Writing Bro Plugins

Bro internally 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 Bro:

  • Bro 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 Bro had the corresponding code built-in. Indeed, internally many of Bro’s pieces are structured as plugins as well, they are just statically compiled into the binary rather than loaded dynamically at runtime.

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 Bro: we’ll add rot13(s: string) : string, a function that rotates every character in a string by 13 places.

Generally, a plugin comes in the form of a directory following a certain structure. To get started, Bro’s distribution provides a helper script aux/bro-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. Bro 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 namespace Bro is reserved for functionality distributed by the Bro 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
module Demo;

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

    for ( char* p = rot13; *p; p++ )
        char b = islower(*p) ? 'a' : 'A';
        *p  = (*p - b + 13) % 26 + b;

    BroString* bs = new BroString(1, reinterpret_cast<byte_vec>(rot13),
    return new StringVal(bs);

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

Now we can already compile our plugin, we just need to tell the configure script (that init-plugin created) where the Bro source tree is located (Bro needs to have been built there first):

# cd rot13-plugin
# ./configure --bro-dist=/path/to/bro/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 Bro to recognize it as a dynamic plugin.

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

# export BRO_PLUGIN_PATH=/path/to/rot13-plugin/build
# bro -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 characters 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
# bro -N | grep Rot13
Demo::Rot13 - Caesar cipher rotating a string's characters by 13 places. (dynamic, version 0.1.0)

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

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

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

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

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

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

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

One can distribute the plugin independently of Bro 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 bro_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 <bro-install-prefix>/lib/bro/plugins/. Alternatively, if you unpack it in another location, then you need to point BRO_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 Bro (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 Bro 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. Bro will load this in dynamically at run-time if OS and architecture match the current platform.
A directory with the plugin’s custom Bro scripts. When the plugin gets activated, this directory will be automatically added to BROPATH, so that any scripts/modules inside can be “@load”ed.
A Bro 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 Bro 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 Bro scripts that declare the plugin’s bif elements. The files here are produced by bifcl.

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

By convention, a plugin should put its custom scripts into sub folders of scripts/, i.e., scripts/<plugin-namespace>/<plugin-name>/<script>.bro to avoid conflicts. As usual, you can then put a __load__.bro 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 Bro 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 Bro’s BRO_PLUGIN_PATH), and when installed alongside Bro (after make install).

make install copies over the lib and scripts directories, as well as the __bro_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 Bro source (--bro-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.

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 BROPATH
  4. Load scripts/__preload__.bro
  5. Make BiF elements available to scripts.
  6. Load scripts/__load__.bro

By default, Bro will automatically activate all dynamic plugins found in its search path BRO_PLUGIN_PATH. However, in bare mode (bro -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 BRO_PLUGIN_ACTIVATE to a list of comma(!)-separated names of plugins to unconditionally activate, even in bare mode.

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

Plugin Components

The following subsections detail providing individual types of functionality via plugins. Note that a single plugin can provide more than one component type. For example, a plugin could provide multiple protocol analyzers at once; or both a logging backend and input reader at the same time.


These subsections are mostly missing right now, as much of their content isn’t actually plugin-specific, but concerns generally writing such functionality for Bro. The best way to get started right now is to look at existing code implementing similar functionality, either as a plugin or inside Bro proper. Also, for each component type there’s a unit test in testing/btest/plugins creating a basic plugin skeleton with a corresponding component.

Bro Scripts

Scripts are easy: 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.

Builtin Language Elements


Protocol Analyzers


File Analyzers


Logging Writer


Input Reader


Packet Sources


Packet Dumpers




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 Bro loads the plugin correctly. It won’t have a baseline yet, so let’s get that in place:

# cd tests
# btest -d
[  0%] rot13.show-plugin ... failed
% 'btest-diff output' failed unexpectedly (exit code 100)
% cat .diag
== File ===============================
Demo::Rot13 - Caesar cipher rotating a string's characters by 13 places. (dynamic, version 0.1.0)
    [Function] Demo::rot13

== Error ===============================
test-diff: no baseline found.

# btest -U
all 1 tests successful

# 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.bro

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

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

Check the output:

# btest -d rot13/bif-rot13.bro
[  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.bro
all 1 tests successful

Run the test-suite:

# btest
all 2 tests successful

Debugging Plugins

If your plugin isn’t loading as expected, Bro’s debugging facilities can help illuminate what’s going on. To enable, recompile Bro with debugging support (./configure --enable-debug), and afterwards rebuild your plugin as well. If you then run Bro 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 Bro’s standard debugging macros (grep for DBG_LOG in Bro’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 Bro’s compiled in debug mode.

Documenting Plugins


Integrate all this with Broxygen.

Copyright 2016, The Bro Project. Last updated on January 10, 2019. Created using Sphinx 1.7.5.