kea/doc/guide/ddns.xml

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  <chapter id="dhcp-ddns-server">
    <title>The DHCP-DDNS Server</title>
    <para>
    The DHCP-DDNS Server (kea-dhcp-ddns, known informally as D2) conducts the client side of
    the DDNS protocol (defined in RFC 2136) on behalf of the DHCPv4 and DHCPv6
    servers (kea-dhcp4 and kea-dhcp6 respectively). The DHCP servers construct
    DDNS update requests, known as NameChangeRequests (NCRs), based upon DHCP
    lease change events and then post these to D2. D2 attempts to match
    each such request to the appropriate DNS server(s) and carry out the
    necessary conversation with those servers to update the DNS data.
    </para>
    <para>
    In order to match a request to appropriate DNS servers, D2 must have a
    catalog of servers from which to select. In fact, D2 has two such catalogs,
    one for forward DNS and one for reverse DNS; these catalogs are referred
    to as DDNS Domain Lists.  Each list consists of one or more named DDNS
    Domains. Further, each DDNS Domain has a list of one or more DNS
    servers that publish the DNS data for that domain.
    </para>
    <para>
    When conducting forward domain matching, D2 will compare the FQDN in
    the request against the name of each forward DDNS Domain. The domain
    whose name matches the longest portion of the FQDN is considered the
    best match.  For example, if the FQDN is "myhost.sample.example.com.",
    and there are two forward domains in the catalog: "sample.example.com."
    and "example.com.", the former is regarded as the best match.  In some
    cases, it may not be possible to find a suitable match. Given the same two
    forward domains there would be no match for the FQDN, "bogus.net", so the
    request would be rejected.   Finally, if there are no forward DDNS Domains
    defined, D2 will simply disregard the forward update portion of requests.
    </para>
    <para>
    When conducting reverse domain matching, D2 constructs a reverse
    FQDN from the lease address in the request and compare that against
    the name of each reverse DDNS Domain. Again, the domain whose name matches
    the longest portion of the FQDN is considered the best match. For instance,
    if the lease address is "172.16.1.40" and there are two reverse domains in
    the catalog: "1.16.172.in-addr.arpa." and "16.172.in-addr.arpa", the
    former is the best match.  As with forward matching, it is possible to not
    find a suitable match.  Given the same two domains, there would be no
    match for the lease address, "192.168.1.50", and the request would be
    rejected. Finally, if there are no reverse DDNS Domains defined, D2 will
    simply disregard the reverse update portion of requests.
    </para>
    <section id="dhcp-ddns-server-start-stop">
      <title>Starting and Stopping the DHCP-DDNS Server</title>

      <para>
      <command>kea-dhcp-ddns</command> is the Kea DHCP-DDNS server
      and, due to the nature of DDNS, it is run alongside either the
      DHCPv4 or DHCPv6 components (or both).  Like other parts of
      Kea, is a separate binary that can be run on its own or through
      <command>keactrl</command> (see <xref linkend="keactrl"/>). In
      normal operation, controlling <command>kea-dhcp-ddns</command>
      with <command>keactrl</command> is recommended. However, it is also
      possible to run the DHCP-DDNS server directly. It accepts the
      following command-line switches:
      </para>

      <itemizedlist>
          <listitem>
            <simpara>
            <command>-c <replaceable>file</replaceable></command> -
            specifies the configuration file. This is the only mandatory
            switch.</simpara>
          </listitem>
          <listitem>
            <simpara>
            <command>-d</command> - specifies whether the server
            logging should be switched to debug/verbose mode. In verbose mode,
            the logging severity and debuglevel specified in a configuration
            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>
          </listitem>
          <listitem>
            <simpara>
              <command>-v</command> - prints out Kea version and exits.
            </simpara>
          </listitem>
          <listitem>
            <simpara>
              <command>-V</command> - prints out Kea extended version with
              additional parameters and exits.
            </simpara>
          </listitem>
      </itemizedlist>

      <para>
      Upon start up the module will load its configuration and begin listening
      for NCRs based on that configuration.
      </para>
    </section> <!-- end start-stop -->
    <section id="d2-configuration">
      <title>Configuring the DHCP-DDNS Server</title>
      <para>
	Before starting <command>kea-dhcp-ddns</command> module for the
	first time, a configuration file needs to be created. The following default
	configuration is a template that can be customised to your requirements.
<screen>
<userinput>"DhcpDdns": {
    "ip_address": "127.0.0.1",
    "port": 53001,
    "dns_server_timeout": 100,
    "ncr_protocol": "UDP",
    "ncr_format": "JSON",
    "tsig_keys": [ ],
    "forward_ddns": {
	"ddns_domains": [ ]
    },
    "reverse_ddns": {
	"ddns_domains": [ ]
    }
}</userinput>
</screen>
      </para>
      <para>
      The configuration can be divided as follows, each of which is described
      in its own section:
      </para>
	<itemizedlist>
	  <listitem>
	    <simpara>
	      <command>Global Server Parameters</command> -
	      values which control connectivity and global server behavior
	    </simpara>
	  </listitem>
	  <listitem>
	    <simpara>
	      <command>TSIG Key Info</command> -
	      defines the TSIG keys used for secure traffic with DNS servers
	    </simpara>
	  </listitem>
	  <listitem>
	    <simpara>
	      <command>Forward DDNS</command> -
	      defines the catalog of Forward DDNS Domains
	    </simpara>
	  </listitem>
	  <listitem>
	    <simpara>
	      <command>Reverse DDNS</command> -
	      defines the catalog of Forward DDNS Domains
	    </simpara>
	  </listitem>
	</itemizedlist>
      <section id="d2-server-parameter-config">
	<title>Global Server Parameters</title>
      <itemizedlist>

      <listitem><simpara>
      <command>ip_address</command> - IP address on which D2
      listens for requests. The default is the local loopback interface at
      address 127.0.0.1. You may specify either an IPv4 or IPv6 address.
      </simpara></listitem>

      <listitem><simpara>
      <command>port</command> - Port on which D2 listens for requests.  The default value
      is 53001.
      </simpara></listitem>

      <listitem><simpara>
      <command>dns_server_timeout</command> - The maximum amount
      of time in milliseconds, that D2 will wait for a response from a
      DNS server to a single DNS update message.
      </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>

      <listitem><simpara>
      <command>ncr_format</command> - Socket protocol to use when sending requests to D2.
      Currently only UDP is supported.  TCP may be available in an upcoming
      release.
      </simpara></listitem>

      </itemizedlist>
	<para>
	D2 must listen for change requests on a known address and port.  By
	default it listens at 127.0.0.1 on port 53001. The following example
	illustrates how to change D2's global parameters so it will listen
	at 192.168.1.10 port 900:
<screen>
"DhcpDdns": {
    <userinput>"ip_address": "192.168.1.10",
    "port": 900,</userinput>
    ...
    }
}</screen>
	</para>
	<warning>
	  <simpara>
	    It is possible for a malicious attacker to send bogus
	    NameChangeRequests to the DHCP-DDNS server.  Addresses
	    other than the IPv4 or IPv6 loopback addresses (127.0.0.1
	    or ::1) should only be used for testing purposes, but
	    note that local users may still communicate with the
	    DHCP-DDNS server.  A future version of Kea will implement
	    authentication to guard against such attacks.
	  </simpara>
<!-- see ticket #3514 -->
	</warning>
<note>
<simpara>
If the ip_address and port are changed, it will be necessary to change the
corresponding values in the DHCP servers' "dhcp-ddns" configuration section.
</simpara>
</note>
      </section> <!-- "d2-server-parameter-config" -->

      <section id="d2-tsig-key-list-config">
	<title>TSIG Key List</title>
	<para>
	A DDNS protocol exchange can be conducted with or without TSIG
	(defined in <ulink url="http://tools.ietf/org/html/rfc2845">RFC
	2845</ulink>). This configuration section allows the administrator
	to define the set of TSIG keys that may be used in such
	exchanges.</para>

	<para>To use TSIG when updating entries in a DNS Domain,
	a key must be defined in the TSIG Key List and referenced by
	name in that domain's configuration entry.  When D2 matches a
	change request to a domain, it checks whether the domain has
	a TSIG key associated with it.  If so, D2 will use that key to
	sign DNS update messages sent to and verify responses received
	from the domain's DNS server(s). For each TSIG key required by
	the DNS servers that D2 will be working with there must be a
	corresponding TSIG key in the TSIG Key list.</para>

	<para>
	As one might gather from the name, the tsig_key section of the
	D2 configuration lists the TSIG keys.  Each entry describes a
	TSIG key used by one or more DNS servers to authenticate requests
	and sign responses.  Every entry in the list has three parameters:
	<itemizedlist>
	  <listitem>
	    <simpara>
	      <command>name</command> -
	      a unique text label used to identify this key within the
	      list.  This value is used to specify which key (if any) should be
	      used when updating a specific domain. So long as it is unique its
	      content is arbitrary, although for clarity and ease of maintenance
	      it is recommended that it match the name used on the DNS server(s).
	      It cannot be blank.
	    </simpara>
	  </listitem>
	  <listitem>
	    <simpara>
	      <command>algorithm</command> -
	      specifies which hashing algorithm should be used with this
	      key.  This value must specify the same algorithm used for the
	      key on the DNS server(s). The supported algorithms are listed below:
	      <itemizedlist>
		<listitem>
		   <command>HMAC-MD5</command>
		</listitem>
		<listitem>
		    <command>HMAC-SHA1</command>
		</listitem>
		<listitem>
		  <command>HMAC-SHA224</command>
	      </listitem>
	      <listitem>
		  <command>HMAC-SHA256</command>
	      </listitem>
	      <listitem>
		  <command>HMAC-SHA384</command>
		  </listitem>
	      <listitem>
		  <command>HMAC-SHA512</command>
	      </listitem>
	      </itemizedlist>
	      This value is not case sensitive.
	    </simpara>
	  </listitem>
	  <listitem>
	    <simpara>
	      <command>secret</command> -
	      is used to specify the shared secret key code for this key.  This value is
	      case sensitive and must exactly match the value specified on the DNS server(s).
	      It is a base64-encoded text value.
	    </simpara>
	  </listitem>
	</itemizedlist>
	</para>
	<para>
	As an example, suppose that a domain D2 will be updating is
	maintained by a BIND9 DNS server which requires dynamic updates
	to be secured with TSIG.  Suppose further that the entry for
	the TSIG key in BIND9's named.conf file looks like this:
<screen>
   :
   key "key.four.example.com." {
       algorithm hmac-sha224;
       secret "bZEG7Ow8OgAUPfLWV3aAUQ==";
   };
   :
</screen>
	By default, the TSIG Key list is empty:
<screen>
"DhcpDdns": {
   <userinput>"tsig_keys": [ ]</userinput>,
   ...
}
</screen>

	We must extend the list with a new key:
<screen>
"DhcpDdns": {
    "tsig_keys": [
    <userinput>    {
	    "name": "key.four.example.com",
	    "algorithm": "HMAC-SHA224",
	    "secret": "bZEG7Ow8OgAUPfLWV3aAUQ=="
	}</userinput>
    ],
    ...
}
</screen>
	</para>

	<para>These steps would be repeated for each TSIG key needed.  Note that
	the same TSIG key can be used with more than one domain.</para>
      </section>
	<!-- "d2-tsig-key-list-config" -->

      <section id="d2-forward-ddns-config">
	<title>Forward DDNS</title>
	<para>
	The Forward DDNS section is used to configure D2's forward update
	behavior. Currently it contains a single parameter, the catalog of
	forward DDNS Domains, which is a list of structures.
<screen>
"DhcpDdns": {
    <userinput>"forward_ddns": {
	"ddns_domains": [ ]
    }</userinput>,
    ...
}
</screen>

	By default, this list is empty, which will cause the server to ignore
	the forward update portions of requests.
	</para>
	<section id="add-forward-ddns-domain">
	  <title>Adding Forward DDNS Domains</title>
	  <para>
	  A forward DDNS Domain maps a forward DNS zone to a set of
	  DNS servers which maintain the forward DNS data (i.e. name to
	  address mapping) for that zone.  You will need one forward DDNS
	  Domain for each zone you wish to service.  It may very well
	  be that some or all of your zones are maintained by the same
	  servers. You will still need one DDNS Domain per zone. Remember
	  that matching a request to the appropriate server(s) is done
	  by zone and a DDNS Domain only defines a single zone.
	  </para>
	  <para>
	  The section describes how to add Forward DDNS Domains. Repeat these
	  steps for each Forward DDNS Domain desired.  Each Forward DDNS Domain
	  has the following parameters:
	  <itemizedlist>
	    <listitem>
	      <simpara>
	      <command>name</command> -
	      The fully qualified domain name (or zone) that this DDNS Domain
	      can update.  This is value used to compare against the request
	      FQDN during forward matching.  It must be unique within the
	      catalog.
	      </simpara>
	    </listitem>
	    <listitem>
	      <simpara>
	      <command>key_name</command> -
	      If TSIG is used with this domain's servers, this
	      value should be the name of the key from within the TSIG Key List
	      to use.  If the value is blank (the default), TSIG will not be
	      used in DDNS conversations with this domain's servers.
	      </simpara>
	    </listitem>
	    <listitem>
	      <simpara>
	      <command>dns_servers</command> -
	      A list of one or more DNS servers which can conduct the server
	      side of the DDNS protocol for this domain.  The servers
	      are used in a first to last preference. In other words, when D2
	      begins to process a request for this domain it will pick the
	      first server in this list and attempt to communicate with it.
	      If that attempt fails, it will move to next one in the list and
	      so on until the it achieves success or the list is exhausted.
	      </simpara>
	    </listitem>
	  </itemizedlist>
	To create a new forward DDNS Domain, one must add a new domain
	element and set its parameters:
<screen>
"DhcpDdns": {
    "forward_ddns": {
	"ddns_domains": [
	    <userinput>{
		"name": "other.example.com",
		"key_name": "",
		"dns_servers": [
		]
	    }</userinput>
	]
    }
}
</screen>

	It is permissible to add a domain without any servers. If that domain
	should be matched to a request, however, the request will fail.  In
	order to make the domain useful though, we must add at least one DNS
	server to it.
	</para>

	<section id="add-forward-dns-servers">
	  <title>Adding Forward DNS Servers</title>
	  <para>
	  The section describes how to add DNS servers to a Forward DDNS Domain.
	  Repeat them for as many servers as desired for a each domain.
	  </para>
	  <para>
	  Forward DNS Server entries represent actual DNS servers which
	  support the server side of the DDNS protocol. Each Forward DNS Server
	  has the following parameters:
	  <itemizedlist>
	    <listitem>
	      <simpara>
	      <command>hostname</command> -
	      The resolvable host name of the DNS server. This value is not
	      yet implemented.
	      </simpara>
	    </listitem>
	    <listitem>
	      <simpara>
	      <command>ip_address</command> -
	      The IP address at which the server listens for DDNS requests.
	      This may be either an IPv4 or an IPv6 address.
	      </simpara>
	    </listitem>
	    <listitem>
	      <simpara>
	      <command>port</command> -
	      The port on which the server listens for DDNS requests. It
	      defaults to the standard DNS service port of 53.
	      </simpara>
	    </listitem>
	  </itemizedlist>
	  To create a new forward DNS Server, one must add a new server
	  element to the domain and fill its parameters.  If for
	example the service is running at "172.88.99.10", then set it as
	follows:
<screen>
"DhcpDdns": {
    "forward_ddns": {
	"ddns_domains": [
	    {
		"name": "other.example.com",
		"key_name": "",
		"dns_servers": [
		    <userinput>{
			"hostname": "",
			"ip_address": "172.88.99.10",
			"port": 53
		    }</userinput>
		]
	    }
	]
    }
}
</screen>
	  </para>

    <note><simpara>
	As stated earlier, "hostname" is not yet supported so, the parameter
	"ip_address" must be set to the address of the DNS server.
    </simpara></note>

	</section> <!-- "add-forward-dns-servers" -->

      </section> <!-- "add-forward-ddns-domains" -->

      </section> <!-- "d2-forward-ddns-config" -->

      <section id="d2-reverse-ddns-config">
	<title>Reverse DDNS</title>
	<para>
	The Reverse DDNS section is used to configure D2's reverse update
	behavior, and the concepts are the same as for the forward DDNS
	section. Currently it contains a single parameter, the catalog of
	reverse DDNS Domains, which is a list of structures.
<screen>
"DhcpDdns": {
    <userinput>"reverse_ddns": {
	"ddns_domains": [ ]
    }</userinput>
    ...
}
</screen>
	By default, this list is empty, which will cause the server to ignore
	the reverse update portions of requests.
	</para>
	<section id="add-reverse-ddns-domain">
	  <title>Adding Reverse DDNS Domains</title>
	  <para>
	  A reverse DDNS Domain maps a reverse DNS zone to a set of DNS
	  servers which maintain the reverse DNS data (address to name
	  mapping) for that zone.  You will need one reverse DDNS Domain
	  for each zone you wish to service.  It may very well be that
	  some or all of your zones are maintained by the same servers;
	  even then, you will still need one DDNS Domain entry for each
	  zone. Remember that matching a request to the appropriate
	  server(s) is done by zone and a DDNS Domain only defines a
	  single zone.
	  </para>
	  <para>
	  The section describes how to add Reverse DDNS Domains. Repeat these
	  steps for each Reverse DDNS Domain desired.  Each Reverse DDNS Domain
	  has the following parameters:
	  <itemizedlist>
	    <listitem>
	      <simpara>
	      <command>name</command> -
	      The fully qualified reverse zone that this DDNS Domain
	      can update.  This is the value used during reverse matching
	      which will compare it with a reversed version of the request's
	      lease address. The zone name should follow the appropriate
	      standards: for example, to to support the IPv4 subnet 172.16.1,
	      the name should be. "1.16.172.in-addr.arpa.".  Similarly,
	      to support an IPv6 subent of 2001:db8:1, the name should be
	      "1.0.0.0.8.B.D.0.1.0.0.2.ip6.arpa."
	      Whatever the name, it must be unique within the catalog.
	      </simpara>
	    </listitem>
	    <listitem>
	      <simpara>
	      <command>key_name</command> -
	      If TSIG should be used with this domain's servers, then this
	      value should be the name of that key from the TSIG Key List.
	      If the value is blank (the default), TSIG will not be
	      used in DDNS conversations with this domain's servers.  Currently
	      this value is not used as TSIG has not been implemented.
	      </simpara>
	    </listitem>
	    <listitem>
	      <simpara>
	      <command>dns_servers</command> -
	      a list of one or more DNS servers which can conduct the server
	      side of the DDNS protocol for this domain.  Currently the servers
	      are used in a first to last preference. In other words, when D2
	      begins to process a request for this domain it will pick the
	      first server in this list and attempt to communicate with it.
	      If that attempt fails, it will move to next one in the list and
	      so on until the it achieves success or the list is exhausted.
	      </simpara>
	    </listitem>
	  </itemizedlist>
	To create a new reverse DDNS Domain, one must add a new domain element
	and set its parameters. For example, to support subnet 2001:db8:1::,
	the following configuration could be used:
<screen>
"DhcpDdns": {
    "reverse_ddns": {
	"ddns_domains": [
	    <userinput>{
		"name": "1.0.0.0.8.B.D.0.1.0.0.2.ip6.arpa.",
		"key_name": "",
		"dns_servers": [
		]
	    }</userinput>
	]
    }
}
</screen>

	It is permissible to add a domain without any servers. If that domain
	should be matched to a request, however, the request will fail.  In
	order to make the domain useful though, we must add at least one DNS
	server to it.
	</para>

	<section id="add-reverse-dns-servers">
	  <title>Adding Reverse DNS Servers</title>
	  <para>
	  The section describes how to add DNS servers to a Reverse DDNS Domain.
	  Repeat them for as many servers as desired for a each domain.
	  </para>
	  <para>
	  Reverse DNS Server entries represents a actual DNS servers which
	  support the server side of the DDNS protocol. Each Reverse DNS Server
	  has the following parameters:
	  <itemizedlist>
	    <listitem>
	      <simpara>
	      <command>hostname</command> -
	      The resolvable host name of the DNS server. This value is
	      currently ignored.
	      </simpara>
	    </listitem>
	    <listitem>
	      <simpara>
	      <command>ip_address</command> -
	      The IP address at which the server listens for DDNS requests.
	      </simpara>
	    </listitem>
	    <listitem>
	      <simpara>
	      <command>port</command> -
	      The port on which the server listens for DDNS requests. It
	      defaults to the standard DNS service port of 53.
	      </simpara>
	    </listitem>
	  </itemizedlist>
	  To create a new reverse DNS Server, one must first add a new server
	  element to the domain and fill its parameters.  If for
	example the service is running at "172.88.99.10", then set it as
	follows:
<screen>
"DhcpDdns": {
    "reverse_ddns": {
	"ddns_domains": [
	    {
		"name": "1.0.0.0.8.B.D.0.1.0.0.2.ip6.arpa.",
		"key_name": "",
		"dns_servers": [
		    <userinput>{
			"hostname": "",
			"ip_address": "172.88.99.10",
			"port": 53
		    }</userinput>
		]
	    }
	]
    }
}
</screen>
</para>

    <note>
    <simpara>
    As stated earlier, "hostname" is not yet supported so, the parameter
    "ip_address" must be set to the address of the DNS server.
    </simpara>
    </note>

	</section> <!-- "add-reverse-dns-servers" -->

      </section> <!-- "add-reverse-ddns-domains" -->

      </section> <!-- "d2-reverse-ddns-config" -->

      <section id="d2-exmaple-config">
	<title>Example DHCP-DDNS Server Configuration</title>
	<para>
	This section provides an example DHCP-DDNS server configuration based
	on a small example network.  Let's suppose our example network has
	three domains, each with their own subnet.

	<table>
	  <title>Our example network</title>
	  <tgroup cols='4' align='left'>
	  <colspec colname='domain'/>
	  <colspec colname='subnet'/>
	  <colspec colname='fservers'/>
	  <colspec colname='rservers'/>
	  <thead>
	    <row>
	      <entry>Domain</entry>
	      <entry>Subnet</entry>
	      <entry>Forward DNS Servers</entry>
	      <entry>Reverse DNS Servers</entry>
	    </row>
	  </thead>
	  <tbody>
	    <row>
	      <entry>four.example.com</entry>
	      <entry>192.0.2.0/24</entry>
	      <entry>172.16.1.5, 172.16.2.5</entry>
	      <entry>172.16.1.5, 172.16.2.5</entry>
	    </row>
	    <row>
	      <entry>six.example.com</entry>
	      <entry>2001:db8:1::/64</entry>
	      <entry>3001:1::50</entry>
	      <entry>3001:1::51</entry>
	    </row>
	    <row>
	      <entry>example.com</entry>
	      <entry>192.0.0.0/16</entry>
	      <entry>172.16.2.5</entry>
	      <entry>172.16.2.5</entry>
	    </row>
	  </tbody>
	  </tgroup>
	</table>
	</para>
	<para>
	We need to construct three forward DDNS Domains:
	<table>
	  <title>Forward DDNS Domains Needed</title>
	  <tgroup cols='3' align='left'>
	  <colspec colname='num'/>
	  <colspec colname='name'/>
	  <colspec colname='servers'/>
	  <thead>
	    <row>
	      <entry>#</entry>
	      <entry>DDNS Domain Name</entry>
	      <entry>DNS Servers</entry>
	    </row>
	  </thead>
	  <tbody>
	    <row>
	      <entry>1.</entry>
	      <entry>four.example.com.</entry>
	      <entry>172.16.1.5, 172.16.2.5</entry>
	    </row>
	    <row>
	      <entry>2.</entry>
	      <entry>six.example.com.</entry>
	      <entry>3001:1::50</entry>
	    </row>
	    <row>
	      <entry>3.</entry>
	      <entry>example.com.</entry>
	      <entry>172.16.2.5</entry>
	    </row>
	  </tbody>
	  </tgroup>
	</table>
	As discussed earlier, FQDN to domain matching is based on the longest
	match. The FQDN, "myhost.four.example.com.", will match the first
	domain ("four.example.com") while "admin.example.com." will match the
	third domain ("example.com"). The
	FQDN, "other.example.net." will fail to match any domain and would
	be rejected.
	</para>
	<para>
	The following example configuration specified the Forward DDNS Domains.
<screen><userinput>
"DhcpDdns": {
    "forward_ddns": {
	"ddns_domains": [
	    {
		"name": "four.example.com.",
		"key_name": "",
		"dns_servers": [
		    { "ip_address": "172.16.1.5" },
		    { "ip_address": "172.16.2.5" }
		]
	    },
	    {
		"name": "six.example.com.",
		"key_name": "",
		"dns_servers": [
		    { "ip_address": "2001:db8::1" }
		]
	    },
	    {
		"name": "example.com.",
		"key_name": "",
		"dns_servers": [
		    { "ip_address": "172.16.2.5" }
		]
	    },

	]
    }
}</userinput>
</screen>

	</para>
	<para>
	Similarly, we need to construct the three reverse DDNS Domains:
	<table>
	  <title>Reverse DDNS Domains Needed</title>
	  <tgroup cols='3' align='left'>
	  <colspec colname='num'/>
	  <colspec colname='DDNS Domain name'/>
	  <colspec colname='DDNS Domain DNS Servers'/>
	  <thead>
	    <row>
	      <entry>#</entry>
	      <entry>DDNS Domain Name</entry>
	      <entry>DNS Servers</entry>
	    </row>
	  </thead>
	  <tbody>
	    <row>
	      <entry>1.</entry>
	      <entry>2.0.192.in-addr.arpa.</entry>
	      <entry>172.16.1.5, 172.16.2.5</entry>
	    </row>
	    <row>
	      <entry>2.</entry>
	      <entry>1.0.0.0.8.d.b.0.1.0.0.2.ip6.arpa.</entry>
	      <entry>3001:1::50</entry>
	    </row>
	    <row>
	      <entry>3.</entry>
	      <entry>0.182.in-addr.arpa.</entry>
	      <entry>172.16.2.5</entry>
	    </row>
	  </tbody>
	  </tgroup>
	</table>
	An address of "192.0.2.150" will match the first domain,
	"2001:db8:1::10" will match the second domain, and "192.0.50.77"
	the third domain.
	</para>
	<para>
	These Reverse DDNS Domains are specified as follows:

<screen><userinput>
"DhcpDdns": {
    "reverse_ddns": {
	"ddns_domains": [
	    {
		"name": "2.0.192.in-addr.arpa.",
		"key_name": "",
		"dns_servers": [
		    { "ip_address": "172.16.1.5" },
		    { "ip_address": "172.16.2.5" }
		]
	    }
	    {
		"name": "1.0.0.0.8.B.D.0.1.0.0.2.ip6.arpa.",
		"key_name": "",
		"dns_servers": [
		    { "ip_address": "2001:db8::1" }
		]
	    }
	    {
		"name": "0.192.in-addr.arpa.",
		"key_name": "",
		"dns_servers": [
		    { "ip_address": "172.16.2.5" }
		]
	    }
	]
    }
}</userinput>
</screen>

	</para>
	</section> <!-- end of "d2-example" -->
    </section> <!-- end of section "d2-configuration" -->
    <section>
      <title>DHCP-DDNS Server Limitations</title>
      <para>The following are the current limitations of the DHCP-DDNS Server.</para>
      <itemizedlist>
	<listitem>
	  <simpara>
	    Requests received from the DHCP servers are placed in a
	    queue until they are processed.  Currently all queued requests
	    are lost when the server shuts down.
	  </simpara>
	</listitem>
      </itemizedlist>
    </section>
  </chapter> <!-- DHCP-DDNS Server -->