'

It works. We next install the plugin along with Zeek itself, so that it will find it directly without needing the ZEEK_PLUGIN_PATH environment variable. If we first unset the variable, the function will no longer be available:

# zeek -e 'print Demo::rot13("Hello")'
error in <command line>, line 1: unknown identifier Demo::rot13, at or near "Demo::rot13"

Once we install it, it works again:

# make install
# zeek -e 'print Demo::rot13("Hello")'

The installed version went into <zeek-install-prefix>/lib/zeek/plugins/Demo_Rot13.

One can distribute the plugin independently of Zeek for others to use. To distribute in source form, just remove the build/ directory (make distclean does that) and then tar up the whole rot13-plugin/ directory. Others then follow the same process as above after unpacking.

To distribute the plugin in binary form, the build process conveniently creates a corresponding tarball in build/dist/. In this case, it’s called Demo_Rot13-0.1.0.tar.gz, with the version number coming out of the VERSION file that init-plugin put into place. The binary tarball has everything needed to run the plugin, but no further source files. Optionally, one can include further files by specifying them in the plugin’s CMakeLists.txt through the zeek_plugin_dist_files macro; the skeleton does that for README, VERSION, CHANGES, and COPYING. To use the plugin through the binary tarball, just unpack it into <zeek-install-prefix>/lib/zeek/plugins/. Alternatively, if you unpack it in another location, then you need to point ZEEK_PLUGIN_PATH there.

Before distributing your plugin, you should edit some of the meta files that init-plugin puts in place. Edit README and VERSION, and update CHANGES when you make changes. Also put a license file in place as COPYING; if BSD is fine, you will find a template in COPYING.edit-me.

Plugin Directory Layout

A plugin’s directory needs to follow a set of conventions so that Zeek (1) recognizes it as a plugin, and (2) knows what to load. While init-plugin takes care of most of this, the following is the full story. We’ll use <base> to represent a plugin’s top-level directory. With the skeleton, <base> corresponds to build/.


A file that marks a directory as containing a Zeek plugin. The file must exist, and its content must consist of a single line with the qualified name of the plugin (e.g., “Demo::Rot13”).


The shared library containing the plugin’s compiled code. Zeek will load this in dynamically at run-time if OS and architecture match the current platform.


A directory with the plugin’s custom Zeek scripts. When the plugin gets activated, this directory will be automatically added to ZEEKPATH, so that any scripts/modules inside can be “@load”ed.


A Zeek script that will be loaded when the plugin gets activated. When this script executes, any BIF elements that the plugin defines will already be available. See below for more information on activating plugins.


A Zeek script that will be loaded when the plugin gets activated, but before any BIF elements become available. See below for more information on activating plugins.


Directory with auto-generated Zeek scripts that declare the plugin’s BIF elements. The files here are produced by bifcl.

Any other files in <base> are ignored by Zeek.

By convention, a plugin should put its custom scripts into sub folders of scripts/, i.e., scripts/<plugin-namespace>/<plugin-name>/<script>.zeek to avoid conflicts. As usual, you can then put a __load__.zeek in there as well so that, e.g., @load Demo/Rot13 could load a whole module in the form of multiple individual scripts.

Note that in addition to the paths above, the init-plugin helper puts some more files and directories in place that help with development and installation (e.g., CMakeLists.txt, Makefile, and source code in src/). However, all these do not have a special meaning for Zeek at runtime and aren’t necessary for a plugin to function.


init-plugin puts a basic plugin structure in place that follows the above layout and augments it with a CMake build and installation system. Plugins with this structure can be used both directly out of their source directory (after make and setting Zeek’s ZEEK_PLUGIN_PATH), and when installed alongside Zeek (after make install).

Upon completion, init-plugin initializes a git repository and stages its produced files for committing, but does not yet commit the files. This allows you to tweak the new plugin as needed prior to the initial commit.

make install copies over the lib and scripts directories, as well as the __zeek_plugin__ magic file and any further distribution files specified in CMakeLists.txt (e.g., README, VERSION). You can find a full list of files installed in build/MANIFEST. Behind the scenes, make install really just unpacks the binary tarball from build/dist into the destination directory.

init-plugin will never overwrite existing files. If its target directory already exists, it will by default decline to do anything. You can run it with -u instead to update an existing plugin, however it will never overwrite any existing files; it will only put in place files it doesn’t find yet. To revert a file back to what init-plugin created originally, delete it first and then rerun with -u.

init-plugin puts a configure script in place that wraps cmake with a more familiar configure-style configuration. By default, the script provides two options for specifying paths to the Zeek source (--zeek-dist) and to the plugin’s installation directory (--install-root). To extend configure with plugin-specific options (such as search paths for its dependencies) don’t edit the script directly but instead extend configure.plugin, which configure includes. That way you will be able to more easily update configure in the future when the distribution version changes. In configure.plugin you can use the predefined shell function append_cache_entry to seed values into the CMake cache; see the installed skeleton version and existing plugins for examples.


In the past init-plugin also generated a zkg.meta file, automatically creating a Zeek package containing a plugin. init-plugin now focuses purely on plugins, as its name suggests. To bootstrap new Zeek packages (possibly containing plugins), use the more featureful templating functionality provided by the zkg create command, explained here.

Activating a Plugin

A plugin needs to be activated to make it available to the user. Activating a plugin will:

  1. Load the dynamic module

  2. Make any BIF items available

  3. Add the scripts/ directory to ZEEKPATH

  4. Load scripts/__preload__.zeek

  5. Make BIF elements available to scripts.

  6. Load scripts/__load__.zeek

By default, Zeek will automatically activate all dynamic plugins found in its search path ZEEK_PLUGIN_PATH. However, in bare mode (zeek -b), no dynamic plugins will be activated by default; instead the user can selectively enable individual plugins in scriptland using the @load-plugin <qualified-plugin-name> directive (e.g., @load-plugin Demo::Rot13). Alternatively, one can activate a plugin from the command-line by specifying its full name (Demo::Rot13), or set the environment variable ZEEK_PLUGIN_ACTIVATE to a list of comma-separated names of plugins to unconditionally activate, even in bare mode.

zeek -N shows activated plugins separately from found but not yet activated plugins. Note that plugins compiled statically into Zeek are always activated, and hence show up as such even in bare mode.

Plugin Components

It’s easy for a plugin to provide custom scripts: just put them into scripts/, as described above. The CMake infrastructure will automatically install them, as well include them into the source and binary plugin distributions.

Any number or combination of other components can be provided by a single plugin. For example a plugin can provide multiple different protocol analyzers, or both a log writer and input reader.

The best place to look for examples or templates for a specific type of plugin component are the source code of Zeek itself since every one of its components uses the same API as any external plugin.

Each component type also has a simple integration test, found in the Zeek source-tree’s testing/btest/plugins/ directory, that can serve useful for creating basic plugin skeletons.

Testing Plugins

A plugin should come with a test suite to exercise its functionality. The init-plugin script puts in place a basic BTest setup to start with. Initially, it comes with a single test that just checks that Zeek loads the plugin correctly:

# cd tests
# btest -A
[  0%] rot13.show-plugin ... ok
all 1 tests successful

You can also run this via the Makefile:

# cd ..
# make test
make -C tests
make[1]: Entering directory `tests'
all 1 tests successful
make[1]: Leaving directory `tests'

Now let’s add a custom test that ensures that our BIF works correctly:

# cd tests
# cat >rot13/bif-rot13.zeek

# @TEST-EXEC: zeek %INPUT >output
# @TEST-EXEC: btest-diff output

event zeek_init()
    print Demo::rot13("Hello");

Check the output:

# btest -d rot13/bif-rot13.zeek
[  0%] rot13.bif-rot13 ... failed
% 'btest-diff output' failed unexpectedly (exit code 100)
% cat .diag
== File ===============================
== Error ===============================
test-diff: no baseline found.

% cat .stderr

1 of 1 test failed

Install the baseline:

# btest -U rot13/bif-rot13.zeek
all 1 tests successful

Run the test-suite:

# btest
all 2 tests successful

Debugging Plugins

If your plugin isn’t loading as expected, Zeek’s debugging facilities can help illuminate what’s going on. To enable, recompile Zeek with debugging support (./configure --enable-debug), and afterwards rebuild your plugin as well. If you then run Zeek with -B plugins, it will produce a file debug.log that records details about the process for searching, loading, and activating plugins.

To generate your own debugging output from inside your plugin, you can add a custom debug stream by using the PLUGIN_DBG_LOG(<plugin>, <args>) macro (defined in DebugLogger.h), where <plugin> is the Plugin instance and <args> are printf-style arguments, just as with Zeek’s standard debugging macros (grep for DBG_LOG in Zeek’s src/ to see examples). At runtime, you can then activate your plugin’s debugging output with -B plugin-<name>, where <name> is the name of the plugin as returned by its Configure() method, yet with the namespace-separator :: replaced with a simple dash. Example: If the plugin is called Demo::Rot13, use -B plugin-Demo-Rot13. As usual, the debugging output will be recorded to debug.log if Zeek’s compiled in debug mode.

Building Plugins Statically

Plugins can be built statically into a Zeek binary using the --include-plugins option passed to configure. This argument takes a colon-separated list of paths to plugin source. Building plugins in this manner includes them directly into the Zeek binary and installation. They are loaded automatically by Zeek at startup without needing to install them separately.

Headers for built-in plugins are installed into a subdirectory of <zeek-install-prefix>/include/zeek/builtin-plugins specific to each plugin. Scripts are installed into a subdirectory of <zeek-install-prefix>/share/zeek/builtin-plugins specific to each plugin. The scripts directory is also automatically added to the default ZEEKPATH.