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  <chapter id="dhcp4">
    <title>The DHCPv4 Server</title>

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

      <!-- @todo Rewrite this section once #3422 is done -->

      <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 the server will listen on. This is only
            useful during testing, as the DHCPv4 server listening on
            ports other than default DHCPv4 ports will not be able to
            handle regular DHCPv4 queries.</simpara>
          </listitem>
          <listitem>
            <simpara>
              <command>-v</command> - prints out Kea version and exits.
            </simpara>
          </listitem>
          <listitem>
            <simpara>
              <command>-V</command> - prints out Kea extended version with
              additional parameters and exits.
            </simpara>
          </listitem>
      </itemizedlist>

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

      <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.
      </para>

      <para>
        Since the DHCPv4 server opens privileged ports, it requires root
        access. Make sure you run this daemon as root.
      </para>

    </section>

    <section 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 this line
}

} </screen>
</para>

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

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

<para>The configuration starts in the first line with the initial
opening curly bracket (or brace). Each configuration consists of
one or more objects. In this specific example, we have only one
object called Dhcp4. This is a simplified configuration, as usually
there will be additional objects, like <command>Logging</command> or
<command>DhcpDns</command>, but we omit them now for clarity. The 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 &mdash; although
legal JSON &mdash; 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 very 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 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
<ulink url="http://tools.ietf.org/html/rfc2131">RFC 2131</ulink>.</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, lease database configuration
is more extensive and contains additional parameters.  Note that
<command>lease-database</command>
is an object and opens up a new scope, using an opening brace.
Its parameters (just one in this example -- <command>type</command>)
follow. Had there been more than one, they would be separated by commas. This
scope is closed with a closing brace. As more parameters follow, a trailing
comma is present.</para>

<para>Finally, we need to define a list of 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>
</para>

<para>After all parameters are 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 three database backends available:
  memfile (which is the default backend), MySQL and PostgreSQL.</para>
<section>
  <title>Memfile, Basic Storage for Leases</title>

  <para>The server is able to store lease data in different repositories. Larger
  deployments may elect to store leases in a database. <xref
  linkend="database-configuration4"/> describes this option. In typical
  smaller deployments though, the server will use a CSV file rather than a database to
  store lease information. As well as requiring less administration, an
  advantage of using a file for storage is that it
  eliminates a dependency on third-party database software.</para>

  <para>The configuration of the file backend (Memfile) is controlled through
  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 presents the remaining, not mandatory
  parameters, which 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 (Memfile backend) will perform a lease file cleanup (LFC),
      which removes the 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>0</userinput>, which disables
      the LFC.</simpara>
    </listitem>

  </itemizedlist>
  </para>

  <para>The 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 when
  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 supersede the existing client's lease, as it would require
  the lookup of the specific lease entry, but simply appends the new lease
  information at the end of the lease 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 larger number
  of lease entries.
  </para>

  <para>The lease file cleanup 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 renewals are, the lesser
  value of the <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, this 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 completing the cleanup before the
  new cleanup is triggered.
  </para>

  <para>The LFC is performed by a separate process (in background) to avoid
  performance impact on the server process. In order to avoid the conflicts
  between the 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:
  <ulink url="http://kea.isc.org/wiki/LFCDesign"/>.
  </para>

</section>

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

  <note>
    <para>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>Database configuration is controlled through the Dhcp4/lease-database
  parameters. The type of the database must be set to "mysql" or "postgresql",
  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 lease database was created (see <xref linkend="mysql-database-create"/>
  or <xref linkend="pgsql-database-create"/>).
<screen>
"Dhcp4": { "lease-database": { <userinput>"name": "<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 (although 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>
  </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>

<section 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 the 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>If it is desired that the server listens on multiple IPv4 addresses assigned
  to the same interface, multiple addresses can be specified for this 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 on the interface yet. 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, it is desired to configure the
  DHCP server to utilize the 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 brodcast
  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 brodcast 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 the IP/UDP sockets.</para>
  </note>
</section>

<section 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 id="dhcp4-address-config">
  <title>Configuration of IPv4 Address Pools</title>
  <para>
    The essential role of DHCPv4 server is address assignment. The server has to
    be configured with at least one subnet and one pool of dynamic addresses to
    be managed. For example, assume that the server is connected to a network
    segment that uses the 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 subnet is defined as a simple string, but the <command>pools</command> parameter is
    actually a list of pools: for this reason, the pools definition is enclosed
    in square brackets, even though only one range of addresses is
    specified in this example.</para>

    <para>Each pool 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>
    The number of pools is not limited, but for performance reasons it is recommended to
    use as few as possible. 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 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 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.  Although there are several options that require
        special behavior, most options are sent by the server only if the client
        explicitly requests them.  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>
      </para>
    <para>
      The <command>name</command> parameter specifies the
      option name. For a complete 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>
        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>
        The currently supported standard DHCPv4 options are
        listed in <xref linkend="dhcp4-std-options-list"/>
        and <xref linkend="dhcp4-std-options-list-part2"/>.
        The "Name" and "Code"
        are the values that should be used as a name in the option-data
        structures. "Type" designates the format of the data: the meanings of
        the various types is given in <xref linkend="dhcp-types"/>.
      </para>
      <para>
        Some options are designated as arrays, which means that more than one
        value is allowed in such an option. For example the option 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 -->

      <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 does not provide a definition yet. 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 has
        no means to validate the format at the moment.
      </para>

      <para>
        <table frame="all" id="dhcp4-std-options-list">
          <title>List of standard DHCPv4 options</title>
          <tgroup cols='4'>
          <colspec colname='name'/>
          <colspec colname='code' align='center'/>
          <colspec colname='type' align='center'/>
          <colspec colname='array' align='center'/>
          <thead>
            <row>
              <entry>Name</entry>
              <entry>Code</entry>
              <entry>Type</entry>
              <entry>Array?</entry>
            </row>
          </thead>
          <tbody>
<row><entry>subnet-mask</entry><entry>1</entry><entry>ipv4-address</entry><entry>false</entry></row>
<row><entry>time-offset</entry><entry>2</entry><entry>int32</entry><entry>false</entry></row>
<row><entry>routers</entry><entry>3</entry><entry>ipv4-address</entry><entry>true</entry></row>
<row><entry>time-servers</entry><entry>4</entry><entry>ipv4-address</entry><entry>true</entry></row>
<row><entry>name-servers</entry><entry>5</entry><entry>ipv4-address</entry><entry>false</entry></row>
<row><entry>domain-name-servers</entry><entry>6</entry><entry>ipv4-address</entry><entry>true</entry></row>
<row><entry>log-servers</entry><entry>7</entry><entry>ipv4-address</entry><entry>true</entry></row>
<row><entry>cookie-servers</entry><entry>8</entry><entry>ipv4-address</entry><entry>true</entry></row>
<row><entry>lpr-servers</entry><entry>9</entry><entry>ipv4-address</entry><entry>true</entry></row>
<row><entry>impress-servers</entry><entry>10</entry><entry>ipv4-address</entry><entry>true</entry></row>
<row><entry>resource-location-servers</entry><entry>11</entry><entry>ipv4-address</entry><entry>true</entry></row>
<row><entry>host-name</entry><entry>12</entry><entry>string</entry><entry>false</entry></row>
<row><entry>boot-size</entry><entry>13</entry><entry>uint16</entry><entry>false</entry></row>
<row><entry>merit-dump</entry><entry>14</entry><entry>string</entry><entry>false</entry></row>
<row><entry>domain-name</entry><entry>15</entry><entry>fqdn</entry><entry>false</entry></row>
<row><entry>swap-server</entry><entry>16</entry><entry>ipv4-address</entry><entry>false</entry></row>
<row><entry>root-path</entry><entry>17</entry><entry>string</entry><entry>false</entry></row>
<row><entry>extensions-path</entry><entry>18</entry><entry>string</entry><entry>false</entry></row>
<row><entry>ip-forwarding</entry><entry>19</entry><entry>boolean</entry><entry>false</entry></row>
<row><entry>non-local-source-routing</entry><entry>20</entry><entry>boolean</entry><entry>false</entry></row>
<row><entry>policy-filter</entry><entry>21</entry><entry>ipv4-address</entry><entry>true</entry></row>
<row><entry>max-dgram-reassembly</entry><entry>22</entry><entry>uint16</entry><entry>false</entry></row>
<row><entry>default-ip-ttl</entry><entry>23</entry><entry>uint8</entry><entry>false</entry></row>
<row><entry>path-mtu-aging-timeout</entry><entry>24</entry><entry>uint32</entry><entry>false</entry></row>
<row><entry>path-mtu-plateau-table</entry><entry>25</entry><entry>uint16</entry><entry>true</entry></row>
<row><entry>interface-mtu</entry><entry>26</entry><entry>uint16</entry><entry>false</entry></row>
<row><entry>all-subnets-local</entry><entry>27</entry><entry>boolean</entry><entry>false</entry></row>
<row><entry>broadcast-address</entry><entry>28</entry><entry>ipv4-address</entry><entry>false</entry></row>
<row><entry>perform-mask-discovery</entry><entry>29</entry><entry>boolean</entry><entry>false</entry></row>
<row><entry>mask-supplier</entry><entry>30</entry><entry>boolean</entry><entry>false</entry></row>
<row><entry>router-discovery</entry><entry>31</entry><entry>boolean</entry><entry>false</entry></row>
<row><entry>router-solicitation-address</entry><entry>32</entry><entry>ipv4-address</entry><entry>false</entry></row>
<row><entry>static-routes</entry><entry>33</entry><entry>ipv4-address</entry><entry>true</entry></row>
<row><entry>trailer-encapsulation</entry><entry>34</entry><entry>boolean</entry><entry>false</entry></row>
<row><entry>arp-cache-timeout</entry><entry>35</entry><entry>uint32</entry><entry>false</entry></row>
<row><entry>ieee802-3-encapsulation</entry><entry>36</entry><entry>boolean</entry><entry>false</entry></row>
<row><entry>default-tcp-ttl</entry><entry>37</entry><entry>uint8</entry><entry>false</entry></row>
<row><entry>tcp-keepalive-interval</entry><entry>38</entry><entry>uint32</entry><entry>false</entry></row>
<row><entry>tcp-keepalive-garbage</entry><entry>39</entry><entry>boolean</entry><entry>false</entry></row>

          </tbody>
          </tgroup>
        </table>
      </para>

      <para>
        <table frame="all" id="dhcp4-std-options-list-part2">
          <title>List of standard DHCPv4 options (continued)</title>
          <tgroup cols='4'>
          <colspec colname='name'/>
          <colspec colname='code'/>
          <colspec colname='type'/>
          <colspec colname='array'/>
          <thead>
            <row>
              <entry>Name</entry>
              <entry>Code</entry>
              <entry>Type</entry>
              <entry>Array?</entry>
            </row>
          </thead>
          <tbody>

<row><entry>nis-domain</entry><entry>40</entry><entry>string</entry><entry>false</entry></row>
<row><entry>nis-servers</entry><entry>41</entry><entry>ipv4-address</entry><entry>true</entry></row>
<row><entry>ntp-servers</entry><entry>42</entry><entry>ipv4-address</entry><entry>true</entry></row>
<row><entry>vendor-encapsulated-options</entry><entry>43</entry><entry>empty</entry><entry>false</entry></row>
<row><entry>netbios-name-servers</entry><entry>44</entry><entry>ipv4-address</entry><entry>true</entry></row>
<row><entry>netbios-dd-server</entry><entry>45</entry><entry>ipv4-address</entry><entry>true</entry></row>
<row><entry>netbios-node-type</entry><entry>46</entry><entry>uint8</entry><entry>false</entry></row>
<row><entry>netbios-scope</entry><entry>47</entry><entry>string</entry><entry>false</entry></row>
<row><entry>font-servers</entry><entry>48</entry><entry>ipv4-address</entry><entry>true</entry></row>
<row><entry>x-display-manager</entry><entry>49</entry><entry>ipv4-address</entry><entry>true</entry></row>
<row><entry>dhcp-requested-address</entry><entry>50</entry><entry>ipv4-address</entry><entry>false</entry></row>
<!-- Lease time should not be configured by a user.
<row><entry>dhcp-lease-time</entry><entry>51</entry><entry>uint32</entry><entry>false</entry></row>
-->
<row><entry>dhcp-option-overload</entry><entry>52</entry><entry>uint8</entry><entry>false</entry></row>
<!-- Message Type, Server Identifier and Parameter Request List should not be configured by a user.
<row><entry>dhcp-message-type</entry><entry>53</entry><entry>uint8</entry><entry>false</entry></row>
<row><entry>dhcp-server-identifier</entry><entry>54</entry><entry>ipv4-address</entry><entry>false</entry></row>
<row><entry>dhcp-parameter-request-list</entry><entry>55</entry><entry>uint8</entry><entry>true</entry></row>
-->
<row><entry>dhcp-message</entry><entry>56</entry><entry>string</entry><entry>false</entry></row>
<row><entry>dhcp-max-message-size</entry><entry>57</entry><entry>uint16</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></row>
<row><entry>dhcp-rebinding-time</entry><entry>59</entry><entry>uint32</entry><entry>false</entry></row>
-->
<row><entry>vendor-class-identifier</entry><entry>60</entry><entry>binary</entry><entry>false</entry></row>
<!-- Client identifier should not be configured by a user.
<row><entry>dhcp-client-identifier</entry><entry>61</entry><entry>binary</entry><entry>false</entry></row>
-->
<row><entry>nwip-domain-name</entry><entry>62</entry><entry>string</entry><entry>false</entry></row>
<row><entry>nwip-suboptions</entry><entry>63</entry><entry>binary</entry><entry>false</entry></row>
<row><entry>tftp-server-name</entry><entry>66</entry><entry>string</entry><entry>false</entry></row>
<row><entry>boot-file-name</entry><entry>67</entry><entry>string</entry><entry>false</entry></row>
<row><entry>user-class</entry><entry>77</entry><entry>binary</entry><entry>false</entry></row>
<row><entry>fqdn</entry><entry>81</entry><entry>record</entry><entry>false</entry></row>
<row><entry>dhcp-agent-options</entry><entry>82</entry><entry>empty</entry><entry>false</entry></row>
<row><entry>authenticate</entry><entry>90</entry><entry>binary</entry><entry>false</entry></row>
<row><entry>client-last-transaction-time</entry><entry>91</entry><entry>uint32</entry><entry>false</entry></row>
<row><entry>associated-ip</entry><entry>92</entry><entry>ipv4-address</entry><entry>true</entry></row>
<row><entry>subnet-selection</entry><entry>118</entry><entry>ipv4-address</entry><entry>false</entry></row>
<row><entry>domain-search</entry><entry>119</entry><entry>binary</entry><entry>false</entry></row>
<row><entry>vivco-suboptions</entry><entry>124</entry><entry>binary</entry><entry>false</entry></row>
<row><entry>vivso-suboptions</entry><entry>125</entry><entry>binary</entry><entry>false</entry></row>
          </tbody>
          </tgroup>
        </table>

      </para>
      <para>
        <table frame="all" 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>binary</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>record</entry><entry>Structured data that may comprise any types (except "record" and "empty")</entry></row>
            <row><entry>string</entry><entry>Any text</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>
          </tbody>
          </tgroup>
       </table>
      </para>
    </section>

    <section 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 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 rather 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>
      <note>
        <para>
          In the current release the default values are not propagated to the
          parser when the new configuration is being set. Therefore, all
          parameters must be specified at all times, even if their values are
          left blank.
        </para>
      </note>

      <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 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>
     <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 Kea versions will accept all those values
       for all boolean parameters.</para>
     </note>
    </section>

    <section id="dhcp4-vendor-opts">
      <title>DHCPv4 Vendor Specific Options</title>
      <para>
      Currently there are three option spaces defined: "dhcp4" (used by the DHCPv4 daemon)
      and "dhcp6" (for the DHCPv6 daemon); there is also "vendor-encapsulated-options-space",
      which is empty by default, but options
      can be defined in it. Those options are called vendor-specific
      information options. The following examples show how to define
      an option "foo" with code 1 that consists of an 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",
            "encapsulates": ""</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 set up a dummy value for "vendor-encapsulated-options", 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",
            "space": "dhcp4",
            "code": 43,
            "csv-format": false,
            "data": ""</userinput>
        }
    ],
    ...
}</screen>
      </para>

      <note>
        <para>
          With this version of Kea, the "vendor-encapsulated-options" option
          must be specified in the configuration although it has no configurable
          parameters. If it is not specified, the server will assume that it is
          not configured and will not send it to a client. In the future there
          will be no need to include this option in the configuration.
        </para>
      </note>

    </section>

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

      <title>Nested DHCPv4 Options (Custom Option Spaces)</title>
      <para>It is sometimes useful to define completely new option
      space. This is the case when user creates new option in the
      standard option space ("dhcp4 or "dhcp6") 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 "encapsulate" field. The "type" 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",
            "space": "isc",
            "code": 1,
            "csv-format": true,
            "data": "192.0.2.3"</userinput>
        },
        }
            <userinput>"name": "subopt2",
            "space": "isc",
            "code": 2,
            "csv-format": true,
            "data": "Hello world"</userinput>
        },
        {
            <userinput>"name": "container",
            "space": "dhcp4",
            "code": 222,
            "csv-format": true,
            "data": ""</userinput>
        }
    ],
    ...
}
</screen>
    Even though the "container" option does not carry any data except
    sub-options, the "data" field must be explicitly set to an empty value.
    This is required because in the current version of Kea, the
    default configuration values are not propagated to the configuration parsers:
    if the "data" is not set the parser will assume that this
    parameter is not specified and an error will be reported.
    </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 "type" value would have to be set to "uint16" in
    the option definition. (Such an option would then have the following
    data structure: DHCP header, uint16 value, sub-options.) The value specified
    with the "data" parameter &mdash; which should be a valid integer enclosed in quotes,
    e.g. "123" &mdash; would then be assigned to the uint16 field in the "container" option.
    </para>
    </section>

    <section 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 specifing 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
          spec 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
          and the definition for the particular option exists, 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. If the definition
          does not exist for this option, the server will assume that the data
          parameter contains the option payload in the binary format (represented
          as a string of hexadecimal digits). Note that not specifying this
          parameter doesn't imply that it defaults to a fixed value, but
          the configuration data interpretation also depends on the presence
          of the option definition. An administrator must be aware if the
          definition for the particular option exists when this parameter
          is not specified. It is generally recommended to not specify this
          parameter only for the options for which the definition exists, e.g.
          standard options. Setting <command>csv-format</command> to an explicit
          value will cause the server to strictly check the format of the option
          data specified.
          </simpara>
        </listitem>
      </itemizedlist>
      </para>

    </section>

    <section 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. DNS servers' addresses.
      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 the particular subnet defined in the server's configuration. The 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 id="dhcp4-client-classifier">
      <title>Client Classification in DHCPv4</title>
      <note>
      <para>
        The DHCPv4 server has been extended to support limited client classification.
        Although the current capability is modest, it is expected to be expanded
        in the future. However, it is envisaged that the majority of client classification
        extensions will be using hooks extensions.
      </para>
      </note>
      <para>In certain cases it is useful to differentiate between different
      types of clients and treat them differently. The process of doing
      classification is conducted in two steps. The first step is to assess an
      incoming packet and assign it to zero or more classes. This classification
      is currently simple, but is expected to grow in capability soon. Currently
      the server checks whether an incoming packet includes the vendor class identifier
      option (60). If it does, the content of that option is prepended with
      &quot;VENDOR_CLASS_&quot; then it is interpreted as a class. For example,
      modern cable modems will send this option with value &quot;docsis3.0&quot;
      and as a result the packet will belong to class &quot;VENDOR_CLASS_docsis3.0&quot;.
      </para>

      <para>It is envisaged that the client classification will be used for changing the
      behavior of almost any part of the DHCP message processing, including assigning
      leases from different pools, assigning different options (or different values of
      the same options) etc. For now, there are only two mechanisms that are taking
      advantage of client classification: specific processing for cable modems and
      subnet selection.</para>

      <para>
        For clients that belong to the VENDOR_CLASS_docsis3.0 class, the siaddr
        field is set to the value of next-server (if specified in a subnet). If
        there is a boot-file-name option specified, its value is also set in the
        file field in the DHCPv4 packet. For eRouter1.0 class, the siaddr is
        always set to 0.0.0.0. That capability is expected to be moved to
        an external hook library that will be dedicated to cable modems.
      </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. 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="dhcp4-subnet-class"/>.
      </para>


    <section id="dhcp4-subnet-class">
      <title>Limiting Access to IPv4 Subnet to Certain Classes</title>
      <para>
        In certain cases it beneficial to restrict access to certain subnets
        only to clients that belong to a given subnet. For details on client
        classes, see <xref linkend="dhcp4-client-classifier"/>. This is an
        extension of a previous example from <xref linkend="dhcp4-address-config"/>.
        Let's 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. Only clients belonging to client class
        VENDOR_CLASS_docsis3.0 are allowed to use this subnet. Such a
        configuration can be achieved in the following way:
        <screen>
"Dhcp4": {
    "subnet4": [
        {
            <userinput>"subnet": "192.0.2.0/24",
            "pools": [ { "pool": "192.0.2.10 - 192.0.2.20" } ],
            "client-class": "VENDOR_CLASS_docsis3.0"</userinput>
        }
    ],
    ...
}</screen>
      </para>

      <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>
    </section>
    </section>

    <section id="dhcp4-ddns-config">
      <title>Configuring DHCPv4 for DDNS</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": false</command>
      </simpara></listitem>
      <listitem><simpara>
      <command>"generated-prefix": "myhost"</command>
      </simpara></listitem>
      </itemizedlist>
      </para>

      <section 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 how it communications with D2:
      <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-format</command> - socket protocol use when sending requests to D2.  Currently
      only UDP is supported.  TCP may be available in an upcoming release.
      </simpara></listitem>

      <listitem><simpara>
      <command>ncr-protocol</command> - packet format to use when sending requests to D2.
      Currently only JSON format is supported.  Other formats may be available
      in future releases.
      </simpara></listitem>

      </itemizedlist>
      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 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
      <ulink url="http://tools.ietf.org/html/rfc4702">RFC 4702</ulink>.
      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 "enable-updates" 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.  The decision
      making involved when granting a new lease (the first case) is more
      involved and is discussed next.
      </para>
      <para>
      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 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 DHCP-DDNS 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
      <ulink url="http://tools.ietf.org/html/rfc4702">RFC 4702</ulink>. 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 your 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 a DDNS update request 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 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 rules for determining the FQDN option are as follows:
      <orderedlist>
      <listitem><para>
      If configured to do, so ignore the REQUEST contents and generate a
      FQDN using a configurable prefix and suffix.
      </para></listitem>
      <listitem><para>
      If the REQUEST contains the client FQDN option, the candidate
      name is taken from there, otherwise it is taken from the Host Name option.
      The candidate name may then be modified:
      <orderedlist>
      <listitem><para>
      If the candidate name is a fully qualified domain name, use it.
      </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 is a empty, generate a FQDN using a
      configurable prefix and suffix.
      </para></listitem>
      </orderedlist>
      </para></listitem>
      </orderedlist>
      To instruct kea-dhcp4 to always generate the FQDN for a client, set the
      parameter <command>replace-client-name</command> to true as follows:
      </para>
<screen>
"Dhcp4": {
    "dhcp-ddns": {
        <userinput>"replace-client-name": true</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>
      </section>
      <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 id="dhcp4-next-server">
      <title>Next Server (siaddr)</title>
      <para>In some cases, clients want to obtain configuration from the 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>

<screen>
"Dhcp4": {
    <userinput>"next-server": "192.0.2.123"</userinput>,
    ...,
    "subnet4": [
        {
            <userinput>"next-server": "192.0.2.234"</userinput>,
            ...
        }
    ]
}
</screen>
    </section>

    <section id="dhcp4-echo-client-id">
      <title>Echoing Client-ID (RFC 6842)</title>
      <para>The original DHCPv4 specification
      (<ulink url="http://tools.ietf.org/html/rfc2131">RFC 2131</ulink>)
      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
      <ulink url="http://tools.ietf.org/html/rfc6842">RFC 6842</ulink>.
      for details. That
      behavior has changed with the publication of
      <ulink url="http://tools.ietf.org/html/rfc6842">RFC 6842</ulink>.
      which updated
      <ulink url="http://tools.ietf.org/html/rfc2131">RFC 2131</ulink>.
      That update now states that the server must
      send client-id if the client sent it. That is the default behaviour
      that Kea offers. However, in some cases older devices that do
      not support
      <ulink url="http://tools.ietf.org/html/rfc6842">RFC 6842</ulink>.
      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> <!-- end of configuring kea-dhcp4 server section with many subsections -->

  <!-- Host reservation is a large topic. There will be many subsections,
   so it should be a section on its own. -->
  <section 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 specific, static
    address for exclusive use by a given client (host) &dash; returning client will
    receive the same address from the server every time, and other clients will
    generally not receive that address. 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 the subsequent sections.
    Once conflict is resolved,the client will keep receiving the reserved
    configuration when it renews.</para>

    <para>Another example when the host reservations are applicable is when a host
    that has specific requirements, e.g. a printer that needs additional DHCP options.
    Yet another possible use case is to define unique names for hosts. Although not all
    of the presented use cases are implemented yet, Kea software will support them in the
    near future.</para>

    <para>Hosts reservations are defined as parameters for each subnet. Each host
    has to be identified by its 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 reservrations for a single host. In particular, such a structure has
    to have an indentifer that uniquely identifies a host.  In DHCPv4 context, such an
    identifier is a hardware or MAC address.  In most cases, also an address
    will be specified. It is possible to specify a hostname. Additional
    capabilities are planned.</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"
            },
            {
                "hw-address": "0a:0b:0c:0d:0e:0f",
                "ip-address": "192.0.2.100",
                "hostname": "alice-laptop"
            }
        ]</userinput>
    }
]
</screen>
    The first entry reserves the 192.0.2.202 address for the client that uses
    MAC adress of 1a:1b:1c:1d:1e:1f. The second entry reserves the address
    192.0.2.100 and the hostname of alice-laptop for client using MAC
    address 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.
    </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. It is a valid configuration, if such definitions
    are specified in different subnets, though.
    </para>

    <para>Adding host reservation incurs a performance penalty. In principle,
    when the 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 (if there is a lease for it), but also whether the address
    could be used by someone else (if there is a reservation for it). That
    additional check incurs performance penalty.</para>

    <section 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, the host reservation can reserve any address
      that belongs to the subnet. The reservations that specify addresses that
      belong to configured pools are called <command>in-pool reservations</command>.
      In contrast, those that do not belong to dynamic pools are called
      <command>out-of-pool reservations</command>. There is no formal difference
      in the reservation syntax. As of 0.9.1, 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 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. The Host A requests an address and gets 19.0.2.10. Now the system
      administrator decides to reserve the address for the Host B. He decides to
      reserve 192.0.2.10 for that purpose. 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 the Host B boots up and requests an address, the server is
      not able to assign the reserved address 192.0.2.10 for the Host B. 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 the Host A renews its address, the server will discover that
      the address being renewed is now reserved for another host - the Host
      B. Therefore the server will inform the Host A that it is no longer
      allowed to use it by sending 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 the Host A, it will then 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, the address 192.0.2.10 will be 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, 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). The 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 the 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 renew-timer value specified.
      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 (note that
      having multiple reservations for the same address is not allowed).
      </para>
    </section>

    <section id="reservation4-hostname">
      <title>Reserving a hostname</title>
      <para>When the reservation for the client includes the <command>hostname
      </command>, the server will assign this hostname to the client and send
      it back in the Client FQDN or Hostname option, depending on which of them
      the client has sent to the server. 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 id="reservation4-options">
      <title>Reserving specific options</title>
      <!-- @todo: replace this with the actual text once #3572 is implemented -->
      <para>Currently it is not possible to specify options in host
      reservation. Such a feature will be added in the upcoming Kea
      releases.</para>
    </section>

    <section id="reservation4-mode">
      <title>Fine Tuning IPv4 Host Reservation</title>

      <note>
        <para><command>reservation-mode</command> configuration parameter in DHCPv4
        server is accepted, but not used in the Kea 0.9.1 beta. Full implementation
        will be available in the upcoming releases.</para>
      </note>

      <para>Host reservation capability introduces additional restrictions for the
      allocation engine 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 be not 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's a reservation for this host, so the exisiting
      (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. 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 0.9.1 does not sanity check the reservations against
      <command>reservation-mode</command>. 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>        
   </section>

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

    <section 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>
        Currently there is no mechanism to override the default server identifiers
        by an administrator. In the future, the configuration mechanism will be used
        to specify the custom server identifier.
      </para>
    </section>

    <section 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 the 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 subnets. 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 id="dhcp4-relay-override">
      <title>Using a Specific Relay Agent for a Subnet</title>
      <para>
        The 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 &mdash;
        valid &mdash; 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>

    </section>

      <section 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="dhcp4-subnet-class"/>.
          One specific example is cable network, where typically modems
          get addresses from a different subnet than all devices connected
          behind them.
        </para>
        <para>
          Let's 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 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>,
            <ulink url="http://tools.ietf.org/html/rfc2131">RFC 2131</ulink>:
            Supported messages are DISCOVER (1), OFFER (2),
            REQUEST (3), RELEASE (7), INFORM (8), ACK (5), and NAK(6).</simpara>
          </listitem>
          <listitem>
            <simpara><emphasis>DHCP Options and BOOTP Vendor Extensions</emphasis>,
            <ulink url="http://tools.ietf.org/html/rfc2132">RFC 2132</ulink>:
            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>,
            <ulink url="http://tools.ietf.org/html/rfc3046">RFC 3046</ulink>:
            Relay Agent Information option is supported.</simpara>
          </listitem>
          <listitem>
            <simpara><emphasis>Vendor-Identifying Vendor Options for
            Dynamic Host Configuration Protocol version 4</emphasis>,
            <ulink url="http://tools.ietf.org/html/rfc3925">RFC 3925</ulink>:
            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>,
            <ulink url="http://tools.ietf.org/html/rfc6842">RFC 6842</ulink>:
            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-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> <!-- see tickets #3234, #3281 -->
            <simpara>
              Removal of a subnet during server reconfiguration may cause renumbering
              of auto-generated subnet identifiers, as described in section
              <xref linkend="ipv4-subnet-id"/>.
            </simpara>
          </listitem>
          <listitem>
            <simpara>Host reservation (static addresses) is not supported yet.</simpara>
          </listitem>
          <listitem>
            <simpara>Full featured client classification is not supported yet.</simpara>
          </listitem>
          <listitem>
            <simpara>
              BOOTP (<ulink url="http://tools.ietf.org/html/rfc951">RFC 951</ulink>)
              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 <ulink url="http://tools.ietf.org/html/rfc2131">RFC 2131</ulink>, the
            allocating server should verify that address is not used by
            sending ICMP echo request.</simpara>
          </listitem>
          <listitem>
            <simpara>Address duplication report (DECLINE) is not supported yet.</simpara>
          </listitem>
          <listitem>
            <simpara>
              The server doesn't act upon expired leases. In particular,
              when a lease expires, the server doesn't request the removal
              of the DNS records associated with it. Expired leases can be
              recycled.
            </simpara>
          </listitem>
      </itemizedlist>
    </section>

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

      <para>
        This section provides easy to use example. Each example can be read
        separately. It is not intended to be read sequentially as there will
        be many repetitions between examples. They are expected to serve as
        easy to use copy-paste solutions to many common deployments.
      </para>

      @todo: add simple configuration for direct clients
      @todo: add configuration for relayed clients
      @todo: add client classification example

    </section> -->

  </chapter>
