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<chapter id="kea-ctrl-agent">
  <title>Kea Control Agent</title>

  <section id="agent-overview">
    <title>Overview</title>
    <para>Kea Control Agent (CA) is a daemon, first included in Kea 1.2, which
    exposes a RESTful control interface for managing Kea servers. The daemon
    can receive control commands over HTTP and either forward these commands
    to the respective Kea servers or handle these commands on its own. The
    determination whether the command should be handled by the CA or forwarded
    is made by checking the value of the 'service' parameter which may be
    included in the command from the controlling client. The details of the
    supported commands as well as their structures are provided in
    <xref linkend="ctrl-channel"/>.</para>
    <para>Hook libraries can be attached to the CA to provide support for
    additional commands or custom behavior of existing commands. Such hook
    libraries must implement callouts for 'control_command_receive' hook point.
    Details about creating new hook libraries and supported hook points can be
    found in
    <ulink url="https://jenkins.isc.org/job/Kea_doc/doxygen/">Kea Developer's Guide</ulink>.
    </para>

    <para>
      The CA processes received commands according to the following algorithm:
      <itemizedlist>
        <listitem>
          <simpara>
            Pass command into any installed hooks (regardless of service value(s)).
            If the command is handled by a hook, return the response.
          </simpara>
        </listitem>

        <listitem>
          <simpara>
            If the service specifies one more or services, the CA will forward the
            command to specified services and return the accumulated responses.
          </simpara>
        </listitem>

        <listitem>
          <simpara>
            If service is not specified or is an empty list, the CA will handle
            the command if it supports it.
          </simpara>
        </listitem>

      </itemizedlist>
    </para>
  </section>

  <section id="agent-configuration">
    <title>Configuration</title>
    <para>The following example demonstrates the basic CA configuration.</para>
    <para>
<screen>
{
    "Control-agent": {
        "http-host": "10.20.30.40",
        "http-port": 8080,

        "control-sockets": {
            "dhcp4": {
                "socket-type": "unix",
                "socket-name": "/path/to/the/unix/socket-v4"
            },
            "dhcp6": {
                "socket-type": "unix",
                "socket-name": "/path/to/the/unix/socket-v4"
            }
        },

        "hooks-libraries": [
        {
            "library": "/opt/local/control-agent-commands.so",
            "parameters": {
                "param1": "foo"
            }
        } ]
    },

    "Logging": {
        "loggers": [ {
            "name": "kea-ctrl-agent",
            "severity": "INFO"
        } ]
    }
}</screen>
    </para>

    <para>
      The <command>http-host</command> and <command>http-port</command>
      specify an IP address and port to which HTTP service will be bound.
      In case of the example configuration provided above, the RESTful
      service will be available under the URL of
      <command>http://10.20.30.40:8080/</command>. If these parameters
      are not specified, the default URL is http://127.0.0.1:8000/
    </para>

    <para>
      It has been mentioned in the <xref linkend="agent-overview"/> that
      CA can forward received commands to the specific Kea servers for
      processing. For example, <command>config-get</command> is sent to
      retrieve configuration of one of the Kea services. When CA receives
      this command, including a <command>service</command> parameter
      indicating that the client desires to retrieve configuration of
      the DHCPv4 server, the CA will forward this command to this server
      and then pass the received response back to the client. More about
      the <command>service</command> parameter and general structure of
      the commands can be found in <xref linkend="ctrl-channel"/>.
    </para>

    <para>
      The CA uses unix domain sockets to forward control commands and receive
      responses from other Kea services. The <command>dhcp4</command>,
      <command>dhcp6</command> and <command>d2</command> maps
      specify the files to which unix domain sockets are bound. In case
      of the configuration above, the CA will connect to the DHCPv4 server
      via <filename>/path/to/the/unix/socket-v4</filename> to forward the
      commands to it. Obviously, the DHCPv4 server must be configured to
      listen to connections via this same socket. In other words, the command
      socket configuration for the DHCPv4 server and CA (for this server)
      must match. Consult the <xref linkend="dhcp4-ctrl-channel"/> and the
      <xref linkend="dhcp6-ctrl-channel"/> to learn how the socket
      configuration is specified for the DHCPv4 and DHCPv6 services.
    </para>

    <warning>
      <simpara>
        We have renamed "dhcp4-server", "dhcp6-server" and "d2-server"
        to "dhcp4", "dhcp6" and "d2" respectively after release of Kea 1.2.
        If you are migrating from Kea 1.2 you need to tweak your CA config
        to use this new naming convention. We have made this incompatible
        change to facilitate future use cases where it will be possible to
        specify additional values of the "service" parameter to point to
        the particular instances of the Kea servers, e.g. "dhcp4/3"
        pointing to the 3rd instance of the DHCPv4 server in the
        multi-processed configuration. This is not yet supported but the
        current renaming lays the ground for it.
      </simpara>
    </warning>

    <para>
      Hooks libraries can be attached to the Control Agent just like to
      DHCPv4 and DHCPv6 servers. It currently supports one hook point
      'control_command_receive' which makes it possible to delegate
      processing of some commands to the hooks library. The
      <command>hooks-libraries</command> list contains the list of hooks
      libraries that should be loaded by the CA, along with their configuration
      information specified with <command>parameters</command>.
    </para>

    <para>
      Please consult <xref linkend="logging"/> for the details how to
      configure logging. The CA's root logger's name is
      <command>kea-ctrl-agent</command> as given in the example above.
    </para>
  </section>

  <section id="agent-secure-connection">
    <title>Secure Connections</title>
    <para>
      Control Agent doesn't natively support secure HTTP connections like
      SSL or TLS. In order to setup secure connection please use one
      of the available third party HTTP servers and configure it to run
      as a reverse proxy to the Control Agent. Kea has been tested with
      two major HTTP server implentations working as a reverse proxy:
      Apache2 and nginx. Example configurations including extensive
      comments are provided in the <filename>doc/examples/https/</filename>
      directory.
    </para>

    <para>
      The reverse proxy forwards HTTP requests received over secure
      connection to the Control Agent using (not secured) HTTP. Typically,
      the reverse proxy and the Control Agent are running on the same machine,
      but it is possible to configure them to run on separate machines as
      well. In this case, security depends on the protection of the
      communications between the reverse proxy and the Control Agent.
    </para>

    <para>Apart from providing the encryption layer for the control channel,
    a reverse proxy server is also often used for authentication of the
    controlling clients. In this case, the client must present a valid
    certificate when it connects via reverse proxy. The proxy server
    authenticates the client by checking if the presented certificate is
    signed by the certificate authority used by the server.</para>

    <para>To illustrate this, we provide a sample configuration for the
    nginx server running as a reverse proxy to the Kea Control Agent.
    The server enables authentication of the clients using
    certificates.</para>

<screen>
#   The server certificate and key can be generated as follows:
#
#   openssl genrsa -des3 -out kea-proxy.key 4096
#   openssl req -new -x509 -days 365 -key kea-proxy.key -out kea-proxy.crt
#
#   The CA certificate and key can be generated as follows:
#
#   openssl genrsa -des3 -out ca.key 4096
#   openssl req -new -x509 -days 365 -key ca.key -out ca.crt
#
#
#   The client certificate needs to be generated and signed:
#
#   openssl genrsa -des3 -out kea-client.key 4096
#   openssl req -new -key kea-client.key -out kea-client.csr
#   openssl x509 -req -days 365 -in kea-client.csr -CA ca.crt \
#           -CAkey ca.key -set_serial 01 -out kea-client.crt
#
#   Note that the 'common name' value used when generating the client
#   and the server certificates must differ from the value used
#   for the CA certificate.
#
#   The client certificate must be deployed on the client system.
#   In order to test the proxy configuration with 'curl' run
#   command similar to the following:
#
#   curl -k --key kea-client.key --cert kea-client.crt -X POST \
#        -H Content-Type:application/json -d '{ "command": "list-commands" }' \
#         https://kea.example.org/kea
#
#
#
#   nginx configuration starts here.

events {
}

http {
        #   HTTPS server
    server {
        #     Use default HTTPS port.
        listen 443 ssl;
        #     Set server name.
        server_name kea.example.org;

        #   Server certificate and key.
        ssl_certificate /path/to/kea-proxy.crt;
        ssl_certificate_key /path/to/kea-proxy.key;

        #   Certificate Authority. Client certificate must be signed by the CA.
        ssl_client_certificate /path/to/ca.crt;

        # Enable verification of the client certificate.
        ssl_verify_client on;

        # For URLs such as https://kea.example.org/kea, forward the
        # requests to http://127.0.0.1:8080.
        location /kea {
            proxy_pass http://127.0.0.1:8080;
        }
    }
}
</screen>

  <note>
    <simpara>Note that the configuration snippet provided above is for testing
    purposes only. Consult security policies and best practices of your
    organization which apply to this setup.</simpara>
  </note>

    <para>When you use an HTTP client without TLS support as <command>
    kea-shell</command> you can use an HTTP/HTTPS translator such as stunnel
    in client mode. A sample configuration is provided in the
    <filename>doc/examples/https/shell/</filename> directory</para>

  </section>

  <section id="agent-limitations">
    <title>Control Agent Limitations</title>
    <para>
      Control Agent is a new component, first released in Kea 1.2. In
      this release it comes with one notable limitation:
      <itemizedlist>
        <listitem>
          <simpara>
            keactrl hasn't been updated to manage the Control Agent (start, stop
            reload). As a result, the CA must be started directly as described in
            <xref linkend="agent-launch"/>
          </simpara>
        </listitem>
      </itemizedlist>
    </para>
  </section>

  <section id="agent-launch">
    <title>Starting Control Agent</title>
    <para>
      The CA is started by running its binary and specifying the configuration file
      it should use. For example:
<screen>
$ ./kea-ctrl-agent -c /usr/local/etc/kea/kea-ctrl-agent.conf
</screen>
    </para>
  </section>

  <section id="agent-clients">
    <title>Connecting to the Control Agent</title>
    <para>For an example of tool that can take advantage of the
    RESTful API, see <xref linkend="kea-shell"/>.</para>
  </section>
</chapter>
