Configuration Framework

Bro includes a “configuration framework” that allows updating script options dynamically at runtime. This functionality consists of several components: an “option” declaration, the ability to specify input files to enable changing the value of options at runtime, a couple of functions, and a log file “config.log” which contains information about every change to option values.


The configuration framework provides an alternative to using Bro script constants to store various Bro settings.

In general, traditional constants can be used when a value is not expected to change at runtime, but they cannot be used for values that need to be modified occasionally. While a “redef” allows a re-definition of an already defined constant in Bro, these redefinitions can only be performed when Bro first starts. Afterwards, constants can no longer be modified.

However, it is clearly desirable to be able to change at runtime many of the configuration options that Bro offers. Having to restart Bro can be time-consuming and causes Bro to lose all connection state and knowledge that it accumulated. Bro’s configuration framework solves this problem by allowing changing configuration options at runtime.

Declaring options

The “option” keyword allows variables to be declared as configuration options.

module TestModule;

export {
    option my_networks: set[subnet] = {};
    option enable_feature = F;
    option hostname = "testsystem";
    option timeout = 1min;
    option my_ports: vector of port = {};

The rules regarding options can be thought of as being in between global variables and constants. Like global variables, options cannot be declared inside a function, hook, or event handler. Like constants, options must be initialized when declared (the type can often be inferred from the initializer but may need to be specified). The value of an option can change at runtime, but options cannot be assigned a new value using normal assignments.

The initial value of an option can be redefined with a redef declaration just like for global variables and constants. The only difference being that there is no need to specify the &redef attribute in the declaration of an option. For example, given the above option declarations, here are some possible redefs:

redef TestModule::enable_feature = T;
redef TestModule::my_networks += {, };

Changing options

The configuration framework facilitates reading in new option values from external files at runtime.

Configuration files contain a mapping between option names and their values. The format for these files looks like this:

[option name][tab/spaces][new value]

Configuration files can be specified by adding them to Config::config_files. Note that in a cluster configuration, only the manager node attempts to read the specified configuration files.

For example, simply add something like this to local.bro:

redef Config::config_files += { "/path/to/config.dat" };

The specified configuration file will then be monitored continuously for changes, so that writing TestModule::enable_feature T into that file will automatically update the option’s value accordingly (in a cluster configuration, the change will be sent from the manager to all other nodes in the cluster). Here is an example configuration file:

TestModule::enable_feature  T
TestModule::hostname  host-1
TestModule::timeout 50.5
TestModule::my_ports 80/tcp,53/udp

Note that as seen in the above example, for options of type interval, the numeric value in the config file is interpreted as seconds and there cannot be any time units (such as sec, min, etc.).

Internally, the configuration framework uses the Bro input framework with a type of input reader specifically for reading config files. Users familiar with the Bro input framework might be aware that the input framework is usually very strict about the syntax of input files. This is not true for configuration files: the files need no header lines and either tabs or spaces are accepted as separators.

If you inspect the configuration framework scripts, you will notice that the scripts simply catch events from the input framework and then a function Config::set_value is called to set an option to the new value. If you want to change an option yourself during runtime, you can call Config::set_value directly from a script (in a cluster configuration, this only needs to happen on the manager, as the change will be automatically sent to all other nodes in the cluster).

Note that some data types (such as pattern, table, and record) are not supported by the config input reader. In that case you would need to use the Config::set_value function to change the value of such an option as shown in the following example.

module TestModule;

export {
    option host_port: table[addr] of port = {};

event bro_init() {
    local t: table[addr] of port = { [] = 123/tcp };
    Config::set_value("TestModule::host_port", t);

Regardless of whether an option change is triggered by a config file or by the Config::set_value function, the change is always logged to the log file “config.log”.

Change handlers

A change handler is a user-defined function that is called automatically each time an option value changes. This example shows how to register a change handler for an option that has a data type of “addr” (for other data types, the return type and 2nd parameter data type must be adjusted accordingly):

module TestModule;

export {
    option testaddr =;

# Note: the data type of 2nd parameter and return type must match
function change_addr(ID: string, new_value: addr): addr
    print fmt("Value of %s changed from %s to %s", ID, testaddr, new_value);
    return new_value;

event bro_init()
    Option::set_change_handler("TestModule::testaddr", change_addr);

Immediately before the specified option value is changed, the change handler function will be called. The value returned by the change handler is the value finally assigned to the option. This allows, for example, checking of values to reject invalid input (the original value can be returned to reject the change).

It is possible define multiple change handlers for a single option. In this case, the change handlers are chained together: the value returned by the first change handler is the “new value” seen by the next change handler, and so on. The built-in function Option::set_change_handler takes an optional third argument that can specify a priority for the handlers.

A change handler function can optionally have a third argument of type string. When a config file triggers a change, then the third argument is the pathname of the config file. When the Config::set_value function triggers a change, then the third argument of the change handler is the value passed to the optional third argument of the Config::set_value function.

Note that change handlers are also used internally by the configuration framework. If you look at the script level source code of the config framework, you can see that change handlers are used for logging the option changes to config.log.

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