dhcp4-srv.xml

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<chapter xmlns="http://docbook.org/ns/docbook" version="5.0" xml:id="dhcp4">
    <title>The DHCPv4 Server</title>

    <section xml:id="dhcp4-start-stop">
      <title>Starting and Stopping the DHCPv4 Server</title>

      <para>
        It is recommended that the Kea DHCPv4 server be started and stopped
        using <command>keactrl</command> (described in <xref linkend="keactrl"/>).
        However, it is also possible to run the server directly: it accepts
        the following command-line switches:
      </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 the 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>-p <replaceable>port</replaceable></command> -
            specifies UDP port on which the server will listen. This is only
            useful during testing, as a DHCPv4 server listening on
            ports other than the standard ones will not be able to
            handle regular DHCPv4 queries.</simpara>
          </listitem>
          <listitem>
            <simpara>
            <command>-t <replaceable>file</replaceable></command> -
            specifies the configuration file to be tested. Kea-dhcp4
            will attempt to load it, and will conduct sanity
            checks. Note that certain checks are possible only while
            running the actual server. The actual status is reported
            with exit code (0 = configuration looks ok, 1 = error
            encountered). Kea will print out log messages to standard
            output and error to standard error when testing
            configuration.</simpara>
          </listitem>
          <listitem>
            <simpara>
              <command>-v</command> - prints out the Kea version and exits.
            </simpara>
          </listitem>
          <listitem>
            <simpara>
              <command>-V</command> - prints out the Kea extended version with
              additional parameters and exits. The listing includes the versions
              of the libraries dynamically linked to Kea.
            </simpara>
          </listitem>
          <listitem>
            <simpara>
              <command>-W</command> - prints out the Kea configuration report
              and exits. The report is a copy of the
              <filename>config.report</filename> file produced by
              <userinput>./configure</userinput>: it is embedded in the
              executable binary.
            </simpara>
          </listitem>
      </itemizedlist>

      <para>
        The <filename>config.report</filename> may also be accessed more
        directly.  The following command may be used to extract this
        information.  The binary <userinput>path</userinput> may be found
        in the install directory or in the <filename>.libs</filename>
        subdirectory in the source tree. For example
        <filename>kea/src/bin/dhcp4/.libs/kea-dhcp4</filename>.

<screen>
strings <userinput>path</userinput>/kea-dhcp4 | sed -n 's/;;;; //p'
</screen>
      </para>

      <para>
        On start-up, the server will detect available network interfaces
        and will attempt to open UDP sockets on all interfaces
        mentioned in the configuration file.
        Since the DHCPv4 server opens privileged ports, it requires root
        access. Make sure you run this daemon as root.
      </para>

      <para>
        During startup the server will attempt to create a PID file of the
        form: localstatedir]/[conf name].kea-dhcp6.pid where:
        <itemizedlist>
            <listitem>
            <simpara><command>localstatedir</command>: The value as passed into the
            build configure script. It defaults to "/usr/local/var".  (Note
            that this value may be overridden at run time by setting the environment
            variable KEA_PIDFILE_DIR.  This is intended primarily for testing purposes.)
            </simpara>
            </listitem>
            <listitem>
            <simpara><command>conf name</command>: The configuration file name
            used to start the server, minus all preceding path and file extension.
            For example, given a pathname of "/usr/local/etc/kea/myconf.txt", the
            portion used would be "myconf".
            </simpara>
            </listitem>
        </itemizedlist>
        If the file already exists and contains the PID of a live process,
        the server will issue a DHCP4_ALREADY_RUNNING log message and exit. It
        is possible, though unlikely, that the file is a remnant of a system crash
        and the process to which the PID belongs is unrelated to Kea.  In such a
        case it would be necessary to manually delete the PID file.
      </para>

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

    <section xml:id="dhcp4-configuration">
      <title>DHCPv4 Server Configuration</title>
<section>
  <title>Introduction</title>
      <para>
        This section explains how to configure the DHCPv4 server using the
        Kea configuration backend. (Kea configuration using any other
        backends is outside of scope of this document.) Before DHCPv4
        is started, its configuration file has to be created. The
        basic configuration is as follows:
<screen>
{
# DHCPv4 configuration starts in this line
"Dhcp4": {

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

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

# And we specify the type of lease database
    "lease-database": {
        "type": "memfile",
        "persist": true,
        "name": "/var/kea/dhcp4.leases"
    },

# Finally, we list the subnets from which we will be leasing addresses.
    "subnet4": [
        {
            "subnet": "192.0.2.0/24",
            "pools": [
                {
                     "pool": "192.0.2.1 - 192.0.2.200"
                }
            ]
        }
    ]
# DHCPv4 configuration ends with the next line
}

} </screen>
</para>

<para>The following paragraphs provide a brief overview of the parameters in
the above example together with
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 Dhcp4. This is a simplified configuration, as usually
there will be additional objects, like <command>Logging</command> or
<command>DhcpDdns</command>, but we omit them now for clarity. The Dhcp4
configuration starts with the <command>"Dhcp4": {</command> line
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 Dhcp4 configuration.</para>

<para>In the general case, the order in which those parameters appear does not
matter. There are two caveats here though. The first one is to remember that
the configuration file must be well formed JSON. That means that the 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
moving a parameter to or from the last position in a given scope may also require
moving the comma. The second caveat is that it is uncommon — although
legal JSON — to
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 DHCPv4 configuration elements, the first few elements
define some global parameters. <command>valid-lifetime</command>
defines for how long the addresses (leases) given out by the
server are valid. If nothing changes, a client that got an address is allowed to
use it for 4000 seconds. (Note that integer numbers are specified as is,
without any quotes around them.) <command>renew-timer</command> and
<command>rebind-timer</command> are values (also in seconds) that
define T1 and T2 timers that govern when the client will begin the renewal and
rebind procedures. Note that <command>renew-timer</command> and
<command>rebind-timer</command> are optional. If they are not specified the
client will select values for T1 and T2 timers according to the
<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://tools.ietf.org/html/rfc2131">RFC 2131</link>.</para>

<para>The <command>interfaces-config</command> map specifies the server
configuration concerning the network interfaces, on which the server should
listen to the DHCP messages. The <command>interfaces</command> parameter
specifies a list of network interfaces on which the server should listen.
Lists are opened and closed with square brackets, with elements separated
by commas. Had we wanted to listen on two interfaces, the
<command>interfaces-config</command> would look like this:
<screen>
"interfaces-config": {
    "interfaces": [ "eth0", "eth1" ]
},
</screen>
</para>

<para>The next couple of lines define the lease database, the place where the server
stores its lease information. This particular example tells the server to use
<command>memfile</command>, which is the simplest (and fastest) database
backend. It uses an in-memory database and stores leases on disk in a CSV
file. This is a very simple configuration. Usually the 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 for the Dhcp4 definition
follow, a trailing comma is present.</para>

<para>Finally, we need to define a list of IPv4 subnets. This is the
most important DHCPv4 configuration structure as the server uses that
information to process clients' requests. It defines all subnets from
which the server is expected to receive DHCP requests. The subnets are
specified with the <command>subnet4</command> parameter.  It is a list,
so it starts and ends with square brackets.  Each subnet definition in
the list has several attributes associated with it, so it is a structure
and is opened and closed with braces. At a minimum, a subnet definition
has to have at least two parameters: <command>subnet</command> (that
defines the whole subnet) and <command>pools</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>subnet4</command> parameter would be specified and
separated by commas. For example, to define three subnets, the following
syntax would be used:
<screen>
"subnet4": [
    {
        "pools": [ { "pool":  "192.0.2.1 - 192.0.2.200" } ],
        "subnet": "192.0.2.0/24"
    },
    {
        "pools": [ { "pool": "192.0.3.100 - 192.0.3.200" } ],
        "subnet": "192.0.3.0/24"
    },
    {
        "pools": [ { "pool": "192.0.4.1 - 192.0.4.254" } ],
        "subnet": "192.0.4.0/24"
    }
]
</screen>
Note that indentation is optional and is used for aesthetic purposes only.
In some cases in may be preferable to use more compact notation.
</para>

<para>After all the parameters have been specified, we have two contexts open:
global and Dhcp4, hence we need two closing curly brackets to close them.
In a real life configuration file there most likely would be additional
components defined such as Logging or DhcpDdns, so the closing brace would
be followed by a comma and another object definition.</para>
</section>

<section>
  <title>Lease Storage</title>
  <para>All leases issued by the server are stored in the lease database.
  Currently there are four database backends available:  memfile (which is the
  default backend), MySQL, PostgreSQL and Cassandra.</para>
  <section>
  	<title>DHCPv4 lease field description</title>
  	<para>
  	Leases issued by the dhcp4 server are stored in the backends with the fields as,
  	<screen>address,hwaddr,client_id,valid_lifetime,expire,subnet_id,fqdn_fwd,fqdn_rev,hostname,state</screen>
  	</para>
  	<para>
  	Let's decipher these fields
  	<itemizedlist>
  		<listitem>
      		<simpara><command>address</command>: This field is the actual IPv4 address
      		offered or assigned to a host.
      		</simpara>
      	</listitem>
      	<listitem>
      		<simpara><command>hwaddr</command>: The hardware address/MAC address of the host or network
      		interface on which the lease will be used. Usually the same as <command>clien_id</command>.
      		</simpara>
      	</listitem>
      	<listitem>
      		<simpara><command>client_id</command>: A unique identifier for the host used by the host
      		to acquire the lease. Clients are not required to send client identifiers, this will only
      		appear if a client, in fact, did send one. Usually the same as <command>hwaddr</command>.
      		</simpara>
      	</listitem>
      	<listitem>
      		<simpara><command>valid_lifetime</command>: The valid-time or lifetime for the lease in seconds for which the lease
      		is assigned (active) to the host, this is by default <command>3600</command> seconds after which it will be available (free)
      		to be assign to possibly same or new host.
      		</simpara>
      	</listitem>
      	<listitem>
      		<simpara><command>expire</command>: The lease end-time, a time in seconds since epoch for which the lease
      		will expire and will be available (free) for re-assignment.
      		</simpara>
      	</listitem>
      	<listitem>
      		<simpara><command>subnet_id</command>: The identifier (unique) for the particular subnet from which the lease was assigned.
      		</simpara>
      	</listitem>
      	<listitem>
      		<simpara><command>fqdn_fwd</command>: A bool value, was forward update performed? Required during lease release or expiration.
      		</simpara>
      	</listitem>
      	<listitem>
      		<simpara><command>fqdn_rev</command>: A bool value, was reverse update performed? Required during lease release or expiration.
      		</simpara>
      	</listitem>
      	<listitem>
      		<simpara><command>hostname</command>: A string, hosts hostname. A host may send their
      		hostname in the host-name option. If the client sends its hostname in this way, the hostname
      		is recorded in the hostname field. A hostname may be shared by possible many leases if a client
      		requests two or more addresses.
      		</simpara>
      	</listitem>
      	<listitem>
      		<simpara><command>state</command>: The state of the lease active, expired or released.
      		</simpara>
      	</listitem>
  	</itemizedlist>
  	</para>
  </section>
<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 linkend="database-configuration4"/> describes this option. In typical
  smaller deployments though, the server will store lease information in a CSV file rather
  than a database. 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
  the Dhcp4/lease-database parameters. The <command>type</command> parameter
  is mandatory and it specifies which storage for leases the server should use.
  The value of <userinput>"memfile"</userinput> indicates that the file should
  be used as the storage. The following list gives additional, optional,
  parameters that can be used to configure the Memfile backend.

  <itemizedlist>
    <listitem>
      <simpara><command>persist</command>: controls whether the new leases and
      updates to existing leases are written to the file. It is strongly
      recommended that the value of this parameter is set to
      <userinput>true</userinput> at all times, during the server's normal
      operation. Not writing leases to disk will mean that if a server is restarted
      (e.g. after a power failure), it will not know what addresses have been
      assigned.  As a result, it may hand out addresses to new clients that are
      already in use. The value of <userinput>false</userinput> is mostly useful
      for performance testing purposes. The default value of the
      <command>persist</command> parameter is <userinput>true</userinput>,
      which enables writing lease updates
      to the lease file.
      </simpara>
    </listitem>

    <listitem>
      <simpara><command>name</command>: specifies an absolute location of the lease
      file in which new leases and lease updates will be recorded. The default value
      for this parameter is <userinput>"[kea-install-dir]/var/kea/kea-leases4.csv"
      </userinput>.</simpara>
    </listitem>

    <listitem>
      <simpara><command>lfc-interval</command>: specifies the interval in seconds, at
      which the server will perform a lease file cleanup (LFC).  This
      removes redundant (historical) information from the lease file
      and effectively reduces the lease file size. The cleanup process is described
      in more detailed fashion further in this section. The default value of the
      <command>lfc-interval</command> is <userinput>3600</userinput>. A value of 0
      disables the LFC.</simpara>
    </listitem>

  </itemizedlist>
  </para>

  <para>An example configuration of the Memfile backend is presented below:

<screen>
"Dhcp4": {
    "lease-database": {
        <userinput>"type": "memfile"</userinput>,
        <userinput>"persist": true</userinput>,
        <userinput>"name": "/tmp/kea-leases4.csv",</userinput>
        <userinput>"lfc-interval": 1800</userinput>
    }
}
</screen>

    This configuration selects the <filename>/tmp/kea-leases4.csv</filename> as
    the storage for lease information and enables persistence (writing lease updates
    to this file). It also configures the backend perform the periodic cleanup
    of the lease files, executed every 30 minutes.
  </para>

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

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

  <para>Lease file cleanup is performed by a separate process (in background) to avoid
  a performance impact on the server process. In order to avoid the conflicts
  between two processes both using the same lease files, the LFC process
  operates on the copy of the original lease file, rather than on the lease
  file used by the server to record lease updates. There are also other files
  being created as a side effect of the lease file cleanup. The detailed
  description of the LFC is located on the Kea wiki:
  <uri xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://kea.isc.org/wiki/LFCDesign">http://kea.isc.org/wiki/LFCDesign</uri>.
  </para>

</section>

<section xml:id="database-configuration4">
  <title>Lease Database Configuration</title>

  <note>
    <para>Lease database access information must be configured for the DHCPv4 server,
    even if it has already been configured for the DHCPv6 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>Lease database configuration is controlled through the Dhcp4/lease-database
  parameters. The type of the database must be set to "memfile", "mysql", "postgresql" or
  "cql", e.g.
<screen>
"Dhcp4": { "lease-database": { <userinput>"type": "mysql"</userinput>, ... }, ... }
</screen>
  Next, the name of the database to hold the leases must be set: this is the
  name used when the database was created
  (see <xref linkend="mysql-database-create"/>,
  <xref linkend="pgsql-database-create"/> or
  <xref linkend="cql-database-create"/>).
<screen>
"Dhcp4": { "lease-database": { <userinput>"name": "<replaceable>database-name</replaceable>" </userinput>, ... }, ... }
</screen>
  For Cassandra:
<screen>
"Dhcp4": { "lease-database": { <userinput>"keyspace": "<replaceable>database-name</replaceable>" </userinput>, ... }, ... }
</screen>
  If the database is located on a different system to the DHCPv4 server, the
  database host name must also be specified. (It should be noted that this
  configuration may have a severe impact on server performance.):
<screen>
"Dhcp4": { "lease-database": { <userinput>"host": "<replaceable>remote-host-name</replaceable>"</userinput>, ... }, ... }
</screen>

  The usual state of affairs will be to have the database on the same machine as
  the DHCPv4 server.  In this case, set the value to the empty string:
<screen>
"Dhcp4": { "lease-database": { <userinput>"host" : ""</userinput>, ... }, ... }
</screen>

  Should the database use a port different than default, it may be
  specified as well:
<screen>
"Dhcp4": { "lease-database": { <userinput>"port" : 12345</userinput>, ... }, ... }
</screen>
  Should the database be located on a different system, you may need to specify a longer interval
  for the connection timeout:
<screen>
"Dhcp4": { "lease-database": { <userinput>"connect-timeout" : <replaceable>timeout-in-seconds</replaceable></userinput>, ... }, ... }
</screen>
The default value of five seconds should be more than adequate for local connections.
If a timeout is given though, it should be an integer greater than zero.
  </para>

  <para>Finally, the credentials of the account under which the server will
  access the database should be set:
<screen>
"Dhcp4": { "lease-database": { <userinput>"user": "<replaceable>user-name</replaceable>"</userinput>,
                               <userinput>"password": "<replaceable>password</replaceable>"</userinput>,
                              ... },
           ... }
</screen>
  If there is no password to the account, set the password to the empty string
  "". (This is also the default.)</para>
</section>

<section xml:id="cassandra-database-configuration4">
  <title>Cassandra specific parameters</title>
  <para>
    Cassandra backend is configured slightly differently. Cassandra has a
    concept of contact points that could be used to contact the cluster,
    instead of a single IP or hostname. It takes a list of comma separated IP
    addresses. This may be specified as:
    <screen>
"Dhcp4": {
    "lease-database": {
        "type": "cql",
        <userinput>"contact-points": "<replaceable>ip-address1, ip-address2 [,...]</replaceable>"</userinput>,
        ...
    },
    ...
}</screen>
  </para>

  <para>
    Cassandra also supports a number of optional parameters:
    <itemizedlist>

      <listitem><simpara>
        <command>reconnect-wait-time</command> - governs how long Kea waits
        before attempting to reconnect. Expressed in milliseconds. The default
        is 2000 [ms].
      </simpara></listitem>

      <listitem><simpara>
        <command>connect-timeout</command> - sets the timeout for connecting to
        a node. Expressed in milliseconds. The default is 5000 [ms].
      </simpara></listitem>

      <listitem><simpara>
        <command>request-timeout</command> - this parameter sets the timeout for
        waiting for a response from a node. Expressed in milliseconds. The
        default is 12000 [ms].
      </simpara></listitem>

      <listitem><simpara>
        <command>tcp-keepalive</command> - This parameter governs the TCP
        keep-alive mechanism. Expressed in seconds of delay. If the parameter
        is not present, the mechanism is disabled.
      </simpara></listitem>

      <listitem><simpara>
        <command>tcp-nodelay</command> - This parameter enables/disabled Nagle's
        algorithm on connections. The default is true.
      </simpara></listitem>

    </itemizedlist>
  </para>

  <para>
    For example, a complex Cassandra configuration with most parameters specified
    could look as follows:
<screen>
"Dhcp4": {
  "lease-database": {
      "type": "cql",
      "keyspace": "keatest",
      "contact-points": "192.0.2.1, 192.0.2.2, 192.0.2.3",
      "port": 9042,
      "reconnect-wait-time": 2000,
      "connect-timeout": 5000,
      "request-timeout": 12000,
      "tcp-keepalive": 1,
      "tcp-nodelay": true
    },
    ...
}</screen>

Similar parameters can be specified for hosts database.
  </para>
</section>

</section>

<section xml:id="hosts4-storage">
  <title>Hosts Storage</title>
    <para>Kea is also able to store information about host reservations in the
    database. The hosts database configuration uses the same syntax as the lease
    database. In fact, a Kea server opens independent connections for each
    purpose, be it lease or hosts information. This arrangement gives the most
    flexibility. Kea can be used to keep leases and host reservations
    separately, but can also point to the same database. Currently the
    supported hosts database types are MySQL, PostgreSQL and Cassandra.</para>

    <para>Please note that usage of hosts storage is optional. A user can define
    all host reservations in the configuration file. That is the recommended way
    if the number of reservations is small. However, when the number of
    reservations grows it's more convenient to use host storage. Please note
    that both storage methods (configuration file and one of the supported databases)
    can be used together. If hosts are defined in both places, the definitions
    from the configuration file are checked first and external storage is checked
    later, if necessary.</para>

    <para>Version 1.4 extends the host storage to multiple storages. Operations
    are performed on host storages in the configuration order with a special
    case for addition: read-only storages must be configured after a
    required read-write storage, or host reservation addition will
    always fail.</para>

<section xml:id="hosts-databases-configuration4">
  <title>DHCPv4 Hosts Database Configuration</title>

  <para>Hosts database configuration is controlled through the Dhcp4/hosts-database
  parameters. If enabled, the type of the database must be set to "mysql" or
  "postgresql". Other hosts backends may be added in later versions of Kea.
<screen>
"Dhcp4": { "hosts-database": { <userinput>"type": "mysql"</userinput>, ... }, ... }
</screen>
  Next, the name of the database to hold the reservations must be set: this is the
  name used when the lease database was created (see <xref linkend="supported-databases"/>
  for instructions how to setup the desired database type).
<screen>
"Dhcp4": { "hosts-database": { <userinput>"name": "<replaceable>database-name</replaceable>" </userinput>, ... }, ... }
</screen>
  If the database is located on a different system than the DHCPv4 server, the
  database host name must also be specified. (Again it should be noted that this
  configuration may have a severe impact on server performance.):
<screen>
"Dhcp4": { "hosts-database": { <userinput>"host": <replaceable>remote-host-name</replaceable></userinput>, ... }, ... }
</screen>
  The usual state of affairs will be to have the database on the same machine as
  the DHCPv4 server.  In this case, set the value to the empty string:
<screen>
"Dhcp4": { "hosts-database": { <userinput>"host" : ""</userinput>, ... }, ... }
</screen>
  Should the database use a port different than default, it may be
  specified as well:
<screen>
"Dhcp4": { "hosts-database": { <userinput>"port" : 12345</userinput>, ... }, ... }
</screen>

  </para>
  <para>Finally, the credentials of the account under which the server will
  access the database should be set:
<screen>
"Dhcp4": { "hosts-database": { <userinput>"user": "<replaceable>user-name</replaceable>"</userinput>,
                               <userinput>"password": "<replaceable>password</replaceable>"</userinput>,
                              ... },
           ... }
</screen>
  If there is no password to the account, set the password to the empty string
  "". (This is also the default.)</para>

  <para>The multiple storage extension uses a similar syntax: a configuration
  is placed into a "hosts-databases" list instead of into a "hosts-database"
  entry as in:
<screen>
"Dhcp4": { "hosts-databases": [ { <userinput>"type": "mysql"</userinput>, ... }, ... ], ... }
</screen>
  </para>

  <para>For additional Cassandra specific parameters, see <xref
  linkend="cassandra-database-configuration4"/>.</para>

</section>

<section xml:id="read-only-database-configuration4">
<title>Using Read-Only Databases for Host Reservations</title>
<para>
In some deployments the database user whose name is specified in the database backend
configuration may not have write privileges to the database. This is often
required by the policy within a given network to secure the data from being
unintentionally modified. In many cases administrators have inventory databases
deployed, which contain substantially more information about the hosts than
static reservations assigned to them. The inventory database can be used to create
a view of a Kea hosts database and such view is often read only.
</para>
<para>
Kea host database backends operate with an implicit configuration to both
read from and write to the database. If the database user does not have
write access to the host database, the backend will fail to start and the
server will refuse to start (or reconfigure). However, if access to a read
only host database is required for retrieving reservations for clients
and/or assign specific addresses and options, it is possible to explicitly
configure Kea to start in "read-only" mode. This is controlled by the
<command>readonly</command> boolean parameter as follows:
<screen>
"Dhcp4": { "hosts-database": { <userinput>"readonly": true</userinput>, ... }, ... }
</screen>
Setting this parameter to <userinput>false</userinput> would configure the
database backend to operate in "read-write" mode, which is also a default
configuration if the parameter is not specified.
</para>
<note><para>The <command>readonly</command> parameter is currently only supported
for MySQL and PostgreSQL databases.</para></note>
</section>

</section>

<section xml:id="dhcp4-interface-configuration">
  <title>Interface Configuration</title>
  <para>The DHCPv4 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>
"Dhcp4": {
    "interfaces-config": {
        "interfaces": [ <userinput>"*"</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>
"Dhcp4": {
    "interfaces-config": {
        "interfaces": [ <userinput>"eth1", "eth3"</userinput> ]
    },
    ...
}
  </screen>
  </para>
  <para>It is possible to use wildcard interface name (asterisk) concurrently
  with explicit interface names:
  <screen>
"Dhcp4": {
    "interfaces-config": {
        "interfaces": [ <userinput>"eth1", "eth3", "*"</userinput> ]
    },
    ...
}
  </screen>
It is anticipated that this form of usage will only be used when it is desired to
temporarily override a list of interface names and listen on all interfaces.
  </para>
  <para>Some deployments of DHCP servers require that the servers listen
  on the interfaces with multiple IPv4 addresses configured. In these situations,
  the address to use can be selected by appending an IPv4 address to the interface
  name in the following manner:
  <screen>
"Dhcp4": {
    "interfaces-config": {
        "interfaces": [ <userinput>"eth1/10.0.0.1", "eth3/192.0.2.3"</userinput> ]
    },
    ...
}
  </screen>
  </para>

  <para>Should the server be required to listen on multiple IPv4 addresses assigned
  to the same interface, multiple addresses can be specified for an interface
  as in the example below:
  <screen>
"Dhcp4": {
    "interfaces-config": {
        "interfaces": [ <userinput>"eth1/10.0.0.1", "eth1/10.0.0.2"</userinput> ]
    },
    ...
}
  </screen>
  </para>

  <para>Alternatively, if the server should listen on all addresses for the particular
  interface, an interface name without any address should be specified.</para>

  <para>Kea supports responding to directly connected clients which don't have
  an address configured. This requires that the server
  injects the hardware address of the destination into the data link layer
  of the packet being sent to the client. The DHCPv4 server utilizes the
  raw sockets to achieve this, and builds the entire IP/UDP stack for the
  outgoing packets. The down side of raw socket use, however, is that incoming
  and outgoing packets bypass the firewalls (e.g. iptables). It is also
  troublesome to handle traffic on multiple IPv4 addresses assigned to the
  same interface, as raw sockets are bound to the interface and advanced
  packet filtering techniques (e.g. using the BPF) have to be used to
  receive unicast traffic on the desired addresses assigned to the interface,
  rather than capturing whole traffic reaching the interface to which the raw
  socket is bound. Therefore, in the deployments where the server doesn't
  have to provision the directly connected clients and only receives the
  unicast packets from the relay agents, the
  DHCP server should be configured to utilize IP/UDP datagram sockets instead of raw sockets.
  The following configuration demonstrates how this can be achieved:

  <screen>
"Dhcp4": {
    "interfaces-config": {
        "interfaces": [ <userinput>"eth1", "eth3"</userinput> ],
        "dhcp-socket-type": "udp"
    },
    ...
}
  </screen>
  The <command>dhcp-socket-type</command> specifies that the IP/UDP sockets will
  be opened on all interfaces on which the server listens, i.e. "eth1" and
  "eth3" in our case. If the <command>dhcp-socket-type</command> is set to
  <userinput>raw</userinput>, it configures the server to use raw sockets
  instead. If the <command>dhcp-socket-type</command> value is not specified, the
  default value <userinput>raw</userinput> is used.
  </para>

  <para>Using UDP sockets automatically disables the reception of broadcast
  packets from directly connected clients. This effectively means that the
  UDP sockets can be used for relayed traffic only. When using the raw sockets,
  both the traffic from the directly connected clients and the relayed traffic
  will be handled. Caution should be taken when configuring the server to open
  multiple raw sockets on the interface with several IPv4 addresses assigned.
  If the directly connected client sends the message to the broadcast address
  all sockets on this link will receive this message and multiple responses
  will be sent to the client. Hence, the configuration with multiple IPv4
  addresses assigned to the interface should not be used when the directly
  connected clients are operating on that link. To use a single address on
  such interface, the "interface-name/address" notation should be used.
  </para>

  <note>
    <para>Specifying the value <userinput>raw</userinput> as the socket type,
    doesn't guarantee that the raw sockets will be used! The use of raw sockets
    to handle the traffic from the directly connected clients is currently
    supported on Linux and BSD systems only. If the raw sockets are not
    supported on the particular OS, the server will issue a warning and
    fall back to use IP/UDP sockets.</para>
  </note>

  <para>In typical environment the DHCP server is expected to send back a
  response on the same network interface on which the query is received. This is
  the default behavior. However, in some deployments it is desired that the
  outbound (response) packets will be sent as regular traffic and the outbound
  interface will be determined by the routing tables. This kind of asymetric
  traffic is uncommon, but valid. Kea now supports a parameter called
  <command>outbound-interface</command> that controls this behavior. It supports
  two values. The first one, <userinput>same-as-inbound</userinput>, tells Kea
  to send back the response on the same inteface the query packet is received. This
  is the default behavior. The second one, <userinput>use-routing</userinput>
  tells Kea to send regular UDP packets and let the kernel's routing table to
  determine most appropriate interface. This only works when
  <command>dhcp-socket-type</command> is  set to <userinput>udp</userinput>.
  An example configuration looks as follows:
  <screen>
"Dhcp4": {
    "interfaces-config": {
        "interfaces": [ "eth1", "eth3" ],
        "dhcp-socket-type": "udp",
        <userinput>"outbound-interface": "use-routing"</userinput>
    },
    ...
}</screen>
  </para>

  <para>Interfaces are re-detected at each reconfiguration. This behavior
  can be disabled by setting <command>re-detect</command> value to
  <userinput>false</userinput>, for instance:

  <screen>
"Dhcp4": {
    "interfaces-config": {
        "interfaces": [ <userinput>"eth1", "eth3"</userinput> ],
        "re-detect": <userinput>false</userinput>
    },
    ...
}
  </screen>
  Note interfaces are not re-detected during <command>config-test</command>.
  </para>

  <para>Usually loopback interfaces (e.g. the "lo" or "lo0" interface)
  may not be configured but if a loopback interface is explicitely configured
  and IP/UDP sockets are specified the loopback interface is accepted.
  </para>

  <para>It can be used for instance to run Kea in a FreeBSD jail having
  only a loopback interface, servicing relayed DHCP request:

  <screen>
"Dhcp4": {
    "interfaces-config": {
        "interfaces": [ <userinput>"lo0"</userinput> ],
        "dhcp-socket-type": "udp"
    },
    ...
}</screen>
  </para>

</section>

<section xml:id="dhcpinform-unicast-issues">
  <title>Issues with Unicast Responses to DHCPINFORM</title>
  <para>The use of UDP sockets has certain benefits in deployments
  where the server receives only relayed traffic; these benefits are
  mentioned in <xref linkend="dhcp4-interface-configuration"/>. From
  the administrator's perspective it is often desirable to
  configure the system's firewall to filter out the unwanted traffic, and
  the use of UDP sockets facilitates this. However, the administrator must
  also be aware of the implications related to filtering certain types
  of traffic as it may impair the DHCP server's operation.
  </para>

  <para>In this section we are focusing on the case when the server
  receives the DHCPINFORM message from the client via a relay. According
  to <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://tools.ietf.org/html/rfc2131">RFC 2131</link>,
  the server should unicast the DHCPACK response to the address carried in
  the "ciaddr" field. When the UDP socket is in use, the DHCP server
  relies on the low level functions of an operating system to build the
  data link, IP and UDP layers of the outgoing message. Typically, the
  OS will first use ARP to obtain the client's link layer address to be
  inserted into the frame's header, if the address is not cached from
  a previous transaction that the client had with the server.
  When the ARP exchange is successful, the DHCP message can be unicast
  to the client, using the  obtained address.
  </para>

  <para>Some system administrators block ARP messages in their network,
  which causes issues for the server when it responds to the
  DHCPINFORM messages, because the server is unable to send the
  DHCPACK if the preceding ARP communication fails. Since the OS is
  entirely responsible for the ARP communication and then sending
  the DHCP packet over the wire, the DHCP server has no means to
  determine that the ARP exchange failed and the DHCP response message
  was dropped. Thus, the server does not log any error messages when
  the outgoing DHCP response is dropped. At the same time, all hooks
  pertaining to the packet sending operation will be called, even
  though the message never reaches its destination.
  </para>

  <para>Note that the issue described in this section is not observed
  when the raw sockets are in use, because, in this case, the DHCP server
  builds all the layers of the outgoing message on its own and does not
  use ARP. Instead, it inserts the value carried in the 'chaddr' field
  of the DHCPINFORM message into the link layer.
  </para>

  <para>Server administrators willing to support DHCPINFORM
  messages via relays should not block ARP traffic in their
  networks or should use raw sockets instead of UDP sockets.
  </para>
</section>

<section xml:id="ipv4-subnet-id">
  <title>IPv4 Subnet Identifier</title>
  <para>
    The subnet identifier is a unique number associated with a particular subnet.
    In principle, it is used to associate clients' leases with their respective subnets.
    When a subnet identifier is not specified for a subnet being configured, it will
    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:
    if there are four subnets and the third is removed the last subnet will be assigned
    the identifier that the third subnet had before removal. As a result, the leases
    stored in the lease database for subnet 3 are now associated with
    subnet 4, something that may have unexpected consequences. It is planned
    to implement a mechanism to preserve auto-generated subnet ids in a
    future version of Kea.  However, the only remedy for this issue
    at present is to
    manually specify a unique identifier for each subnet.
  </para>
      <para>
        The following configuration will assign the specified subnet
        identifier to the newly configured subnet:

        <screen>
"Dhcp4": {
    "subnet4": [
        {
            "subnet": "192.0.2.0/24",
            <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.
  </para>
    <!-- @todo: describe whether database needs to be updated after changing
      id -->
</section>

<section xml:id="dhcp4-address-config">
  <title>Configuration of IPv4 Address Pools</title>
  <para>
    The main role of a DHCPv4 server is address assignment. For this, the server has to
    be configured with at least one subnet and one pool of dynamic addresses for it to manage.
    For example, assume that the server is connected to a network
    segment that uses the 192.0.2.0/24 prefix. The Administrator of that network
    has decided that addresses from range 192.0.2.10 to 192.0.2.20 are going to
    be managed by the Dhcp4 server. Such a configuration can be achieved in the
    following way:
    <screen>
"Dhcp4": {
    <userinput>"subnet4": [
        {
            "subnet": "192.0.2.0/24",
            "pools": [
                { "pool": "192.0.2.10 - 192.0.2.20" }
            ],
            ...
        }
    ]</userinput>
}</screen>

    Note that <command>subnet</command> is defined as a simple string, but
    the <command>pools</command> parameter is actually a list of pools: for
    this reason, the pool definition is enclosed in square brackets, even
    though only one range of addresses is specified.</para>

    <para>Each <command>pool</command> is a structure that contains the
    parameters that describe a single pool. Currently there is only one
    parameter, <command>pool</command>, which gives the range of addresses
    in the pool. Additional parameters will be added in future releases of
    Kea.</para>

    <para>It is possible to define more than one pool in a subnet: continuing
    the previous example, further assume that 192.0.2.64/26 should be also be
    managed by the server. It could be written as 192.0.2.64 to
    192.0.2.127. Alternatively, it can be expressed more simply as
    192.0.2.64/26. Both formats are supported by Dhcp4 and can be mixed in the
    pool list.  For example, one could define the following pools:
<screen>
"Dhcp4": {
    "subnet4": [
        {
            "subnet": "192.0.2.0/24",
            <userinput>"pools": [
                { "pool": "192.0.2.10-192.0.2.20" },
                { "pool": "192.0.2.64/26" }
            ]</userinput>,
            ...
        }
    ],
    ...
}
</screen>
    White space in pool definitions is ignored, so spaces before and after the hyphen are optional.
    They can be used to improve readability.
  </para>
  <para>
    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:
<screen>
"Dhcp4": {
    "subnet4": [
        {
            "subnet": "192.0.2.0/24",
            "pools": [ { "pool": "192.0.2.1 - 192.0.2.200" } ],
            ...
        },
        {
            "subnet": "192.0.3.0/24",
            "pools": [ { "pool": "192.0.3.100 - 192.0.3.200" } ],
            ...
        },
        {
            "subnet": "192.0.4.0/24",
            "pools": [ { "pool": "192.0.4.1 - 192.0.4.254" } ],
            ...
        }
    ]
}
</screen>
  </para>
  <para>
    When configuring a DHCPv4 server using prefix/length notation, please pay
    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
    address) and the last (typically broadcast address) address from that pool.
    In the aforementioned example of pool 192.0.3.0/24, both 192.0.3.0 and
    192.0.3.255 addresses may be assigned as well. This may be invalid in some
    network configurations. If you want to avoid this, please use the "min-max" notation.
  </para>
</section>

    <section xml:id="dhcp4-std-options">
      <title>Standard DHCPv4 Options</title>
      <para>
        One of the major features of the DHCPv4 server is to provide configuration
        options to clients. Most of the options are sent by the server only if the
        client explicitly requests them using the Parameter Request List option.
        Those that do not require inclusion in the Parameter Request List
        option are commonly used options, e.g. "Domain Server", and options which
        require special behavior, e.g. "Client FQDN" is returned to the client
        if the client has included this option in its message to the server.
      </para>

      <para>
        <xref linkend="dhcp4-std-options-list"/> comprises the list of the
        standard DHCPv4 options whose values can be configured using the
        configuration structures described in this section. This table excludes
        the options which require special processing and thus cannot be configured
        with some fixed values. The last column of the table indicates which
        options can be sent by the server even when they are not requested in
        the Parameter Request list option, and those which are sent only when
        explicitly requested.
      </para>

      <para>
        The following example shows how to configure the addresses of DNS
        servers, which is one of the most frequently used options. Options
        specified in this way are considered global and apply to all
        configured subnets.

        <screen>
"Dhcp4": {
    "option-data": [
        {
           <userinput>"name": "domain-name-servers",
           "code": 6,
           "space": "dhcp4",
           "csv-format": true,
           "data": "192.0.2.1, 192.0.2.2"</userinput>
        },
        ...
    ]
}
        </screen>
        Note that only one of name or code is required, you don't need to
        specify both. Space has a default value of "dhcp4", so you can skip this
        as well if you define a regular (not encapsulated) DHCPv4 option.
        Finally, csv-format defaults to true, so it too can be skipped, unless
        you want to specify the option value as hexstring. Therefore the
        above example can be simplified to:
        <screen>
"Dhcp4": {
    "option-data": [
        {
           <userinput>"name": "domain-name-servers",
           "data": "192.0.2.1, 192.0.2.2"</userinput>
        },
        ...
    ]
}
        </screen>
        Defined options are added to response when the client requests them
        at a few exceptions which are always added. To enforce the addition
        of a particular option set the always-send flag to true as in:
        <screen>
"Dhcp4": {
    "option-data": [
        {
           <userinput>"name": "domain-name-servers",
           "data": "192.0.2.1, 192.0.2.2",
           "always-send": true</userinput>
        },
        ...
    ]
}
        </screen>
        The effect is the same as if the client added the option code in the
        Parameter Request List option (or its equivalent for vendor
        options) so in:
        <screen>
"Dhcp4": {
    "option-data": [
        {
           <userinput>"name": "domain-name-servers",
           "data": "192.0.2.1, 192.0.2.2",
           "always-send": true</userinput>
        },
        ...
    ],
    "subnet4": [
        {
           "subnet": "192.0.3.0/24",
           "option-data": [
               {
                   <userinput>"name": "domain-name-servers",
                   "data": "192.0.3.1, 192.0.3.2"</userinput>
               },
               ...
           ],
           ...
        },
        ...
    ],
    ...
}
        </screen>
        The Domain Name Servers option is always added to responses
        (the always-send is "sticky") but the value is the subnet one
        when the client is localized in the subnet.
      </para>
    <para>
      The <command>name</command> parameter specifies the option name. For a
      list of currently supported names, see <xref linkend="dhcp4-std-options-list"/> below.  The <command>code</command>
      parameter specifies the option code, which must match one of the values
      from that list. The next line specifies the option space, which must
      always be set to "dhcp4" as these are standard DHCPv4 options. For other
      option spaces, including custom option spaces, see <xref linkend="dhcp4-option-spaces"/>. The next line specifies the format in
      which the data will be entered: use of CSV (comma separated values) is
      recommended. The sixth line gives the actual value to be sent to
      clients. Data is specified as normal text, with values separated by commas
      if more than one value is allowed.
    </para>

    <para>
      Options can also be configured as hexadecimal values. If
      <command>csv-format</command> is
      set to false, option data must be specified as a hexadecimal string. The
      following commands configure the domain-name-servers option for all
      subnets with the following addresses: 192.0.3.1 and 192.0.3.2.
      Note that <command>csv-format</command> is set to false.
      <screen>
"Dhcp4": {
    "option-data": [
        {
            <userinput>"name": "domain-name-servers",
            "code": 6,
            "space": "dhcp4",
            "csv-format": false,
            "data": "C0 00 03 01 C0 00 03 02"</userinput>
        },
        ...
    ],
    ...
}</screen>
    </para>

       <para>
       Care should be taken
       to use proper encoding when using hexadecimal format as Kea's ability
       to validate data correctness in hexadecimal is limited.
      </para>

      <para>
        Most of the parameters in the "option-data" structure are optional and
        can be omitted in some circumstances as discussed in the
        <xref linkend="dhcp4-option-data-defaults"/>.
      </para>

      <para>
        It is possible to specify or 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
        (Dhcp4/option-data), rather you should set only subnet-specific values
        (Dhcp4/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 192.0.2.3.
<screen>
"Dhcp4": {
    "subnet4": [
        {
            <userinput>"option-data": [
                {
                    "name": "domain-name-servers",
                    "code": 6,
                    "space": "dhcp4",
                    "csv-format": true,
                    "data": "192.0.2.3"
                },
                ...
            ]</userinput>,
            ...
        },
        ...
    ],
    ...
}
</screen>
      </para>

      <para>
        In some cases it is useful to associate some options with an
        address pool from which a client is assigned a lease. Pool
        specific option values override subnet specific and global
        option values. The server's administrator must not try to
        prioritize assignment of pool specific options by trying to
        order pools declarations in the server configuration. Future
        Kea releases may change the order in which options are
        assigned from the pools without any notice.
      </para>

     <para>
       The following configuration snippet demonstrates how to specify the
       DNS servers option, which will be assigned to a client only if the
       client obtains an address from the given pool:
<screen>
"Dhcp4": {
    "subnet4": [
        {
            "pools": [
                {
                    "pool": "192.0.2.1 - 192.0.2.200",
                    <userinput>"option-data": [
                        {
                            "name": "domain-name-servers",
                            "data": "192.0.2.3"
                         },
                         ...
                    ]</userinput>,
                    ...
                },
                ...
            ],
            ...
        },
        ...
    ],
    ...
}
</screen>
      </para>

      <para>
        The currently supported standard DHCPv4 options are
        listed in <xref linkend="dhcp4-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>When a data field is a string, and that string contains the comma
      (,; U+002C) character, the comma must be escaped with a double reverse solidus
      character (\; U+005C). This double escape is required, because both the
      routine splitting CSV data into fields and JSON use the same escape
      character: a single escape (\,) would make the JSON invalid.
      For example, the string "foo,bar" would be represented as:
      <screen>
"Dhcp4": {
    "subnet4": [
        {
            "pools": [
                {
                    <userinput>"option-data": [
                        {
                            "name": "boot-file-name",
                            "data": "foo\\,bar"
                        }
                    ]</userinput>
                },
                ...
            ],
            ...
        },
        ...
    ],
    ...
}
</screen>
      </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 time-servers
        allows the specification of more than one IPv4 address, so allowing
        clients to obtain the addresses of multiple NTP servers.
      </para>
      <!-- @todo: describe record types -->
      <!-- @todo: describe array in record types -->

      <para>
        The <xref linkend="dhcp4-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 currently does not provide a definition. In order to use such options,
        a server administrator must create a definition as described in
        <xref linkend="dhcp4-custom-options"/> in the 'dhcp4' option space. This
        definition should match the option format described in the relevant
        RFC but the configuration mechanism will allow any option format as it
        presently has no means to validate it.
      </para>

      <para>
        <table frame="all" xml:id="dhcp4-std-options-list">
          <title>List of standard DHCPv4 options</title>
          <tgroup cols="5">
          <colspec colname="name"/>
          <colspec colname="code" align="center"/>
          <colspec colname="type" align="center"/>
          <colspec colname="array" align="center"/>
          <colspec colname="always-returned" align="center"/>
          <thead>
            <row>
              <entry>Name</entry>
              <entry>Code</entry>
              <entry>Type</entry>
              <entry>Array?</entry>
              <entry>Returned if not requested?</entry>
            </row>
          </thead>
          <tbody>
<!-- Subnet Mask option is not configured by the user
<row><entry>subnet-mask</entry><entry>1</entry><entry>ipv4-address</entry><entry>false</entry><entry>true</entry></row>
-->
<row><entry>time-offset</entry><entry>2</entry><entry>int32</entry><entry>false</entry><entry>false</entry></row>
<row><entry>routers</entry><entry>3</entry><entry>ipv4-address</entry><entry>true</entry><entry>true</entry></row>
<row><entry>time-servers</entry><entry>4</entry><entry>ipv4-address</entry><entry>true</entry><entry>false</entry></row>
<row><entry>name-servers</entry><entry>5</entry><entry>ipv4-address</entry><entry>true</entry><entry>false</entry></row>
<row><entry>domain-name-servers</entry><entry>6</entry><entry>ipv4-address</entry><entry>true</entry><entry>true</entry></row>
<row><entry>log-servers</entry><entry>7</entry><entry>ipv4-address</entry><entry>true</entry><entry>false</entry></row>
<row><entry>cookie-servers</entry><entry>8</entry><entry>ipv4-address</entry><entry>true</entry><entry>false</entry></row>
<row><entry>lpr-servers</entry><entry>9</entry><entry>ipv4-address</entry><entry>true</entry><entry>false</entry></row>
<row><entry>impress-servers</entry><entry>10</entry><entry>ipv4-address</entry><entry>true</entry><entry>false</entry></row>
<row><entry>resource-location-servers</entry><entry>11</entry><entry>ipv4-address</entry><entry>true</entry><entry>false</entry></row>
<!-- Hostname option value is not explicitly configured by the user.
This rather belong to the DDNS configuration
<row><entry>host-name</entry><entry>12</entry><entry>string</entry><entry>false</entry><entry>true</entry></row>
-->
<row><entry>boot-size</entry><entry>13</entry><entry>uint16</entry><entry>false</entry><entry>false</entry></row>
<row><entry>merit-dump</entry><entry>14</entry><entry>string</entry><entry>false</entry><entry>false</entry></row>
<row><entry>domain-name</entry><entry>15</entry><entry>fqdn</entry><entry>false</entry><entry>true</entry></row>
<row><entry>swap-server</entry><entry>16</entry><entry>ipv4-address</entry><entry>false</entry><entry>false</entry></row>
<row><entry>root-path</entry><entry>17</entry><entry>string</entry><entry>false</entry><entry>false</entry></row>
<row><entry>extensions-path</entry><entry>18</entry><entry>string</entry><entry>false</entry><entry>false</entry></row>
<row><entry>ip-forwarding</entry><entry>19</entry><entry>boolean</entry><entry>false</entry><entry>false</entry></row>
<row><entry>non-local-source-routing</entry><entry>20</entry><entry>boolean</entry><entry>false</entry><entry>false</entry></row>
<row><entry>policy-filter</entry><entry>21</entry><entry>ipv4-address</entry><entry>true</entry><entry>false</entry></row>
<row><entry>max-dgram-reassembly</entry><entry>22</entry><entry>uint16</entry><entry>false</entry><entry>false</entry></row>
<row><entry>default-ip-ttl</entry><entry>23</entry><entry>uint8</entry><entry>false</entry><entry>false</entry></row>
<row><entry>path-mtu-aging-timeout</entry><entry>24</entry><entry>uint32</entry><entry>false</entry><entry>false</entry></row>
<row><entry>path-mtu-plateau-table</entry><entry>25</entry><entry>uint16</entry><entry>true</entry><entry>false</entry></row>
<row><entry>interface-mtu</entry><entry>26</entry><entry>uint16</entry><entry>false</entry><entry>false</entry></row>
<row><entry>all-subnets-local</entry><entry>27</entry><entry>boolean</entry><entry>false</entry><entry>false</entry></row>
<row><entry>broadcast-address</entry><entry>28</entry><entry>ipv4-address</entry><entry>false</entry><entry>false</entry></row>
<row><entry>perform-mask-discovery</entry><entry>29</entry><entry>boolean</entry><entry>false</entry><entry>false</entry></row>
<row><entry>mask-supplier</entry><entry>30</entry><entry>boolean</entry><entry>false</entry><entry>false</entry></row>
<row><entry>router-discovery</entry><entry>31</entry><entry>boolean</entry><entry>false</entry><entry>false</entry></row>
<row><entry>router-solicitation-address</entry><entry>32</entry><entry>ipv4-address</entry><entry>false</entry><entry>false</entry></row>
<row><entry>static-routes</entry><entry>33</entry><entry>ipv4-address</entry><entry>true</entry><entry>false</entry></row>
<row><entry>trailer-encapsulation</entry><entry>34</entry><entry>boolean</entry><entry>false</entry><entry>false</entry></row>
<row><entry>arp-cache-timeout</entry><entry>35</entry><entry>uint32</entry><entry>false</entry><entry>false</entry></row>
<row><entry>ieee802-3-encapsulation</entry><entry>36</entry><entry>boolean</entry><entry>false</entry><entry>false</entry></row>
<row><entry>default-tcp-ttl</entry><entry>37</entry><entry>uint8</entry><entry>false</entry><entry>false</entry></row>
<row><entry>tcp-keepalive-interval</entry><entry>38</entry><entry>uint32</entry><entry>false</entry><entry>false</entry></row>
<row><entry>tcp-keepalive-garbage</entry><entry>39</entry><entry>boolean</entry><entry>false</entry><entry>false</entry></row>
<row><entry>nis-domain</entry><entry>40</entry><entry>string</entry><entry>false</entry><entry>false</entry></row>
<row><entry>nis-servers</entry><entry>41</entry><entry>ipv4-address</entry><entry>true</entry><entry>false</entry></row>
<row><entry>ntp-servers</entry><entry>42</entry><entry>ipv4-address</entry><entry>true</entry><entry>false</entry></row>
<row><entry>vendor-encapsulated-options</entry><entry>43</entry><entry>empty</entry><entry>false</entry><entry>false</entry></row>
<row><entry>netbios-name-servers</entry><entry>44</entry><entry>ipv4-address</entry><entry>true</entry><entry>false</entry></row>
<row><entry>netbios-dd-server</entry><entry>45</entry><entry>ipv4-address</entry><entry>true</entry><entry>false</entry></row>
<row><entry>netbios-node-type</entry><entry>46</entry><entry>uint8</entry><entry>false</entry><entry>false</entry></row>
<row><entry>netbios-scope</entry><entry>47</entry><entry>string</entry><entry>false</entry><entry>false</entry></row>
<row><entry>font-servers</entry><entry>48</entry><entry>ipv4-address</entry><entry>true</entry><entry>false</entry></row>
<row><entry>x-display-manager</entry><entry>49</entry><entry>ipv4-address</entry><entry>true</entry><entry>false</entry></row>
<!-- Lease time and requested address should not be configured by a user.
<row><entry>dhcp-requested-address</entry><entry>50</entry><entry>ipv4-address</entry><entry>false</entry><entry>true</entry></row>
<row><entry>dhcp-lease-time</entry><entry>51</entry><entry>uint32</entry><entry>false</entry><entry>true</entry></row>
-->
<row><entry>dhcp-option-overload</entry><entry>52</entry><entry>uint8</entry><entry>false</entry><entry>false</entry></row>
<!-- Message Type  should not be configured by a user.
<row><entry>dhcp-message-type</entry><entry>53</entry><entry>uint8</entry><entry>false</entry><entry>false</entry></row>
-->
<row><entry>dhcp-server-identifier</entry><entry>54</entry><entry>ipv4-address</entry><entry>false</entry><entry>true</entry></row>
<!-- Parameter Request List should not be configured by a user.
<row><entry>dhcp-parameter-request-list</entry><entry>55</entry><entry>uint8</entry><entry>true</entry><entry>true</entry></row>
-->
<row><entry>dhcp-message</entry><entry>56</entry><entry>string</entry><entry>false</entry><entry>false</entry></row>
<row><entry>dhcp-max-message-size</entry><entry>57</entry><entry>uint16</entry><entry>false</entry><entry>false</entry></row>
<!-- Renewal and rebinding time should not be configured by a user.
<row><entry>dhcp-renewal-time</entry><entry>58</entry><entry>uint32</entry><entry>false</entry><entry>true</entry></row>
<row><entry>dhcp-rebinding-time</entry><entry>59</entry><entry>uint32</entry><entry>false</entry><entry>true</entry></row>
-->
<row><entry>vendor-class-identifier</entry><entry>60</entry><entry>hex</entry><entry>false</entry><entry>false</entry></row>
<!-- Client identifier should not be configured by a user.
<row><entry>dhcp-client-identifier</entry><entry>61</entry><entry>hex</entry><entry>false</entry><entry>true</entry></row>
-->
<row><entry>nwip-domain-name</entry><entry>62</entry><entry>string</entry><entry>false</entry><entry>false</entry></row>
<row><entry>nwip-suboptions</entry><entry>63</entry><entry>hex</entry><entry>false</entry><entry>false</entry></row>
<row><entry>nisplus-domain-name</entry><entry>64</entry><entry>string</entry><entry>false</entry><entry>false</entry></row>
<row><entry>nisplus-servers</entry><entry>65</entry><entry>ipv4-address</entry><entry>true</entry><entry>false</entry></row>
<row><entry>tftp-server-name</entry><entry>66</entry><entry>string</entry><entry>false</entry><entry>false</entry></row>
<row><entry>boot-file-name</entry><entry>67</entry><entry>string</entry><entry>false</entry><entry>false</entry></row>
<row><entry>mobile-ip-home-agent</entry><entry>68</entry><entry>ipv4-address</entry><entry>true</entry><entry>false</entry></row>
<row><entry>smtp-server</entry><entry>69</entry><entry>ipv4-address</entry><entry>true</entry><entry>false</entry></row>
<row><entry>pop-server</entry><entry>70</entry><entry>ipv4-address</entry><entry>true</entry><entry>false</entry></row>
<row><entry>nntp-server</entry><entry>71</entry><entry>ipv4-address</entry><entry>true</entry><entry>false</entry></row>
<row><entry>www-server</entry><entry>72</entry><entry>ipv4-address</entry><entry>true</entry><entry>false</entry></row>
<row><entry>finger-server</entry><entry>73</entry><entry>ipv4-address</entry><entry>true</entry><entry>false</entry></row>
<row><entry>irc-server</entry><entry>74</entry><entry>ipv4-address</entry><entry>true</entry><entry>false</entry></row>
<row><entry>streettalk-server</entry><entry>75</entry><entry>ipv4-address</entry><entry>true</entry><entry>false</entry></row>
<row><entry>streettalk-directory-assistance-server</entry><entry>76</entry><entry>ipv4-address</entry><entry>true</entry><entry>false</entry></row>
<row><entry>user-class</entry><entry>77</entry><entry>hex</entry><entry>false</entry><entry>false</entry></row>
<row><entry>slp-directory-agent</entry><entry>78</entry><entry>record (boolean, ipv4-address)</entry><entry>true</entry><entry>false</entry></row>
<row><entry>slp-service-scope</entry><entry>79</entry><entry>record (boolean, string)</entry><entry>false</entry><entry>false</entry></row>
<!-- The Client FQDN option value is not explicitly configured.
It is a part of the DDNS/D2 configuration
<row><entry>fqdn</entry><entry>81</entry><entry>record</entry><entry>false</entry><entry>true</entry></row>
-->
<!-- Relay Agent Information is not configured by the user.
It is merely echoed by the server
<row><entry>dhcp-agent-options</entry><entry>82</entry><entry>empty</entry><entry>false</entry><entry>false</entry></row>
-->
<row><entry>nds-server</entry><entry>85</entry><entry>ipv4-address</entry><entry>true</entry><entry>false</entry></row>
<row><entry>nds-tree-name</entry><entry>86</entry><entry>string</entry><entry>false</entry><entry>false</entry></row>
<row><entry>nds-context</entry><entry>87</entry><entry>string</entry><entry>false</entry><entry>false</entry></row>
<row><entry>bcms-controller-names</entry><entry>88</entry><entry>fqdn</entry><entry>true</entry><entry>false</entry></row>
<row><entry>bcms-controller-address</entry><entry>89</entry><entry>ipv4-address</entry><entry>true</entry><entry>false</entry></row>
<!-- Authentication option requires special processing
<row><entry>authenticate</entry><entry>90</entry><entry>hex</entry><entry>false</entry><entry>false</entry></row>
-->
<!-- Last transaction time and associated IP is dynamically calculated
<row><entry>client-last-transaction-time</entry><entry>91</entry><entry>uint32</entry><entry>false</entry><entry>false</entry></row>
<row><entry>associated-ip</entry><entry>92</entry><entry>ipv4-address</entry><entry>true</entry><entry>false</entry></row>
-->
<row><entry>client-system</entry><entry>93</entry><entry>uint16</entry><entry>true</entry><entry>false</entry></row>
<row><entry>client-ndi</entry><entry>94</entry><entry>record (uint8, uint8, uint8)</entry><entry>false</entry><entry>false</entry></row>
<row><entry>uuid-guid</entry><entry>97</entry><entry>record (uint8, hex)</entry><entry>false</entry><entry>false</entry></row>
<row><entry>uap-servers</entry><entry>98</entry><entry>string</entry><entry>false</entry><entry>false</entry></row>
<row><entry>geoconf-civic</entry><entry>99</entry><entry>hex</entry><entry>false</entry><entry>false</entry></row>
<row><entry>pcode</entry><entry>100</entry><entry>string</entry><entry>false</entry><entry>false</entry></row>
<row><entry>tcode</entry><entry>101</entry><entry>string</entry><entry>false</entry><entry>false</entry></row>
<row><entry>netinfo-server-address</entry><entry>112</entry><entry>ipv4-address</entry><entry>true</entry><entry>false</entry></row>
<row><entry>netinfo-server-tag</entry><entry>113</entry><entry>string</entry><entry>false</entry><entry>false</entry></row>
<row><entry>default-url</entry><entry>114</entry><entry>string</entry><entry>false</entry><entry>false</entry></row>
<row><entry>auto-config</entry><entry>116</entry><entry>uint8</entry><entry>false</entry><entry>false</entry></row>
<row><entry>name-service-search</entry><entry>117</entry><entry>uint16</entry><entry>true</entry><entry>false</entry></row>
<row><entry>subnet-selection</entry><entry>118</entry><entry>ipv4-address</entry><entry>false</entry><entry>false</entry></row>
<row><entry>domain-search</entry><entry>119</entry><entry>fqdn</entry><entry>true</entry><entry>false</entry></row>
<row><entry>vivco-suboptions</entry><entry>124</entry><entry>hex</entry><entry>false</entry><entry>false</entry></row>
<row><entry>vivso-suboptions</entry><entry>125</entry><entry>hex</entry><entry>false</entry><entry>false</entry></row>
<row><entry>pana-agent</entry><entry>136</entry><entry>ipv4-address</entry><entry>true</entry><entry>false</entry></row>
<row><entry>v4-lost</entry><entry>137</entry><entry>fqdn</entry><entry>false</entry><entry>false</entry></row>
<row><entry>capwap-ac-v4</entry><entry>138</entry><entry>ipv4-address</entry><entry>true</entry><entry>false</entry></row>
<row><entry>sip-ua-cs-domains</entry><entry>142</entry><entry>fqdn</entry><entry>true</entry><entry>false</entry></row>
<row><entry>rdnss-selection</entry><entry>146</entry><entry>record (uint8, ipv4-address, ipv4-address, fqdn)</entry><entry>true</entry><entry>false</entry></row>
<row><entry>v4-portparams</entry><entry>159</entry><entry>record (uint8, psid)</entry><entry>false</entry><entry>false</entry></row>
<row><entry>v4-captive-portal</entry><entry>160</entry><entry>string</entry><entry>false</entry><entry>false</entry></row>
<row><entry>option-6rd</entry><entry>212</entry><entry>record (uint8, uint8, ipv6-address, ipv4-address)</entry><entry>true</entry><entry>false</entry></row>
<row><entry>v4-access-domain</entry><entry>213</entry><entry>fqdn</entry><entry>false</entry><entry>false</entry></row>
          </tbody>
          </tgroup>
        </table>

      </para>
      <para>
        <table frame="all" xml:id="dhcp-types">
          <title>List of standard DHCP option types</title>
          <tgroup cols="2">
          <colspec colname="name"/>
          <colspec colname="meaning"/>
          <thead>
            <row><entry>Name</entry><entry>Meaning</entry></row>
          </thead>
          <tbody>
            <row><entry>hex</entry><entry>An arbitrary string of bytes, specified as a set of hexadecimal digits.</entry></row>
            <row><entry>boolean</entry><entry>Boolean value with allowed values true or false</entry></row>
            <row><entry>empty</entry><entry>No value, data is carried in suboptions</entry></row>
            <row><entry>fqdn</entry><entry>Fully qualified domain name (e.g. www.example.com)</entry></row>
            <row><entry>ipv4-address</entry><entry>IPv4 address in the usual dotted-decimal notation (e.g. 192.0.2.1)</entry></row>
            <row><entry>ipv6-address</entry><entry>IPv6 address in the usual colon notation (e.g. 2001:db8::1)</entry></row>
            <row><entry>ipv6-prefix</entry><entry>IPv6 prefix and prefix length specified using CIDR notation, e.g. 2001:db8:1::/64. This data type is used to represent an 8-bit field conveying a prefix length and the variable length prefix value</entry></row>
            <row><entry>psid</entry><entry>PSID and PSID length separated by a slash, e.g. 3/4 specifies PSID=3 and PSID length=4. In the wire format it is represented by an 8-bit field carrying PSID length (in this case equal to 4) and the 16-bits long PSID value field (in this case equal to "0011000000000000b" using binary notation). Allowed values for a PSID length are 0 to 16. See <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://tools.ietf.org/html/rfc7597">RFC 7597</link> for the details about the PSID wire representation</entry></row>
            <row><entry>record</entry><entry>Structured data that may be comprised of any types (except "record" and "empty"). The array flag applies to the last field only.</entry></row>
            <row><entry>string</entry><entry>Any text</entry></row>
            <row><entry>tuple</entry><entry>A length encoded as a 8 (16 for DHCPv6) bit unsigned integer followed by a string of this length</entry></row>
            <row><entry>uint8</entry><entry>8 bit unsigned integer with allowed values 0 to 255</entry></row>
            <row><entry>uint16</entry><entry>16 bit unsigned integer with allowed values 0 to 65535</entry></row>
            <row><entry>uint32</entry><entry>32 bit unsigned integer with allowed values 0 to 4294967295</entry></row>
            <row><entry>int8</entry><entry>8 bit signed integer with allowed values -128 to 127</entry></row>
            <row><entry>int16</entry><entry>16 bit signed integer with allowed values -32768 to 32767</entry></row>
            <row><entry>int32</entry><entry>32 bit signed integer with allowed values -2147483648 to 2147483647</entry></row>
          </tbody>
          </tgroup>
       </table>
      </para>
    </section>

    <section xml:id="dhcp4-custom-options">
      <title>Custom DHCPv4 options</title>
      <para>Kea supports custom (non-standard) DHCPv4 options. Assume
      that we want to define a new DHCPv4 option called "foo" which
      will have a code 222 and will convey a single unsigned 32 bit
      integer value. We can define such an option by using the
      following entry in the configuration file:
<screen>
"Dhcp4": {
    "option-def": [
        {
            <userinput>"name": "foo",
            "code": 222,
            "type": "uint32",
            "array": false,
            "record-types": "",
            "space": "dhcp4",
            "encapsulate": ""</userinput>
        }, ...
    ],
    ...
}
</screen>
      The <command>false</command> value of the <command>array</command>
      parameter determines that the option does NOT comprise an array of
      "uint32" values but is, instead, a single value.  Two other parameters have been
      left blank: <command>record-types</command> and
      <command>encapsulate</command>.  The former specifies the comma separated
      list of option data fields if the option comprises a record of data
      fields. This should be non-empty if the <command>type</command> 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 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.
      </para>

      <para>The <command>name</command>, <command>code</command> and
      <command>type</command> parameters are required, all others are
      optional. The <command>array</command> default value is
      <command>false</command>. The <command>record-types</command>
      and <command>encapsulate</command> default values are blank
      (i.e. ""). The default <command>space</command> is "dhcp4".
      </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.
<screen>
"Dhcp4": {
    "option-data": [
        {
            <userinput>"name": "foo",
            "code": 222,
            "space": "dhcp4",
            "csv-format": true,
            "data": "12345"</userinput>
        }, ...
    ],
    ...
}
</screen>
      </para>

      <para>New options can take more complex forms than simple use of
      primitives (uint8, string, ipv4-address etc): it is possible to
      define an option comprising a number of existing primitives.
      Assume we want to define a new option that will consist of
      an IPv4 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>
"Dhcp4": {
    "option-def": [
        {
            <userinput>"name": "bar",
            "code": 223,
            "space": "dhcp4",
            "type": "record",
            "array": false,
            "record-types": "ipv4-address, uint16, boolean, string",
            "encapsulate": ""</userinput>
        }, ...
    ],
    ...
}
</screen>
      The <command>type</command> 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 <command>record-types</command> field and should be ones from those listed in <xref linkend="dhcp-types"/>.
      </para>
      <para>
      The values of the option are set as follows:
<screen>
"Dhcp4": {
    "option-data": [
        {
            <userinput>"name": "bar",
            "space": "dhcp4",
            "code": 223,
            "csv-format": true,
            "data": "192.0.2.100, 123, true, Hello World"</userinput>
        }
    ],
    ...
}</screen>
      <command>csv-format</command> is set to <command>true</command> to indicate
      that the <command>data</command> field comprises a command-separated list
      of values.  The values in the <command>data</command> must correspond to
      the types set in the <command>record-types</command> field of the option
      definition.
     </para>
     <para>
       When <command>array</command> is set to <command>true</command>
       and <command>type</command> is set to "record", the last field
       is an array, i.e., it can contain more than one value as in:
<screen>
"Dhcp4": {
    "option-def": [
        {
            <userinput>"name": "bar",
            "code": 223,
            "space": "dhcp4",
            "type": "record",
            "array": true,
            "record-types": "ipv4-address, uint16",
            "encapsulate": ""</userinput>
        }, ...
    ],
    ...
}
</screen>
      The new option content is one IPv4 address followed by one or more 16
      bit unsigned integers.
     </para>
     <note>
       <para>In the general case, boolean values are specified as <command>true</command> or
       <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 versions of Kea will accept all those values
       for all boolean parameters.</para>
     </note>
     <note>
       <para>Numbers can be specified in decimal or hexadecimal format.
        The hexadecimal format can be either plain (e.g. abcd) or
        prefixed with 0x (e.g. 0xabcd).
       </para>
     </note>
    </section>

    <section id="dhcp4-private-opts">
      <title>DHCPv4 Private Options</title>
      <para>
      Options with code between 224 and 254 are reserved for private use.
      They can be defined at the global scope or at client class local
      scope: this allows to use option definitions depending on context
      and to set option data accordingly. For instance to configure
      an old PXEClient vendor:
<screen>
"Dhcp4": {
    "client-class": [
        {
            <userinput>"name": "pxeclient",
            "test": "option[vendor-class-identifier].text == 'PXEClient'",
            "option-def": [
                {
                    "name": "configfile",
                    "code": 209,
                    "type": "string"
                }
            ],</userinput>
            ...
        }, ...
    ],
    ...
}
</screen>
      </para>
      <para>
      As the Vendor Specific Information option (code 43) has vendor
      specific format, i.e. can carry either raw binary value or
      sub-options, this mechanism is available for this option too.
      </para>
      <para>
      In the following example taken from a real configuration two vendor
      classes use the option 43 for different and incompatible purposes:
<screen>
"Dhcp4": {
    "option-def": [
        {
            <userinput>"name": "cookie",
            "code": 1,
            "type": "string",
            "space": "APC"
        },
        {
            "name": "mtftp-ip",
            "code": 1,
            "type": "ipv4-address",
            "space": "PXE"
        },</userinput>
        ...
    ],
    "client-class": [
        {
            <userinput>"name": "APC",
            "test": "(option[vendor-class-identifier].text == 'APC'",
            "option-def": [
                {
                    "name": "vendor-encapsulated-options",
                    "type": "empty",
                    "encapsulate": "APC"
                }
            ],
            "option-data": [
                {
                    "name": "cookie",
                    "space": "APC",
                    "data": "1APC"
                },
                {
                    "name": "vendor-encapsulated-options"
                },</userinput>
                ...
            ],
            ...
        },
        {
            <userinput>"name": "PXE",
            "test": "(option[vendor-class-identifier].text == 'PXE'",
            "option-def": [
                {
                    "name": "vendor-encapsulated-options",
                    "type": "empty",
                    "encapsulate": "PXE"
                }
            ],
            "option-data": [
                {
                    "name": "mtftp-ip",
                    "space": "PXE",
                    "data": "0.0.0.0"
                },
                {
                    "name": "vendor-encapsulated-options"
                },</userinput>
                ...
            ],
            ...
        },
        ...
    ],
    ...
}
</screen>
      </para>
      <para>
      The definition used to decode a VSI option is:
      <orderedlist>
      <listitem><para>
      The local definition of a client class the incoming packet belongs to
      </para></listitem>
      <listitem><para>
      If none, the global definition
      </para></listitem>
      <listitem><para>
      If none, the last resort definition described in the next section
      <xref linkend="dhcp4-vendor-opts"/> (backward compatible with
      previous Kea versions).
      </para></listitem>
      </orderedlist>
      </para>
      <note>
      <para>
      This last resort definition for the Vendor Specific Information
      option (code 43) is not compatible with a raw binary value.
      So when there are some known cases where a raw binary value
      will be used, a client class must be defined with a classification
      expression matching these cases and an option definition for
      the VSI option with a binary type and no encapsulation.
      </para>
      </note>
      <note>
      <para>
      Option definitions in client classes is allowed only for these
      limited option set (codes 43 and from 224 to 254), and only
      for DHCPv4.
      </para>
      </note>
    </section>

    <section xml:id="dhcp4-vendor-opts">
      <title>DHCPv4 Vendor Specific Options</title>
      <para>
      Currently there are two option spaces defined for the DHCPv4 daemon:
      "dhcp4" (for the top level DHCPv4 options) and
      "vendor-encapsulated-options-space", which is empty by default but
      in which options can be defined. Such options will be carried in the
      Vendor Specific Information option (code 43). The following examples
      show how to define an option "foo" in that space that has a code 1,
      and comprises an
      IPv4 address, an unsigned 16 bit integer and a string. The "foo"
      option is conveyed in a Vendor Specific Information option.
      </para>
      <para>
      The first step is to define the format of the option:
<screen>
"Dhcp4": {
    "option-def": [
        {
            <userinput>"name": "foo",
            "code": 1,
            "space": "vendor-encapsulated-options-space",
            "type": "record",
            "array": false,
            "record-types": "ipv4-address, uint16, string",
            "encapsulate": ""</userinput>
        }
    ],
    ...
}</screen>
     (Note that the option space is set to "vendor-encapsulated-options-space".)
     Once the option format is defined, the next step is to define actual values
     for that option:
<screen>
"Dhcp4": {
    "option-data": [
        {
            <userinput>"name": "foo",
            "space": "vendor-encapsulated-options-space",
            "code": 1,
            "csv-format": true,
            "data": "192.0.2.3, 123, Hello World"</userinput>
        }
    ],
    ...
}</screen>
    We also include the Vendor Specific Information option, the option
    that conveys our sub-option "foo". This is required, else the option
    will not be included in messages sent to the client.
<screen>
"Dhcp4": {
    "option-data": [
        {
            <userinput>"name": "vendor-encapsulated-options"</userinput>
        }
    ],
    ...
}</screen>
    Alternatively, the option can be specified using its code.

<screen>
"Dhcp4": {
    "option-data": [
        {
            <userinput>"code": 43</userinput>
        }
    ],
    ...
}</screen>
      </para>

      <para>
      Another possibility, added in Kea 1.3, is to redefine the option,
      see <xref linkend="dhcp4-private-opts"/>.
      </para>
    </section>

    <section xml:id="dhcp4-option-spaces">

      <title>Nested DHCPv4 Options (Custom Option Spaces)</title>
      <para>It is sometimes useful to define a completely new option
      space. This is the case when user creates new option in the
      standard option space ("dhcp4") and wants this option
      to convey sub-options. Since they are in a separate space,
      sub-option codes will have a separate numbering scheme and may
      overlap with the codes of standard options.
      </para>
      <para>Note that creation of a new option space when defining
      sub-options for a standard option is not required, because it is
      created by default if the standard option is meant to convey any
      sub-options (see <xref linkend="dhcp4-vendor-opts"/>).
      </para>
      <para>
      Assume that we want to have a DHCPv4 option called "container" with
      code 222 that conveys two sub-options with codes 1 and 2.
      First we need to define the new sub-options:
<screen>
"Dhcp4": {
    "option-def": [
        {
            <userinput>"name": "subopt1",
            "code": 1,
            "space": "isc",
            "type": "ipv4-address",
            "record-types": "",
            "array": false,
            "encapsulate": ""
        },
        {
            "name": "subopt2",
            "code": 2,
            "space": "isc",
            "type": "string",
            "record-types": "",
            "array": false,
            "encapsulate": ""</userinput>
        }
    ],
    ...
}</screen>
    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 DHCPv4 option with our desired
    code and specify that it should include options from the new option space:
<screen>
"Dhcp4": {
    "option-def": [
        ...,
        {
            <userinput>"name": "container",
            "code": 222,
            "space": "dhcp4",
            "type": "empty",
            "array": false,
            "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 is set to "empty"
    to indicate that this option does not carry any data other than
    sub-options.
    </para>
    <para>
    Finally, we can set values for the new options:
<screen>
"Dhcp4": {
    "option-data": [
        {
            <userinput>"name": "subopt1",
            "code": 1,
            "space": "isc",
            "data": "192.0.2.3"</userinput>
        },
        }
            <userinput>"name": "subopt2",
            "code": 2,
            "space": "isc",
            "data": "Hello world"</userinput>
        },
        {
            <userinput>"name": "container",
            "code": 222,
            "space": "dhcp4"</userinput>
        }
    ],
    ...
}
</screen>
    </para>
    <para>Note that it is possible to create an option which carries some data
    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 <command>type</command> 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 value, sub-options.) The value specified
    with the <command>data</command> parameter — which should be a valid integer enclosed in quotes,
    e.g. "123" — would then be assigned to the uint16 field in the "container" option.
    </para>
    </section>

    <section xml:id="dhcp4-option-data-defaults">
      <title>Unspecified Parameters for DHCPv4 Option Configuration</title>
      <para>In many cases it is not required to specify all parameters for
      an option configuration and the default values may be used. However, it is
      important to understand the implications of not specifying some of them
      as it may result in configuration errors. The list below explains
      the behavior of the server when a particular parameter is not explicitly
      specified:

      <itemizedlist>
        <listitem>
          <simpara><command>name</command> - the server requires an option name or
          option code to identify an option. If this parameter is unspecified, the
          option code must be specified.
          </simpara>
        </listitem>

        <listitem>
          <simpara><command>code</command> - the server requires an option name or
          option code to identify an option. This parameter may be left unspecified if
          the <command>name</command> parameter is specified. However, this also
          requires that the particular option has its definition (it is either a
          standard option or an administrator created a definition for the option
          using an 'option-def' structure), as the option definition associates an
          option with a particular name. It is possible to configure an option
          for which there is no definition (unspecified option format).
          Configuration of such options requires the use of option code.
          </simpara>
        </listitem>

        <listitem>
          <simpara><command>space</command> - if the option space is unspecified it
          will default to 'dhcp4' which is an option space holding DHCPv4 standard
          options.
          </simpara>
        </listitem>

        <listitem>
          <simpara><command>data</command> - if the option data is unspecified it
          defaults to an empty value. The empty value is mostly used for the
          options which have no payload (boolean options), but it is legal to specify
          empty values for some options which carry variable length data and which
          the specification allows for the length of 0. For such options, the data parameter
          may be omitted in the configuration.</simpara>
        </listitem>

        <listitem>
          <simpara><command>csv-format</command> - if this value is not
          specified the server will assume that the option data is specified as
          a list of comma separated values to be assigned to individual fields
          of the DHCP option. This behavior has changed in Kea 1.2. Older
          versions used additional logic to determine whether the csv-format
          should be true or false. That is no longer the case.
          </simpara>
        </listitem>
      </itemizedlist>
      </para>

    </section>

    <section xml:id="dhcp4-stateless-configuration">
      <title>Stateless Configuration of DHCPv4 Clients</title>
      <para>The DHCPv4 server supports the stateless client configuration whereby the
      client has an IP address configured (e.g. using manual configuration) and only
      contacts the server to obtain other configuration parameters, e.g. addresses of DNS servers.
      In order to obtain the stateless configuration parameters the client sends the
      DHCPINFORM message to the server with the "ciaddr" set to the address that the
      client is currently using. The server unicasts the DHCPACK message to the
      client that includes the stateless configuration ("yiaddr" not set).
      </para>

      <para>The server will respond to the DHCPINFORM when the client is associated
      with a subnet defined in the server's configuration. An example
      subnet configuration will look like this:
        <screen>
"Dhcp4": {
    "subnet4": [
        {
            "subnet": "192.0.2.0/24"
            "option-data": [ {
                "name": "domain-name-servers",
                "code": 6,
                "data": "192.0.2.200,192.0.2.201",
                "csv-format": true,
                "space": "dhcp4"
            } ]
        }
    ]
}</screen>
      </para>
      <para>This subnet specifies the single option which will be included in
      the DHCPACK message to the client in response to DHCPINFORM. Note that
      the subnet definition does not require the address pool configuration
      if it will be used solely for the stateless configuration.
      </para>

      <para>This server will associate the subnet with the client if one of
      the following conditions is met:
      <itemizedlist>
          <listitem>
            <simpara>The DHCPINFORM is relayed and the giaddr matches the
            configured subnet.</simpara>
          </listitem>
          <listitem>
            <simpara>The DHCPINFORM is unicast from the client and the ciaddr
            matches the configured subnet.</simpara>
          </listitem>
          <listitem>
            <simpara>The DHCPINFORM is unicast from the client, the ciaddr is
            not set but the source address of the IP packet matches the
            configured subnet.</simpara>
          </listitem>
          <listitem>
            <simpara>The DHCPINFORM is not relayed and the IP address on the
            interface on which the message is received matches the configured
            subnet.</simpara>
          </listitem>
      </itemizedlist>
      </para>
    </section>

    <section xml:id="dhcp4-client-classifier">
      <title>Client Classification in DHCPv4</title>
      <para>
      The DHCPv4 server includes support for client classification.  For a deeper
      discussion of the classification process see <xref linkend="classify"/>.
      </para>

      <para>In certain cases it is useful to configure the server to differentiate between
      DHCP clients types and treat them accordingly. It is envisaged that client
      classification will be used for modifying the behavior of almost any part of
      the DHCP message processing. In the current release of Kea, there are four
      mechanisms that take advantage of the client classification in DHCPv4: subnet
      selection, address pool selection, DHCP options assignment, and, for cable modems,
      there are specific options for use with the TFTP server address and boot file field.
      </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 is cable networks. Here, there are two
      classes of devices: the cable modem itself, which should be handed a lease
      from subnet A and all other devices behind the modem 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="classification-subnets"/>.
      </para>

      <para>
      Client classification can also be used to restrict access to specific
      pools within a subnet. This is useful when to segregate clients belonging
      to the same subnet into different address ranges.
      </para>

      <para>
      The process of doing classification is conducted in three steps. The first step
      is to assess an incoming packet and assign it to zero or more classes.  The
      second step is to choose a subnet, possibly based on the class information.
      The third step is to assign options, again possibly based on the class
      information.
      </para>

      <para>
      There are two methods of doing classification. The first is automatic and relies
      on examining the values in the vendor class options. Information from these
      options is extracted and a class name is constructed from it and added to
      the class list for the packet. The second allows you to specify an expression
      that is evaluated for each packet. If the result is true the packet is
      a member of the class.
      </para>

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

      <section>
        <title>Setting Fixed Fields in Classification</title>
        <para>
          It is possible to specify that clients belonging to a particular class
          should receive packets with specific values in certain fixed fields.
          In particular, three fixed fields are supported:
          <command>next-server</command> (that conveys an IPv4 address, which is
          set in the siaddr field), <command>server-hostname</command> (that
          conveys a server hostname, can be up to 64 bytes long and will be sent
          in the sname field) and <command>boot-file-name</command> (that
          conveys the configuration file, can be up to 128 bytes long and will
          be sent using file field).
        </para>
        <para>
          Obviously, there are many ways to assign clients to specific classes,
          but for the PXE clients the client architecture type option (code 93)
          seems to be particularly suited to
          make the distinction. The following example checks if the client
          identifies itself as PXE device with architecture EFI x86-64, and
          sets several fields if it does. See
          <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://tools.ietf.org/html/rfc4578#section-2.1">Section 2.1 of RFC 4578</link>)
          or the documentation of your client for specific values.
        </para>
          <screen>
"Dhcp4": {
    "client-classes": [
        {
            "name": "ipxe_efi_x64",
            "test": "option[93].hex == 0x0009",
            <userinput>"next-server": "192.0.2.254",
            "server-hostname": "hal9000",
            "boot-file-name": "/dev/null"</userinput>
        },
        ...
    ],
    ...
          }</screen>

          <para>
            If there are multiple classes defined and an incoming packet is matched
            to multiple classes, the class whose name is alphabetically the first
            is used.
          </para>
        </section>

      <section>
        <title>Using Vendor Class Information in Classification</title>
        <para>
        The server checks whether an incoming packet includes the vendor class identifier
        option (60). If it does, the content of that option is prepended with
        "VENDOR_CLASS_", it is interpreted as a class. For example,
        modern cable modems will send this option with value "docsis3.0"
        and as a result the packet will belong to class "VENDOR_CLASS_docsis3.0".
        </para>

        <note>
        <para>
          Kea 1.0 and earlier versions performed special actions for
          clients that were in VENDOR_CLASS_docsis3.0. This is no longer the
          case in Kea 1.1 and later. In these versions the old behavior
          can be achieved by defining VENDOR_CLASS_docsis3.0 and setting
          its next-server and boot-file-name values appropriately.
        </para>
        </note>

        <para>
        This example shows a configuration using an automatically generated
        "VENDOR_CLASS_" class. The administrator of the network has
        decided that addresses from range 192.0.2.10 to 192.0.2.20 are
        going to be managed by the Dhcp4 server and only clients belonging to the
        docsis3.0 client class are allowed to use that pool.

        <screen>
"Dhcp4": {
    "subnet4": [
        {
            "subnet": "192.0.2.0/24",
            "pools": [ { "pool": "192.0.2.10 - 192.0.2.20" } ],
            <userinput>"client-class": "VENDOR_CLASS_docsis3.0"</userinput>
        }
    ],
    ...
}</screen>
        </para>

      </section>

      <section>
        <title>Defining and Using Custom Classes</title>
        <para>
        The following example shows how to configure a class using an expression
        and a subnet that makes use of the class. This configuration defines the
        class named "Client_foo".
        It is comprised of all clients who's client ids (option 61) start with the
        string "foo". Members of this class will be given addresses from
        192.0.2.10 to 192.0.2.20 and the addresses of their DNS servers
        set to 192.0.2.1 and 192.0.2.2.

          <screen>
"Dhcp4": {
    "client-classes": [
        {<userinput>
            "name": "Client_foo",
            "test": "substring(option[61].hex,0,3) == 'foo'",
            "option-data": [
                {
                    "name": "domain-name-servers",
                    "code": 6,
                    "space": "dhcp4",
                    "csv-format": true,
                    "data": "192.0.2.1, 192.0.2.2"
                }
            ]</userinput>
        },
        ...
    ],
    "subnet4": [
        {
            "subnet": "192.0.2.0/24",
            "pools": [ { "pool": "192.0.2.10 - 192.0.2.20" } ],
            <userinput>"client-class": "Client_foo"</userinput>
        },
        ...
    ],
    ...
}</screen>
        </para>
      </section>
    </section>

    <section xml:id="dhcp4-ddns-config">
      <title>DDNS for DHCPv4</title>
      <para>
      As mentioned earlier, kea-dhcp4 can be configured to generate requests to the
      DHCP-DDNS server (referred to here as "D2" ) to update 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 (A records), reverse
      DNS updates (PTR records), or both.
      </para></listitem>
      <listitem><para>
      The FQDN, lease address, and DHCID
      </para></listitem>
      </orderedlist>
      The parameters for controlling the generation of NCRs for submission to D2
      are contained in the <command>dhcp-ddns</command> section of the kea-dhcp4 server
      configuration. The mandatory parameters for the DHCP DDNS configuration
      are <command>enable-updates</command> which is unconditionally
      required, and <command>qualifying-suffix</command> which has no
      default value and is required when <command>enable-updates</command>
      is set to <command>true</command>.

      The two (disabled and enabled) minimal DHCP DDNS configurations are:
<screen>
"Dhcp4": {
    "dhcp-ddns": {
        <userinput>"enable-updates": false</userinput>
    },
    ...
}
</screen>
      and for example:
<screen>
"Dhcp4": {
    "dhcp-ddns": {
        <userinput>"enable-updates": true,
        "qualifying-suffix": "example."</userinput>
    },
    ...
}
</screen>

      The default values for the "dhcp-ddns" section are as follows:
      <itemizedlist>
      <listitem><simpara>
      <command>"server-ip": "127.0.0.1"</command>
      </simpara></listitem>
      <listitem><simpara>
      <command>"server-port": 53001</command>
      </simpara></listitem>
      <listitem><simpara>
      <command>"sender-ip": ""</command>
      </simpara></listitem>
      <listitem><simpara>
      <command>"sender-port": 0</command>
      </simpara></listitem>
      <listitem><simpara>
      <command>"max-queue-size": 1024</command>
      </simpara></listitem>
      <listitem><simpara>
      <command>"ncr-protocol": "UDP"</command>
      </simpara></listitem>
      <listitem><simpara>
      <command>"ncr-format": "JSON"</command>
      </simpara></listitem>
      <listitem><simpara>
      <command>"override-no-update": false</command>
      </simpara></listitem>
      <listitem><simpara>
      <command>"override-client-update": false</command>
      </simpara></listitem>
      <listitem><simpara>
      <command>"replace-client-name": "never"</command>
      </simpara></listitem>
      <listitem><simpara>
      <command>"generated-prefix": "myhost"</command>
      </simpara></listitem>
      </itemizedlist>
      </para>

      <section xml:id="dhcpv4-d2-io-config">
      <title>DHCP-DDNS Server Connectivity</title>
      <para>
      In order for NCRs to reach the D2 server, kea-dhcp4 must be able
      to communicate with it.  kea-dhcp4 uses the following configuration
      parameters to control this communication:
      <itemizedlist>
      <listitem><simpara>
      <command>enable-updates</command> - determines whether or not kea-dhcp4 will
      generate NCRs.  By default, this value is 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
      the local loopback interface at address 127.0.0.1. You may specify
      either an IPv4 or IPv6 address.
      </simpara></listitem>
      <listitem><simpara>
      <command>server-port</command> - port on which D2 listens for requests.  The default value
      is 53001.
      </simpara></listitem>
      <listitem><simpara>
      <command>sender-ip</command> - IP address which kea-dhcp4 should use to send requests to D2.
      The default value is blank which instructs kea-dhcp4 to select a suitable
      address.
      </simpara></listitem>
      <listitem><simpara>
      <command>sender-port</command> - port which kea-dhcp4 should use to send requests to D2. The
      default value of 0 instructs kea-dhcp4 to select a suitable port.
      </simpara></listitem>
      <listitem><simpara>
      <command>max-queue-size</command> - maximum number of requests allowed to queue waiting to
      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
      been sufficiently reduced.  The intention is to allow the kea-dhcp4 server to
      continue lease operations without running the risk that its memory usage
      grows without limit.  The default value is 1024.
      </simpara></listitem>
      <listitem><simpara>
      <command>ncr-protocol</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-format</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>
      By default, kea-dhcp-ddns is assumed to be running on the same machine as kea-dhcp4, and
      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 be altered accordingly. For example, if D2 has been
      configured to listen on 192.168.1.10 port 900, the following configuration
      would be required:
<screen>
"Dhcp4": {
    "dhcp-ddns": {
        <userinput>"server-ip": "192.168.1.10",
        "server-port": 900</userinput>,
        ...
    },
    ...
}
</screen>
      </para>
      </section>
      <section xml:id="dhcpv4-d2-rules-config">
      <title>When Does the kea-dhcp4 Server Generate DDNS Requests?</title>
      <para>kea-dhcp4 follows the behavior prescribed for DHCP servers in
      <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://tools.ietf.org/html/rfc4702">RFC 4702</link>.
      It is important to keep in mind that kea-dhcp4 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 within the purview of D2 itself (<xref linkend="dhcp-ddns-server"/>).
      This section describes when kea-dhcp4 will generate NCRs and the
      configuration parameters that can be used to influence this decision.
      It assumes that the <command>enable-updates</command> parameter is true.
      </para>
      <para>
      In general, kea-dhcp4 will generate DDNS update requests when:
      <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.
      </para>
      <para>
      The decision making involved when granting a new lease (the first case) is more
      involved.  When a new lease is granted, kea-dhcp4 will generate a DDNS
      update request if the DHCP REQUEST contains either the FQDN option
      (code 81) or the Host Name option (code 12). If both are present,
      the server will use the FQDN option. By default kea-dhcp4
      will respect the FQDN N and S flags specified by the client as shown
      in the following table:
      </para>
        <table xml:id="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
      the server should do the reverse updates.  By default, kea-dhcp4 will honor
      the client's wishes and generate a DDNS request to the D2 server to update only
      reverse DNS data.  The parameter <command>override-client-update</command> can be used
      to instruct the server to override client delegation requests.  When
      this parameter is true, kea-dhcp4 will disregard requests for client
      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
      <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://tools.ietf.org/html/rfc4702">RFC 4702</link>. If such a
      combination is received from the client, the packet will be dropped by kea-dhcp4.)
      </para>
      <para>
      To override client delegation, set the following values in the configuration file:
      </para>
<screen>
"Dhcp4": {
    "dhcp-ddns": {
        <userinput>"override-client-update": true</userinput>,
        ...
    },
    ...
}
</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, <command>override-no-update</command>,
      can be used to instruct the server to disregard the client's wishes. When
      this parameter is true, kea-dhcp4 will generate DDNS update requests to kea-dhcp-ddns
      even if the client requests that no updates be done.  The N-S-O flags in the
      server's response to the client will be 0-1-1.
      </para>
      <para>
      To override client delegation, the following values should be set in your configuration:
      </para>
<screen>
"Dhcp4": {
    "dhcp-ddns": {
        <userinput>"override-no-update": true</userinput>,
        ...
    },
    ...
}
</screen>
      <para>
      kea-dhcp4 will always generate DDNS update requests if the client request
      only contains the Host Name option. In addition it will include an FQDN
      option in the response to the client with the FQDN N-S-O flags set to
      0-1-0 respectively. The domain name portion of the FQDN option will be
      the name submitted to D2 in the DDNS update request.
      </para>
      </section>

      <section xml:id="dhcpv4-fqdn-name-generation">
      <title>kea-dhcp4 name generation for DDNS update requests</title>
      <para>Each NameChangeRequest must of course include the fully qualified domain
      name whose DNS entries are to be affected.  kea-dhcp4 can be configured to
      supply a portion or all of that name based upon what it receives from
      the client in the DHCP REQUEST.</para>
      <para>
       The default rules for constructing the FQDN that will be used for DNS
       entries are:
      <orderedlist>
      <listitem><para>
        If the DHCPREQUEST contains the client FQDN option, the candidate name
        is taken from there, otherwise it is taken from the Host Name option.
      </para></listitem>
      <listitem><para>
        If the candidate name is a partial (i.e. unqualified) name then add a
        configurable suffix to the name and use the result as the FQDN.
      </para></listitem>
      <listitem><para>
        If the candidate name provided is empty, generate a FQDN using a
        configurable prefix and suffix.
      </para></listitem>
      <listitem><para>
        If the client provided neither option, then no DNS action will be taken.
      </para></listitem>
      </orderedlist>
        These rules can amended by setting the
        <command>replace-client-name</command> parameter which provides the
        following modes of behavior:
      <itemizedlist>
      <listitem><para>
        <command>never</command> - Use the name the client sent.  If the client
        sent no name, do not generate one.  This is the default mode.
      </para></listitem>
      <listitem><para>
        <command>always</command> - Replace the name the client sent. If the
        client sent no name, generate one for the client.
      </para></listitem>
      <listitem><para>
        <command>when-present</command> - Replace the name the client sent.
        If the client sent no name, do not generate one.
      </para></listitem>
      <listitem><para>
        <command>when-not-present</command> - Use the name the client sent.
        If the client sent no name, generate one for the client.
      </para></listitem>
      </itemizedlist>
      <note>
        <para>
    Note that formerly, this parameter was a boolean and permitted only values
    of <command>true</command> and <command>false</command>.  Boolean values
    have been deprecated and are no longer accepted.  If you are currently using
    booleans, you must replace them with the desired mode name. A value of
    <command>true</command> maps to <command>"when-present"</command>, while
    <command>false</command> maps to <command>"never"</command>.
        </para>
    </note>

      For example, To instruct kea-dhcp4 to always generate the FQDN for a
      client, set the parameter <command>replace-client-name</command> to
      <command>always</command> as follows:
      </para>
<screen>
"Dhcp4": {
    "dhcp-ddns": {
        <userinput>"replace-client-name": "always"</userinput>,
        ...
    },
    ...
}
</screen>
      <para>
      The prefix used in the generation of a FQDN is specified by the
      <command>generated-prefix</command> parameter.  The default value is "myhost".  To alter
      its value, simply set it to the desired string:
      </para>
<screen>
"Dhcp4": {
    "dhcp-ddns": {
        <userinput>"generated-prefix": "another.host"</userinput>,
        ...
    },
    ...
}
</screen>
      <para>
      The suffix used when generating a FQDN or when qualifying a
      partial name is specified by
      the <command>qualifying-suffix</command> parameter. This
      parameter has no default value, thus it is mandatory when
      DDNS updates are enabled.
      To set its value simply set it to the desired string:
      </para>
<screen>
"Dhcp4": {
    "dhcp-ddns": {
        <userinput>"qualifying-suffix": "foo.example.org"</userinput>,
        ...
    },
    ...
}
</screen>
      <para>
      When generating a name, kea-dhcp4 will construct name of the format:
      </para>
      <para>
        [generated-prefix]-[address-text].[qualifying-suffix].
      </para>
      <para>
      where address-text is simply the lease IP address converted to a
      hyphenated string.  For example, if the lease address is 172.16.1.10,
      the qualifying suffix "example.com", and the default value is used for
      <command>generated-prefix</command>, the generated FQDN would be:
      </para>
      <para>
        myhost-172-16-1-10.example.com.
      </para>
      </section>
    </section>

    <section xml:id="dhcp4-next-server">
      <title>Next Server (siaddr)</title>
      <para>In some cases, clients want to obtain configuration from a TFTP server.
      Although there is a dedicated option for it, some devices may use the siaddr field
      in the DHCPv4 packet for that purpose. That specific field can be configured
      using <command>next-server</command> directive. It is possible to define it in the global scope or
      for a given subnet only. If both are defined, the subnet value takes precedence.
      The value in subnet can be set to 0.0.0.0, which means that <command>next-server</command> should
      not be sent. It may also be set to an empty string, which means the same as if
      it was not defined at all, i.e. use the global value.
      </para>
      <para>
      The <command>server-hostname</command> (that conveys a server hostname,
      can be up to 64 bytes long and will be sent in the sname field) and
      <command>boot-file-name</command> (that conveys the configuration file,
      can be up to 128 bytes long and will be sent using file field)
      directives are handled the same way as <command>next-server</command>.
      </para>

<screen>
"Dhcp4": {
    <userinput>"next-server": "192.0.2.123",
    "boot-file-name": "/dev/null"</userinput>,
    ...,
    "subnet4": [
        {
            <userinput>"next-server": "192.0.2.234",
            "server-hostname": "some-name.example.org",
            "boot-file-name": "bootfile.efi"</userinput>,
            ...
        }
    ]
}
</screen>
    </section>

    <section xml:id="dhcp4-echo-client-id">
      <title>Echoing Client-ID (RFC 6842)</title>
      <para>The original DHCPv4 specification
      (<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://tools.ietf.org/html/rfc2131">RFC 2131</link>)
      states that the DHCPv4
      server must not send back client-id options when responding to
      clients. However, in some cases that confused clients that did
      not have MAC address or client-id; see
      <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://tools.ietf.org/html/rfc6842">RFC 6842</link>.
      for details. That
      behavior has changed with the publication of
      <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://tools.ietf.org/html/rfc6842">RFC 6842</link>
      which updated
      <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://tools.ietf.org/html/rfc2131">RFC 2131</link>.
      That update states that the server must
      send client-id if the client sent it. That is Kea's default behavior.
      However, in some cases older devices that do not support
      <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://tools.ietf.org/html/rfc6842">RFC 6842</link>.
      may refuse to accept responses that include the
      client-id option. To enable backward compatibility, an optional
      configuration parameter has been introduced. To configure it,
      use the following configuration statement:</para>

<screen>
"Dhcp4": {
    <userinput>"echo-client-id": false</userinput>,
    ...
}
</screen>
    </section>

    <section xml:id="dhcp4-match-client-id">
      <title>Using Client Identifier and Hardware Address</title>
      <para>The DHCP server must be able to identify the client (and distinguish it from
      other clients) from which it receives the message. There are many reasons
      why this identification is required and the most important ones are:
      <itemizedlist>
        <listitem><simpara>When the client contacts the server to allocate a new
        lease, the server must store the client identification information in
        the lease database as a search key.</simpara></listitem>
        <listitem><simpara>When the client is trying to renew or release the existing
        lease, the server must be able to find the existing lease entry in the
        database for this client, using the client identification information as a
        search key.</simpara></listitem>
        <listitem><simpara>Some configurations use static reservations for the IP
        addresses and other configuration information. The server's administrator
        uses client identification information to create these static assignments.
        </simpara></listitem>
        <listitem><simpara>In the dual stack networks there is often a need to
        correlate the lease information stored in DHCPv4 and DHCPv6 server for
        a particular host. Using common identification information by the DHCPv4
        and DHCPv6 client allows the network administrator to achieve this
        correlation and better administer the network.</simpara></listitem>
      </itemizedlist>
      </para>

      <para>DHCPv4 makes use of two distinct identifiers which are placed
      by the client in the queries sent to the server and copied by the server
      to its responses to the client: "chaddr" and "client identifier". The
      former was introduced as a part of the BOOTP specification and it is also
      used by DHCP to carry the hardware address of the interface used to send
      the query to the server (MAC address for the Ethernet). The latter is
      carried in the Client-identifier option, introduced in
      <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://tools.ietf.org/html/rfc2132">RFC 2132</link>.
      </para>

      <para><link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://tools.ietf.org/html/rfc2131">RFC 2131</link>
      indicates that the server may use both of these identifiers to identify
      the client but the "client identifier", if present, takes precedence
      over "chaddr". One of the reasons for this is that "client identifier"
      is independent from the hardware used by the client to communicate with
      the server. For example, if the client obtained the lease using one
      network card and then the network card is moved to another host, the
      server will wrongly identify this host is the one which has obtained
      the lease. Moreover,
      <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://tools.ietf.org/html/rfc4361">RFC 4361</link> gives
      the recommendation to use a DUID
      (see <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://tools.ietf.org/html/rfc3315">RFC 3315</link>,
      the DHCPv6 specification)
      carried as "client identifier" when dual stack networks are in use
      to provide consistent identification information of the client, regardless
      of the protocol type it is using. Kea adheres to these specifications and
      the "client identifier" by default takes precedence over the value carried
      in "chaddr" field when the server searches, creates, updates or removes
      the client's lease.
      </para>

      <para>When the server receives a DHCPDISCOVER or DHCPREQUEST message from the
      client, it will try to find out if the client already has a lease in the
      database and will hand out that lease rather than allocate
      a new one. Each lease in the lease database is associated with the
      "client identifier" and/or "chaddr". The server will first use the
      "client identifier" (if present) to search the lease. If the lease is
      found, the server will treat this lease as belonging to the client
      even if the current "chaddr" and the "chaddr" associated with
      the lease do not match. This facilitates the scenario when the network card
      on the client system has been replaced and thus the new MAC address
      appears in the messages sent by the DHCP client. If the server fails
      to find the lease using the "client identifier" it will perform another lookup
      using the "chaddr". If this lookup returns no result, the client is
      considered as not having a lease and the new lease will be created.
      </para>

      <para>A common problem reported by network operators is that poor
      client implementations do not use stable client identifiers, instead
      generating a new "client identifier" each time the client connects
      to the network. Another well known case is when the client changes its
      "client identifier" during the multi-stage boot process (PXE). In such
      cases, the MAC address of the client's interface remains stable and
      using "chaddr" field to identify the client guarantees that the
      particular system is considered to be the same client, even though its
      "client identifier" changes.
      </para>

      <para>To address this problem, Kea includes a configuration option
      which enables client identification using "chaddr" only by instructing
      the server to disregard server to "ignore" the "client identifier" during
      lease lookups and allocations for a particular subnet. Consider the following
      simplified server configuration:</para>
<screen>
"Dhcp4": {
    ...
    <userinput>"match-client-id": true,</userinput>
    ...
    "subnet4": [
    {
        "subnet": "192.0.10.0/24",
        "pools": [ { "pool": "192.0.2.23-192.0.2.87" } ],
        <userinput>"match-client-id": false</userinput>
    },
    {
        "subnet": "10.0.0.0/8",
        "pools": [ { "pool": "10.0.0.23-10.0.2.99" } ],
    }
    ]
}
</screen>

     <para>The <command>match-client-id</command> is a boolean value which
     controls this behavior. The default value of <userinput>true</userinput>
     indicates that the server will use the "client identifier" for lease
     lookups and "chaddr" if the first lookup returns no results. The
     <command>false</command> means that the server will only
     use the "chaddr" to search for client's lease. Whether the DHCID for
     DNS updates is generated from the "client identifier" or "chaddr" is
     controlled through the same parameter accordingly.</para>

     <para>The <command>match-client-id</command> parameter may appear
     both in the global configuration scope and/or under any subnet
     declaration. In the example shown above, the effective value of the
     <command>match-client-id</command> will be <userinput>false</userinput>
     for the subnet 192.0.10.0/24, because the subnet specific setting
     of the parameter overrides the global value of the parameter. The
     effective value of the <command>match-client-id</command> for the subnet
     10.0.0.0/8 will be set to <userinput>true</userinput> because the
     subnet declaration lacks this parameter and the global setting is
     by default used for this subnet. In fact, the global entry for this
     parameter could be omitted in this case, because
     <userinput>true</userinput> is the default value.
     </para>

     <para>It is important to explain what happens when the client obtains
     its lease for one setting of the <command>match-client-id</command>
     and then renews when the setting has been changed. First consider
     the case when the client obtains the lease when the
     <command>match-client-id</command> is set to <userinput>true</userinput>.
     The server will store the lease information including "client identifier"
     (if supplied) and "chaddr" in the lease database. When the setting is
     changed and the client renews the lease the server will determine that
     it should use the "chaddr" to search for the existing lease. If the
     client hasn't changed its MAC address the server should successfully
     find the existing lease. The "client identifier" associated with the
     returned lease is ignored and the client is allowed to use this lease.
     When the lease is renewed only the "chaddr" is recorded for this
     lease according to the new server setting.
     </para>

     <para>In the second case the client has the lease with only a "chaddr"
     value recorded. When the setting is changed to
     <command>match-client-id</command> set to <userinput>true</userinput>
     the server will first try to use the "client identifier" to find the
     existing client's lease. This will return no results because the
     "client identifier" was not recorded for this lease. The server will
     then use the "chaddr" and the lease will be found. If the lease appears
     to have no "client identifier" recorded, the server will assume that
     this lease belongs to the client and that it was created with the previous
     setting of the <command>match-client-id</command>.
     However, if the lease contains "client identifier" which is different
     from the "client identifier" used by the client the lease will be
     assumed to belong to another client and the new lease will be
     allocated.
     </para>

    </section>

     <section xml:id="dhcp4-dhcp4o6-config">
      <title>DHCPv4-over-DHCPv6: DHCPv4 Side</title>
      <para>
      The support of DHCPv4-over-DHCPv6 transport is described in
      <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://tools.ietf.org/html/rfc7341">RFC 7341</link>
      and is implemented using cooperating DHCPv4 and DHCPv6 servers.
      This section is about the configuration of the DHCPv4 side
      (the DHCPv6 side is described in <xref linkend="dhcp6-dhcp4o6-config"/>).
      </para>
      <note>
        <para>DHCPv4-over-DHCPv6 support is experimental and the
        details of the inter-process communication can change: both
        the DHCPv4 and DHCPv6 sides should be running the same version
        of Kea.</para>
      </note>
      <para>
      The <command>dhcp4o6-port</command> global parameter specifies
      the first of the two consecutive ports of the UDP sockets used
      for the communication between the DHCPv6 and DHCPv4 servers
      (the DHCPv4 server is bound to ::1 on <command>port</command> + 1
      and connected to ::1 on <command>port</command>).
      </para>
      <para>
      With DHCPv4-over-DHCPv6 the DHCPv4 server does not have access
      to several of the identifiers it would normally use to select a
      subnet.  In order to address this issue three new configuration
      entries have been added.  The presence of any of these allows the
      subnet to be used with DHCPv4-over-DHCPv6.  These entries are:
      <itemizedlist>
        <listitem>
        <simpara><command>4o6-subnet</command>: Takes a prefix (i.e., an
        IPv6 address followed by a slash and a prefix length) which is
        matched against the source address.
        </simpara>
        </listitem>
        <listitem>
        <simpara><command>4o6-interface-id</command>: Takes a relay interface
        ID option value.
        </simpara>
        </listitem>
        <listitem>
        <simpara><command>4o6-interface</command>: Takes an interface name
        which is matched against the incoming interface name.
        </simpara>
        </listitem>
      </itemizedlist>
      </para>
      <para>
      The following configuration was used during some tests:
<screen>
{

# DHCPv4 conf
"Dhcp4": {
    "interfaces-config": {
        "interfaces": [ "eno33554984" ]
    },

    "lease-database": {
        "type": "memfile",
        "name": "leases4"
    },

    "valid-lifetime": 4000,

    "subnet4": [ {
        "subnet": "10.10.10.0/24",
        <userinput>"4o6-interface": "eno33554984",</userinput>
        <userinput>"4o6-subnet": "2001:db8:1:1::/64",</userinput>
        "pools": [ { "pool": "10.10.10.100 - 10.10.10.199" } ]
    } ],

    <userinput>"dhcp4o6-port": 6767</userinput>

},

"Logging": {
    "loggers": [ {
        "name": "kea-dhcp4",
        "output_options": [ {
            "output": "/tmp/kea-dhcp4.log"
        } ],
        "severity": "DEBUG",
        "debuglevel": 0
    } ]
}

}
</screen>
    </para>
    </section>

  </section> <!-- end of configuring kea-dhcp4 server section -->

  <!-- Host reservation is a large topic. There will be many subsections,
   so it should be a section on its own. -->
  <section xml:id="host-reservation-v4">
    <title>Host Reservation in DHCPv4</title>

    <para>There are many cases where it is useful to provide a configuration on
    a per host basis. The most obvious one is to reserve a specific, static
    address for exclusive use by a given client (host) ‐ the returning client will
    receive the same address from the server every time, and other clients will
    generally not receive that address.
    Another example when the host reservations are applicable is when a host
    has specific requirements, e.g. a printer that needs additional DHCP options.
    Yet another possible use case is to define unique names for hosts.</para>

    <para>Note that there may be cases when the
    new reservation has been made for the client for the address being currently
    in use by another client. We call this situation a "conflict". The conflicts
    get resolved automatically over time as described in subsequent sections.
    Once the conflict is resolved, the client will keep receiving the reserved
    configuration when it renews.</para>

    <para>Host reservations are defined as parameters for each subnet. Each host
    has to be identified by an identifier, for example the hardware/MAC address. There is an optional
    <command>reservations</command> array in the <command>Subnet4</command>
    element. Each element in that array is a structure that holds information
    about reservations for a single host. In particular, the structure has
    to have an identifier that uniquely identifies a host.  In the DHCPv4 context, the
    identifier is usually a hardware or MAC address.  In most cases an IP address
    will be specified. It is also possible to specify a hostname, host
    specific options or fields carried within DHCPv4 message such as siaddr,
    sname or file.</para>

    <para>In Kea 1.0.0 it was only possible to create host reservations
    using client's hardware address. Host reservations by client
    identifier, DUID and circuit-id have been added in Kea 1.1.0.</para>

    <para>The following example shows how to reserve addresses for specific
    hosts:

<screen>
"subnet4": [
    {
        "pools": [ { "pool":  "192.0.2.1 - 192.0.2.200" } ],
        "subnet": "192.0.2.0/24",
        "interface": "eth0",
        <userinput>"reservations": [
            {
                "hw-address": "1a:1b:1c:1d:1e:1f",
                "ip-address": "192.0.2.202"
            },
            {
                "duid": "0a:0b:0c:0d:0e:0f",
                "ip-address": "192.0.2.100",
                "hostname": "alice-laptop"
            },
            {
                "circuit-id": "'charter950'",
                "ip-address": "192.0.2.203"
            },
            {
                "client-id": "01:11:22:33:44:55:66",
                "ip-address": "192.0.2.204"
            }
        ]</userinput>
    }
]
</screen>
    The first entry reserves the 192.0.2.202 address for the client that uses
    a MAC address of 1a:1b:1c:1d:1e:1f. The second entry reserves the address
    192.0.2.100 and the hostname of alice-laptop for the client using a DUID
    0a:0b:0c:0d:0e:0f. (Note that if you plan to do DNS updates, it
    is strongly recommended for the hostnames to be unique.) The third
    example reserves address 192.0.3.203 to a client whose request
    would be relayed by a relay agent that inserts a circuit-it option
    with the value 'charter950'. The fourth entry reserves address
    192.0.2.204 for a client that uses a client identifier with value
    01:11:22:33:44:55:66.</para>

    <para>The above example is used for illustrational purposes only
    and in actual deployments it is recommended to use as few types as possible
    (preferably just one). See <xref linkend="reservations4-tuning"/> for a detailed
    discussion of this point.</para>

    <para>Making a reservation for a mobile host that may visit multiple subnets
    requires a separate host definition in each subnet it is expected to visit.
    It is not allowed to define multiple host definitions with the same hardware
    address in a single subnet. Multiple host definitions with the same hardware
    address are valid if each is in a different subnet.
    </para>

    <para>Adding host reservation incurs a performance penalty. In principle,
    when a server that does not support host reservation responds to a query,
    it needs to check whether there is a lease for a given address being
    considered for allocation or renewal. The server that also supports host
    reservation has to perform additional checks: not only if the address is
    currently used (i.e. if there is a lease for it), but also whether the address
    could be used by someone else (i.e. there is a reservation for it). That
    additional check incurs additional overhead.</para>

    <section xml:id="reservation4-types">
      <title>Address Reservation Types</title>

      <para>In a typical scenario there is an IPv4 subnet defined,
      e.g. 192.0.2.0/24, with certain part of it dedicated for dynamic allocation
      by the DHCPv4 server. That dynamic part is referred to as a dynamic pool or
      simply a pool. In principle, a host reservation can reserve any address
      that belongs to the subnet. The reservations that specify addresses that
      belong to configured pools are called "in-pool reservations".
      In contrast, those that do not belong to dynamic pools are called
      "out-of-pool reservations". There is no formal difference
      in the reservation syntax and both reservation types are
      handled uniformly. However, upcoming releases may offer improved performance
      if there are only out-of-pool reservations as the server will be able
      to skip reservation checks when dealing with existing leases. Therefore,
      system administrators are encouraged to use out-of-pool reservations if
      possible.</para>
    </section>

    <section xml:id="reservation4-conflict">
      <title>Conflicts in DHCPv4 Reservations</title>
      <para>As the reservations and lease information are stored separately,
      conflicts may arise. Consider the following series of events. The server
      has configured the dynamic pool of addresses from the range of 192.0.2.10 to
      192.0.2.20. Host A requests an address and gets 192.0.2.10. Now the system
      administrator decides to reserve address 192.0.2.10 for Host B.
      In general, reserving an address that
      is currently assigned to someone else is not recommended, but there are
      valid use cases where such an operation is warranted.</para>

      <para>The server now has a conflict to resolve. Let's analyze the
      situation here. If Host B boots up and requests an address, the server is
      not able to assign the reserved address 192.0.2.10. A naive approach
      would to be immediately remove the existing lease for the Host A
      and create a new one for the Host B. That would not solve the problem,
      though, because as soon as the Host B gets the address, it will detect
      that the address is already in use by the Host A and would send
      the DHCPDECLINE message. Therefore, in this situation, the server has
      to temporarily assign a different address (not matching what has been
      reserved) to the Host B.</para>

      <!-- let's keep this text around. It describes how that is working in v6
      <para>When the Host A renews its address, the server will discover that
      the address being renewed is now reserved for someone else (host
      B). Therefore the server will remove the lease and will inform the Host A
      that it is no longer allowed to use it by sending DHCPNAK message. Host A
      will then revert to server discovery and will eventually get a different
      address.  The address 192.0.2.10 is now no longer used. When host B tries
      to renew its temporarily assigned  address, the server will detect that
      it has a valid lease, but there is a reservation for a different address.
      The server will send DHCPNAK to inform host B that its address is no
      longer usable. The server will also remove its temporary lease. It will
      revert to the server discovery phase and will eventually send a
      DHCPREQUEST message. This time the server will find out that there is a
      reservation for that host and the reserved address 192.0.2.10 is not used,
      so it will be granted.</para> -->

      <para>When Host A renews its address, the server will discover that
      the address being renewed is now reserved for another host - Host
      B. Therefore the server will inform the Host A that it is no longer
      allowed to use it by sending a DHCPNAK message. The server will not remove the
      lease, though, as there's small chance that the DHCPNAK may be lost if the
      network is lossy. If that happens, the client will not receive any
      responses, so it will retransmit its DHCPREQUEST packet. Once the
      DHCPNAK is received by Host A, it will revert to the server
      discovery and will eventually get a different address. Besides
      allocating a new lease, the server will also remove the old one. As
      a result, address 192.0.2.10 will become free . When Host B
      tries to renew its temporarily assigned address, the server will detect
      that it has a valid lease, but there is a reservation for a different
      address. The server will send DHCPNAK to inform Host B that its address
      is no longer usable, but will keep its lease (again, the DHCPNAK may be
      lost, so the server will keep it, until the client returns for a new
      address). Host B will revert to the server discovery phase and will
      eventually send a DHCPREQUEST message. This time the server will find
      out that there is a reservation for that host and the reserved address
      192.0.2.10 is not used, so it will be granted. It will also remove the
      lease for the temporarily assigned address that Host B previously
      obtained.</para>

      <para>This recovery will succeed, even if other hosts will attempt to get
      the reserved address. Had the Host C requested address 192.0.2.10 after
      the reservation was made, the server will either offer a different
      address (when responding to DHCPDISCOVER) or would send DHCPNAK
      (when responding to DHCPREQUEST).</para>

      <para>This recovery mechanism allows the server to fully recover from a
      case where reservations conflict with the existing leases. This procedure
      takes time and will roughly take as long as the value set for of renew-timer.
      The best way to avoid such recovery is to not define new reservations that
      conflict with existing leases. Another recommendation is to use
      out-of-pool reservations. If the reserved address does not belong to a
      pool, there is no way that other clients could get this address.
      </para>
    </section>

    <section xml:id="reservation4-hostname">
      <title>Reserving a Hostname</title>
      <para>When the reservation for a client includes the <command>hostname</command>,
      the server will return this hostname to the client in
      the Client FQDN or Hostname options. The server responds with the Client
      FQDN option only if the client has included Client FQDN option in its
      message to the server. The server will respond with the Hostname option
      if the client included Hostname option in its message to the server
      or when the client requested Hostname option using Parameter Request
      List option. The server will return the Hostname option even if it is not
      configured to perform DNS updates. The reserved hostname always takes
      precedence over the hostname supplied by the client or the autogenerated
      (from the IPv4 address) hostname.</para>

      <para>The server qualifies the reserved hostname with the value
      of the <command>qualifying-suffix</command> parameter. For example, the
      following subnet configuration:
<screen>
    {
        "subnet4": [ {
            "subnet": "10.0.0.0/24",
            "pools": [ { "pool": "10.0.0.10-10.0.0.100" } ],
            "reservations": [
               {
                 "hw-address": "aa:bb:cc:dd:ee:ff",
                 "hostname": "alice-laptop"
               }
            ]
         }],
        "dhcp-ddns": {
            "enable-updates": true,
            "qualifying-suffix": "example.isc.org."
        }
    }
</screen>
      will result in assigning the "alice-laptop.example.isc.org." hostname to the
      client using the MAC address "aa:bb:cc:dd:ee:ff". If the <command>qualifying-suffix
      </command> is not specified, the default (empty) value will be used, and
      in this case the value specified as a <command>hostname</command> will
      be treated as fully qualified name. Thus, by leaving the
      <command>qualifying-suffix</command> empty it is possible to qualify
      hostnames for the different clients with different domain names:
<screen>
    {
        "subnet4": [ {
            "subnet": "10.0.0.0/24",
            "pools": [ { "pool": "10.0.0.10-10.0.0.100" } ],
            "reservations": [
               {
                 "hw-address": "aa:bb:cc:dd:ee:ff",
                 "hostname": "alice-laptop.isc.org."
               },
               {
                 "hw-address": "12:34:56:78:99:AA",
                 "hostname": "mark-desktop.example.org."
               }

            ]
         }],
        "dhcp-ddns": {
            "enable-updates": true,
        }
    }
</screen>

      </para>
    </section>

    <section xml:id="reservation4-options">
      <title>Including Specific DHCPv4 Options in Reservations</title>
      <para>Kea 1.1.0 introduced the ability to specify options on a
      per host basis. The options follow the same rules as any other
      options. These can be standard options (see <xref linkend="dhcp4-std-options"/>), custom options (see <xref linkend="dhcp4-custom-options"/>) or vendor specific options
      (see <xref linkend="dhcp4-vendor-opts"/>). The following
      example demonstrates how standard options can be defined.</para>

      <screen>
{
    "subnet4": [ {
        "reservations": [
        {
            "hw-address": "aa:bb:cc:dd:ee:ff",
            "ip-address": "192.0.2.1",
            <userinput>"option-data": [
            {
                "name": "cookie-servers",
                "data": "10.1.1.202,10.1.1.203"
            },
            {
                "name": "log-servers",
                "data": "10.1.1.200,10.1.1.201"
            } ]</userinput>
        } ]
    } ]
}</screen>

    <para>Vendor specific options can be reserved in a similar manner:</para>

    <screen>
{
    "subnet4": [ {
        "reservations": [
        {
            "hw-address": "aa:bb:cc:dd:ee:ff",
            "ip-address": "10.0.0.7",
            <userinput>"option-data": [
            {
                "name": "vivso-suboptions",
                "data": "4491"
            },
            {
                "name": "tftp-servers",
                "space": "vendor-4491",
                "data": "10.1.1.202,10.1.1.203"
            } ]</userinput>
        } ]
    } ]
}</screen>

<para>
 Options defined on host level have the highest priority. In other words,
 if there are options defined with the same type on global, subnet, class and
 host level, the host specific values will be used.
</para>

    </section>

    <section xml:id="reservation4-message-fields">
      <title>Reserving Next Server, Server Hostname and Boot File Name</title>
      <para>BOOTP/DHCPv4 messages include "siaddr", "sname" and "file" fields.
      Even though, DHCPv4 includes corresponding options, such as option 66 and
      option 67, some clients may not support these options. For this reason, server
      administrators often use the "siaddr", "sname" and "file" fields instead.</para>

      <para>With Kea, it is possible to make static reservations for these DHCPv4
      message fields:</para>

    <screen>
{
    "subnet4": [ {
        "reservations": [
        {
            "hw-address": "aa:bb:cc:dd:ee:ff",
            <userinput>"next-server": "10.1.1.2",
            "server-hostname": "server-hostname.example.org",
            "boot-file-name": "/tmp/bootfile.efi"</userinput>
        } ]
    } ]
}</screen>

    <para>Note that those parameters can be specified in combination with
    other parameters for a reservation, e.g. reserved IPv4 address. These
    parameters are optional, i.e. a subset of them can specified, or all of
    them can be omitted.</para>
    </section>

    <section xml:id="reservation4-client-classes">
      <title>Reserving Client Classes in DHCPv4</title>
      <para><xref linkend="classification-using-expressions"/> explains how
      to configure the server to assign classes to a client based on the content
      of the options that this client sends to the server. Host reservations
      mechanisms also allow for statically assigning classes to the clients.
      The definitions of these classes must exist in the Kea
      configuration. The following configuration snippet shows how to specify
      that a client belongs to classes <command>reserved-class1</command>
      and <command>reserved-class2</command>. Those classes are associated with
      specific options being sent to the clients which belong to them.
      </para>

<screen>
{
    "client-classes": [
    {
       "name": "reserved-class1",
       "option-data": [
       {
           "name": "routers",
           "data": "10.0.0.200"
       }
       ]
    },
    {
       "name": "reserved-class2",
       "option-data": [
       {
           "name": "domain-name-servers",
           "data": "10.0.0.201"
       }
       ]
    }
    ],
    "subnet4": [ {
        "subnet": "10.0.0.0/24",
        "pools": [ { "pool": "10.0.0.10-10.0.0.100" } ],
        "reservations": [
        {
            "hw-address": "aa:bb:cc:dd:ee:ff",
            <userinput>
            "client-classes": [ "reserved-class1", "reserved-class2" ]
            </userinput>
        }
        ]
    } ]
}

</screen>

    <para>Static class assignments, as shown above, can be used in conjunction
    with classification using expressions.</para>
    </section>

    <section id="reservations4-mysql-pgsql-cql">
      <title>Storing Host Reservations in MySQL, PostgreSQL or Cassandra</title>

      <para>
        It is possible to store host reservations in MySQL, PostgreSQL or Cassandra. See
        <xref linkend="hosts6-storage" /> for information on how to configure Kea to use
        reservations stored in MySQL, PostgreSQL or Cassandra. Kea provides dedicated hook for
        managing reservations in a database, section <xref linkend="host-cmds" /> provide
        detailed information. <uri
        xmlns:xlink="http://www.w3.org/1999/xlink"
        xlink:href="http://kea.isc.org/wiki/HostReservationsHowTo">http://kea.isc.org/wiki/HostReservationsHowTo</uri>
        provides some examples how to conduct common host reservation operations.
      </para>

      <note><simpara>In Kea maximum length of an option specified per host is
      arbitrarily set to 4096 bytes.</simpara></note>
    </section>

    <section xml:id="reservations4-tuning">
      <title>Fine Tuning DHCPv4 Host Reservation</title>

      <para>The host reservation capability introduces additional restrictions for the
      allocation engine (the component of Kea that selects an address for a client)
      during lease selection and renewal. In particular, three
      major checks are necessary. First, when selecting a new lease, it is not
      sufficient for a candidate lease to not be used by another DHCP client. It
      also must not be reserved for another client. Second, when renewing a lease,
      additional check must be performed whether the address being renewed is not
      reserved for another client. Finally, when a host renews an address, the server
      has to check whether there is a reservation for this host, so the existing
      (dynamically allocated) address should be revoked and the reserved one be
      used instead.
      </para>
      <para>Some of those checks may be unnecessary in certain deployments and not
      performing them may improve performance. The Kea server provides the
      <command>reservation-mode</command> configuration parameter to select the
      types of reservations allowed for the particular subnet. Each reservation
      type has different constraints for the checks to be performed by the
      server when allocating or renewing a lease for the client.
      Allowed values are:

      <itemizedlist>
      <listitem><simpara> <command>all</command> - enables all host reservation
      types. This is the default value. This setting is the safest and the most
      flexible. It allows in-pool and out-of-pool reservations. As all checks
      are conducted, it is also the slowest.
      </simpara></listitem>

      <listitem><simpara> <command>out-of-pool</command> - allows only out of
      pool host reservations.  With this setting in place, the server may assume
      that all host reservations are for addresses that do not belong to the
      dynamic pool. Therefore it can skip the reservation checks when dealing
      with in-pool addresses, thus improving performance. Do not use this mode
      if any of your reservations use in-pool address. Caution is advised when
      using this setting: Kea does not sanity check the reservations against
      <command>reservation-mode</command> and misconfiguration may cause problems.
      </simpara></listitem>

      <listitem><simpara>
      <command>disabled</command> - host reservation support is disabled. As there
      are no reservations, the server will skip all checks. Any reservations defined
      will be completely ignored. As the checks are skipped, the server may
      operate faster in this mode.
      </simpara></listitem>

      </itemizedlist>
      </para>

      <para>
        An example configuration that disables reservation looks like follows:
<screen>
"Dhcp4": {
    "subnet4": [
    {
        "subnet": "192.0.2.0/24",
        <userinput>"reservation-mode": "disabled"</userinput>,
        ...
    }
    ]
}
</screen>
      </para>

      <para>Another aspect of the host reservations are the different types of
      identifiers. Kea 1.1.0 supports four types of identifiers
      (hw-address, duid, client-id and circuit-id), but more identifier types
      are likely to be added in the future. This is beneficial from a
      usability perspective. However, there is a drawback. For each incoming
      packet Kea has to to extract each identifier type and then query the
      database to see if there is a reservation done by this particular
      identifier. If nothing is found, the next identifier is extracted and the next
      query is issued. This process continues until either a reservation is
      found or all identifier types have been checked. Over time with an increasing
      number of supported identifier types, Kea would become slower and
      slower.</para>

      <para>To address this problem, a parameter called
      <command>host-reservation-identifiers</command> has been introduced. It
      takes a list of identifier types as a parameter. Kea will check only those
      identifier types enumerated in host-reservation-identifiers. From a
      performance perspective the number of identifier types should be kept to a
      minimum, ideally limited to one. If your deployment uses several
      reservation types, please enumerate them from most to least frequently
      used as this increases the chances of Kea finding the reservation using the
      fewest number of queries. An example of host reservation identifiers looks
      as follows:

<screen>
<userinput>"host-reservation-identifiers": [ "circuit-id", "hw-address", "duid", "client-id" ],</userinput>
"subnet4": [
    {
        "subnet": "192.0.2.0/24",
        ...
    }
]</screen>
</para>

<para>
If not specified, the default value is:
<screen>
<userinput>"host-reservation-identifiers": [ "hw-address", "duid", "circuit-id", "client-id" ]</userinput>
</screen>
<!-- see CfgHostOperations::createConfig4() in
src/lib/dhcpsrv/cfg_host_operations.cc -->

</para>
   </section>

  </section>
  <!-- end of host reservations section -->


  <!-- shared networks -->

  <section id="shared-network4">
    <title>Shared networks in DHCPv4</title>

    <para>DHCP servers use subnet information in two ways. First, it is used
    to determine the point of attachment, or simply put, where the client is
    connected to the network. Second, the subnet information is used to group
    information pertaining to specific location in the network. This approach
    works well in general case, but the are scenarios where the boundaries are
    blurred. Sometimes it is useful to have more than one logical IP subnet
    being deployed on the same physical link. The need to understand
    that two or more subnets are used on the same link requires additional logic
    in the DHCP server. This capability has been added in Kea 1.3.0. It is
    called "shared networks" in Kea and ISC DHCP projects. It is sometimes also
    called "shared subnets". In Microsoft's nomenclature it is called "multinet".
    </para>

    <para>There are many use cases where the feature is useful. This paragraph
    explains just a handful of the most common ones. The first and by far the most
    common use case is an existing network that has grown and is running out of
    available address space. Rather than migrating all devices to a new, larger
    subnet, it is easier to simply configure additional subnet on top of the
    existing one. Sometimes, due to address space fragmentation (e.g. only many
    disjoint /24s are available) this is the only choice. Also, configuring
    additional subnet has the advantage of not disrupting the operation of
    existing devices.</para>

    <para>Another very frequent use case comes from cable networks. There are two types
    of devices in cable networks: cable modems and the end user devices behind
    them. It is a common practice to use different subnet for cable modems to
    prevent users from tinkering with their cable modems. In this case, the
    distinction is based on the type of device, rather than address space
    exhaustion.</para>

    <para>A client connected to a shared network may be assigned an address from
    any of the address pools defined within the subnets belonging to the shared
    network. Internally, the server selects one of the subnets belonging to a
    shared network and tries to allocate an address from this subnet. If the
    server is unable to allocate an address from the selected subnet (e.g. due
    to address pools exhaustion) it will use another subnet from the same shared
    network and try to allocate an address from this subnet etc. Therefore, in the
    typical case, the server will allocate all addresses available for a given
    subnet before it starts allocating addresses from other subnets belonging to
    the same shared network. However, in certain situations the client can be
    allocated an address from the other subnets before the address pools in the
    first subnet get exhausted, e.g. when the client provides a hint that
    belongs to another subnet or the client has reservations in a different than
    default subnet.
    </para>

    <note>
    <para>It is strongly discouraged for the Kea deployments to assume that the
      server doesn't allocate addresses from other subnets until it uses all
      the addresses from the first subnet in the shared network. Apart from the
      fact that hints, host reservations and client classification affect subnet
      selection, it is also foreseen that we will enhance allocation strategies
      for shared networks in the future versions of Kea, so as the selection
      of subnets within a shared network is equally probable (unpredictable).</para>
    </note>

    <para>In order to define a shared network an additional configuration scope
    is introduced:
<screen>
{
"Dhcp4": {
    <userinput>"shared-networks": [
        {
            // Name of the shared network. It may be an arbitrary string
            // and it must be unique among all shared networks.
            "name": "my-secret-lair-level-1",

            // Subnet selector can be specifed on the shared network level.
            // Subnets from this shared network will be selected for directly
            // connected clients sending requests to server's "eth0" interface.
            "interface": "eth0",

            // This starts a list of subnets in this shared network.
            // There are two subnets in this example.
            "subnet4": [
                {
                    "subnet": "10.0.0.0/8",
                    "pools": [ { "pool":  "10.0.0.1 - 10.0.0.99" } ],
                },
                {
                    "subnet": "192.0.2.0/24",
                    "pools": [ { "pool":  "192.0.2.100 - 192.0.2.199" } ]
                }
            ],
        } ]</userinput>, // end of shared-networks

    // It is likely that in your network you'll have a mix of regular,
    // "plain" subnets and shared networks. It is perfectly valid to mix
    // them in the same config file.
    //
    // This is regular subnet. It's not part of any shared-network.
    "subnet4": [
        {
            "subnet": "192.0.3.0/24",
            "pools": [ { "pool":  "192.0.3.1 - 192.0.3.200" } ],
            "interface": "eth1"
        }
    ]

} // end of Dhcp4
}
</screen>
    </para>
    <para>As you see in the example, it is possible to mix shared and regular
    ("plain") subnets. Each shared network must have a unique name. This is
    similar to ID for subnets, but gives you more flexibility. This is used
    for logging, but also internally for identifying shared networks.</para>

    <para>In principle it makes sense to define only shared networks that
    consist of two or more subnets. However, for testing purposes it is allowed
    to define a shared network with just one subnet or even an empty one.  This
    is not a recommended practice in production networks, as the shared network
    logic requires additional processing and thus lowers server's performance.
    To avoid unnecessary performance degradation the shared subnets should only
    be defined when required by the deployment.
    </para>

    <para>Shared networks provide an ability to specify many parameters in
    the shared network scope that will apply to all subnets within it. If
    necessary, you can specify a parameter on the shared network scope and then
    override its value in the subnet scope. For example:
<screen>
"shared-networks": [
    {
        "name": "lab-network3",

        "interface": "eth0",

        // This applies to all subnets in this shared network, unless
        // values are overridden on subnet scope.
        <userinput>"valid-lifetime": 600</userinput>,

        // This option is made available to all subnets in this shared
        // network.
        <userinput>"option-data": [ {
            "name": "log-servers",
            "data": "1.2.3.4"
        } ]</userinput>,

        "subnet4": [
            {
                "subnet": "10.0.0.0/8",
                "pools": [ { "pool":  "10.0.0.1 - 10.0.0.99" } ],

                // This particular subnet uses different values.
                <userinput>"valid-lifetime": 1200,
                "option-data": [
                {
                    "name": "log-servers",
                    "data": "10.0.0.254"
                },
                {
                    "name": "routers",
                    "data": "10.0.0.254"
                } ]</userinput>
            },
            {
                 "subnet": "192.0.2.0/24",
                 "pools": [ { "pool":  "192.0.2.100 - 192.0.2.199" } ],

                 // This subnet does not specify its own valid-lifetime value,
                 // so it is inherited from shared network scope.
                 <userinput>"option-data": [
                 {
                     "name": "routers",
                     "data": "192.0.2.1"
                 } ]</userinput>
            }
        ]
    } ]</screen>
    In this example, there is a log-servers option defined that is available to
    clients in both subnets in this shared network. Also, a valid lifetime is
    set to 10 minutes (600s). However, the first subnet overrides some of the values
    (valid lifetime is 20 minutes, different IP address for log-servers), but
    also adds its own option (router address). Assuming a client asking for
    router and log servers options is assigned a lease from this subnet, he will
    get a lease for 20 minutes and log-servers and routers value of 10.0.0.254.
    If the same client is assigned to the second subnet, he will get a 10
    minutes long lease, log-servers value of 1.2.3.4 and routers set to 192.0.2.1.
    </para>

    <section>
      <title>Local and relayed traffic in shared networks</title>

    <para>It is possible to specify interface name in the shared network scope to
    tell the server that this specific shared network is reachable directly (not
    via relays) using local network interface. It is sufficient to specify
    it once on the shared network level. As all subnets in a shared network are
    expected to be used on the same physical link, it is a configuration error
    to attempt to define a shared network using subnets that are reachable over
    different interfaces. It is allowed to specify interface parameter on each
    subnet, although its value must be the same for each subnet. Thus it's
    usually more convenient to specify it once on the shared network level.
<screen>
"shared-networks": [
    {
        "name": "office-floor-2",

        // This tells Kea that the whole shared networks is reachable over
        // local interface. This applies to all subnets in this network.
        <userinput>"interface": "eth0"</userinput>,

        "subnet4": [
            {
                "subnet": "10.0.0.0/8",
                "pools": [ { "pool":  "10.0.0.1 - 10.0.0.99" } ],
                <userinput>"interface": "eth0"</userinput>
            },
            {
                 "subnet": "192.0.2.0/24",
                 "pools": [ { "pool":  "192.0.2.100 - 192.0.2.199" } ]

                 // Specifying a different interface name is configuration
                 // error:
                 // "interface": "eth1"
            }
        ]
    } ]
</screen>
</para>

<para>Somewhat similar to interface names, also relay IP addresses can be
specified for the whole shared network. However, depending on your relay
configuration, it may use different IP addresses depending on which subnet
is being used. Thus there is no requirement to use the same IP relay address
for each subnet. Here's an example:

<screen>
"shared-networks": [
    {"
        "name": "kakapo",
        <userinput>"relay": {
            "ip-address": "192.3.5.6"
        }</userinput>,
        "subnet4": [
            {
                "subnet": "192.0.2.0/26",
                <userinput>"relay": {
                    "ip-address": "192.1.1.1"
                }</userinput>,
                "pools": [ { "pool": "192.0.2.63 - 192.0.2.63" } ]
            },
            {
                "subnet": "10.0.0.0/24",
                <userinput>"relay": {
                    "ip-address": "192.2.2.2"
                }</userinput>,
                "pools": [ { "pool": "10.0.0.16 - 10.0.0.16" } ]
            }
        ]
    }
]</screen>
In this particular case the relay IP address specified on network level doesn't
have much sense, as it is overridden in both subnets, but it was left there
as an example of how one could be defined on network level. Note that the
relay agent IP address typically belongs to the subnet it relays packets from,
but this is not a strict requirement. Therefore Kea accepts any value here
as long as it is valid IPv4 address.</para>

    </section>
    <section>
      <title>Client classification in shared networks</title>

      <para>Sometimes it is desired to segregate clients into specific subnets
      based on some properties. This mechanism is called client classification
      and is described in <xref linkend="classify"/>. Client classification
      can be applied to subnets belonging to shared networks in the same way
      as it is used for subnets specified outside of shared networks.
      It is important to understand how the server selects subnets for
      the clients when client classification is in use, to assure that the
      desired subnet is selected for a given client type.</para>

      <para>If a subnet is associated with some classes, only the clients
      belonging to any of these classes can use this subnet. If there are no
      classes specified for a subnet, any client connected to a given shared
      network can use this subnet. A common mistake is to assume that the
      subnet including client classes is preferred over subnets without
      client classes. Consider the following example:

<screen>
{
    "client-classes": [
        {
            "name": "b-devices",
            "test": "option[93].hex == 0x0002"
        }
    ],
    "shared-networks": [
        {
            "name": "galah",
            "interface": "eth0",
            "subnet4": [
                {
                    "subnet": "192.0.2.0/26",
                    "pools": [ { "pool": "192.0.2.1 - 192.0.2.63" } ],
                },
                {
                    "subnet": "10.0.0.0/24",
                    "pools": [ { "pool": "10.0.0.2 - 10.0.0.250" } ],
                    <userinput>"client-class": "b-devices"</userinput>
                }
            ]
        }
    ]
}
</screen>

        If the client belongs to "b-devices" class (because it includes option
        93 with a value of 0x0002) it doesn't guarantee that the subnet 10.0.0.0/24
        will be used (or preferred) for this client. The server can use any of
        the two subnets because the subnet 192.0.2.0/26 is also allowed for
        this client. The client classification used in this case should be pereceived
        as a way to restrict access to certain subnets, rather than a way to express
        subnet preference. For example, if the client doesn't belong to the
        "b-devices" class it may only use the subnet 192.0.2.0/26 and will
        never use the subnet 10.0.0.0/24.
      </para>

      <para>A typical use case for client classification is in the cable network,
      where cable modems should use one subnet and other devices should use
      another subnet within the same shared network. In this case it is required
      to apply classification on all subnets. The following example defines two
      classes of devices. The subnet selection is  made based on option 93 values.
<screen>
{
    "client-classes": [
        {

            "name": "a-devices",
            "test": "option[93].hex == 0x0001"
        },
        {
            "name": "b-devices",
            "test": "option[93].hex == 0x0002"
        }
    ],
    "shared-networks": [
        {
            "name": "galah",
            "interface": "eth0",
            "subnet4": [
                {
                    "subnet": "192.0.2.0/26",
                    "pools": [ { "pool": "192.0.2.1 - 192.0.2.63" } ],
                    <userinput>"client-class": "a-devices"</userinput>
                },
                {
                    "subnet": "10.0.0.0/24",
                    "pools": [ { "pool": "10.0.0.2 - 10.0.0.250" } ],
                    <userinput>"client-class": "b-devices"</userinput>
                }
            ]
        }
    ]
}
</screen>
In this example each class has its own restriction. Only clients that belong to
class "a-devices" will be able to use subnet 192.0.2.0/26 and only clients
belonging to b-devices will be able to use subnet 10.0.0.0/24. Care should be
taken to not define too restrictive classification rules, as clients that are
unable to use any subnets will be refused service. Although, this may be a
desired outcome if one desires to service only clients of known properties
(e.g. only VoIP phones allowed on a given link).</para>

    <para>
      Note that it is possible to achieve similar effect as presented in this
      section without the use of shared networks. If the subnets are placed in
      the global subnets scope, rather than in the shared network, the server
      will still use classification rules to pick the right subnet for a given
      class of devices. The major benefit of placing subnets within the
      shared network is that common parameters for the logically grouped
      subnets can be specified once, in the shared network scope, e.g.
      "interface" or "relay" parameter. All subnets belonging to this shared
      network will inherit those parameters.
    </para>

    </section>

    <section>
      <title>Host reservations in shared networks</title>

<para>
  Subnets being part of a shared network allow host reservations, similar to
  regular subnets:
  <screen>
{
    "shared-networks": [
    {
        "name": "frog",
        "interface": "eth0",
        "subnet4": [
            {
                "subnet": "192.0.2.0/26",
                "id": 100,
                "pools": [ { "pool": "192.0.2.1 - 192.0.2.63" } ],
                <userinput>"reservations": [
                    {
                        "hw-address": "aa:bb:cc:dd:ee:ff",
                        "ip-address": "192.0.2.28"
                    }
                ]</userinput>
            },
            {
                "subnet": "10.0.0.0/24",
                "id": 101,
                "pools": [ { "pool": "10.0.0.1 - 10.0.0.254" } ],
                <userinput>"reservations": [
                    {
                        "hw-address": "11:22:33:44:55:66",
                        "ip-address": "10.0.0.29"
                    }
                ]</userinput>
            }
        ]
    }
    ]
}
  </screen>
</para>
<para>It is worth noting that Kea conducts additional checks when processing a
packet if shared networks are defined. First, instead of simply checking if
there's a reservation for a given client in his initially selected subnet, it
goes through all subnets in a shared network looking for a reservation. This is
one of the reasons why defining a shared network may impact performance. If
there is a reservation for a client in any subnet, that particular subnet will
be picked for the client. Although it's technically not an error, it is
considered a bad practice to define reservations for the same host in multiple
subnets belonging to the same shared network.</para>

<para>While not strictly mandatory, it is strongly recommended to use explicit
"id" values for subnets if you plan to use database storage for host
reservations. If ID is not specified, the values for it be autogenerated,
i.e. it will assign increasing integer values starting from 1. Thus, the
autogenerated IDs are not stable across configuration changes.</para>
    </section>

  </section>

    <section xml:id="dhcp4-serverid">
      <title>Server Identifier in DHCPv4</title>
      <para>
        The DHCPv4 protocol uses a "server identifier" to allow clients
        to discriminate between several servers present on the same link: this
        value is an IPv4 address of the server. The server chooses the IPv4 address
        of the interface on which the message from the client (or relay) has been
        received. A single server instance will use multiple server identifiers
        if it is receiving queries on multiple interfaces.
      </para>

      <para>
        It is possible to override default server identifier values by specifying
        "dhcp-server-identifier" option. This option is only supported on the
        global, shared network and subnet level. It must not be specified
        on client class and host reservation level.
      </para>

      <para>
        The following example demonstrates how to override server identifier for
        a subnet:
<screen>
"subnet4": [
    {
        "subnet": "192.0.2.0/24",
        "option-data": [
            {
                "name": "dhcp-server-identifier",
                "data": "10.2.5.76"
            }
        ],
        ...
    }
]</screen>
</para>
    </section>

    <section xml:id="dhcp4-subnet-selection">
      <title>How the DHCPv4 Server Selects a Subnet for the Client</title>
      <para>
        The DHCPv4 server differentiates between the directly connected clients,
        clients trying to renew leases and clients sending their messages through
        relays. For directly connected clients, the server will check the
        configuration for the interface on which the message has been received and,
        if the server configuration doesn't match any configured subnet, the
        message is discarded.</para>
        <para>Assuming that the server's interface is configured with the
        IPv4 address 192.0.2.3, the server will only process messages received through
        this interface from a directly connected client if there is a subnet
        configured to which this IPv4 address belongs, e.g. 192.0.2.0/24.
        The server will use this subnet to assign IPv4 address for the client.
      </para>
      <para>
        The rule above does not apply when the client unicasts its message, i.e.
        is trying to renew its lease. Such a message is accepted through any
        interface. The renewing client sets ciaddr to the currently used IPv4
        address. The server uses this address to select the subnet for the client
        (in particular, to extend the lease using this address).
      </para>
      <para>
        If the message is relayed it is accepted through any interface. The giaddr
        set by the relay agent is used to select the subnet for the client.
      </para>
      <para>
        It is also possible to specify a relay IPv4 address for a given subnet. It
        can be used to match incoming packets into a subnet in uncommon configurations,
        e.g. shared networks. See <xref linkend="dhcp4-relay-override"/> for details.
      </para>
      <note>
        <para>The subnet selection mechanism described in this section is based
        on the assumption that client classification is not used. The classification
        mechanism alters the way in which a subnet is selected for the client,
        depending on the classes to which the client belongs.</para>
      </note>

    <section xml:id="dhcp4-relay-override">
      <title>Using a Specific Relay Agent for a Subnet</title>
      <para>
        A relay has to have an interface connected to the link on which
        the clients are being configured. Typically the relay has an IPv4
        address configured on that interface that belongs to the subnet from which
        the server will assign addresses. In the typical case, the
        server is able to use the IPv4 address inserted by the relay (in the giaddr
        field of the DHCPv4 packet) to select the appropriate subnet.
      </para>
      <para>
        However, that is not always the case. In certain uncommon —
        but valid — deployments, the relay address may not match the subnet. This
        usually means that there is more than one subnet allocated for a given
        link. The two most common examples where this is the case are long lasting
        network renumbering (where both old and new address space is still being
        used) and a cable network. In a cable network both cable modems and the
        devices behind them are physically connected to the same link, yet
        they use distinct addressing. In such a case, the DHCPv4 server needs
        additional information (the IPv4 address of the relay) to properly select
        an appropriate subnet.
      </para>
      <para>
        The following example assumes that there is a subnet 192.0.2.0/24
        that is accessible via a relay that uses 10.0.0.1 as its IPv4 address.
        The server will be able to select this subnet for any incoming packets
        that came from a relay that has an address in 192.0.2.0/24 subnet.
        It will also select that subnet for a relay with address 10.0.0.1.
<screen>
"Dhcp4": {
    "subnet4": [
        {
            "subnet": "192.0.2.0/24",
            "pools": [ { "pool": "192.0.2.10 - 192.0.2.20" } ],
            <userinput>"relay": {
                "ip-address": "10.0.0.1"
            }</userinput>,
            ...
        }
    ],
    ...
}
</screen>
      </para>

      <para>If "relay" is specified, the "ip-address" parameter within
      it is mandatory.</para>

    </section>

      <section xml:id="dhcp4-srv-example-client-class-relay">
        <title>Segregating IPv4 Clients in a Cable Network</title>
        <para>
          In certain cases, it is useful to mix relay address information,
          introduced in <xref linkend="dhcp4-relay-override"/> with client
          classification, explained in <xref linkend="classify"/>.
          One specific example is cable network, where typically modems
          get addresses from a different subnet than all devices connected
          behind them.
        </para>
        <para>
          Let us assume that there is one CMTS (Cable Modem Termination System)
          with one CM MAC (a physical link that modems are connected to).
          We want the modems to get addresses from the 10.1.1.0/24 subnet, while
          everything connected behind modems should get addresses from another
          subnet (192.0.2.0/24). The CMTS that acts as a relay uses address
          10.1.1.1. The following configuration can serve that configuration:
<screen>
"Dhcp4": {
    "subnet4": [
        {
            "subnet": "10.1.1.0/24",
            "pools":  [ { "pool": "10.1.1.2 - 10.1.1.20" } ],
            <userinput>"client-class" "docsis3.0",
            "relay": {
                "ip-address": "10.1.1.1"
            }</userinput>
        },
        {
            "subnet": "192.0.2.0/24",
            "pools": [ { "pool": "192.0.2.10 - 192.0.2.20" } ],
            <userinput>"relay": {
                "ip-address": "10.1.1.1"
            }</userinput>
        }
    ],
    ...
}
</screen>
      </para>
      </section>

    </section>

    <section xml:id="dhcp4-decline">
      <title>Duplicate Addresses (DHCPDECLINE Support)</title>

      <para>The DHCPv4 server is configured with a certain pool of addresses
      that it is expected to hand out to the DHCPv4 clients.  It is
      assumed that the server is authoritative and has complete jurisdiction
      over those addresses. However, due to various reasons, such as
      misconfiguration or a faulty client implementation that retains its
      address beyond the valid lifetime, there may be devices connected that use
      those addresses without the server's approval or knowledge.</para>

      <para>Such an
      unwelcome event can be detected by legitimate clients (using ARP or ICMP
      Echo Request mechanisms) and reported to the DHCPv4 server using a DHCPDECLINE
      message. The server will do a sanity check (if the client declining an
      address really was supposed to use it), and then will conduct a clean up
      operation. Any DNS entries related to that address will be removed, the
      fact will be logged and hooks will be triggered. After that is done, the
      address will be marked as declined (which indicates that it is used by an
      unknown entity and thus not available for assignment to anyone) and a
      probation time will be set on it. Unless otherwise configured, the
      probation period lasts 24 hours. After that period, the server will
      recover the lease (i.e. put it back into the available state) and the address will
      be available for assignment again. It should be noted that if the
      underlying issue of a misconfigured device is not resolved, the duplicate
      address scenario will repeat. On the other hand, it provides an
      opportunity to recover from such an event automatically, without any
      sysadmin intervention.</para>

      <para>To configure the decline probation period to a value other
      than the default, the following syntax can be used:
<screen>
  "Dhcp4": {
    <userinput>"decline-probation-period": 3600</userinput>,
    "subnet4": [ ... ],
    ...
}
</screen>
      The parameter is expressed in seconds, so the example above will instruct
      the server to recycle declined leases after an hour.</para>

      <para>There are several statistics and hook points associated with the
      Decline handling procedure. The lease4_decline hook is triggered after the
      incoming DHCPDECLINE message has been sanitized and the server is about to
      decline the lease. The declined-addresses statistic is increased after the
      hook returns (both global and subnet specific variants). (See
      <xref linkend="dhcp4-stats"/> and <xref linkend="hooks-libraries"/> for more details
      on DHCPv4 statistics and Kea hook points.)</para>

      <para>Once the probation time elapses, the declined lease is recovered
      using the standard expired lease reclamation procedure, with several
      additional steps. In particular, both declined-addresses statistics
      (global and subnet specific) are decreased. At the same time,
      reclaimed-declined-addresses statistics (again in two variants, global and
      subnet specific) are increased.</para>

      <para>Note about statistics: The server does not decrease the
      assigned-addresses statistics when a DHCPDECLINE is received and processed
      successfully. While technically a declined address is no longer assigned,
      the primary usage of the assigned-addresses statistic is to monitor pool
      utilization. Most people would forget to include declined-addresses in the
      calculation, and simply do assigned-addresses/total-addresses. This would
      have a bias towards under-representing pool utilization. As this has a
      potential for major issues, we decided not to decrease assigned addresses
      immediately after receiving DHCPDECLINE, but to do it later when we
      recover the address back to the available pool.</para>

    </section>

    <section xml:id="dhcp4-stats">
      <title>Statistics in the DHCPv4 Server</title>
      <note>
        <para>This section describes DHCPv4-specific statistics. For a general
        overview and usage of statistics, see <xref linkend="stats"/>.</para>
      </note>

      <para>
        The DHCPv4 server supports the following statistics:
      </para>
        <table frame="all" xml:id="dhcp4-statistics">
          <title>DHCPv4 Statistics</title>
          <tgroup cols="3">
          <colspec colname="statistic" align="center"/>
          <colspec colname="type" align="center"/>
          <colspec colname="description" align="left"/>
          <thead>
            <row>
              <entry>Statistic</entry>
              <entry>Data Type</entry>
              <entry>Description</entry>
            </row>
          </thead>
          <tbody>

            <row>
            <entry>pkt4-received</entry>
            <entry>integer</entry>
            <entry>
            Number of DHCPv4 packets received. This includes all packets: valid,
            bogus, corrupted, rejected etc.  This statistic is expected to grow
            rapidly.
            </entry>
            </row>

            <row>
            <entry>pkt4-discover-received</entry>
            <entry>integer</entry>
            <entry>
            Number of DHCPDISCOVER packets received. This statistic is expected to grow.
            Its increase means that clients that just booted started their configuration process
            and their initial packets reached your server.
            </entry>
            </row>

            <row>
            <entry>pkt4-offer-received</entry>
            <entry>integer</entry>
            <entry>
            Number of DHCPOFFER packets received. This statistic
            is expected to remain zero at all times, as DHCPOFFER packets are sent
            by the server and the server is never expected to receive them. Non-zero
            value indicates an error. One likely cause would be a misbehaving relay
            agent that incorrectly forwards DHCPOFFER messages towards the server,
            rather back to the clients.
            </entry>
            </row>

            <row>
            <entry>pkt4-request-received</entry>
            <entry>integer</entry>
            <entry>
            Number of DHCPREQUEST packets received. This statistic
            is expected to grow. Its increase means that clients that just booted
            received server's response (DHCPOFFER), accepted it and now requesting
            an address (DHCPREQUEST).
            </entry>
            </row>

            <row>
            <entry>pkt4-ack-received</entry>
            <entry>integer</entry>
            <entry>
            Number of DHCPACK packets received. This statistic
            is expected to remain zero at all times, as DHCPACK packets are sent
            by the server and the server is never expected to receive them. Non-zero
            value indicates an error. One likely cause would be a misbehaving relay
            agent that incorrectly forwards DHCPACK messages towards the server,
            rather back to the clients.
            </entry>
            </row>

            <row>
            <entry>pkt4-nak-received</entry>
            <entry>integer</entry>
            <entry>
            Number of DHCPNAK packets received. This statistic
            is expected to remain zero at all times, as DHCPNAK packets are sent
            by the server and the server is never expected to receive them. Non-zero
            value indicates an error. One likely cause would be a misbehaving relay
            agent that incorrectly forwards DHCPNAK messages towards the server,
            rather back to the clients.
            </entry>
            </row>

            <row>
            <entry>pkt4-release-received</entry>
            <entry>integer</entry>
            <entry>
            Number of DHCPRELEASE packets received. This statistic
            is expected to grow. Its increase means that clients that had an address
            are shutting down or stop using their addresses.
            </entry>
            </row>

            <row>
            <entry>pkt4-decline-received</entry>
            <entry>integer</entry>
            <entry>
            Number of DHCPDECLINE packets received. This statistic
            is expected to remain close to zero. Its increase means that a client
            that leased an address, but discovered that the address is currently
            used by an unknown device in your network.
            </entry>
            </row>

            <row>
            <entry>pkt4-inform-received</entry>
            <entry>integer</entry>
            <entry>
            Number of DHCPINFORM packets received. This statistic
            is expected to grow. Its increase means that there are clients that
            either do not need an address or already have an address and are
            interested only in getting additional configuration parameters.
            </entry>
            </row>

            <row>
            <entry>pkt4-unknown-received</entry>
            <entry>integer</entry>
            <entry>
            Number of packets received of an unknown type. Non-zero
            value of this statistic indicates that the server received a packet
            that it wasn't able to recognize: either with unsupported type
            or possibly malformed (without message type option).
            </entry>
            </row>

            <row>
            <entry>pkt4-sent</entry>
            <entry>integer</entry>
            <entry>
            Number of DHCPv4 packets sent. This statistic is expected to grow
            every time the server transmits a packet. In general, it should
            roughly match pkt4-received, as most incoming packets cause
            server to respond. There are exceptions (e.g. DHCPRELEASE), so
            do not worry, if it is lesser than pkt4-received.
            </entry>
            </row>

            <row>
            <entry>pkt4-offer-sent</entry>
            <entry>integer</entry>
            <entry>
            Number of DHCPOFFER packets sent. This statistic is expected to
            grow in most cases after a DHCPDISCOVER is processed. There are
            certain uncommon, but valid cases where incoming DHCPDISCOVER is
            dropped, but in general this statistic is expected to be close to
            pkt4-discover-received.
            </entry>
            </row>

            <row>
            <entry>pkt4-ack-sent</entry>
            <entry>integer</entry>
            <entry>
            Number of DHCPACK packets sent. This statistic is expected to
            grow in most cases after a DHCPREQUEST is processed. There are
            certain cases where DHCPNAK is sent instead. In general, the sum of
            pkt4-ack-sent and pkt4-nak-sent should be close to
            pkt4-request-received.
            </entry>
            </row>

            <row>
            <entry>pkt4-nak-sent</entry>
            <entry>integer</entry>
            <entry>
            Number of DHCPNAK packets sent. This statistic is expected
            to grow when the server chooses to not honor the address
            requested by a client. In general, the sum of
            pkt4-ack-sent and pkt4-nak-sent should be close to
            pkt4-request-received.
            </entry>
            </row>

            <row>
            <entry>pkt4-parse-failed</entry>
            <entry>integer</entry>
            <entry>
            Number of incoming packets that could not be parsed.  A non-zero value of
            this statistic indicates that the server received malformed or truncated packet.
            This may indicate problems in your network, faulty clients or a bug in the server.
            </entry>
            </row>

            <row>
            <entry>pkt4-receive-drop</entry>
            <entry>integer</entry>
            <entry>
            Number of incoming packets that were dropped. The
            exact reason for dropping packets is logged, but the most common
            reasons may be: an unacceptable packet type, direct responses are
            forbidden, or the server-id sent by the client does not match
            the server's server-id.
            </entry>
            </row>

            <row>
              <entry>subnet[id].total-addresses</entry>
              <entry>integer</entry>
              <entry>The total number of addresses available for DHCPv4
              management. In other words, this is the sum of all addresses in
              all configured pools. This statistic changes only during
              configuration changes. Note it does not take into account any
              addresses that may be reserved due to host reservation. The
              <emphasis>id</emphasis> is the subnet-id of a given subnet. This
              statistic is exposed for each subnet separately. This statistic is
              reset during reconfiguration event.</entry>
            </row>
            <row>
              <entry>subnet[id].assigned-addresses</entry>
              <entry>integer</entry>
              <entry>This statistic shows the number of assigned addresses in a
              given subnet. It increases every time a new lease is
              allocated (as a result of receiving a DHCPREQUEST message) and is
              decreased every time a lease is released (a DHCPRELEASE message is
              received) or expires. The <emphasis>id</emphasis> is the subnet-id
              of the subnet. This statistic is exposed for each subnet
              separately. This statistic is reset during reconfiguration event.
              </entry>
            </row>
            <row>
              <entry>reclaimed-leases</entry>
              <entry>integer</entry>
              <entry>This statistic is the number of expired leases that have
              been reclaimed since server startup. It is incremented each time
              an expired lease is reclaimed and is reset when the server is
              reconfigured.
              </entry>
            </row>
            <row>
              <entry>subnet[id].reclaimed-leases</entry>
              <entry>integer</entry>
              <entry>This statistic is the number of expired leases associated
              with a given subnet (<emphasis>id</emphasis> is the subnet-id)
              that have been reclaimed since server startup. It is incremented
              each time an expired lease is reclaimed and is reset when the
              server is reconfigured.
              </entry>
            </row>
            <row>
            <entry>declined-addresses</entry>
            <entry>integer</entry>
            <entry>
              This statistic shows the number of IPv4 addresses that are
              currently declined, so counting the number of leases
              currently unavailable. Once a lease is recovered, this
              statistic will be decreased. Ideally, this statistic should be
              zero. If this statistic is non-zero (or worse increasing),
              a network administrator should investigate if there is
              a misbehaving device in his network. This is a global statistic
              that covers all subnets.
            </entry>
            </row>

            <row>
            <entry>subnet[id].declined-addresses</entry>
            <entry>integer</entry>
            <entry>
              This statistic shows the number of IPv4 addresses that are
              currently declined in a given subnet, so is a count of the
              number of leases currently unavailable. Once a lease is
              recovered, this statistic will be decreased. Ideally, this
              statistic should be zero. If this statistic is
              non-zero (or worse increasing), a network administrator should
              investigate if there is a misbehaving device in his network. The
              <emphasis>id</emphasis> is the subnet-id of a given subnet. This
              statistic is exposed for each subnet separately.
            </entry>
            </row>

            <row>
            <entry>reclaimed-declined-addresses</entry>
            <entry>integer</entry>
            <entry>
              This statistic shows the number of IPv4 addresses that were
              declined, but have now been recovered. Unlike
              declined-addresses, this statistic never decreases. It can be used
              as a long term indicator of how many actual valid Declines were
              processed and recovered from. This is a global statistic that
              covers all subnets.
            </entry>
            </row>

            <row>
            <entry>subnet[id].reclaimed-declined-addresses</entry>
            <entry>integer</entry>
            <entry>
              This statistic shows the number of IPv4 addresses that were
              declined, but have now been recovered. Unlike
              declined-addresses, this statistic never decreases. It can be used
              as a long term indicator of how many actual valid Declines were
              processed and recovered from. The
              <emphasis>id</emphasis> is the subnet-id of a given subnet. This
              statistic is exposed for each subnet separately.
            </entry>
            </row>

        </tbody>
        </tgroup>
        </table>
    </section>

    <section xml:id="dhcp4-ctrl-channel">
      <title>Management API for the DHCPv4 Server</title>
      <para>
        The management API allows the issuing of specific
        management commands, such as statistics retrieval, reconfiguration or shutdown.
        For more details, see <xref linkend="ctrl-channel"/>. Currently the only
        supported communication channel type is UNIX stream socket. By default there
        are no sockets open. To instruct Kea to open a socket, the following entry
        in the configuration file can be used:
<screen>
"Dhcp4": {
    "control-socket": {
        "socket-type": "unix",
        "socket-name": <userinput>"/path/to/the/unix/socket"</userinput>
    },

    "subnet4": [
        ...
    ],
    ...
}
</screen>
      </para>

      <para>
        The length of the path specified by the <command>socket-name</command>
        parameter is restricted by the maximum length for the unix socket name
        on your operating system, i.e. the size of the <command>sun_path</command>
        field in the <command>sockaddr_un</command> structure, decreased by 1.
        This value varies on different operating systems between 91 and 107
        characters. Typical values are 107 on Linux and 103 on FreeBSD.
      </para>

      <para>
        Communication over control channel is conducted using JSON structures.
        See the Control Channel section in the Kea Developer's Guide for more
        details.
      </para>

      <para>The DHCPv4 server supports the following operational commands:
        <itemizedlist>
            <listitem>build-report</listitem>
            <listitem>config-get</listitem>
            <listitem>config-reload</listitem>
            <listitem>config-set</listitem>
            <listitem>config-test</listitem>
            <listitem>config-write</listitem>
            <listitem>dhcp-disable</listitem>
            <listitem>dhcp-enable</listitem>
            <listitem>leases-reclaim</listitem>
            <listitem>list-commands</listitem>
            <listitem>shutdown</listitem>
            <listitem>version-get</listitem>
        </itemizedlist>
         as described in <xref linkend="commands-common"/>.  In addition,
         it supports the following statistics related commands:
        <itemizedlist>
            <listitem>statistic-get</listitem>
            <listitem>statistic-reset</listitem>
            <listitem>statistic-remove</listitem>
            <listitem>statistic-get-all</listitem>
            <listitem>statistic-reset-all</listitem>
            <listitem>statistic-remove-all</listitem>
        </itemizedlist>
        as described here <xref linkend="command-stats"/>.
      </para>

    </section>

    <section xml:id="dhcp4-std">
      <title>Supported DHCP Standards</title>
      <para>The following standards are currently supported:</para>
      <itemizedlist>
          <listitem>
            <simpara><emphasis>Dynamic Host Configuration Protocol</emphasis>,
            <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://tools.ietf.org/html/rfc2131">RFC 2131</link>:
            Supported messages are DHCPDISCOVER (1), DHCPOFFER (2),
            DHCPREQUEST (3), DHCPRELEASE (7), DHCPINFORM (8), DHCPACK (5), and
            DHCPNAK(6).</simpara>
          </listitem>
          <listitem>
            <simpara><emphasis>DHCP Options and BOOTP Vendor Extensions</emphasis>,
            <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://tools.ietf.org/html/rfc2132">RFC 2132</link>:
            Supported options are: PAD (0),
            END(255), Message Type(53), DHCP Server Identifier (54),
            Domain Name (15), DNS Servers (6), IP Address Lease Time
            (51), Subnet mask (1), and Routers (3).</simpara>
          </listitem>
          <listitem>
            <simpara><emphasis>DHCP Relay Agent Information Option</emphasis>,
            <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://tools.ietf.org/html/rfc3046">RFC 3046</link>:
            Relay Agent Information option is supported.</simpara>
          </listitem>
          <listitem>
            <simpara><emphasis>Vendor-Identifying Vendor Options for
            Dynamic Host Configuration Protocol version 4</emphasis>,
            <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://tools.ietf.org/html/rfc3925">RFC 3925</link>:
            Vendor-Identifying Vendor Class and Vendor-Identifying Vendor-Specific
            Information options are supported.</simpara>
          </listitem>
          <listitem>
            <simpara><emphasis>Client Identifier Option in DHCP Server Replies</emphasis>,
            <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://tools.ietf.org/html/rfc6842">RFC 6842</link>:
            Server by default sends back client-id option. That capability may be
            disabled. See <xref linkend="dhcp4-echo-client-id"/> for details.
            </simpara>
          </listitem>
      </itemizedlist>
    </section>

      <section id="dhcp4-user-contexts">
        <title>User contexts in IPv4</title>
        <para>
          Kea allows loading hook libraries that sometimes could benefit from
          additional parameters. If such a parameter is specific to the whole
          library, it is typically defined as a parameter for the hook library.
          However, sometimes there is a need to specify parameters that are
          different for each pool.
        </para>

        <para>
          User contexts can store arbitrary data as long as it is valid JSON
          syntax and its top level element is a map (i.e. the data must be
          enclosed in curly brackets). Some hook libraries may expect specific
          formatting, though.  Please consult specific hook library
          documentation for details.
        </para>

        <para>
          User contexts can be specified on either global scope,
          shared network, subnet, pool, client class, option data or
          definition level, and host reservation. One other useful
          usage is the ability to store comments or descriptions.
        </para>

        <para>
          Let's consider an imaginary case of devices that have color LED
          lights. Depending on their location, they should glow red, blue or
          green. It would be easy to write a hook library that would send
          specific values as maybe a vendor option. However, the server has to
          have some way to specify that value for each pool. This need is
          addressed by user contexts. In essence, any user data can specified
          in the user context as long as it is a valid JSON map. For example,
          the forementioned case of LED devices could be configured in the
          following way:

          <screen>
"Dhcp4": {
    "subnet4": [
        {
            "subnet": "192.0.2.0/24",
            "pools": [ {
                "pool": "192.0.2.10 - 192.0.2.20",
                // This is pool specific user context
                <userinput>"user-context": { "colour": "red" }</userinput>
            } ],

            // This is a subnet specific user context. You can put whatever type
            // of information you want as long as it is a valid JSON.
            <userinput>"user-context": {
                "comment": "network on the second floor",
                "last-modified": "2017-09-04 13:32",
                "description": "you can put here anything you like",
                "phones": [ "x1234", "x2345" ],
                "devices-registered": 42,
                "billing": false
            }</userinput>
        },
        ...
    ],
    ...
}</screen>
        </para>

        <para>
          It should be noted that Kea will not use that information, but will
          simply store and make it available to hook libraries. It is up to the
          hook library to extract that information and make use of it.
          The parser translates a "comment" entry into a user-context
          with the entry, this allows to attach a comment inside the
          configuration itself.
        </para>
        <para>
          For more background information, see <xref linkend="user-context"/>.
        </para>
    </section>

    <section xml:id="dhcp4-limit">
      <title>DHCPv4 Server Limitations</title>
      <para>These are the current limitations of the DHCPv4 server
      software. Most of them are reflections of the current stage of
      development and should be treated as <quote>not implemented
      yet</quote>, rather than actual limitations. However, some of them
      are implications of the design choices made. Those are clearly
      marked as such.</para>
      <itemizedlist>
          <listitem>
            <simpara>
              BOOTP (<link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://tools.ietf.org/html/rfc951">RFC 951</link>)
              is not supported. This is a design choice: BOOTP support is not planned.
            </simpara>
          </listitem>
          <listitem>
            <simpara>On Linux and BSD system families the DHCP messages are sent
            and received over the raw sockets (using LPF and BPF) and all packet
            headers (including data link layer, IP and UDP headers) are created and
            parsed by Kea, rather than the system kernel. Currently, Kea can
            only parse the data link layer headers with a format adhering to
            IEEE 802.3 standard and assumes this data link layer header format
            for all interfaces. Hence, Kea will fail to work on interfaces
            which use different data link layer header formats (e.g. Infiniband).
            </simpara>
          </listitem>
          <listitem>
            <simpara>The DHCPv4 server does not  verify that
            assigned address is unused. According to <link xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="http://tools.ietf.org/html/rfc2131">RFC 2131</link>, the
            allocating server should verify that address is not used by
            sending ICMP echo request.</simpara>
          </listitem>
      </itemizedlist>
    </section>

    <section id="dhcp4-srv-examples">
      <title>Kea DHCPv4 server examples</title>

      <para>
        A collection of simple to use examples for DHCPv4 component of Kea is
        available with the sources. It is located in doc/examples/kea4
        directory. At the time of writing this text there were 15 examples,
        but the number is growing slowly with each release.
      </para>

    </section>

  </chapter>