Commit ae405fc3 authored by Stephen Morris's avatar Stephen Morris
Browse files

[3635] Updated details of how to configure hooks libraries

parent abbe8b20
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd" [
<!ENTITY mdash "&#x2014;" >
]>
<chapter id="hooks-libraries">
<title>Hooks Libraries</title>
<section id="hooks-libraries-introduction">
<title>Introduction</title>
<para>
Although Kea offers a lot of flexibility, there may be cases where
its behavior needs customisation. To accommodate this possibility,
Kea includes the idea of "Hooks". This feature lets Kea load one
or more dynamically-linked libraries (known as "hooks libraries")
and, at various points in its processing ("hook points"), call
functions in them. Those functions perform whatever custom
processing is required.
</para>
<para>
Hooks libraries are attached to individual Kea processes, not to
Kea as a whole. This means (for example) that it is possible
to associate one set of libraries with the DHCP4 server and a
different set to the DHCP6 server.
</para>
<para>
Another point to note is that it is possible for a process to
load multiple libraries. At every hook point, Kea calls all
hooks library functions attached it. If multiple libraries have
attached a function to a given hook point, Kea calls all of them.
The order in which the functions are called is the order in which
the libraries are specified in the configuration file. This may
be important: consult the documentation of the libraries to see
if this is the case.
</para>
<para>
The next section describes how to configure hooks libraries. If you
are interested in writing your own hooks library, information can be
found in the <ulink url="http://git.kea.isc.org/~tester/kea/doxygen">Kea
Developer's Guide</ulink>.
</para>
</section> <!-- end Introduction -->
<section>
<title>Configuring Hooks Libraries</title>
<para>
The hooks libraries for a given process are configured using the
<command>hooks-libraries</command> keyword in the
configuration for that process. (Note that
the word "hooks" is plural). The value of the keyword
is an array of strings, each string corresponding to a hooks library.
For example, to set up two hooks libraries for the DHCPv4 server, the
configuration would be:
<screen>
<userinput>"Dhcp4": {
:
"hooks-libraries": [
"/opt/charging.so",
"/opt/local/notification.so"
]
:
}</userinput>
</screen>
</para>
<note><para>
At present, the libraries are specified as a simple list. A future
version of Kea will support the capability of specifying a set of
parameters for each library. When that is added, it is likely
that the syntax for specifying hooks libraries will change.
</para></note>
<para>
Notes:
<itemizedlist mark='bullet'>
<listitem><para>
The full path to each library should be given.
</para></listitem>
<listitem><para>
As noted above, order may be important - consult the documentation for
each library.
</para></listitem>
<listitem><para>
An empty list has the same effect as omitting the
<command>hooks-libraries</command> configuration element all together.
</para></listitem>
</itemizedlist>
</para>
<para>
At the present time, only the kea-dhcp4 and kea-dhcp6 processes support
hooks libraries.
</para>
</section>
</chapter> <!-- hooks-libraries -->
......@@ -67,6 +67,8 @@
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="ddns.xml" />
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="hooks.xml" />
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="libdhcp.xml" />
<xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="logging.xml" />
......
// Copyright (C) 2013 Internet Systems Consortium, Inc. ("ISC")
// Copyright (C) 2013-2014 Internet Systems Consortium, Inc. ("ISC")
//
// Permission to use, copy, modify, and/or distribute this software for any
// purpose with or without fee is hereby granted, provided that the above
......@@ -412,7 +412,7 @@ usage is intuitive:
handle.setSkip(true);
}
return;
@endcode
Like arguments, the "skip" flag is passed to all callouts on a hook. Callouts
......@@ -607,21 +607,25 @@ which they are attached (e.g. hook code for DNS will need to link against
the libkea-dns++ library). Depending on operating system, you may also need
to explicitly list libraries on which the Kea libraries depend.
@subsection hooksdgConfiguration Configuring the Hook Library
@subsection hooksdgConfiguration Configuring the Hooks Library
The final step is to make the library known to Kea. All Kea modules to
which hooks can be added contain the "hook_library" element, and user
libraries are added to this. (The Kea hooks system can handle multiple libraries
- this is discussed below.).
The final step is to make the library known to Kea. The configuration
keywords of all Kea modules to which hooks can be added contain the
"hooks-libraries" element and user libraries are added to this. (The Kea
hooks system can handle multiple libraries - this is discussed below.)
To add the example library (assumed to be in /usr/local/lib) to the DHCPv4
module, the following bindctl commands must be executed:
To add the example library (assumed to be in /usr/local/lib) to the
DHCPv4 module, it must be listed in the "hooks-libraries" element of the
"Dhcp4" part of the configuration file:
@code
> config add Dhcp4/hook_libraries
> config set Dhcp4/hook_libraries[0] "/usr/local/lib/example.so"
> config commit
"Dhcp4": {
:
"hooks-libraries": [ "/usr/local/lib/example.so" ]
:
}
@endcode
(Note that "hooks" is plural.)
The DHCPv4 server will load the library and execute the callouts each time a
request is received.
......@@ -841,7 +845,7 @@ ones with non-standard names need to be registered manually.
@subsubsection hooksdgMultipleCallouts Multiple Callouts on a Hook
The Kea hooks framework allows multiple callouts to be attached to
The Kea hooks framework allows multiple callouts to be attached to
a hook point. Although it is likely to be rare for user code to need to
do this, there may be instances where it make sense.
......@@ -1003,6 +1007,17 @@ without worrying about the presence of other libraries. Other libraries
may be present, but will not affect the context values set by a library's
callouts.
Configuring multiple libraries just requires listing the libraries
as separate elements of the hooks-libraries configuration element, e.g.
@code
"Dhcp4": {
:
"hooks-libraries": [ "/usr/lib/library1.so", "/opt/library2.so" ]
:
}
@endcode
@subsection hooksdgInterLibraryData Passing Data Between Libraries
In rare cases, it is possible that one library may want to pass
......
Markdown is supported
0% or .
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment