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<?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="dhcp6">
    <title>The DHCPv6 Server</title>

    <section id="dhcp6-start-stop">
      <title>Starting and Stopping the DHCPv6 Server</title>

      <para>
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        It is recommended that the Kea DHCPv6 server be started and stopped
        using <command>keactrl</command> (described in <xref linkend="keactrl"/>).
        However, it is also possible to run the server directly: it accepts
        the following command-line switches:
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      </para>

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      <itemizedlist>
          <listitem>
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            <simpara>
            <command>-c <replaceable>file</replaceable></command> -
            specifies the configuration file. This is the only mandatory
            switch.</simpara>
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          </listitem>
          <listitem>
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            <simpara>
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            <command>-d</command> - specifies whether the server
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            logging should be switched to verbose mode. In verbose mode,
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            the logging severity and debuglevel specified in the configuration
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            file are ignored and "debug" severity and the maximum debuglevel
            (99) are assumed. The flag is convenient, for temporarily
            switching the server into maximum verbosity, e.g. when
            debugging.</simpara>
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          </listitem>
          <listitem>
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            <simpara>
            <command>-p <replaceable>port</replaceable></command> -
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            specifies UDP port on which the server will listen. This is only
            useful during testing, as a DHCPv6 server listening on
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            ports other than the standard ones will not be able to
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            handle regular DHCPv6 queries.</simpara>
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          </listitem>
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          <listitem>
            <simpara>
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              <command>-v</command> - prints out the Kea version and exits.
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            </simpara>
          </listitem>
          <listitem>
            <simpara>
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              <command>-V</command> - prints out the Kea extended version with
              additional parameters and exits. The listing includes the versions
              of the libraries dynamically linked to Kea.
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            </simpara>
          </listitem>
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          <listitem>
            <simpara>
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              <command>-W</command> - prints out the Kea configuration report
              and exits. The report is a copy of the
              <filename>config.report</filename> file produced by
              <userinput>./configure</userinput>: it is embedded in the
              executable binary.
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            </simpara>
          </listitem>
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      </itemizedlist>

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      <para>
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        The <filename>config.report</filename> may also be accessed more
        directly.  The following command may be used to extract this
        information.  The binary <userinput>path</userinput> may be found
        in the install directory or in the <filename>.libs</filename>
        subdirectory in the source tree. For example
        <filename>kea/src/bin/dhcp6/.libs/kea-dhcp6</filename>.
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<screen>
strings <userinput>path</userinput>/kea-dhcp6 | sed -n 's/;;;; //p'
</screen>
      </para>

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      <para>
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        On start-up, the server will detect available network interfaces
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        and will attempt to open UDP sockets on all interfaces
        mentioned in the configuration file.
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        Since the DHCPv6 server opens privileged ports, it requires root
        access. Make sure you run this daemon as root.
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      </para>

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      <para>
        During startup the server will attempt to create a PID file of the
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        form: localstatedir]/[conf name].kea-dhcp6.pid where:
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        <itemizedlist>
            <listitem>
            <simpara><command>localstatedir</command>: The value as passed into the
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            build configure script. It defaults to "/usr/local/var".  Note
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            that this value may be overridden at run time by setting the environment
            variable KEA_PIDFILE_DIR.  This is intended primarily for testing purposes.
            </simpara>
            </listitem>
            <listitem>
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            <simpara><command>conf name</command>: The configuration file name
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            used to start the server, minus all preceding path and file extension.
            For example, given a pathname of "/usr/local/etc/kea/myconf.txt", the
            portion used would be "myconf".
            </simpara>
            </listitem>
        </itemizedlist>
        If the file already exists and contains the PID of a live process,
        the server will issue a DHCP6_ALREADY_RUNNING log message and exit. It
        is possible, though unlikely, that the file is a remnant of a system crash
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        and the process to which the PID belongs is unrelated to Kea.  In such a
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        case it would be necessary to manually delete the PID file.
      </para>
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      <para>
        The server can be stopped using the <command>kill</command> command.
        When running in a console, the server can be shut down by
        pressing ctrl-c. It detects the key combination and shuts
        down gracefully.
      </para>
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    </section>

    <section id="dhcp6-configuration">
      <title>DHCPv6 Server Configuration</title>
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<section>
  <title>Introduction</title>
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      <para>
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        This section explains how to configure the DHCPv6 server using the
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        Kea configuration backend. (Kea configuration using any other
        backends is outside of scope of this document.) Before DHCPv6
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        is started, its configuration file has to be created. The
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        basic configuration is as follows:
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<screen>
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{
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# DHCPv6 configuration starts on the next line
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"Dhcp6": {

# First we set up global values
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    "valid-lifetime": 4000,
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    "renew-timer": 1000,
    "rebind-timer": 2000,
    "preferred-lifetime": 3000,

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# Next we setup the interfaces to be used by the server.
    "interfaces-config": {
        "interfaces": [ "eth0" ]
    },

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# And we specify the type of lease database
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    "lease-database": {
        "type": "memfile",
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        "persist": true,
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        "name": "/var/kea/dhcp6.leases"
    },

# Finally, we list the subnets from which we will be leasing addresses.
    "subnet6": [
        {
            "subnet": "2001:db8:1::/64",
            "pools": [
                 {
                     "pool": "2001:db8:1::1-2001:db8:1::ffff"
                 }
             ]
        }
    ]
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# DHCPv6 configuration ends with the next line
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}

} </screen>
</para>

<para>The following paragraphs provide a brief overview of the parameters in
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the above example together with
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their format. Subsequent sections of this chapter go into much greater detail
for these and other parameters.</para>

<para>The lines starting with a hash (#) are comments and are ignored by
the server; they do not impact its
operation in any way.</para>

<para>The configuration starts in the first line with the initial
opening curly bracket (or brace). Each configuration consists of
one or more objects. In this specific example, we have only one
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object, called Dhcp6. This is a simplified configuration, as usually
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there will be additional objects, like <command>Logging</command> or
<command>DhcpDns</command>, but we omit them now for clarity. The Dhcp6
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configuration starts with the <command>"Dhcp6": {</command> line
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and ends with the corresponding closing brace (in the above example,
the brace after the last comment).  Everything defined between those
lines is considered to be the Dhcp6 configuration.</para>

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<para>In the general case, the order in which those parameters appear does not
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matter. There are two caveats here though. The first one is to remember that
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the configuration file must be well formed JSON. That means that parameters
for any given scope must be separated by a comma and there must not be a comma
after the last parameter. When reordering a configuration file, keep in mind that
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moving a parameter to or from the last position in a given scope may also require
moving the comma. The second caveat is that it is uncommon &mdash; although
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legal JSON &mdash; to
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repeat the same parameter multiple times. If that happens, the last occurrence of a
given parameter in a given scope is used while all previous instances are
ignored. This is unlikely to cause any confusion as there are no real life
reasons to keep multiple copies of the same parameter in your configuration
file.</para>

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<para>Moving onto the DHCPv6 configuration elements, the very first few elements
define some global parameters. <command>valid-lifetime</command>
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defines for how long the addresses (leases) given out by the server are valid. If
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nothing changes, a client that got an address is allowed to use it for 4000
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seconds. (Note that integer numbers are specified as is, without any quotes
around them.) The address will become deprecated in 3000 seconds (clients are
allowed to keep old connections, but can't use this address for creating new
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connections). <command>renew-timer</command> and
<command> rebind-timer</command> are values that define T1 and T2 timers that
govern when the client will begin the renewal and rebind procedures.</para>
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<para>The <command>interfaces-config</command> map specifies the server
configuration concerning the network interfaces, on which the server should
listen to the DHCP messages. The <command>interfaces</command> parameter
specifies a list of network interfaces on which the server should listen.
Lists are opened and closed with square brackets, with elements separated
by commas. Had we wanted to listen on two interfaces, the
<command>interfaces-config</command> would look like this:
<screen>
"interfaces-config": {
    "interfaces": [ "eth0", "eth1" ]
},
</screen>
</para>

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<para>The next couple of lines define the lease database, the place where the server
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stores its lease information. This particular example tells the server to use
<command>memfile</command>, which is the simplest (and fastest) database
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backend. It uses an in-memory database and stores leases on disk in a CSV
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file. This is a very simple configuration. Usually the lease database configuration
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is more extensive and contains additional parameters.  Note that
<command>lease-database</command>
is an object and opens up a new scope, using an opening brace.
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Its parameters (just one in this example - <command>type</command>)
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follow. Had there been more than one, they would be separated by commas. This
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scope is closed with a closing brace. As more parameters for the Dhcp6 definition
follow, a trailing comma is present.</para>
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<para>Finally, we need to define a list of IPv6 subnets. This is the
most important DHCPv6 configuration structure as the server uses that
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information to process clients' requests. It defines all subnets from
which the server is expected to receive DHCP requests. The subnets are
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specified with the <command>subnet6</command> parameter.  It is a list,
so it starts and ends with square brackets.  Each subnet definition in
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the list has several attributes associated with it, so it is a structure
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and is opened and closed with braces. At minimum, a subnet definition
has to have at least two parameters: <command>subnet</command> (that
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defines the whole subnet) and <command>pools</command> (which is a list of
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dynamically allocated pools that are governed by the DHCP server).</para>

<para>The example contains a single subnet. Had more than one been defined,
additional elements
in the <command>subnet6</command> parameter would be specified and
separated by commas. For example, to define two subnets, the following
syntax would be used:
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<screen>
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"subnet6": [
    {
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        "pools": [ { "pool": "2001:db8:1::/112" } ],
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        "subnet": "2001:db8:1::/64"
    },
    {
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        "pools": [ { "pool": "2001:db8:2::1-2001:db8:2::ffff" } ],
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        "subnet": "2001:db8:2::/64"
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    }
]
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</screen>
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Note that indentation is optional and is used for aesthetic purposes only.
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In some cases in may be preferable to use more compact notation.
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</para>
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<para>After all parameters are specified, we have two contexts open:
global and Dhcp6, hence we need two closing curly brackets to close them.
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In a real life configuration file there most likely would be additional
components defined such as Logging or DhcpDdns, so the closing brace would
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be followed by a comma and another object definition.</para>
</section>

<section>
  <title>Lease Storage</title>
  <para>All leases issued by the server are stored in the lease database.
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  Currently there are four database backends available:  memfile (which is the
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  default backend), MySQL, PostgreSQL and Cassandra.</para>
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<section>
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  <title>Memfile - Basic Storage for Leases</title>
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  <para>The server is able to store lease data in different repositories. Larger
  deployments may elect to store leases in a database. <xref
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  linkend="database-configuration6"/> describes this option. In typical
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  smaller deployments though, the server will store lease information in a CSV file rather
  than a database. As well as requiring less administration, an
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  advantage of using a file for storage is that it
  eliminates a dependency on third-party database software.</para>

  <para>The configuration of the file backend (Memfile) is controlled through
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  the Dhcp6/lease-database parameters. The <command>type</command> parameter
  is mandatory and it specifies which storage for leases the server should use.
  The value of <userinput>"memfile"</userinput> indicates that the file should
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  be used as the storage. The following list gives additional, optional,
  parameters that can be used to configure the Memfile backend.
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  <itemizedlist>
    <listitem>
      <simpara><command>persist</command>: controls whether the new leases and
      updates to existing leases are written to the file. It is strongly
      recommended that the value of this parameter is set to
      <userinput>true</userinput> at all times, during the server's normal
      operation. Not writing leases to disk will mean that if a server is restarted
      (e.g. after a power failure), it will not know what addresses have been
      assigned.  As a result, it may hand out addresses to new clients that are
      already in use. The value of <userinput>false</userinput> is mostly useful
      for performance testing purposes. The default value of the
      <command>persist</command> parameter is <userinput>true</userinput>,
      which enables writing lease updates
      to the lease file.
      </simpara>
    </listitem>

    <listitem>
      <simpara><command>name</command>: specifies an absolute location of the lease
      file in which new leases and lease updates will be recorded. The default value
      for this parameter is <userinput>"[kea-install-dir]/var/kea/kea-leases6.csv"
      </userinput>.</simpara>
    </listitem>

    <listitem>
      <simpara><command>lfc-interval</command>: specifies the interval in seconds, at
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      which the server will perform a lease file cleanup (LFC).  This
      removes redundant (historical) information from the lease file
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      and effectively reduces the lease file size. The cleanup process is described
      in more detailed fashion further in this section. The default value of the
      <command>lfc-interval</command> is <userinput>0</userinput>, which disables
      the LFC.</simpara>
    </listitem>

  </itemizedlist>
  </para>

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  <para>An example configuration of the Memfile backend is presented below:
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<screen>
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"Dhcp6": {
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    "lease-database": {
        <userinput>"type": "memfile"</userinput>,
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        <userinput>"persist": true</userinput>,
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        <userinput>"name": "/tmp/kea-leases6.csv"</userinput>,
        <userinput>"lfc-interval": 1800</userinput>
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    }
}
</screen>
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    This configuration selects the <filename>/tmp/kea-leases6.csv</filename> as
    the storage for lease information and enables persistence (writing lease updates
    to this file). It also configures the backend perform the periodic cleanup
    of the lease files, executed every 30 minutes.
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  </para>

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  <para>It is important to know how the lease file contents are organized
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  to understand why the periodic lease file cleanup is needed. Every time
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  the server updates a lease or creates a new lease for the client, the new
  lease information must be recorded in the lease file. For performance reasons,
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  the server does not update the existing client's lease in the file, as it would
  potentially require rewriting the entire file. Instead, it simply appends the new lease
  information to the end of the file: the previous lease entries for the
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  client are not removed. When the server loads leases from the lease file, e.g.
  at the server startup, it assumes that the latest lease entry for the client
  is the valid one. The previous entries are discarded. This means that the
  server can re-construct the accurate information about the leases even though
  there may be many lease entries for each client. However, storing many entries
  for each client results in bloated lease file and impairs the performance of
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  the server's startup and reconfiguration as it needs to process a larger number
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  of lease entries.
  </para>

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  <para>Lease file cleanup (LFC) removes all previous entries for each client and
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  leaves only the latest ones. The interval at which the cleanup is performed
  is configurable, and it should be selected according to the frequency of lease
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  renewals initiated by the clients. The more frequent the renewals, the smaller
  the value of <command>lfc-interval</command> should be. Note however, that the
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  LFC takes time and thus it is possible (although unlikely) that new cleanup
  is started while the previous cleanup instance is still running, if the
  <command>lfc-interval</command> is too short. The server would recover from
  this by skipping the new cleanup when it detects that the previous cleanup
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  is still in progress. But it implies that the actual cleanups will be
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  triggered more rarely than configured. Moreover, triggering a new cleanup
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  adds an overhead to the server which will not be able to respond to new
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  requests for a short period of time when the new cleanup process is spawned.
  Therefore, it is recommended that the <command>lfc-interval</command> value
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  is selected in a way that would allow for the LFC to complete the cleanup before a
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  new cleanup is triggered.
  </para>

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  <para>Lease file cleanup is performed by a separate process (in background) to avoid
  a performance impact on the server process. In order to avoid the conflicts
  between two processes both using the same lease files, the LFC process
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  operates on the copy of the original lease file, rather than on the lease
  file used by the server to record lease updates. There are also other files
  being created as a side effect of the lease file cleanup. The detailed
  description of the LFC is located on the Kea wiki:
  <ulink url="http://kea.isc.org/wiki/LFCDesign"/>.
  </para>
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</section>

<section id="database-configuration6">
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  <title>Lease Database Configuration</title>
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  <note>
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    <para>Lease database access information must be configured for the DHCPv6 server,
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    even if it has already been configured for the DHCPv4 server.  The servers
    store their information independently, so each server can use a separate
    database or both servers can use the same database.</para>
  </note>

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  <para>Lease database configuration is controlled through the
  Dhcp6/lease-database parameters. The type of the database must be set to
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  "memfile", "mysql", "postgresql" or "cql", e.g.
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<screen>
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"Dhcp6": { "lease-database": { <userinput>"type": "mysql"</userinput>, ... }, ... }
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</screen>
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  Next, the name of the database is to hold the leases must be set: this is the
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  name used when the database was created
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  (see <xref linkend="mysql-database-create"/>,
  <xref linkend="pgsql-database-create"/>
  or <xref linkend="cql-database-create"/>).
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<screen>
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"Dhcp6": { "lease-database": { <userinput>"name": "<replaceable>database-name</replaceable>" </userinput>, ... }, ... }
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</screen>
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  If the database is located on a different system to the DHCPv6 server, the
  database host name must also be specified. (It should be noted that this
  configuration may have a severe impact on server performance.):
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<screen>
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"Dhcp6": { "lease-database": { <userinput>"host": <replaceable>remote-host-name</replaceable></userinput>, ... }, ... }
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</screen>
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  The usual state of affairs will be to have the database on the same machine as
  the DHCPv6 server.  In this case, set the value to the empty string:
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<screen>
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"Dhcp6": { "lease-database": { <userinput>"host" : ""</userinput>, ... }, ... }
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</screen>
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  Should the database be located on a different system, you may need to specify a longer interval
  for the connection timeout:
<screen>
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"Dhcp6": { "lease-database": { <userinput>"connect-timeout" : <replaceable>timeout-in-seconds</replaceable></userinput>, ... }, ... }
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</screen>
The default value of five seconds should be more than adequate for local connections.
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If a timeout is given though, it should be an integer greater than zero.
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  </para>
  <para>Finally, the credentials of the account under which the server will
  access the database should be set:
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<screen>
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"Dhcp6": { "lease-database": { <userinput>"user": "<replaceable>user-name</replaceable>"</userinput>,
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                               <userinput>"password": "<replaceable>password</replaceable>"</userinput>,
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                              ... },
           ... }
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</screen>
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  If there is no password to the account, set the password to the empty string
  "". (This is also the default.)</para>
</section>
</section>
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<section id="hosts6-storage">
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  <title>Hosts Storage</title>
    <para>Kea is also able to store information about host reservations in the
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    database. The hosts database configuration uses the same syntax as the lease
    database. In fact, a Kea server opens independent connections for each
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    purpose, be it lease or hosts information. This arrangement gives the most
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    flexibility. Kea can be used to keep leases and host reservations
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    separately, but can also point to the same database. Currently the
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    supported hosts database types are MySQL and PostgreSQL. The Cassandra
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    backend does not support host reservations yet.</para>
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    <para>Please note that usage of hosts storage is optional. A user can define
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    all host reservations in the configuration file. That is the recommended way
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    if the number of reservations is small. However, when the number of
    reservations grows it's more convenient to use host storage. Please note
    that both storage methods (configuration file and one of the supported databases)
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    can be used together. If hosts are defined in both places, the definitions
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    from the configuration file are checked first and external storage is checked
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    later, if necessary.</para>
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<section id="hosts-database-configuration6">
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  <title>DHCPv6 Hosts Database Configuration</title>
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  <para>Hosts database configuration is controlled through the Dhcp6/hosts-database
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  parameters. If enabled, the type of the database must be set to "mysql" or
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  "postgresql". Other hosts backends may be added in later version of Kea.
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<screen>
"Dhcp6": { "hosts-database": { <userinput>"type": "mysql"</userinput>, ... }, ... }
</screen>
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  Next, the name of the database to hold the reservations must be set: this is the
  name used when the database was created  (see <xref linkend="supported-databases"/>
  for instructions how to setup desired database type).
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<screen>
"Dhcp6": { "hosts-database": { <userinput>"name": "<replaceable>database-name</replaceable>" </userinput>, ... }, ... }
</screen>
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  If the database is located on a different system than the DHCPv6 server, the
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  database host name must also be specified. (Again it should be noted that this
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  configuration may have a severe impact on server performance):
<screen>
"Dhcp6": { "hosts-database": { <userinput>"host": <replaceable>remote-host-name</replaceable></userinput>, ... }, ... }
</screen>
  The usual state of affairs will be to have the database on the same machine as
  the DHCPv6 server.  In this case, set the value to the empty string:
<screen>
"Dhcp6": { "hosts-database": { <userinput>"host" : ""</userinput>, ... }, ... }
</screen>
  </para>
  <para>Finally, the credentials of the account under which the server will
  access the database should be set:
<screen>
"Dhcp6": { "hosts-database": { <userinput>"user": "<replaceable>user-name</replaceable>"</userinput>,
                               <userinput>"password": "<replaceable>password</replaceable>"</userinput>,
                              ... },
           ... }
</screen>
  If there is no password to the account, set the password to the empty string
  "". (This is also the default.)</para>
</section>
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<section id="read-only-database-configuration6">
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<title>Using Read-Only Databases for Host Reservations</title>
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<para>
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In some deployments the database user whose name is specified in the database backend
configuration may not have write privileges to the database. This is often
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required by the policy within a given network to secure the data from being
unintentionally modified. In many cases administrators have inventory databases
deployed, which contain substantially more information about the hosts than
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static reservations assigned to them. The inventory database can be used to create
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a view of a Kea hosts database and such view is often read only.
</para>
<para>
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Kea host database backends operate with an implicit configuration to both
read from and write to the database. If the database user does not have
write access to the host database, the backend will fail to start and the
server will refuse to start (or reconfigure). However, if access to a read
only host database is required for retrieving reservations for clients
and/or assign specific addresses and options, it is possible to explicitly
configure Kea to start in "read-only" mode. This is controlled by the
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<command>readonly</command> boolean parameter as follows:
<screen>
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"Dhcp6": { "hosts-database": { <userinput>"readonly": true</userinput>, ... }, ... }
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</screen>
Setting this parameter to <userinput>false</userinput> would configure the
database backend to operate in "read-write" mode, which is also a default
configuration if the parameter is not specified.
</para>
<note><para>The <command>readonly</command> parameter is currently only supported
for MySQL and PostgreSQL databases.</para></note>
</section>

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</section>
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<section id="dhcp6-interface-selection">
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  <title>Interface Selection</title>
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  <para>The DHCPv6 server has to be configured to listen on specific network
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  interfaces.  The simplest network interface configuration instructs the server to
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  listen on all available interfaces:
  <screen>
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"Dhcp6": {
    "interfaces-config": {
        "interfaces": [ <userinput>"*"</userinput> ]
    }
    ...
}
</screen>
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  The asterisk plays the role of a wildcard and means "listen on all interfaces".
  However, it is usually a good idea to explicitly specify interface names:
  <screen>
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"Dhcp6": {
    "interfaces-config": {
        "interfaces": [ <userinput>"eth1", "eth3"</userinput> ]
    },
    ...
}
  </screen>
</para>
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  <para>It is possible to use wildcard interface name (asterisk) concurrently
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  with the actual interface names:
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  <screen>
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"Dhcp6": {
    "interfaces-config": {
        "interfaces": [ <userinput>"eth1", "eth3", "*"</userinput> ]
    },
    ...
}
  </screen>
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It is anticipated that this will form of usage only be used where it is desired to
temporarily override a list of interface names and listen on all interfaces.
  </para>
</section>
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    <section id="ipv6-subnet-id">
      <title>IPv6 Subnet Identifier</title>
      <para>
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        The subnet identifier is a unique number associated with a particular subnet.
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        In principle, it is used to associate clients' leases with their respective subnets.
        When a subnet identifier is not specified for a subnet being configured, it will
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        be automatically assigned by the configuration mechanism. The identifiers
        are assigned from 1 and are monotonically increased for each subsequent
        subnet: 1, 2, 3 ....
      </para>
      <para>
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        If there are multiple subnets configured with auto-generated identifiers and
        one of them is removed, the subnet identifiers may be renumbered. For example:
        if there are four subnets and the third is removed the last subnet will be assigned
        the identifier that the third subnet had before removal. As a result, the leases
        stored in the lease database for subnet 3 are now associated with
        subnet 4, something that may have unexpected consequences. It is planned
        to implement a mechanism to preserve auto-generated subnet ids in a
        future version of Kea.  However, the only remedy for this issue
        at present is to
        manually specify a unique identifier for each subnet.
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      </para>
      <para>
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        The following configuration will assign the specified subnet
        identifier to the newly configured subnet:
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        <screen>
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"Dhcp6": {
    "subnet6": [
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        {
            "subnet": "2001:db8:1::/64",
            <userinput>"id": 1024</userinput>,
            ...
        }
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    ]
}
</screen>
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        This identifier will not change for this subnet unless the "id" parameter is
        removed or set to 0. The value of 0 forces auto-generation of the subnet
        identifier.
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      </para>
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      <!-- @todo: describe whether database needs to be updated after changing
      id -->
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    </section>

    <section id="dhcp6-unicast">
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      <title>Unicast Traffic Support</title>
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      <para>
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        When the DHCPv6 server starts, by default it listens to the DHCP traffic
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        sent to multicast address ff02::1:2 on each interface that it is
        configured to listen on (see <xref linkend="dhcp6-interface-selection"/>).
        In some cases it is useful to configure a server to handle incoming
        traffic sent to the global unicast addresses as well. The most common
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        reason for this is to have relays send their traffic to the server
        directly. To configure the server to listen on a specific unicast address,
        nn interface name can be
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        optionally followed by a slash, followed by the global unicast address on which
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        the server should listen. The server listens to this address in addition to normal
        link-local binding and listening on ff02::1:2 address. The sample configuration
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        below shows how to listen on 2001:db8::1 (a global address)
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        configured on the eth1 interface.
      </para>
      <para>
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 <screen>
"Dhcp6": {
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    "interfaces-config": {
        "interfaces": [ <userinput>"eth1/2001:db8::1"</userinput> ]
    },
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    ...
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    "option-data": [
        {
            "name": "unicast",
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            "data": "2001:db8::1"
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        } ],
    ...
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}
 </screen>
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        This configuration will cause the server to listen on
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        eth1 on the link-local address, the multicast group (ff02::1:2) and 2001:db8::1.
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      </para>
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      <para>
        Usually unicast support is associated with a server unicast option
        which allows clients to send unicast messages to the server.
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        The example above includes a server unicast option specification
        which will cause the client to send messages to the specified
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        unicast address.
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      </para>
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      <para>
        It is possible to mix interface names, wildcards and interface name/addresses
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        in the list of interfaces. It is not possible however to specify more than one
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        unicast address on a given interface.
      </para>
      <para>
        Care should be taken to specify proper unicast addresses. The server will
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        attempt to bind to the addresses specified without any additional checks.
        This approach has selected on purpose to allow the software to
        communicate over uncommon addresses if so desired.
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      </para>
    </section>

    <section id="dhcp6-address-config">
      <title>Subnet and Address Pool</title>
      <para>
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        The main role of a DHCPv6 server is address assignment. For this,
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        the server has to be configured with at least one subnet and one pool of dynamic
        addresses to be managed. For example, assume that the server
        is connected to a network segment that uses the 2001:db8:1::/64
        prefix. The Administrator of that network has decided that addresses from range
        2001:db8:1::1 to 2001:db8:1::ffff are going to be managed by the Dhcp6
        server. Such a configuration can be achieved in the following way:
        <screen>
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"Dhcp6": {
    <userinput>"subnet6": [
       {
           "subnet": "2001:db8:1::/64",
           "pools": [
               {
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                   "pool": "2001:db8:1::1-2001:db8:1::ffff"
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               }
           ],
           ...
       }
    ]</userinput>
}</screen>

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        Note that <command>subnet</command> is defined as a simple string, but
        the <command>pools</command> parameter is actually a list of pools: for
        this reason, the pool definition is enclosed in square brackets, even
        though only one range of addresses is specified.</para>
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        <para>Each <command>pool</command> is a structure that contains the
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        parameters that describe a single pool. Currently there is only one
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        parameter, <command>pool</command>, which gives the range of addresses
        in the pool. Additional parameters will be added in future releases of
        Kea.</para>

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        <para>It is possible to define more than one pool in a
        subnet: continuing the previous example, further assume that
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        2001:db8:1:0:5::/80 should also be managed by the server. It could be written as
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        2001:db8:1:0:5:: to 2001:db8:1::5:ffff:ffff:ffff, but typing so many 'f's
        is cumbersome. It can be expressed more simply as 2001:db8:1:0:5::/80. Both
        formats are supported by Dhcp6 and can be mixed in the pool list.
        For example, one could define the following pools:
        <screen>
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"Dhcp6": {
    <userinput>"subnet6": [
    {
        "subnet": "2001:db8:1::/64",
        "pools": [
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            { "pool": "2001:db8:1::1-2001:db8:1::ffff" },
            { "pool": "2001:db8:1:05::/80" }
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        ]</userinput>,
        ...
    }
    ]
}</screen>
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        White space in pool definitions is ignored, so spaces before and after the hyphen are optional.
        They can be used to improve readability.
      </para>
      <para>
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        The number of pools is not limited, but for performance reasons it is recommended to
        use as few as possible.
      </para>
      <para>
         The server may be configured to serve more than one subnet. To add a second subnet,
         use a command similar to the following:
        <screen>
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"Dhcp6": {
    <userinput>"subnet6": [
    {
        "subnet": "2001:db8:1::/64",
        "pools": [
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            { "pool": "2001:db8:1::1-2001:db8:1::ffff" }
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        ]
    },
    {
        "subnet": "2001:db8:2::/64",
        "pools": [
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            { "pool": "2001:db8:2::/64" }
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        ]
    },
</userinput>
        ...
    ]
}</screen>
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        In this example, we allow the server to
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        dynamically assign all addresses available in the whole subnet. Although
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        rather wasteful, it is certainly a valid configuration to dedicate the
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        whole /64 subnet for that purpose. Note that the Kea server does not preallocate
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        the leases, so there is no danger in using gigantic address pools.
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      </para>
      <para>
        When configuring a DHCPv6 server using prefix/length notation, please pay
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        attention to the boundary values. When specifying that the server can use
        a given pool, it will also be able to allocate the first (typically network
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        address) address from that pool. For example, for pool 2001:db8:2::/64 the
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        2001:db8:2:: address may be assigned as well. If you want to avoid this,
        use the "min-max" notation.
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      </para>
    </section>

    <section>
      <title>Subnet and Prefix Delegation Pools</title>
      <para>
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        Subnets may also be configured to delegate prefixes, as defined in
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        <ulink url="http://tools.ietf.org/html/rfc3633">RFC 3633</ulink>.  A
        subnet may have one or more prefix delegation pools.  Each pool has a
        prefixed address, which is specified as a prefix
        (<command>prefix</command>) and a prefix length
        (<command>prefix-len</command>), as well as a delegated prefix length
        (<command>delegated-len</command>). The delegated length must not be
        shorter (that is it must be numerically greater or equal) than the
        prefix length.  If both the delegated and prefix lengths are equal, the
        server will be able to delegate only one prefix. The delegated prefix
        does not have to match the subnet prefix.
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      </para>
      <para> Below is a sample subnet configuration which enables prefix
      delegation for the subnet:
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      <screen>
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"Dhcp6": {
    "subnet6": [
        {
            "subnet": "2001:d8b:1::/64",
            <userinput>"pd-pools": [
                {
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                    "prefix": "3000:1::",
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                    "prefix-len": 64,
                    "delegated-len": 96
                }
            ]</userinput>
        }
    ],
    ...
}</screen>
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      </para>
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    </section>

    <section id="pd-exclude-option">
        <title>Prefix Exclude Option</title>
        <para>
          For each delegated prefix the delegating router may choose to exclude
          a single prefix out of the delegated prefix as specified in the
          <ulink url="http://tools.ietf.org/html/rfc6603"> RFC 6603</ulink>.
          The requesting router must not assign the excluded prefix to any
          of its downstream interfaces and it is intended to be used on a
          link through which the delegating router exchanges DHCPv6 messages with
          the requesting router. The configuration example below demonstrates how
          to specify an excluded prefix within a prefix pool definition. The
          excluded prefix "2001:db8:1:babe:cafe:80::/72" will be sent to a
          requesting router which includes Prefix Exclude option in the ORO, and
          which is delegated a prefix from this pool.
        </para>
<screen>
"Dhcp6": {
    "subnet6": [
        {
            "subnet": "2001:db8:1::/48",
            "pd-pools": [
                {
                    "prefix": "2001:db8:1:8000::",
                    "prefix-len": 48,
                    "delegated-len": 64,
                    "excluded-prefix": "2001:db8:1:babe:cafe:80::",
                    "excluded-prefix-len": 72
                }
            ]
        }
    ]
}
</screen>
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    </section>

    <section id="dhcp6-std-options">
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      <title>Standard DHCPv6 Options</title>
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      <para>
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        One of the major features of a DHCPv6 server is to provide configuration
        options to clients.  Although there are several options that require
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        special behavior, most options are sent by the server only if the client
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        explicitly requests them.  The following example shows how to
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        configure DNS servers, one of the most frequently used
        options.  Options specified in this way are considered
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        global and apply to all configured subnets.

        <screen>
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"Dhcp6": {
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    "option-data": [
        {
           <userinput>"name": "dns-servers",
           "code": 23,
           "space": "dhcp6",
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           "csv-format": true,
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           "data": "2001:db8::cafe, 2001:db8::babe"</userinput>
        },
        ...
    ]
}
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</screen>
      </para>
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    <para>
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      The <command>option-data</command> line creates a new entry in
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      the option-data table.  This table contains
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      information on all global options that the server is supposed to configure
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      in all subnets.  The <command>name</command> line specifies the option name.
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      (For a complete list
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      of currently supported names, see <xref
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      linkend="dhcp6-std-options-list"/>.)  The next line specifies the option code,
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      which must match one of the values from that list. The line beginning with
      <command>space</command> specifies the option space, which must always be set
      to "dhcp6" as these are standard DHCPv6 options.  For other name spaces,
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      including custom option spaces, see <xref
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      linkend="dhcp6-option-spaces"/>. The following line specifies the format in
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      which the data will be entered: use of CSV (comma separated values) is
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      recommended. Finally, the <command>data</command> line gives the actual value to be sent to
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      clients.  Data is specified as normal text, with values separated by
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      commas if more than one value is allowed.
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    </para>

    <para>
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      Options can also be configured as hexadecimal values.  If "csv-format" is
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      set to false, the option data must be specified as a string of hexadecimal
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      numbers.  The
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      following commands configure the DNS-SERVERS option for all
      subnets with the following addresses: 2001:db8:1::cafe and
      2001:db8:1::babe.
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        <screen>
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"Dhcp6": {
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    "option-data": [
        {
           <userinput>"name": "dns-servers",
           "code": 23,
           "space": "dhcp6",
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           "csv-format": false,
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           "data": "2001 0DB8 0001 0000 0000 0000 0000 CAFE
                    2001 0DB8 0001 0000 0000 0000 0000 BABE"</userinput>
        },
        ...
    ]
}
</screen>
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       </para>
       <note><para>
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       The value for the setting of the "data" element is split across two
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       lines in this example for clarity: when entering the command, the
       whole string should be entered on the same line.
       </para></note>
       <para>
       Care should be taken
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       to use proper encoding when using hexadecimal format as Kea's ability
       to validate data correctness in hexadecimal is limited.
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      </para>

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      <para>
        Most of the parameters in the "option-data" structure are optional and
        can be omitted in some circumstances as discussed in the
        <xref linkend="dhcp6-option-data-defaults"/>.
      </para>


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    <para>
      It is possible to override options on a per-subnet basis.  If
      clients connected to most of your subnets are expected to get the
      same values of a given option, you should use global options: you
      can then override specific values for a small number of subnets.
      On the other hand, if you use different values in each subnet,
      it does not make sense to specify global option values
      (Dhcp6/option-data), rather you should set only subnet-specific values
      (Dhcp6/subnet[X]/option-data[Y]).
     </para>
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     <para>
      The following commands override the global
      DNS servers option for a particular subnet, setting a single DNS
      server with address 2001:db8:1::3.
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<screen>
"Dhcp6": {
    "subnet6": [
        {
            <userinput>"option-data": [
                {
                    "name": "dns-servers",
                    "code": 23,
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                    "space": "dhcp6",
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                    "csv-format": true,
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                    "data": "2001:db8:1::3"
                },
                ...
            ]</userinput>,
            ...
        },
        ...
    ],
    ...
}
</screen>
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    </para>

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     <para>
       In some cases it is useful to associate some options with an
       address or prefix pool from which a client is assigned a lease. Pool
       specific option values override subnet specific and global option
       values. If the client is assigned multiple leases from different
       pools, the server will assign options from all pools from which the
       leases have been obtained. However, if the particular option is specified
       in multiple pools from which the client obtains the leases, only one
       instance of this option will be handed out to the client. The server's
       administrator must not try to prioritize assignment of pool specific
       options by trying to order pools declarations in the server
       configuration. Future Kea releases may change the order in which
       options are assigned from the pools without any notice.
     </para>

     <para>
       The following configuration snippet demonstrates how to specify the
       DNS servers option, which will be assigned to a client only if the
       client obtains an address from the given pool:
<screen>
"Dhcp6": {
    "subnet6": [
        {
            "pools": [
                {
                    "pool": "2001:db8:1::100-2001:db8:1::300",
                    <userinput>"option-data": [
                        {
                            "name": "dns-servers",
                            "data": "2001:db8:1::10"
                        }
                    ]</userinput>
                }
            ]
        },
        ...
    ],
    ...
}
</screen>
     </para>

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