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   Copyright (C) Internet Systems Consortium, Inc. ("ISC")
   
   This Source Code Form is subject to the terms of the Mozilla Public
   License, v. 2.0. If a copy of the MPL was not distributed with this
   file, you can obtain one at https://mozilla.org/MPL/2.0/.
   
   See the COPYRIGHT file distributed with this work for additional
   information regarding copyright ownership.

.. _module-info:

Plugins
-------

Plugins are a mechanism to extend the functionality of ``named`` using
dynamically loadable libraries. By using plugins, core server
functionality can be kept simple for the majority of users; more complex
code implementing optional features need only be installed by users that
need those features.

The plugin interface is a work in progress, and is expected to evolve as
more plugins are added. Currently, only "query plugins" are supported;
these modify the name server query logic. Other plugin types may be
added in the future.

The only plugin currently included in BIND is ``filter-aaaa.so``, which
replaces the ``filter-aaaa`` feature that previously existed natively as
part of ``named``. The code for this feature has been removed from
``named`` and can no longer be configured using standard ``named.conf``
syntax, but linking in the ``filter-aaaa.so`` plugin provides identical
functionality.

Configuring Plugins
~~~~~~~~~~~~~~~~~~~

A plugin is configured with the ``plugin`` statement in ``named.conf``:

::

       plugin query "library.so" {
           parameters
       };


In this example, file ``library.so`` is the plugin library. ``query``
indicates that this is a query plugin.

Multiple ``plugin`` statements can be specified, to load different
plugins or multiple instances of the same plugin.

``parameters`` are passed as an opaque string to the plugin's initialization
routine. Configuration syntax differs depending on the module.

Developing Plugins
~~~~~~~~~~~~~~~~~~

Each plugin implements four functions:

-  ``plugin_register``
   to allocate memory, configure a plugin instance, and attach to hook
   points within
   ``named``
   ,
-  ``plugin_destroy``
   to tear down the plugin instance and free memory,
-  ``plugin_version``
   to check that the plugin is compatible with the current version of
   the plugin API,
-  ``plugin_check``
   to test syntactic correctness of the plugin parameters.

At various locations within the ``named`` source code, there are "hook
points" at which a plugin may register itself. When a hook point is
reached while ``named`` is running, it is checked to see whether any
plugins have registered themselves there; if so, the associated "hook
action" - a function within the plugin library - is called. Hook
actions may examine the runtime state and make changes: for example,
modifying the answers to be sent back to a client or forcing a query to
be aborted. More details can be found in the file
``lib/ns/include/ns/hooks.h``.
