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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 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 the DHCPv4 and DHCPv6 servers
    include support for a mechanism that allows
    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 in Kea 1.2.0 is limited,
    the number is expected to grow over time.</para>

    <para>The DHCPv4 and DHCPv6 servers receive commands over the
    unix domain sockets. The details how to configure these sockets,
    see <xref linkend="dhcp4-ctrl-channel" /> and <xref
    linkend="dhcp6-ctrl-channel" />. While it is possible control
    the servers directly using unix domain sockets it requires that
    the controlling client be running on the same machine as
    the server. In order to connect remotely SSH is usually used to
    connect to the controlled machine.</para>

    <para>The network administrators usually prefer using some form
    of a RESTful API to control the servers, rather than using unix
    domain sockets directly. Therefore, as of Kea 1.2.0 release,
    Kea includes a new component called Control Agent (or CA) which
    exposes a RESTful API to the controlling clients and can forward
    commands to the respective Kea services over the unix domain
    sockets. The CA configuration has been described in
    <xref linkend="agent-configuration"/>.</para>

    <para>The HTTP requests received by the CA contain the control
    commands encapsulated within HTTP requests. Simply speaking,
    the CA is responsible for stripping the HTTP layer from the
    received commands and forwarding the commands in a JSON format
    over the unix domain sockets to respective services. Because the
    CA receives commands for all services it requires additional
    "forwarding" information to be included in the client's messages.
    This "forwarding" information is carried within the
    <command>service</command> parameter of the received command.
    If the <command>service</command> parameter is not included or if
    the parameter is a blank list the CA will assume that the control
    command is targetted at the CA itself and will try to handle
    it on its own.
    </para>

    <para>Control connections over both HTTP and unix domain sockets are
    guarded with timeouts. The default timeout value is set to 10s
    and is not configurable. The timeout configuration will be
    implemented in the future.
    </para>

    <note>
      <simpara>Kea 1.2.0 release and earlier had a limitation of 64kB
      on the maximum size of a command and a response sent over the unix
      domain socket. This limitation has been removed in Kea 1.3.0
      release.
      </simpara>
    </note>

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

<screen>
{
    "command": "foo",
    "service": [ "dhcp4" ]
    "arguments": {
        "param1": "value1",
        "param2": "value2",
        ...
    }
}
</screen>

    The same command sent over the RESTful interface to the CA will have
    the following structure.

<screen>
    POST / HTTP/1.1\r\n
    Content-Type: application/json\r\n
    Content-Length: 147\r\n\r\n
    {
        "command": "foo",
        "service": [ "dhcp4" ]
        "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 of the map is command specific.</para>

    <para>
      <command>service</command> is a list of the servers at which the control
      command is targetted. In the example above, the control command is
      targetted at the DHCPv4 server. In most cases, the CA will simply forward this
      command to the DHCPv4 server for processing via unix domain socket.
      Sometimes, the command including a service value may also be processed by the
      CA, if the CA is running a hooks library which handles such command for the
      given server. As an example, the hooks library attached to the CA
      may perform some operations on the database (like adding host reservations,
      modifying leases etc.). An advantage of performing DHCPv4 specific
      administrative operations in the CA rather than forwarding it to
      the DHCPv4 server is the ability to perform these operations without
      disrupting the DHCPv4 service (DHCPv4 server doesn't have to stop
      processing DHCP messages to apply changes to the database). Nevertheless,
      these situations are rather rare and, in most cases, when the
      <command>service</command> parameter contains a name of the service
      the commands are simply forwarded by the CA. The forwarded command
      includes the <command>service</command> parameter but this parameter
      is ignored by the receiving server. This parameter is only meaningful
      to the CA.
    </para>

    <para>
      If the command received by the CA does not include a <command>service</command>
      parameter or this list is empty, the CA will simply process this message
      on its own. For example, the <command>config-get</command> command which
      doesn't include service parameter will return Control Agent's own
      configuration. The <command>config-get</command> including a service
      value "dhcp4" will be forwarded to the DHCPv4 server and will return
      DHCPv4 server's configuration and so on.
    </para>

    <para>
      The following list contains a mapping of the values carried within the
      <command>service</command> parameter to the servers to which the commands
      are forwarded:

      <itemizedlist>
        <listitem>
          <simpara><command>dhcp4</command> - the command is forwarded to the
          <command>kea-dhcp4</command> server,</simpara>
        </listitem>

        <listitem>
          <simpara><command>dhcp6</command> - the command is forwarded to the
          <command>kea-dhcp6</command> server,</simpara>
        </listitem>

        <listitem>
          <simpara><command>d2</command> - the command is forwarded to the
          <command>kea-d2</command> server.</simpara>
        </listitem>
      </itemizedlist>
    </para>

    <para>The server processing the incoming command will send a response of
    the form:
<screen>
{
    "result": 0|1|2|3,
    "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 or at least a
    failure to complete the requested action. Currently 1 is used as a generic
    error, 2 means that a command is not supported and 3 means that the
    requested operation was completed, but the requested object was not
    found. Additional error codes may be added in the future. For example a well
    formed command that requests a subnet that exists in server's configuration
    would return result 0. If the server encounters an error condition, it would
    return 1. If the command was asking for IPv6 subnet, but was sent to DHCPv4
    server, it would return 2. If the query was asking for a subnet-id and there
    is no subnet with such id, the result would be set to 3.</para>

    <para>
    The <command>text</command> field typically appears when result is non-zero
    and contains a description of the error encountered, but it often also appears
    for successful outcomes. The exact text is command specific, but in general
    uses plain English to describe the outcome of the command.
    <command>arguments</command> is a map of additional data values
    returned by the server which is specific to the command issued. The map is
    may be present, but that depends on specific command.
    </para>

    <note>
      <simpara>
        When sending commands via Control Agent, it is possible to specify
        multiple services at which the command is targetted. CA will forward this
        command to each service individually. Thus, the CA response to the
        controlling client will contain an array of individual responses.
      </simpara>
    </note>

    </section>

    <section id="ctrl-channel-client">
    <title>Using the Control Channel</title>

    <para>Kea development team is actively working on providing client applications
    which can be used to control the servers. These applications are, however, in the
    early stages of development and as of Kea 1.2.0 release have certain limitations.
    The easiest way to start interacting with the control API is to use common Unix/Linux tools
    such as <command>socat</command> and <command>curl</command>.</para>

    <para>In order to control the given Kea service via unix domain socket, use
    <command>socat</command> as follows:
<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. Text passed to <command>socat</command>
will be sent to Kea and the responses received from Kea printed to standard
    output. This approach communicates with the specific server directly and
    bypasses Control Agent.</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>

    <para>In order to use Kea's RESTful API with <command>curl</command> you may
    use the following:
<screen>
$ curl -X POST -H "Content-Type: application/json" -d '{ "command": "config-get", "service": [ "dhcp4" ] }' http://ca.example.org:8000/
</screen>

    This assumes that the Control Agent is running on host
    <filename>ca.example.org</filename> and runs RESTful service on port 8000.
    </para>

    </section>

    <section id="commands-common">
      <title>Commands Supported by Both the DHCPv4 and DHCPv6 Servers</title>

      <section id="command-build-report">
        <title>build-report</title>
        <para>
          The <emphasis>build-report</emphasis> command returns
          on the control channel what the command line
          <command>-W</command> argument displays, i.e. the embedded
          content of the <filename>config.report</filename> file.
          This command does not take any parameters.
        </para>
<screen>
{
    "command": "build-report"
}
</screen>
      </section> <!-- end of command-build-report -->

      <section id="command-config-get">
        <title>config-get</title>

        <para>The <emphasis>config-get</emphasis> command retrieves the current
        configuration used by the server. This command does not take any
        parameters. The configuration returned is roughly equal to the
        configuration that was loaded using -c command line option during server
        start-up or later set using config-set command. However, there may be
        certain differences. Comments are not retained. If the original
        configuration used file inclusion, the returned configuration will
        include all parameters from all the included files.</para>

        <para> Note that returned configuration is not redacted, i.e. it will
        contain database passwords in plain text if those were specified in the
        original configuration. Care should be taken to not expose the command
        channel to unprivileged users.</para>

        <para>
          An example command invocation looks like this:
<screen>
{
    "command": "config-get"
}
</screen>
        </para>
      </section> <!-- end of command-config-get -->

      <section id="command-config-reload">
        <title>config-reload</title>

        <para>The <emphasis>config-reload</emphasis> command instructs
        Kea to load again the configuration file that was used
        previously. This operation is useful if the configuration file
        has been changed by some external sources. For example, a
        sysadmin can tweak the configuration file and use this command
        to force Kea pick up the changes.</para>

        <para>Caution should be taken when mixing this with config-set
        commands. Kea remembers the location of the configuration file
        it was started with. This configuration can be significantly
        changed using config-set command. When config-reload is issued
        after config-set, Kea will attempt to reload its original
        configuration from the file, possibly losing all changes
        introduced using config-set or other commands.</para>

        <para><emphasis>config-reload</emphasis> does not take any parameters.
        An example command invocation looks like this:
<screen>
{
    "command": "config-reload"
}
</screen>
        </para>
      </section> <!-- end of command-config-reload -->


      <section id="command-config-test">
        <title>config-test</title>

      <para>
     The <emphasis>config-test</emphasis> command instructs the server to check
     whether the new configuration supplied in the command's arguments can
     be loaded. The supplied configuration is expected to be the full
     configuration for the target server along with an optional Logger
     configuration. As for the <command>-t</command> command some sanity checks
     are not performed so it is possible a configuration which successfully
     passes this command will still fail in <command>config-set</command>
     command or at launch time.
     The structure of the command is as follows:
      </para>
<screen>
{
    "command": "config-test",
    "arguments":  {
        "&#60;server&#62;": {
        },
        "Logging": {
        }
     }
}
</screen>
      <para>
    where &#60;server&#62; is the configuration element name for a given server
    such as "Dhcp4" or "Dhcp6".  For example:
      </para>
<screen>
{
    "command": "config-test",
    "arguments":  {
        "Dhcp6": {
            :
        },
        "Logging": {
            :
        }
     }
}
</screen>
      <para>
        The server's response will contain a numeric code, "result" (0 for success,
    non-zero on failure), and  a string, "text", describing the outcome:
<screen>
    {"result": 0, "text": "Configuration seems sane..." }

    or

    {"result": 1, "text": "unsupported parameter: BOGUS (&#60;string&#62;:16:26)" }
</screen>
      </para>
    </section> <!-- end of command-config-test -->

      <section id="command-config-write">
        <title>config-write</title>

        <para>The <emphasis>config-write</emphasis> command instructs Kea server
        to write its current configuration to a file on disk. It takes one
        optional argument called <emphasis>filename</emphasis> that specifies
        the name of the file to write configuration to. If not specified, the
        name used when starting Kea (passed as -c argument) will be
        used. If relative path is specified, Kea will write its files
        only in the directory it is running.</para>
        <para>
          An example command invocation looks like this:
<screen>
{
    "command": "config-write",
    "arguments": {
        "filename": "config-modified-2017-03-15.json"
    }
}
</screen>
        </para>
      </section> <!-- end of command-config-write -->

      <section id="command-leases-reclaim">
        <title>leases-reclaim</title>
        <para>
          The <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-libreload">
        <title>libreload</title>

        <para>
          The <emphasis>libreload</emphasis> command will first unload and then
          load all currently loaded hook libraries.  This is primarily intended
          to allow one or more hook libraries to be replaced with newer versions
          without requiring Kea servers to be reconfigured or restarted.  Note
          the hook libraries will be passed the same parameter values (if any)
          they were passed when originally loaded.
<screen>
{
    "command": "libreload",
    "arguments": { }
}
</screen>
       </para>
       <para>
         The server will respond with a result of 0 indicating success, or 1
         indicating a failure.
       </para>
     </section> <!-- end of command-libreload -->

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

      <para>
        The <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-config-set">
      <title>config-set</title>

      <para>
    The <emphasis>config-set</emphasis> command instructs the server to replace
    its current configuration with the new configuration supplied in the
    command's arguments. The supplied configuration is expected to be the full
    configuration for the target server along with an optional Logger
    configuration.  While optional, the Logger configuration is highly
    recommended as without it the server will revert to its default logging
    configuration. The structure of the command is as follows:
      </para>
<screen>
{
    "command": "config-set",
    "arguments":  {
        "&#60;server&#62;": {
        },
        "Logging": {
        }
     }
}
</screen>
      <para>
    where &#60;server&#62; is the configuration element name for a given server
    such as "Dhcp4" or "Dhcp6".  For example:
      </para>
<screen>
{
    "command": "config-set",
    "arguments":  {
        "Dhcp6": {
            :
        },
        "Logging": {
            :
        }
     }
}
</screen>
      <para>
    If the new configuration proves to be invalid the server will retain
    its current configuration.  Please note that the new configuration is
    retained in memory only.  If the server is restarted or a configuration
    reload is triggered via a signal, the server will use the configuration
    stored in its configuration file.

        The server's response will contain a numeric code, "result" (0 for success,
    non-zero on failure), and  a string, "text", describing the outcome:
<screen>
    {"result": 0, "text": "Configuration successful." }

    or

    {"result": 1, "text": "unsupported parameter: BOGUS (&#60;string&#62;:16:26)" }
</screen>
      </para>
    </section> <!-- end of command-config-set -->

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

      <para>
        The <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"
}
</screen>
      </para>
      <para>
        The server will respond with a confirmation that the shutdown procedure
        has been initiated.
      </para>
    </section> <!-- end of command-shutdown -->

      <section id="command-version-get">
        <title>version-get</title>
        <para>
          The <emphasis>version-get</emphasis> command returns on the control
          channel what the command line <command>-v</command> argument
          displays with in arguments the extended version, i.e., what
          the command line <command>-V</command> argument displays. This command
          does not take any parameters.
        </para>
<screen>
{
    "command": "version-get"
}
</screen>
      </section> <!-- end of command-version-get -->

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

    <section id="agent-commands">
      <title>Commands Supported by Control Agent</title>
      <para>The following commands listed in <xref linkend="commands-common"/>
      are also supported by the Control Agent, i.e. when the
      <command>service</command> parameter is blank the commands are handled
      by the CA and they relate to the CA process itself:
      <itemizedlist>
        <listitem>
          <simpara>build-report</simpara>
        </listitem>
        <listitem>
          <simpara>config-get</simpara>
        </listitem>
        <listitem>
          <simpara>config-test</simpara>
        </listitem>
        <listitem>
          <simpara>config-write</simpara>
        </listitem>
        <listitem>
          <simpara>list-commands</simpara>
        </listitem>
        <listitem>
          <simpara>shutdown</simpara>
        </listitem>
        <listitem>
          <simpara>version-get</simpara>
        </listitem>
      </itemizedlist>
      </para>
    </section>

  </chapter>
