dhcp6-srv.xml 77.5 KB
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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 DHCPv4 server be started and stopped
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      using <command>keactrl</command> (described in <xref linkend="keactrl"/>).
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      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 default DHCPv6 ports will not be able to
            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 Kea version and exits.
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            </simpara>
          </listitem>
          <listitem>
            <simpara>
              <command>-V</command> - prints out Kea extended version with
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              additional parameters and exits.
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            </simpara>
          </listitem>
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      </itemizedlist>

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      <para>
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	    When running in a console, the server can be shut down by
	    pressing ctrl-c. It detects the key combination and shuts
	    down gracefully.
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      </para>

      <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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      </para>

      <para>
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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>

    </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
        basic configuration looks 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
    "interfaces": [ "eth0" ],
    "renew-timer": 1000,
    "rebind-timer": 2000,
    "preferred-lifetime": 3000,
    "valid-lifetime": 4000,

# Next we specify the type of lease database
    "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
the above example and
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
object called Dhcp6. This is a simplified configuration, as usually
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 require
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moving the comma as well. The second caveat is that it is uncommon &mdash; although
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>

<para>Moving onto the DHCPv6 configuration elements,
the line defining <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 line could
look like this:
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<screen>
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"interfaces": [ "eth0", "eth1" ],
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</screen>

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As "<command>interfaces</command>" is not the last parameter in the
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configuration, a trailing comma is required.</para>
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<para>A number of other parameters follow. <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
connections). <command>renew-timer</command> and <command>
rebind-timer</command> are values that define T1 and T2 timers that govern when
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the client will begin the renewal and rebind procedures.</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, lease database configuration
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.
Its parameters (just one in this example -- <command>type</command>)
follow. Had there been more than one, they would be separated by commas. This
scope is closed with a closing brace. As more parameters follow, a trailing
comma is present.</para>

<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
defines the whole subnet) and <command>pool</command> (which is a list of
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": [
    {
        "pools": [
        {
            "pool": "2001:db8:1::/112"
        }
        ],
        "subnet": "2001:db8:1::/64"
    },
    {
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        "pools": [ { "pool": "2001:db8:2::1-2001:db8:2::ffff" } ],
        "subnet": "2001:db8:2::/64",
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        "interface": "eth0"
    }
]
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</screen>
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Note that indentation is optional and is used for aesthetic purposes only.
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>

<para>Kea 0.9 does not have configuration syntax validation
implemented yet. Such a feature is planned for the near future. For
the time being, it is convenient to use on-line JSON validators and/or
viewers to check whether the syntax is correct. One example of such a
JSON validator is available at <ulink url="http://jsonviewer.stack.hu/"/>.
</para>
</section>

<section>
  <title>Lease Storage</title>
  <para>All leases issued by the server are stored in the lease database.
  Currently there are three database backends available:
  memfile (which is the default backend), MySQL and PostgreSQL.</para>
<section>
  <title>Memfile - Basic Storage for Leases</title>

  <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 use a CSV file rather than a database to
  store lease information. As well as requiring less administration, an
  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. <!-- @todo: we don't have default
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  parameters. Let's comment this out When default parameters are used, the
  Memfile backend will write leases to a disk in the
  [kea-install-dir]/var/kea/kea-leases4.csv. -->
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  The following configuration sets the name of the lease file to
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  <filename>/tmp/kea-leases6.csv</filename>:
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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>
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    }
    ...
}
</screen>
  </para>

  <para>The "persist" parameter controls whether the leases are written to disk.
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  It is strongly recommended that this parameter is set to true at all times
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  during the normal operation of the server. (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.)
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          </para>
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</section>

<section id="database-configuration6">
  <title>Database Configuration</title>

  <note>
    <para>Database access information must be configured for the DHCPv6 server,
    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>

  <para>Database configuration is controlled through the Dhcp6/lease-database
  parameters. The type of the database must be set to "mysql" or "postgresql",
  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
  name used when the lease database was created (see <xref linkend="dhcp-mysql-database-create"/>
  or <xref linkend="dhcp-pgsql-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 than the DHCPv6 server, the
  database host name must also be specified (although 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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  </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="dhcp6-interface-selection">
  <title>Interface selection</title>
  <para>The DHCPv6 server has to be configured to listen on specific network
  interfaces.  The simplest network interface configuration tells the server to
  listen on all available interfaces:
  <screen>
"Dhcp6": { <userinput>"interfaces": ["*"]</userinput>, ... }</screen>
  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>
"Dhcp6": { <userinput>"interfaces": [ "eth1", "eth3" ]</userinput>, ... }</screen>
  </para>
  <para>It is possible to use wildcard interface name (asterisk) concurrently
  with explicit interface names:
  <screen>
"Dhcp6": { <userinput>"interfaces": [ "eth1", "eth3", "*" ]</userinput>, ... }</screen>
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 respective subnets.
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        When the 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>
       If there are multiple subnets configured with auto-generated identifiers and
       one of them is removed, the subnet identifiers may be renumbered. For example:
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       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
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       subnet 4, which may have unexpected consequences. In the future it is planned
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       to implement a mechanism to preserve auto-generated subnet ids upon removal
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       of one of the subnets. Currently, the only remedy for this issue is to
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       manually specify a unique subnet identifier for each subnet.
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      </para>
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      <para>
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	The following configuration will assign the specified subnet
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	identifier to the newly configured subnet:

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	<screen>
"Dhcp6": {
    "subnet6": [
        "subnet": "2001:db8:1::/64",
        <userinput>"id": 1024</userinput>,
        ...
    ]
}
</screen>
	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">
      <title>Unicast traffic support</title>
      <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
        reason for that is to have relays send their traffic to the server
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        directly. To configure the server to listen on a specific unicast address, the
        notation to specify interfaces has been extended.  An interface name can be
        optionally followed by a slash, followed by the global unicast address on which
        the server should listen. This will be done in addition to normal
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        link-local binding + listening on ff02::1:2 address. The sample commands
        listed below show how to listen on 2001:db8::1 (a global address)
        configured on the eth1 interface.
      </para>
      <para>
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 <screen>
"Dhcp6": {
    <userinput>"interfaces": [ "eth1/2001:db8::1"],</userinput>
    ...
}</screen>

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        When this configuration gets committed, the server will start to listen on
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        eth1 on link-local address, multicast group (ff02::1:2) and 2001:db8::1.
      </para>
      <para>
        It is possible to mix interface names, wildcards and interface name/addresses
        on the Dhcp6/interface list. It is not possible to specify more than one
        unicast address on a given interface.
      </para>
      <para>
        Care should be taken to specify proper unicast addresses. The server will
        attempt to bind to those addresses specified, without any additional checks.
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        This approach is selected on purpose, so the software can be used to
        communicate over uncommon addresses if the administrator so desires.
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      </para>
    </section>

    <section id="dhcp6-address-config">
      <title>Subnet and Address Pool</title>
      <para>
        The essential role of a DHCPv6 server is address assignment. For this,
        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 subnet is defined as a simple string, but the pool 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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        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
        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>
<!-- @todo: add real meat to the prefix delegation config this is just place holder stuff -->
      <title>Subnet and Prefix Delegation Pools</title>
      <para>
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        Subnets may also be configured to delegate prefixes, as defined in
	<ulink url="http://tools.ietf.org/html/rfc3633">RFC 3633</ulink>.
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        A subnet may have one or more prefix delegation pools.  Each pool has
        a prefixed address, which is specified as a prefix and a prefix length,
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        as well as a delegated prefix length. <command>delegated-len</command>
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	must not be shorter (that is it must be numerically greater or equal)
	than <command>prefix-len</command>.
	If both <command>delegated-len</command>
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	and <command>prefix-len</command> are equal, the server will be able to
	delegate only one prefix. A sample configuration is shown below:
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      <screen>
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"Dhcp6": {
    "subnet6": [
        {
            "subnet": "2001:d8b:1::/64",
            <userinput>"pd-pools": [
                {
                    "prefix": "2001:db8:1::",
                    "prefix-len": 64,
                    "delegated-len": 96
                }
            ]</userinput>
        }
    ],
    ...
}</screen>
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      </para>
    </section>

    <section id="dhcp6-std-options">
      <title>Standard DHCPv6 options</title>
      <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, which is one of the most frequently used
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        options.  Numbers in the first column are added for easier reference and
        will not appear on screen.  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
      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 next 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. The <command>data</command> line gives the actual value to be sent to
      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>

       The value for the setting of the "data" element is split across two
       lines in this document for clarity: when entering the command, the
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       whole string should be entered on the same line.  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>

    <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>
     <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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Jeremy C. Reed committed
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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>

    <note>
      <para>
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        In future versions of Kea, it will not be necessary to specify
        the <command>code</command>, <command>space</command>
        and <command>csv-format</command> fields, as they will
        be set automatically.
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      </para>
    </note>


    <para>
      The currently supported standard DHCPv6 options are
      listed in <xref linkend="dhcp6-std-options-list"/>.
      The "Name" and "Code"
      are the values that should be used as a name in the option-data
      structures. "Type" designates the format of the data: the meanings of
      the various types is given in <xref linkend="dhcp-types"/>.
    </para>
    <para>
      Some options are designated as arrays, which means that more than one
      value is allowed in such an option. For example the option dns-servers
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      allows the specification of more than one IPv6 address, allowing
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      clients to obtain the addresses of multiple DNS servers.
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    </para>

<!-- @todo: describe record types -->

      <para>
        The <xref linkend="dhcp6-custom-options"/> describes the configuration
        syntax to create custom option definitions (formats). It is generally not
        allowed to create custom definitions for standard options, even if the
        definition being created matches the actual option format defined in the
        RFCs. There is an exception from this rule for standard options for which
        Kea does not provide a definition yet. In order to use such options,
        a server administrator must create a definition as described in
        <xref linkend="dhcp6-custom-options"/> in the 'dhcp6' option space. This
        definition should match the option format described in the relevant
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        RFC but the configuration mechanism would allow any option format as it has
        no means to validate the format at the moment.
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      </para>


    <para>
      <table frame="all" id="dhcp6-std-options-list">
        <title>List of standard DHCPv6 options</title>
        <tgroup cols='4'>
        <colspec colname='name'/>
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        <colspec colname='code' align='center'/>
        <colspec colname='type' align='center'/>
        <colspec colname='array' align='center'/>
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        <thead>
          <row><entry>Name</entry><entry>Code</entry><entry>Type</entry><entry>Array?</entry></row>
        </thead>
        <tbody>
<!-- Our engine uses those options on its own, admin must not configure them on his own
<row><entry>clientid</entry><entry>1</entry><entry>binary</entry><entry>false</entry></row>
<row><entry>serverid</entry><entry>2</entry><entry>binary</entry><entry>false</entry></row>
<row><entry>ia-na</entry><entry>3</entry><entry>record</entry><entry>false</entry></row>
<row><entry>ia-ta</entry><entry>4</entry><entry>uint32</entry><entry>false</entry></row>
<row><entry>iaaddr</entry><entry>5</entry><entry>record</entry><entry>false</entry></row>
<row><entry>oro</entry><entry>6</entry><entry>uint16</entry><entry>true</entry></row> -->
<row><entry>preference</entry><entry>7</entry><entry>uint8</entry><entry>false</entry></row>

<!-- Our engine uses those options on its own, admin must not configure them on his own
<row><entry>elapsed-time</entry><entry>8</entry><entry>uint16</entry><entry>false</entry></row>
<row><entry>relay-msg</entry><entry>9</entry><entry>binary</entry><entry>false</entry></row>
<row><entry>auth</entry><entry>11</entry><entry>binary</entry><entry>false</entry></row>
<row><entry>unicast</entry><entry>12</entry><entry>ipv6-address</entry><entry>false</entry></row>
<row><entry>status-code</entry><entry>13</entry><entry>record</entry><entry>false</entry></row>
<row><entry>rapid-commit</entry><entry>14</entry><entry>empty</entry><entry>false</entry></row>
<row><entry>user-class</entry><entry>15</entry><entry>binary</entry><entry>false</entry></row>
<row><entry>vendor-class</entry><entry>16</entry><entry>record</entry><entry>false</entry></row>
<row><entry>vendor-opts</entry><entry>17</entry><entry>uint32</entry><entry>false</entry></row>
<row><entry>interface-id</entry><entry>18</entry><entry>binary</entry><entry>false</entry></row>
<row><entry>reconf-msg</entry><entry>19</entry><entry>uint8</entry><entry>false</entry></row>
<row><entry>reconf-accept</entry><entry>20</entry><entry>empty</entry><entry>false</entry></row> -->
<row><entry>sip-server-dns</entry><entry>21</entry><entry>fqdn</entry><entry>true</entry></row>
<row><entry>sip-server-addr</entry><entry>22</entry><entry>ipv6-address</entry><entry>true</entry></row>
<row><entry>dns-servers</entry><entry>23</entry><entry>ipv6-address</entry><entry>true</entry></row>
<row><entry>domain-search</entry><entry>24</entry><entry>fqdn</entry><entry>true</entry></row>
<!-- <row><entry>ia-pd</entry><entry>25</entry><entry>record</entry><entry>false</entry></row> -->
<!-- <row><entry>iaprefix</entry><entry>26</entry><entry>record</entry><entry>false</entry></row> -->
<row><entry>nis-servers</entry><entry>27</entry><entry>ipv6-address</entry><entry>true</entry></row>
<row><entry>nisp-servers</entry><entry>28</entry><entry>ipv6-address</entry><entry>true</entry></row>
<row><entry>nis-domain-name</entry><entry>29</entry><entry>fqdn</entry><entry>true</entry></row>
<row><entry>nisp-domain-name</entry><entry>30</entry><entry>fqdn</entry><entry>true</entry></row>
<row><entry>sntp-servers</entry><entry>31</entry><entry>ipv6-address</entry><entry>true</entry></row>
<row><entry>information-refresh-time</entry><entry>32</entry><entry>uint32</entry><entry>false</entry></row>
<row><entry>bcmcs-server-dns</entry><entry>33</entry><entry>fqdn</entry><entry>true</entry></row>
<row><entry>bcmcs-server-addr</entry><entry>34</entry><entry>ipv6-address</entry><entry>true</entry></row>
<row><entry>geoconf-civic</entry><entry>36</entry><entry>record</entry><entry>false</entry></row>
<row><entry>remote-id</entry><entry>37</entry><entry>record</entry><entry>false</entry></row>
<row><entry>subscriber-id</entry><entry>38</entry><entry>binary</entry><entry>false</entry></row>
<row><entry>client-fqdn</entry><entry>39</entry><entry>record</entry><entry>false</entry></row>
<row><entry>pana-agent</entry><entry>40</entry><entry>ipv6-address</entry><entry>true</entry></row>
<row><entry>new-posix-timezone</entry><entry>41</entry><entry>string</entry><entry>false</entry></row>
<row><entry>new-tzdb-timezone</entry><entry>42</entry><entry>string</entry><entry>false</entry></row>
<row><entry>ero</entry><entry>43</entry><entry>uint16</entry><entry>true</entry></row>
<row><entry>lq-query</entry><entry>44</entry><entry>record</entry><entry>false</entry></row>
<row><entry>client-data</entry><entry>45</entry><entry>empty</entry><entry>false</entry></row>
<row><entry>clt-time</entry><entry>46</entry><entry>uint32</entry><entry>false</entry></row>
<row><entry>lq-relay-data</entry><entry>47</entry><entry>record</entry><entry>false</entry></row>
<row><entry>lq-client-link</entry><entry>48</entry><entry>ipv6-address</entry><entry>true</entry></row>
        </tbody>
        </tgroup>
      </table>
    </para>
    </section>

    <section id="dhcp6-custom-options">
      <title>Custom DHCPv6 options</title>
      <para>It is also possible to define options other than the standard ones.
      Assume that we want to define a new DHCPv6 option called "foo" which will have
      code 100 and will convey a single unsigned 32 bit integer value. We can define
      such an option by using the following commands:
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<screen>
"Dhcp6": {
    "option-def": [
        {
            <userinput>"name": "foo",
            "code": 100,
            "type": "uint32",
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            "array": false,
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            "record-types": "",
            "space": "dhcp6",
            "encapsulate": ""</userinput>
        }, ...
    ],
    ...
}
</screen>
      The "false" value of the "array" parameter determines that the option does
      NOT comprise an array of "uint32" values but rather a single value.  Two
      other parameters have been left blank: "record-types" and "encapsulate".
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      The former specifies the comma separated list of option data fields if the
      option comprises a record of data fields. The "record-fields" value should
      be non-empty if the "type" is set to "record". Otherwise it must be left
      blank. The latter parameter specifies the name of the option space being
      encapsulated by the particular option. If the particular option does not
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      encapsulate any option space it should be left blank.  Note that the above
      set of comments define the format of the new option and do not set its
      values.
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      </para>
      <para>Once the new option format is defined, its value is set
      in the same way as for a standard option. For example the following
      commands set a global value that applies to all subnets.
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<screen>
"Dhcp6": {
    "option-data": [
        {
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            <userinput>"name": "foo",
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            "code": 100,
            "space": "dhcp6",
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            "csv-format": true,
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            "data": "12345"</userinput>
        }, ...
    ],
    ...
}
</screen>
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      </para>

      <para>New options can take more complex forms than simple use of
      primitives (uint8, string, ipv6-address etc): it is possible to
      define an option comprising a number of existing primitives.
      </para>
      <para>
      Assume we want to define a new option that will consist of an IPv6
      address, followed by an unsigned 16 bit integer, followed by a
      boolean value, followed by a text string. Such an option could
      be defined in the following way:
<screen>
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"Dhcp6": {
    "option-def": [
        {
            <userinput>"name": "bar",
            "code": 101,
            "space": "dhcp6",
            "type": "record",
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            "array": false,
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            "record-types": "ipv4-address, uint16, boolean, string",
            "encapsulate": ""</userinput>
        }, ...
    ],
    ...
}
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</screen>
      The "type" is set to "record" to indicate that the option contains
      multiple values of different types.  These types are given as a comma-separated
      list in the "record-types" field and should be those listed in <xref linkend="dhcp-types"/>.
      </para>
      <para>
      The values of the option are set as follows:
<screen>
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"Dhcp6": {
    "option-data": [
        {
            <userinput>"name": "bar",
            "space": "dhcp6",
            "code": 101,
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            "csv-format": true,
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            "data": "2001:db8:1::10, 123, false, Hello World"</userinput>
        }
    ],
    ...
}</screen>
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      <command>csv-format</command> is set <command>true</command> to indicate
      that the <command>data</command> field comprises a command-separated list
      of values.  The values in the "data" must correspond to the types set in
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      the "record-types" field of the option definition.
      </para>
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      <note>
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       <para>In the general case, boolean values are specified as <command>true</command> or
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       <command>false</command>, without quotes. Some specific boolean parameters may
       accept also <command>"true"</command>, <command>"false"</command>,
       <command>0</command>, <command>1</command>, <command>"0"</command> and
       <command>"1"</command>. Future Kea versions will accept all those values
       for all boolean parameters.</para>
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      </note>
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    </section>

    <section id="dhcp6-vendor-opts">
      <title>DHCPv6 vendor specific options</title>
      <para>
      Currently there are three option spaces defined: dhcp4 (to be used
      in DHCPv4 daemon) and dhcp6 (for the DHCPv6 daemon); there is also
      vendor-opts-space, which is empty by default, but options can be
      defined in it. Those options are called vendor-specific information
      options. The following examples show how to define an option "foo"
      with code 1 that consists of an IPv6 address, an unsigned 16 bit integer
      and a string. The "foo" option is conveyed in a vendor specific
      information option. This option comprises a single uint32 value
      that is set to "12345". The sub-option "foo" follows the data
      field holding this value.
      <screen>
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"Dhcp6": {
    "option-def": [
        {
            <userinput>"name": "foo",
            "code": 1,
            "space": "vendor-encapsulated-options-space",
            "type": "record",
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            "array": false,
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            "record-types": "ipv6-address, uint16, string",
            "encapsulates": ""</userinput>
        }
    ],
    ...
}</screen>
     (Note that the option space is set to <command>vendor-opts-space</command>.)
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     Once the option format is defined, the next step is to define actual values
     for that option:
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<screen>
"Dhcp6": {
    "option-data": [
        {
            <userinput>"name": "foo"
            "space": "vendor-encapsulated-options-space",
            "code": 1,
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            "csv-format": true,
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            "data": "2001:db8:1::10, 123, Hello World"</userinput>
        },
        ...
    ],
    ...
}</screen>
    We should also define values for the vendor-opts, that will convey our
    option foo.
<screen>
"Dhcp6": {
    "option-data": [
        ...,
        {
            <userinput>"name": "vendor-encapsulated-options"
            "space": "dhcp6",
            "code": 17,
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            "csv-format": true,
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            "data": "12345"</userinput>
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        }
    ],
    ...
}</screen>
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      </para>
    </section>

    <section id="dhcp6-option-spaces">
      <title>Nested DHCPv6 options (custom option spaces)</title>
      <para>It is sometimes useful to define completely new option
      spaces.  This is useful if the user wants his new option to
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      convey sub-options that use a separate numbering scheme, for
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      example sub-options with codes 1 and 2. Those option codes
      conflict with standard DHCPv6 options, so a separate option
      space must be defined.
      </para>
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      <para>Note that it is not required to create a new option space when
      defining sub-options for a standard option because it is
      created by default if the standard option is meant to convey
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      any sub-options (see <xref linkend="dhcp6-vendor-opts"/>).
      </para>
      <para>
      Assume that we want to have a DHCPv6 option called "container"
      with code 102 that conveys two sub-options with codes 1 and 2.
      First we need to define the new sub-options:
<screen>
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"Dhcp6": {
    "option-def": [
        {
            <userinput>"name": "subopt1",
            "code": 1,
            "space": "isc",
            "type": "ipv6-address".
            "record-types": "",
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            "array": false,
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            "encapsulate ""
        },
        {
            "name": "subopt2",
            "code": 2,
            "space": "isc",
            "type": "string",
            "record-types": "",
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            "array": false
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            "encapsulate": ""</userinput>
        }
    ],
    ...
}</screen>
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    Note that we have defined the options to belong to a new option space
    (in this case, "isc").
    </para>
    <para>
The next step is to define a regular DHCPv6 option and specify that it
should include options from the isc option space:
<screen>
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"Dhcp6": {
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    "option-def": [
        ...,
        {
            <userinput>"name": "container",
            "code": 102,
            "space": "dhcp6",
            "type": "empty",
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            "array": false,
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            "record-types": "",
            "encapsulate": "isc"</userinput>
        }
    ],
    ...
}</screen>

    The name of the option space in which the sub-options are defined is set in
    the <command>encapsulate</command> field. The <command>type</command> field
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    is set to <command>empty</command> which limits this option to only carrying
    data in sub-options.
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    </para>
    <para>
    Finally, we can set values for the new options:
<screen>
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"Dhcp6": {
    "option-data": [
        {
            <userinput>"name": "subopt1",
            "space": "isc",
            "code": 1,
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            "csv-format": true,
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            "data": "2001:db8::abcd"</userinput>
        },
        }
            <userinput>"name": "subopt2",
            "space": "isc",
            "code": 2,
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            "csv-format": true,
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            "data": "Hello world"</userinput>
        },
        {
            <userinput>"name": "container",
            "space": "dhcp6",
            "code": 102,
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            "csv-format": true,
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            "data": ""</userinput>
        }
    ],
    ...
}
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</screen>
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    Even though the "container" option does not carry any data except
    sub-options, the "data" field must be explicitly set to an empty value.
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    This is required because in the current version of Kea, the default
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    configuration values are not propagated to the configuration parsers: if the
    "data" is not set the parser will assume that this parameter is not
    specified and an error will be reported.
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    </para>
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    <para>Note that it is possible to create an option which carries some data
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    in addition to the sub-options defined in the encapsulated option space.
    For example, if the "container" option from the previous example was
    required to carry an uint16 value as well as the sub-options, the "type"
    value would have to be set to "uint16" in the option definition. (Such an
    option would then have the following data structure: DHCP header, uint16
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    value, sub-options.) The value specified with the "data" parameter &mdash; which
    should be a valid integer enclosed in quotes, e.g. "123" &mdash; would then be
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    assigned to the uint16 field in the "container" option.
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    </para>
    </section>

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    <section id="dhcp6-config-subnets">
      <title>IPv6 Subnet Selection</title>
      <para>
        The DHCPv6 server may receive requests from local (connected to the
        same subnet as the server) and remote (connecting via relays) clients.
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        As the server may have many subnet configurations defined, it must select
        an appropriate subnet for a given request.
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      </para>
      <para>
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        The server can not assume which of the configured subnets are local. In IPv4
        it is possible as there is a reasonable expectation that the
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        server will have a (global) IPv4 address configured on the interface,
        and can use that information to detect whether a subnet is local or
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        not. That assumption is not true in IPv6, the DHCPv6 server must be able
        to operate while only having link-local addresses. Therefore an optional
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        &quot;interface&quot; parameter is available within a subnet definition
        to designate that a given subnet is local, i.e. reachable directly over
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        the specified interface. For example the server that is intended to serve
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        a local subnet over eth0 may be configured as follows:
	<screen>
"Dhcp6": {
    "subnet6": [
        {
            "subnet": "2001:db8:beef::/48",
            "pools": [
                 {
                     "pool": "2001:db8:beef::/48"
                 }
             ],
            <userinput>"interface": "eth0"</userinput>
        }
    ],
    ...
}
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</screen>
        </para>
      </section>

      <section id="dhcp6-relays">
        <title>DHCPv6 Relays</title>
        <para>
          A DHCPv6 server with multiple subnets defined must select the
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          appropriate subnet when it receives a request from a client.  For clients
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          connected via relays, two mechanisms are used:
        </para>
        <para>
          The first uses the linkaddr field in the RELAY_FORW message. The name
          of this field is somewhat misleading in that it does not contain a link-layer
          address: instead, it holds an address (typically a global address) that is
          used to identify a link. The DHCPv6 server checks if the address belongs
          to a defined subnet and, if it does, that subnet is selected for the client's
          request.
        </para>
        <para>
          The second mechanism is based on interface-id options. While forwarding a client's
          message, relays may insert an interface-id option into the message that
          identifies the interface on the relay that received the message. (Some
          relays allow configuration of that parameter, but it is sometimes
          hardcoded and may range from the very simple (e.g. "vlan100") to the very cryptic:
          one example seen on real hardware was "ISAM144|299|ipv6|nt:vp:1:110"). The
          server can use this information to select the appropriate subnet.
          The information is also returned to the relay which then knows the
          interface to use to transmit the response to the client. In order for
          this to work successfully, the relay interface IDs must be unique within
          the network and the server configuration must match those values.
        </para>
        <para>
          When configuring the DHCPv6 server, it should be noted that two
          similarly-named parameters can be configured for a subnet:
          <itemizedlist>
            <listitem><simpara>
              "interface" defines which local network interface can be used
              to access a given subnet.
            </simpara></listitem>
            <listitem><simpara>
              "interface-id" specifies the content of the interface-id option
              used by relays to identify the interface on the relay to which
              the response packet is sent.
            </simpara></listitem>
          </itemizedlist>
          The two are mutually exclusive: a subnet cannot be both reachable locally
          (direct traffic) and via relays (remote traffic). Specifying both is a
          configuration error and the DHCPv6 server will refuse such a configuration.
        </para>

        <para>
          To specify interface-id with value "vlan123", the following commands can
          be used:
          <screen>
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"Dhcp6": {
    "subnet6": [
        {
            "subnet": "2001:db8:beef::/48",
            "pools": [
                 {
                     "pool": "2001:db8:beef::/48"
                 }
             ],
            <userinput>"interface-id": "vlan123"</userinput>
        }
    ],
    ...
}
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</screen>
        </para>
      </section>

    <section id="dhcp6-client-classifier">
      <title>Client Classification in DHCPv6</title>
      <note>
      <para>
        DHCPv6 server has been extended to support limited client classification.
        Although the current capability is modest, it is expected to be expanded
        in the future. It is envisaged that the majority of client classification
        extensions will be using hooks extensions.
      </para>
      </note>
      <para>In certain cases it is useful to differentiate between different types
      of clients and treat them differently. The process of doing classification
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      is conducted in two steps. The first step is to assess an incoming packet and
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      assign it to zero or more classes. This classification is currently simple,
      but is expected to grow in capability soon. Currently the server checks whether
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      the incoming packet includes vendor class option (16). If it has, the content
      of that option is prepended with &quot;VENDOR_CLASS_&quot; then it is interpreted as a
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      class. For example, modern cable modems will send this option with value
      &quot;docsis3.0&quot; and as a result the packet will belong to class
      &quot;VENDOR_CLASS_docsis3.0&quot;.
      </para>

      <para>It is envisaged that the client classification will be used for changing
      behavior of almost any part of the DHCP engine processing, including assigning
      leases from different pools, assigning different option (or different values of
      the same options) etc. For now, there is only one mechanism that is taking
      advantage of client classification: subnet selection.</para>

      <para>
        Kea can be instructed to limit access to given subnets based on class information.
        This is particularly useful for cases where two types of devices share the
        same link and are expected to be served from two different subnets. The
        primary use case for such a scenario are cable networks. There are two
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        classes of devices: the cable modem itself, which should be handed a lease
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        from subnet A and all other devices behind modems that should get a lease
        from subnet B. That segregation is essential to prevent overly curious
        users from playing with their cable modems. For details on how to set up
        class restrictions on subnets, see <xref linkend="dhcp6-subnet-class"/>.
      </para>

    </section>

    <section id="dhcp6-subnet-class">
      <title>Limiting access to IPv6 subnet to certain classes</title>
      <para>
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        In certain cases it beneficial to restrict access to certain subnets
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        only to clients that belong to a given class. For details on client
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        classes, see <xref linkend="dhcp6-client-classifier"/>. This is an
        extension of a previous example from <xref linkend="dhcp6-address-config"/>.

        Let's 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. Only clients belonging to the
        eRouter1.0 client class are allowed to use that pool. Such a
        configuration can be achieved in the following way:

        <screen>
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"Dhcp6": {
    "subnet6": [
        {
            "subnet": "2001:db8:1::/64",
            "pools": [
                 {
                     "pool": "2001:db8:1::-2001:db8:1::ffff"
                 }
             ],
            <userinput>"client-class": "VENDOR_CLASS_eRouter1.0"</userinput>
        }
    ],
    ...
}
</screen>
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      </para>

      <para>
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        Care should be taken with client classification as it is easy for
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        clients that do not meet class criteria to be denied any service altogether.
      </para>
    </section>


    <section id="dhcp6-ddns-config">
      <title>Configuring DHCPv6 for DDNS</title>
      <para>
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      As mentioned earlier, kea-dhcp6 can be configured to generate requests to
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      the DHCP-DDNS server (referred to here as "D2") to update
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      DNS entries.  These requests are known as NameChangeRequests or NCRs.
      Each NCR contains the following information:
      <orderedlist>
      <listitem><para>
      Whether it is a request to add (update) or remove DNS entries
      </para></listitem>
      <listitem><para>
      Whether the change requests forward DNS updates (AAAA records), reverse
      DNS updates (PTR records), or both.
      </para></listitem>
      <listitem><para>
      The FQDN, lease address, and DHCID
      </para></listitem>
      </orderedlist>
      The parameters controlling the generation of NCRs for submission to D2
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      are contained in the "dhcp-ddns" section of kea-dhcp6
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      configuration. The default values for this section appears as follows:
<screen>
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"Dhcp6": {
    "dhcp-ddns": {
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        <userinput>"enable-updates": true,
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        "server-ip": "127.0.0.1",
        "server-port": 53001,
        "sender-ip": "",
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        "sender-port": 0,
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        "max-queue-size": 1024,
        "ncr-protocol": "UDP",
        "ncr-format": "JSON",
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        "override-no-update": false,
        "override-client-update": false,
        "replace-client-name": false,
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        "generated-prefix": "myhost",
        "qualifying-suffix": "example.com"</userinput>
    },
    ...
}
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</screen>
      </para>
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      <section id="dhcpv6-d2-io-config">
      <title>DHCP-DDNS Server Connectivity</title>
      <para>
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      In order for NCRs to reach the D2 server, kea-dhcp6 must be able
      to communicate with it.  kea-dhcp6 uses the following configuration
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      parameters to control how it communications with D2:
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      <itemizedlist>
      <listitem><simpara>
      <command>enable-updates</command> - determines whether or not kea-dhcp6 will
      generate NCRs.  If missing, this value is assumed to be false hence DDNS updates
      are disabled.  To enable DDNS updates set this value to true:
      </simpara></listitem>
      <listitem><simpara>
      <command>server-ip</command> - IP address on which D2 listens for requests. The default is
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      the local loopback interface at address 127.0.0.1. You may specify
      either an IPv4 or IPv6 address.
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      </simpara></listitem>
      <listitem><simpara>
      <command>server-port</command> - port on which D2 listens for requests.  The default value
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      is 53001.
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      </simpara></listitem>
      <listitem><simpara>
      <command>sender-ip</command> - IP address which kea-dhcp6 should use to send requests to D2.
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      The default value is blank which instructs kea-dhcp6 to select a suitable
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      address.
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      </simpara></listitem>
      <listitem><simpara>
      <command>sender-port</command> - port which kea-dhcp6 should use to send requests to D2. The
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      default value of 0 instructs kea-dhcp6 to select a suitable port.
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      </simpara></listitem>
      <listitem><simpara>
      <command>max-queue-size</command> - maximum number of requests allowed to queue waiting to
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      be sent to D2. This value guards against requests accumulating
      uncontrollably if they are being generated faster than they can be
      delivered.  If the number of requests queued for transmission reaches
      this value, DDNS updating will be turned off until the queue backlog has
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      been sufficiently reduced.  The intent is to allow kea-dhcp6 to
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      continue lease operations.  The default value is 1024.
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      </simpara></listitem>
      <listitem><simpara>
      <command>ncr-format</command> - Socket protocol use when sending requests to D2.  Currently
      only UDP is supported.  TCP may be available in an upcoming release.
      </simpara></listitem>
      <listitem><simpara>
      <command>ncr-protocol</command> - Packet format to use when sending requests to D2.
      Currently only JSON format is supported.  Other formats may be available
      in future releases.
      </simpara></listitem>
      </itemizedlist>
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      By default, kea-dhcp-ddns is assumed to running on the same machine as kea-dhcp6, and
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      all of the default values mentioned above should be sufficient.
      If, however, D2 has been configured to listen on a different address or
      port, these values must altered accordingly. For example, if D2 has been
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      configured to listen on 2001:db8::5 port 900, the following commands
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      would be required:
<screen>
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"Dhcp6": {
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    "dhcp-ddns": {
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        <userinput>"server-ip": "2001:db8::5",
        "server-port": 900</userinput>,
        ...
    },
    ...
}
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</screen>
      </para>
      </section>
      <section id="dhcpv6-d2-rules-config">
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      <title>When does kea-dhcp6 generate DDNS request</title>
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      <para>kea-dhcp6 follows the behavior prescribed for DHCP servers in
      <ulink url="http://tools.ietf.org/html/rfc4704">RFC 4704</ulink>.
      It is important to keep in mind that kea-dhcp6 provides the initial
      decision making of when and what to update and forwards that
      information to D2 in the form of NCRs. Carrying out the actual
      DNS updates and dealing with such things as conflict resolution
      are the purview of D2 (<xref linkend="dhcp-ddns-server"/>).</para>
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      <para>
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      This section describes when kea-dhcp6 will generate NCRs and the
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      configuration parameters that can be used to influence this decision.
      It assumes that the "enable-updates" parameter is true.
      </para>
      <note>
        <para>
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        Currently the interface between kea-dhcp6 and D2 only supports requests
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        which update DNS entries for a single IP address.  If a lease grants
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        only the first of these addresses.  Support for multiple address
        mappings may be provided in a future release.
        </para>
      </note>
      <para>
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      In general, kea-dhcp6 will generate DDNS update requests when:
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      <orderedlist>
      <listitem><para>
      A new lease is granted in response to a DHCP REQUEST
      </para></listitem>
      <listitem><para>
      An existing lease is renewed but the FQDN associated with it has
      changed.
      </para></listitem>
      <listitem><para>
      An existing lease is released in response to a DHCP RELEASE
      </para></listitem>
      </orderedlist>
      In the second case, lease renewal, two  DDNS requests will be issued: one
      request to remove entries for the previous FQDN and a second request to
      add entries for the new FQDN.  In the last case, a lease release, a
      single DDNS request to remove its entries will be made.  The decision
      making involved when granting a new lease is more involved and is
      discussed next.
      </para>
      <para>
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      kea-dhcp6 will generate a DDNS update request only if the DHCP REQUEST
      contains the FQDN option (code 39). By default kea-dhcp6 will
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      respect the FQDN N and S flags specified by the client as shown in the
      following table:
      </para>
        <table id="dhcp6-fqdn-flag-table">
          <title>Default FQDN Flag Behavior</title>
          <tgroup cols='4' align='left'>
          <colspec colname='cflags'/>
          <colspec colname='meaning'/>
          <colspec colname='response'/>
          <colspec colname='sflags'/>
          <thead>
              <row>
                <entry>Client Flags:N-S</entry>
                <entry>Client Intent</entry>
                <entry>Server Response</entry>
                <entry>Server Flags:N-S-O</entry>
              </row>
          </thead>
          <tbody>
            <row>
                <entry>0-0</entry>
                <entry>
                Client wants to do forward updates, server should do reverse updates
                </entry>
                <entry>Server generates reverse-only request</entry>
                <entry>1-0-0</entry>
            </row>
            <row>
                <entry>0-1</entry>
                <entry>Server should do both forward and reverse updates</entry>
                <entry>Server generates request to update both directions</entry>
                <entry>0-1-0</entry>
            </row>
            <row>
                <entry>1-0</entry>
                <entry>Client wants no updates done</entry>
                <entry>Server does not generate a request</entry>
                <entry>1-0-0</entry>
            </row>
          </tbody>
          </tgroup>
        </table>
      <para>
      The first row in the table above represents "client delegation". Here
      the DHCP client states that it intends to do the forward DNS updates and
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      the client's wishes and generate a DDNS request to D2 to update only
      reverse DNS data.  The parameter, "override-client-update", can be used
      to instruct the server to override client delegation requests.  When
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      delegation and generate a DDNS request to update both forward and
      reverse DNS data.  In this case, the N-S-O flags in the server's
      response to the client will be 0-1-1 respectively.
      </para>
      <para>
      (Note that the flag combination N=1, S=1 is prohibited according to
      RFC 4702. If such a combination is received from the client, the packet
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      </para>
      <para>
      To override client delegation, issue the following commands:
      </para>
<screen>
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"Dhcp6": {
    "dhcp-ddns": {
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        <userinput>"override-client-update": true</userinput>,
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        ...
    },
    ...
}
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</screen>
      <para>
      The third row in the table above describes the case in which the client
      requests that no DNS updates be done. The parameter, "override-no-update",
      can be used to instruct the server to disregard the client's wishes. When
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      this parameter is true, kea-dhcp6 will generate DDNS update requests to 
      kea-dhcp-ddns even if the client requests no updates be done.  The N-S-O 
      flags in the server's response to the client will be 0-1-1.