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 withinnamed
,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
.