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  <chapter id="ctrl-channel">
    <title>Management API</title>

    <para>A classic approach to daemon configuration assumes that
    the server's configuration is stored in the configuration files
    and when the configuration is changed, the daemon is restarted.
    This approach has the significant disadvantage of introducing periods
    of downtime, when client traffic is not handled. Another risk
    is that if the new configuration is invalid for whatever reason,
    the server may refuse to start, which will further extend the
    downtime period, until the issue is resolved.</para>

    <para>To avoid such problems, both DHCPv4 and DHCPv6 servers
    introduced support for a mechanism that will allow
    on-line reconfiguration, without requiring server shutdown.
    Both servers can be instructed to open control sockets, which
    is a communication channel. The server is able to receive
    commands on that channel, act on them and report back status.
    While the set of commands supported in Kea 0.9.2 is limited,
    the number is expected to grow over time.</para>

    <para>Currently the only supported type of control channel
    is UNIX stream socket. For details how to configure it, see
    <xref linkend="dhcp4-ctrl-channel" /> and <xref
    linkend="dhcp6-ctrl-channel" />. It is likely that support
    for other control channel types will be added in the future.
    </para>

    <section id="ctrl-channel-syntax">
    <title>Data syntax</title>
    <para>Communication over control channel is conducted using JSON
    structures. If configured, Kea will open a socket and will listen
    for any incoming connections. A process connecting to this socket
    is expected to send JSON commands structured as follows:

<screen>
{
    "command": "foo",
    "arguments": {
	"param1": "value1",
	"param2": "value2",
	...
    }
}
</screen>

    <command>command</command> is the name of command to execute and
    is mandatory. <command>arguments</command> is a map of parameters
    required to carry out the given command.  The exact content and
    format is command specific.</para>

    <para>The server will process the incoming command and then send a
    response of the form:
<screen>
{
    "result": 0|1,
    "text": "textual description",
    "arguments": {
	"argument1": "value1",
	"argument2": "value2",
	...
    }
}
</screen>
    <command>result</command> indicates the outcome of the command. A value of 0
    means success while any non-zero value designates an error. Currently 1 is
    used as a generic error, but additional error codes may be added in the
    future. <command>text</command> field typically appears when result is
    non-zero and contains a description of the error encountered, but it may
    also appear for successful results. That's command specific.
    <command>arguments</command> is a map of additional data values returned by
    the server, specific to the command issued. The map is always present, even
    if it contains no data values.</para>
    </section>

    <section id="ctrl-channel-client">
    <title>Using control channel</title>

    <para>ISC does not provide a client for using control channel.  The primary
    reason for that is the expectation is that the entity using control channel
    is typically an IPAM or similar network management/monitoring software which
    may have quite varied expectations regarding the client and is even likely to
    be written in languages different than C or C++. Therefore we only provide
    examples how one can take advantage of the API.</para>

    <para>The easiest way is to use a tool called <command>socat</command>,
    a tool available from <ulink url="http://www.dest-unreach.org/socat/">socat
    homepage</ulink>, but it is also widely available in Linux and BSD
    distributions. Once Kea is started, one could connect to the control
    interface using the following command:
<screen>
$ socat UNIX:/path/to/the/kea/socket -
</screen>
where <command>/path/to/the/kea/socket</command> is the path specified in the
<command>Dhcp4/control-socket/socket-name</command> parameter in the Kea
configuration file.</para>

    <para>It is also easy to open UNIX socket programmatically. An example of
    such a simplistic client written in C is available in the Kea Developer's
    Guide, chapter Control Channel Overview, section Using Control Channel.</para>

    </section>

    <section id="commands-common">
      <title>Commands supported by both DHCPv4 and DHCPv6 servers</title>

      <section id="command-leases-reclaim">
        <title>leases-reclaim command</title>
        <para>
          <emphasis>leases-reclaim</emphasis> command instructs the server to
          reclaim all expired leases immediately. The command has the following
          JSON syntax:
<screen>
{
    "command": "leases-reclaim",
    "arguments": {
        "remove": true
    }
}
</screen>
        </para>

        <para>The <emphasis>remove</emphasis> boolean parameter is mandatory
        and it indicates whether the reclaimed leases should be removed from
        the lease database (if true), or they should be left in the
        <emphasis>expired-reclaimed</emphasis> state (if false). The latter
        facilitates lease affinity, i.e. ability to re-assign expired lease to
        the same client which used this lease before. See
        <xref linkend="lease-affinity"/> for the details. Also, see
        <xref linkend="lease-reclamation"/> for the general information
        about the processing of expired leases (leases reclamation).</para>
      </section>

      <section id="command-list-commands">
      <title>list-commands command</title>

      <para>
	<emphasis>list-commands</emphasis> command retrieves a list of all
	commands supported by the server. It does not take any arguments.
	An example command may look like this:
<screen>
{
    "command": "list-commands",
    "arguments": { }
}
</screen>
      </para>
      <para>
	The server will respond with a list of all supported commands. The
	arguments element will be a list of strings. Each string will convey
	one supported command.
      </para>
    </section> <!-- end of command-list-commands -->

    <section id="command-shutdown">
      <title>shutdown command</title>

      <para>
	<emphasis>shutdown</emphasis> command instructs the server to initiate
	its shutdown procedure. It is the equivalent of sending a SIGTERM signal
	to the process. This command does not take any arguments.  An example
	command may look like this:
<screen>
{
    "command": "shutdown",
    "arguments": { }
}
</screen>
      </para>
      <para>
	The server will respond with a confirmation that the shutdown procedure
	has been initiated.
      </para>
    </section> <!-- end of command-shutdown -->

    </section> <!-- end of commands supported by both servers -->

  </chapter>
