kea/doc/guide/dhcp4-srv.xml

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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>
        <command>b10-dhcp4</command> is the Kea DHCPv4 server and is configured
        through the <command>bindctl</command> program.
      </para>
      <para>
        After starting <command>bind10</command> and entering bindctl, the first step
        in configuring the server is to add it to the list of running services.
<screen>
&gt; <userinput>config add Init/components b10-dhcp4</userinput>
&gt; <userinput>config set Init/components/b10-dhcp4/kind dispensable</userinput>
&gt; <userinput>config commit</userinput>
</screen>
      </para>
      <para>
         To remove <command>b10-dhcp4</command> from the set of running services,
         the <command>b10-dhcp4</command> is removed from list of Init components:
<screen>
&gt; <userinput>config remove Init/components b10-dhcp4</userinput>
&gt; <userinput>config commit</userinput>
</screen>
      </para>
      <para>
        Note that the server was only removed from the list, so BIND10 will not
        restart it, but the server itself is still running. Hence it is usually
        desired to stop it:
<screen>
&gt; <userinput>Dhcp4 shutdown</userinput>
</screen>
      </para>

      <para>
        On start-up, the server will detect available network interfaces
        and will attempt to open UDP sockets on all interfaces that
        are up, running, are not loopback, and have IPv4 address
        assigned.

        The server will then listen to incoming traffic. Currently
        supported client messages are DISCOVER and REQUEST. The server
        will respond to them with OFFER and ACK, respectively.

        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>Configuring the DHCPv4 Server</title>
      <para>
        This section explains how to configure DHCPv4 server that uses
        Kea configuration backend. Kea configuration using any other
        backends is outside of scope for this document. Before DHCPv4
        is started, its configuration file has to be prepared. The
        basic configuration looks as follows:
<screen>
 1. # This is an example configuration file for the DHCPv4 server in Kea.
 2. # It is a basic scenario with one IPv4 subnet configured. The subnet
 3. # contains a single pool of dynamically allocated addresses.

 5. { "Dhcp4":

 7. {
 8. # Kea is told to listen on eth0 interface only.
 9.   "interfaces": [ "eth0" ],

11. # We need to specify lease type. As of May 2014, three backends are supported:
12. # memfile, mysql and pgsql. We'll just use memfile, because it doesn't require
13. # any prior set up.
14.  "lease-database": {
15.    "type": "memfile"
16.  },

17. # Addresses will be assigned with valid lifetimes being 4000. Client
18. # is told to start renewing after 1000 seconds. If the server does not respond
19. # after 2000 seconds since the lease was granted, client is supposed
20. # to start REBIND procedure (emergency renewal that allows switching
21. # to a different server).
22.   "valid-lifetime": 4000,

24. # Renew and rebind timers are commented out. This implies that options
25. # 58 and 59 will not be sent to the client. In this case it is up to
26. # the client to pick the timer values according to RFC2131. Uncomment the
27. # timers to send these options to the client.
28. #  "renew-timer": 1000,
29. #  "rebind-timer": 2000,

31. # The following list defines subnets. We have only one subnet
32. # here.
33.  "subnet4": [
34.  {    "pool": [ "192.0.2.1 - 192.0.2.200" ],
35.       "subnet": "192.0.2.0/24"  } ]
36. }

38. }
</screen>Note that line numbers are specified for easier reference only and
are not part of the configuration. The examples in following sections do
not have reference numbers.</para>

<para>The following paragraphs provide brief overview of the parameters
and their format. Following sections in this chapter go into much greater
details for aforementioned parameters and also introduce new ones.</para>

<para>The lines 1-3 are comments and do not impact the server
operation in any way. The configuration starts in line 5 with the
initial opening curly bracket. Each configuration consists of one or
more objects. In this specific example, we have only one object called
Dhcp4. This is simplified configuration, as usually there will be
additional objects, like Logging or DhcpDns, but we omit them now for
clarity. The Dhcp4 configuration starts in line 7 and finished in line
36. Everything defined between those lines is considered Dhcp4
configuration.</para>

<para>In general case, the order in which those parameters appear
doesn't matter. For example swapping line 9 and lines 14-16 does not
change the configuration in any way. There are two caveats here,
though. The first one is to remember that the configuration file must
be a well formed JSON. That means that parameters for any given scope
must be separate by a coma and there must not be a coma after the last
parameter. When reordering configuration file, keep in mind that
moving a parameter to or from the last position in a given scope may
require moving the coma as well. The second caveat is that it is
uncommon, but legal to repeat the same parameter multiple times. In
that case the last occurrence of a given parameter is used while all
previous instances are ignored. That is unliekly 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>Line 9 contains the first parameter that specifies a list of
network interfaces, over which the server should listen. Please note
the notation. Lists are defined as square brackets, with elements
separated by comas. Had we wanted to listen on two interfaces, the
line could look like this:
<screen>
"interfaces": [ "eth0", "eth1" ],
</screen>
As "interfaces" is not the last parameter and there are others that
follow, a trailing coma is required.</para>

<para>Lines 14-16 define lease database. It informs the server where
to store its leases information. This specific example tells the
server to use memfile, which is the simplest (and fastest) database
backend. It uses in-memory database and stores leases on disk using
CSV file. This is a very simple configuration. Usually, lease database
configuration is more extensive and contains additional parameters.
Note that lease-database is defined as object or list and it opens up
a new scope, using opening curly bracket. Its parameters (just one --
called &quot;type&quot;) follow. Had there been more than one, they
would be separated by comas. This scope is closed in line 16. As more
parameters follow, trailing coma is present.</para>

<para>Line 22 has a simple definition of a valid lifetime. That value
defines how long the addresses (leases) given out by the server are
valid. If nothing changes, client that got the address is allowed to
use it for 4000 seconds. Please note that integer numbers are specified
as is, without any quotes around them.</para>

<para>The next paragraph, metions parameters that are optional. In
particular, renew-timer and rebind-timer are values that may or may
not appear. If they are not present, the server will say nothing about
renewal (T1) and rebing (T2) timers and it will be up to the client to
choose appropriate timer values. <ulink
url="http://tools.ietf.org/html/rfc2131">RFC 2131</ulink> says that in
such cases client is supposed to use default values of 0.5 *
valid-lifetime for renewal (T1) and 0.875 * valid-lifetime for rebind
(T2). Administrator may want to decide on different values and specify
those parameters explicitly.</para>

<para>Lines 33 to 36 define a list of IPv4 subnets. This is the most
important DHCPv4 configuration structure as this is the essense of the
network topology. It defines all subnets that the server is expected
to receive DHCP requests from. It is a list, so it start and ends with
square brackets. In this example we have only one subnet
defined. Subnet itself has several parameters, hence it is a
structure, so it is opened and closed using curly brackets. Each
subnet has to have at least two parameters: subnet (that defines the
whole subnet) and pool (which is a list of dynamically allocated pools
that are governed by the DHCP server. Subnet4 list is closed with
closing square bracket at the end of line 35. As this is the last
parameter in Dhcp4 context, there is no trailing coma.</para>

<para>Had there been more than one subnet defined, additional subnet4
objects would be specified and separated by comas. For example, to
define 3 subnets, the following syntax should be used:
<screen>
  "subnet4": [
  {    "pool": [ "192.0.2.1 - 192.0.2.200" ],
       "subnet": "192.0.2.0/24"  },
  {    "pool": [ "192.0.3.100 - 192.0.3.200" ],
       "subnet": "192.0.3.0/24"  },
  {    "pool": [ "192.0.4.1 - 192.0.4.254" ],
       "subnet": "192.0.4.0/24"  } ]
</screen>
</para>

<para>Line 36 closes Dhcp4 context. In a real life configuration file
there likely would be additional components defined, like Logging or
DhcpDdns, so line 36 would have a coma behind the closing curly
bracket.</para>

<para>The whole configuration ends in line 38, which closes the global
configuration scope, opened in line 5.</para>

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

  <section>
    <title>Default 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 one way to do it.
      By default, the server will use a CSV file rather than a database to store
      lease information. One of the advantages of using a file is that it eliminates
      dependency on third party database software.
    </para>
    <para>
        The configuration of the file backend (Memfile)
        is controlled through the Dhcp4/lease-database parameters. When default
        parameters are used, the Memfile backend will write leases to a disk in the
        [bind10-install-dir]/var/bind10/kea-leases4.csv.
      </para>
      <para>
        It is possible to alter the default location of the lease file. The following
        configuration:
<screen>
&gt; <userinput>config set Dhcp4/lease-database/type "memfile"</userinput>
&gt; <userinput>config set Dhcp4/lease-database/persist true</userinput>
&gt; <userinput>config set Dhcp4/lease-database/name "/tmp/kea-leases4.csv"</userinput>
&gt; <userinput>config commit</userinput>
</screen>
        will change the default location of the lease file to /tmp/kea-leases4.csv.
      </para>
      <para>
        The "persist" parameter controls whether the leases are written to disk.
        It is strongly recommended that this parameter is set to "true" at all times
        during the normal operation of the server
      </para>
      </section>

      <section id="database-configuration4">
      <title>Database Configuration</title>
      <para>
      All leases issued by the server are stored in the lease database.  Currently
      there are 3 database backends available: MySQL, PostgreSQL and memfile (which
      is the default backend).
      </para>
        <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", "postgresql" or "memfile":
<screen>
<userinput>"Dhcp4": { "lease-database": { "type": "memfile" } }</userinput>
</screen>
      Next, the name of the database is to hold the leases must be set: this is the
      name used when the lease database was created (see <xref linkend="dhcp-mysql-database-create"/>
      or <xref linkend="dhcp-pgsql-database-create"/>).
<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 note that this configuration
      may have a severe impact on server performance):
<screen>
<userinput>"Dhcp4": { "lease-database": { "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>
<userinput>"Dhcp4": { "lease-database": { "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>
      <note>
      <para>The password is echoed when entered and is stored in clear text in the configuration
      database.  Improved password security will be added in a future version of Kea.</para>
      </note>
      </section>

      <section id="dhcp4-interface-selection">
      <title>Interface selection</title>
      <para>
        When DHCPv4 server starts up, by default it will listen to the DHCP
        traffic and respond to it on all interfaces detected during startup.
        However, in many cases it is desired to configure the server to listen and
        respond on selected interfaces only. The sample commands in this section
        show how to make interface selection using bindctl.
      </para>
      <para>
        The default configuration can be presented with the following command:
        <screen>
&gt; <userinput>config show Dhcp4/interfaces</userinput>
<userinput>Dhcp4/interfaces[0] "*" string</userinput></screen>
        An asterisk sign plays a role of the wildcard and means "listen on all interfaces".
      </para>
      <para>
        In order to override the default configuration, the existing entry can be replaced
        with the actual interface name:
        <screen>
&gt; <userinput>config set Dhcp4/interfaces[0] eth1</userinput>
&gt; <userinput>config commit</userinput></screen>
        Other interface names can be added on one-by-one basis:
        <screen>
&gt; <userinput>config add Dhcp4/interfaces eth2</userinput>
&gt; <userinput>config commit</userinput></screen>
        Configuration will now contain two interfaces which can be presented as follows:
        <screen>
&gt; <userinput>config show Dhcp4/interfaces</userinput>
<userinput>Dhcp4/interfaces[0]	"eth1"	string</userinput>
<userinput>Dhcp4/interfaces[1]	"eth2"	string</userinput></screen>
        When configuration gets committed, the server will start to listen on
        eth1 and eth2 interfaces only.
      </para>
      <para>
        It is possible to use wildcard interface name (asterisk) concurrently with explicit
        interface names:
        <screen>
&gt; <userinput>config add Dhcp4/interfaces *</userinput>
&gt; <userinput>config commit</userinput></screen>
        This will result in the following configuration:
        <screen>
&gt; <userinput>config show Dhcp4/interfaces</userinput>
<userinput>Dhcp4/interfaces[0]	"eth1"	string</userinput>
<userinput>Dhcp4/interfaces[1]	"eth2"	string</userinput>
<userinput>Dhcp4/interfaces[2]	"*"	string</userinput></screen>
        The presence of the wildcard name implies that server will listen on all interfaces.
        In order to fall back to the previous configuration when server listens on eth1 and eth2:
        <screen>
&gt; <userinput>config remove Dhcp4/interfaces[2]</userinput>
&gt; <userinput>config commit</userinput></screen>
      </para>
      </section>

      <section id="ipv4-subnet-id">
      <title>IPv4 Subnet Identifier</title>
      <para>
        Subnet identifier is a unique number associated with a particular subnet.
        In principle, it is used to associate clients' leases with respective subnets.
        When 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 4 subnets and 3rd is removed the last subnet will be assigned
       identifier that the 3rd subnet had before removal. As a result, the leases
       stored in the lease database for subnet 3 are now associated with the
       subnet 4, which may have unexpected consequences. In the future it is planned
       to implement the mechanism to preserve auto-generated subnet ids upon removal
       of one of the subnets. Currently, the only remedy for this issue is to
       manually specify the unique subnet identifier for each subnet.
      </para>
      <para>
        The following configuration:
        <screen>
&gt; <userinput>config add Dhcp4/subnet4</userinput>
&gt; <userinput>config set Dhcp4/subnet4[0]/subnet "192.0.2.0/24"</userinput>
&gt; <userinput>config set Dhcp4/subnet4[0]/id 1024</userinput>
&gt; <userinput>config commit</userinput>
        </screen>
        will assign the arbitrary subnet identifier to the newly configured subnet.
        This identifier will not change for this subnet until "id" parameter is
        removed or set to 0. The value of 0 forces auto-generation of subnet
        identifier.
      </para>
      </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>
&gt; <userinput>config add Dhcp4/subnet4</userinput>
&gt; <userinput>config set Dhcp4/subnet4[0]/subnet "192.0.2.0/24"</userinput>
&gt; <userinput>config set Dhcp4/subnet4[0]/pool [ "192.0.2.10 - 192.0.2.20" ]</userinput>
&gt; <userinput>config commit</userinput></screen>
        Note that subnet is defined as a simple string, but the pool parameter
        is actually a list of pools: for this reason, the pool definition is
        enclosed in square brackets, even though only one range of addresses
        is specified.</para>
        <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>
&gt; <userinput>config set Dhcp4/subnet4[0]/pool [ "192.0.2.10-192.0.2.20", "192.0.2.64/26" ]</userinput>
&gt; <userinput>config commit</userinput></screen>
        The number of pools is not limited, but for performance reasons it is recommended to
        use as few as possible. Space and tabulations in pool definitions are ignored, so
        spaces before and after hyphen are optional. They can be used to improve readability.
      </para>
      <para>
         The server may be configured to serve more than one subnet. To add a second subnet,
         use a command similar to the following:
        <screen>
&gt; <userinput>config add Dhcp4/subnet4</userinput>
&gt; <userinput>config set Dhcp4/subnet4[1]/subnet "192.0.3.0/24"</userinput>
&gt; <userinput>config set Dhcp4/subnet4[1]/pool [ "192.0.3.0/24" ]</userinput>
&gt; <userinput>config commit</userinput></screen>
        Arrays are counted from 0. subnet[0] refers to the subnet defined in the
        previous example.  The <command>config add Dhcp4/subnet4</command> command adds
        another (second) subnet. It can be referred to as
        <command>Dhcp4/subnet4[1]</command>. In this example, we allow server to
        dynamically assign all addresses available in the whole subnet.
      </para>
      <para>
        When configuring a DHCPv4 server using prefix/length notation, please pay
        attention to the boundary values. When specifying that the server should use
        a given pool, it will be able to allocate also 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 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 requested them.  The following example shows how to
        configure 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>
&gt; <userinput>config add Dhcp4/option-data</userinput>
&gt; <userinput>config set Dhcp4/option-data[0]/name "domain-name-servers"</userinput>
&gt; <userinput>config set Dhcp4/option-data[0]/code 6</userinput>
&gt; <userinput>config set Dhcp4/option-data[0]/space "dhcp4"</userinput>
&gt; <userinput>config set Dhcp4/option-data[0]/csv-format true</userinput>
&gt; <userinput>config set Dhcp4/option-data[0]/data "192.0.3.1, 192.0.3.2"</userinput>
&gt; <userinput>config commit</userinput>
</screen>
      </para>
    <para>
      The first line creates new entry in option-data table. It
      contains information on all global options that the server is
      supposed to configure in all subnets. The second line specifies
      option name. For a complete list of currently supported names,
      see <xref linkend="dhcp4-std-options-list"/> below.
      The third line specifies option code, which must match one of the
      values from that list. Line 4 specifies 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 fifth 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 a 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 csv-format is
      set to false, option data must be specified as a hex 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 csv-format is set to false.
      <screen>
&gt; <userinput>config add Dhcp4/option-data</userinput>
&gt; <userinput>config set Dhcp4/option-data[0]/name "domain-name-servers"</userinput>
&gt; <userinput>config set Dhcp4/option-data[0]/code 6</userinput>
&gt; <userinput>config set Dhcp4/option-data[0]/space "dhcp4"</userinput>
&gt; <userinput>config set Dhcp4/option-data[0]/csv-format false</userinput>
&gt; <userinput>config set Dhcp4/option-data[0]/data "C0 00 03 01 C0 00 03 02"</userinput>
&gt; <userinput>config commit</userinput>
        </screen>
      </para>

      <para>
        It is possible to override options on a per-subnet basis.  If
        clients connected to most of your subnets are expected to get the
        same values of a given option, you should use global options: you
        can then override specific values for a small number of subnets.
        On the other hand, if you use different values in each subnet,
        it does not make sense to specify global option values
        (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>
&gt; <userinput>config add Dhcp4/subnet4[0]/option-data</userinput>
&gt; <userinput>config set Dhcp4/subnet4[0]/option-data[0]/name "domain-name-servers"</userinput>
&gt; <userinput>config set Dhcp4/subnet4[0]/option-data[0]/code 6</userinput>
&gt; <userinput>config set Dhcp4/subnet4[0]/option-data[0]/space "dhcp4"</userinput>
&gt; <userinput>config set Dhcp4/subnet4[0]/option-data[0]/csv-format true</userinput>
&gt; <userinput>config set Dhcp4/subnet4[0]/option-data[0]/data "192.0.2.3"</userinput>
&gt; <userinput>config commit</userinput></screen>
      </para>

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

      <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 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 configuration mechanism would allow any option format as it has
        no means to validate it 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'/>
          <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>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-internal</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 unsinged 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>It is also possible to define options other than the standard ones.
      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 commands:
      <screen>
&gt; <userinput>config add Dhcp4/option-def</userinput>
&gt; <userinput>config set Dhcp4/option-def[0]/name "foo"</userinput>
&gt; <userinput>config set Dhcp4/option-def[0]/code 222</userinput>
&gt; <userinput>config set Dhcp4/option-def[0]/type "uint32"</userinput>
&gt; <userinput>config set Dhcp4/option-def[0]/array false</userinput>
&gt; <userinput>config set Dhcp4/option-def[0]/record-types ""</userinput>
&gt; <userinput>config set Dhcp4/option-def[0]/space "dhcp4"</userinput>
&gt; <userinput>config set Dhcp4/option-def[0]/encapsulate ""</userinput>
&gt; <userinput>config commit</userinput></screen>
      The "false" value of the "array" parameter determines that the option
      does NOT comprise an array of "uint32" values but rather a single value.
      Two other parameters have been left blank: "record-types" and "encapsulate".
      The former specifies the comma separated list of option data fields if the
      option comprises a record of data fields. The "record-fields" value should
      be non-empty if the "type" is set to "record". Otherwise it must be left
      blank. The latter parameter specifies the name of the option space being
      encapsulated by the particular option. If the particular option does not
      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>
&gt; <userinput>config add Dhcp4/option-data</userinput>
&gt; <userinput>config set Dhcp4/option-data[0]/name "foo"</userinput>
&gt; <userinput>config set Dhcp4/option-data[0]/code 222</userinput>
&gt; <userinput>config set Dhcp4/option-data[0]/space "dhcp4"</userinput>
&gt; <userinput>config set Dhcp4/option-data[0]/csv-format true</userinput>
&gt; <userinput>config set Dhcp4/option-data[0]/data "12345"</userinput>
&gt; <userinput>config commit</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.
      </para>
      <para>Assume we want to define a new option that will consist of
      an IPv4 address, followed by 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>
&gt; <userinput>config add Dhcp4/option-def</userinput>
&gt; <userinput>config set Dhcp4/option-def[0]/name "bar"</userinput>
&gt; <userinput>config set Dhcp4/option-def[0]/code 223</userinput>
&gt; <userinput>config set Dhcp4/option-def[0]/space "dhcp4"</userinput>
&gt; <userinput>config set Dhcp4/option-def[0]/type "record"</userinput>
&gt; <userinput>config set Dhcp4/option-def[0]/array false</userinput>
&gt; <userinput>config set Dhcp4/option-def[0]/record-types "ipv4-address, uint16, boolean, string"</userinput>
&gt; <userinput>config set Dhcp4/option-def[0]/encapsulate ""</userinput>
</screen>
      The "type" is set to "record" to indicate that the option contains
      multiple values of different types.  These types are given as a comma-separated
      list in the "record-types" field and should be those listed in <xref linkend="dhcp-types"/>.
      </para>
      <para>
      The values of the option are set as follows:
<screen>
&gt; <userinput>config add Dhcp4/option-data</userinput>
&gt; <userinput>config set Dhcp4/option-data[0]/name "bar"</userinput>
&gt; <userinput>config set Dhcp4/option-data[0]/space "dhcp4"</userinput>
&gt; <userinput>config set Dhcp4/option-data[0]/code 223</userinput>
&gt; <userinput>config set Dhcp4/option-data[0]/csv-format true</userinput>
&gt; <userinput>config set Dhcp4/option-data[0]/data "192.0.2.100, 123, true, Hello World"</userinput>
&gt; <userinput>config commit</userinput></screen>
      "csv-format" is set "true" to indicate that the "data" field comprises a command-separated
      list of values.  The values in the "data" must correspond to the types set in
      the "record-types" field of the option definition.
     </para>
     <note>
       <para>
         It is recommended that boolean values are specified using "true" and "false"
         strings. This helps to prevent errors when typing multiple comma separated
         values, as it make it easier to identify the type of the value being typed,
         and compare it with the order of data fields. Nevertheless, it is possible
         to use integer values: "1" and "0", instead of "true" and "false"
         accordingly. If other integer value is specified, the configuration is
         rejected.
       </para>
     </note>
    </section>

    <section id="dhcp4-vendor-opts">
      <title>DHCPv4 vendor specific options</title>
      <para>
      Currently there are three option spaces defined: dhcp4 (to
      be used in DHCPv4 daemon) and dhcp6 (for the DHCPv6 daemon); there
      is also vendor-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>
&gt; <userinput>config add Dhcp4/option-def</userinput>
&gt; <userinput>config set Dhcp4/option-def[0]/name "foo"</userinput>
&gt; <userinput>config set Dhcp4/option-def[0]/code 1</userinput>
&gt; <userinput>config set Dhcp4/option-def[0]/space "vendor-encapsulated-options-space"</userinput>
&gt; <userinput>config set Dhcp4/option-def[0]/type "record"</userinput>
&gt; <userinput>config set Dhcp4/option-def[0]/array false</userinput>
&gt; <userinput>config set Dhcp4/option-def[0]/record-types "ipv4-address, uint16, string"</userinput>
&gt; <userinput>config set Dhcp4/option-def[0]/encapsulates ""</userinput>
&gt; <userinput>config commit</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>
&gt; <userinput>config add Dhcp4/option-data</userinput>
&gt; <userinput>config set Dhcp4/option-data[0]/name "foo"</userinput>
&gt; <userinput>config set Dhcp4/option-data[0]/space "vendor-encapsulated-options-space"</userinput>
&gt; <userinput>config set Dhcp4/option-data[0]/code 1</userinput>
&gt; <userinput>config set Dhcp4/option-data[0]/csv-format true</userinput>
&gt; <userinput>config set Dhcp4/option-data[0]/data "192.0.2.3, 123, Hello World"</userinput>
&gt; <userinput>config commit</userinput></screen>
    We also set up a dummy value for vendor-opts, 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>
&gt; <userinput>config add Dhcp4/option-data</userinput>
&gt; <userinput>config set Dhcp4/option-data[1]/name "vendor-encapsulated-options"</userinput>
&gt; <userinput>config set Dhcp4/option-data[1]/space "dhcp4"</userinput>
&gt; <userinput>config set Dhcp4/option-data[1]/code 43</userinput>
&gt; <userinput>config set Dhcp4/option-data[1]/csv-format false</userinput>
&gt; <userinput>config set Dhcp4/option-data[1]/data ""</userinput>
&gt; <userinput>config commit</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. Thanks to being in the separate space,
      sub-option codes will have a separate numbering scheme and may
      overlap with 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>
&gt; <userinput>config add Dhcp4/option-def</userinput>
&gt; <userinput>config set Dhcp4/option-def[0]/name "subopt1"</userinput>
&gt; <userinput>config set Dhcp4/option-def[0]/code 1</userinput>
&gt; <userinput>config set Dhcp4/option-def[0]/space "isc"</userinput>
&gt; <userinput>config set Dhcp4/option-def[0]/type "ipv4-address"</userinput>
&gt; <userinput>config set Dhcp4/option-def[0]/record-types ""</userinput>
&gt; <userinput>config set Dhcp4/option-def[0]/array false</userinput>
&gt; <userinput>config set Dhcp4/option-def[0]/encapsulate ""</userinput>
&gt; <userinput>config commit</userinput>

&gt; <userinput>config add Dhcp4/option-def</userinput>
&gt; <userinput>config set Dhcp4/option-def[1]/name "subopt2"</userinput>
&gt; <userinput>config set Dhcp4/option-def[1]/code 2</userinput>
&gt; <userinput>config set Dhcp4/option-def[1]/space "isc"</userinput>
&gt; <userinput>config set Dhcp4/option-def[1]/type "string"</userinput>
&gt; <userinput>config set Dhcp4/option-def[1]/record-types ""</userinput>
&gt; <userinput>config set Dhcp4/option-def[1]/array false</userinput>
&gt; <userinput>config set Dhcp4/option-def[1]/encapsulate ""</userinput>
&gt; <userinput>config commit</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>
&gt; <userinput>add Dhcp4/option-def</userinput>
&gt; <userinput>set Dhcp4/option-def[2]/name "container"</userinput>
&gt; <userinput>set Dhcp4/option-def[2]/code 222</userinput>
&gt; <userinput>set Dhcp4/option-def[2]/space "dhcp4"</userinput>
&gt; <userinput>set Dhcp4/option-def[2]/type "empty"</userinput>
&gt; <userinput>set Dhcp4/option-def[2]/array false</userinput>
&gt; <userinput>set Dhcp4/option-def[2]/record-types ""</userinput>
&gt; <userinput>set Dhcp4/option-def[2]/encapsulate "isc"</userinput>
&gt; <userinput>commit</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>
&gt; <userinput>config add Dhcp4/option-data</userinput>
&gt; <userinput>config set Dhcp4/option-data[0]/name "subopt1"</userinput>
&gt; <userinput>config set Dhcp4/option-data[0]/space "isc"</userinput>
&gt; <userinput>config set Dhcp4/option-data[0]/code 1</userinput>
&gt; <userinput>config set Dhcp4/option-data[0]/csv-format true</userinput>
&gt; <userinput>config set Dhcp4/option-data[0]/data "192.0.2.3"</userinput>
&gt; <userinput>config commit</userinput>
<userinput></userinput>
&gt; <userinput>config add Dhcp4/option-data</userinput>
&gt; <userinput>config set Dhcp4/option-data[1]/name "subopt2"</userinput>
&gt; <userinput>config set Dhcp4/option-data[1]/space "isc"</userinput>
&gt; <userinput>config set Dhcp4/option-data[1]/code 2</userinput>
&gt; <userinput>config set Dhcp4/option-data[1]/csv-format true</userinput>
&gt; <userinput>config set Dhcp4/option-data[1]/data "Hello world"</userinput>
&gt; <userinput>config commit</userinput>
<userinput></userinput>
&gt; <userinput>config add Dhcp4/option-data</userinput>
&gt; <userinput>config set Dhcp4/option-data[2]/name "container"</userinput>
&gt; <userinput>config set Dhcp4/option-data[2]/space "dhcp4"</userinput>
&gt; <userinput>config set Dhcp4/option-data[2]/code 222</userinput>
&gt; <userinput>config set Dhcp4/option-data[2]/csv-format true</userinput>
&gt; <userinput>config set Dhcp4/option-data[2]/data ""</userinput>
&gt; <userinput>config commit</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 BIND 10 DHCP, 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 - which should be a valid integer enclosed in quotes,
    e.g. "123" - would then be assigned to the uint16 field in the "container" option.
    </para>
    </section>

    <section id="dhcp4-client-classifier">
      <title>Client Classification in DHCPv4</title>
      <note>
      <para>
        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. 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
      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 incoming packet has vendor class identifier
      option (60). If it has, content of that option is prepended with
      &quot;VENDOR_CLASS_&quot; then 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
      behavior of almost any part of the DHCP message processing, including assigning
      leases from different pools, assigning different option (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 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
        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: cable modem itself, which should be handled a lease
        from subnet A and all other devices behind modems that should get a lease
        from subnet B. That segregation is essential to prevent overly curious
        users from playing with their cable modems. For details on how to set up
        class restrictions on subnets, see <xref linkend="dhcp4-subnet-class"/>.
      </para>

    </section>

    <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>
&gt; <userinput>config add Dhcp4/subnet4</userinput>
&gt; <userinput>config set Dhcp4/subnet4[0]/subnet "192.0.2.0/24"</userinput>
&gt; <userinput>config set Dhcp4/subnet4[0]/pool [ "192.0.2.10 - 192.0.2.20" ]</userinput>
&gt; <userinput>config set Dhcp4/subnet4[0]/client-class "VENDOR_CLASS_docsis3.0"</userinput>
&gt; <userinput>config commit</userinput></screen>
      </para>

      <para>
        Care should be taken with client classification as it is easy to prevent
        clients that do not meet class criteria to be denied any service altogether.
      </para>
    </section>
    <section id="dhcp4-ddns-config">
      <title>Configuring DHCPv4 for DDNS</title>
      <para>
      As mentioned earlier, b10-dhcp4 can be configured to generate requests to the
      DHCP-DDNS server 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 "dhcp-ddns" section of the b10-dhcp4 server
      configuration. The default values for this section appears as follows:
<screen>
&gt; <userinput>config show Dhcp4/dhcp-ddns</userinput>
Dhcp4/dhcp-ddns/enable-updates	true	boolean
Dhcp4/dhcp-ddns/server-ip	"127.0.0.1"	string
Dhcp4/dhcp-ddns/server-port	53001	integer
Dhcp4/dhcp-ddns/sender-ip	""	string
Dhcp4/dhcp-ddns/sender-port	0	integer
Dhcp4/dhcp-ddns/max-queue-size	1024	integer
Dhcp4/dhcp-ddns/ncr-protocol	"UDP"	string
Dhcp4/dhcp-ddns/ncr-format	"JSON"	string
Dhcp4/dhcp-ddns/override-no-update	false	boolean
Dhcp4/dhcp-ddns/override-client-update	false	boolean
Dhcp4/dhcp-ddns/replace-client-name	false	boolean
Dhcp4/dhcp-ddns/generated-prefix	"myhost"	string
Dhcp4/dhcp-ddns/qualifying-suffix	"example.com"	string
</screen>
      </para>
      <para>
      The "enable-updates" parameter determines whether or not b10-dhcp4 will
      generate NCRs.  By default, this value is false hence DDNS updates are
      disabled.  To enable DDNS updates set this value to true:
      </para>
<screen>
&gt; <userinput>config set Dhcp4/dhcp-ddns/enable-updates true</userinput>
&gt; <userinput>config commit</userinput>
</screen>
      <section id="dhcpv4-d2-io-config">
      <title>DHCP-DDNS Server Connectivity</title>
      <para>
      In order for NCRs to reach the D2 server, b10-dhcp4 must be able
      to communicate with it.  b10-dhcp4 uses the following configuration
      parameters to control how it communications with D2:
      <orderedlist>
      <listitem><para>
      server-ip - 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.
      </para></listitem>
      <listitem><para>
      server-port - port on which D2 listens for requests.  The default value
      is 53001.
      </para></listitem>
      <listitem><para>
      sender-ip - IP address which b10-dhcp4 should use to send requests to D2.
      The default value is blank which instructs b10-dhcp4 to select a suitable
      address.
      </para></listitem>
      <listitem><para>
      sender-port - port which b10-dhcp4 should use to send requests to D2. The
      default value of 0 instructs b10-dhcp4 to select suitable port.
      </para></listitem>
      <listitem><para>
      ncr-format - Socket protocol use when sending requests to D2.  Currently
      only UDP is supported.  TCP may be available in an upcoming release.
      </para></listitem>
      <listitem><para>
      ncr-protocol - Packet format to use when sending requests to D2.
      Currently only JSON format is supported.  Other formats may be available
      in future releases.
      </para></listitem>
      <listitem><para>
      max-queue-size - 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 intent is allow the b10-dhcp4 server to
      continue lease operations.  The default value is 1024.
      </para></listitem>
      </orderedlist>
      By default, D2 is assumed to running on the same machine as b10-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 altered accordingly. For example, if D2 has been
      configured to listen on 198.162.1.10 port 900, the following commands
      would be required:
<screen>
&gt; <userinput>config set Dhcp4/dhcp-ddns/server-ip "198.162.1.10"</userinput>
&gt; <userinput>config set Dhcp4/dhcp-ddns/server-port 900</userinput>
&gt; <userinput>config commit</userinput>
</screen>
      </para>
      </section>
      <section id="dhcpv4-d2-rules-config">
      <title>When does the b10-dhcp4 server generate DDNS requests?</title>
      b10-dhcp4 follows the behavior prescribed for DHCP servers in RFC 4702.
      It is important to keep in mind that b10-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 the purview of D2 (<xref linkend="dhcp-ddns-server"/>).
      <para>
      This section describes when b10-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, b10-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, b10-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 b10-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, b10-dhcp4 will honor
      the client's wishes and generate a DDNS request to D2 to update only
      reverse DNS data.  The parameter, "override-client-update", can be used
      to instruct the server to override client delegation requests.  When
      this parameter is true, b10-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
      RFC 4702. If such a combination is received from the client, the packet
      will be dropped by the b10-dhcp4.)
      </para>
      <para>
      To override client delegation, issue the following commands:
      </para>
<screen>
&gt; <userinput>config set Dhcp4/dhcp-ddns/override-client-update true</userinput>
&gt; <userinput>config commit</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, "override-no-update",
      can be used to instruct the server to disregard the client's wishes. When
      this parameter is true, b10-dhcp4 will generate DDNS update request to D2
      even if the client requests no updates be done.  The N-S-O flags in the
      server's response to the client will be 0-1-1.
      </para>
      <para>
      To override client delegation, issue the following commands:
      </para>
<screen>
&gt; <userinput>config set Dhcp4/dhcp-ddns/override-no-update true</userinput>
&gt; <userinput>config commit</userinput>
</screen>
      <para>
      b10-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>b10-dhcp4 name generation for DDNS update requests</title>
      Each NameChangeRequest must of course include the fully qualified domain
      name whose DNS entries are to be affected.  b10-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>
      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 candiate 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 b10-dhcp4 to always generate the FQDN for a client, set the
      parameter "replace-client-name" to true as follows:
      </para>
<screen>
&gt; <userinput>config set Dhcp4/dhcp-ddns/replace-client-name true</userinput>
&gt; <userinput>config commit</userinput>
</screen>
      <para>
      The prefix used in the generation of a FQDN is specified by the
      "generated-prefix" parameter.  The default value is "myhost".  To alter
      its value simply set it to the desired string:
      </para>
<screen>
&gt; <userinput>config set Dhcp4/dhcp-ddns/generated-prefix "another.host"</userinput>
&gt; <userinput>config commit</userinput>
</screen>
      <para>
      The suffix used when generating a FQDN or when qualifying a partial
      name is specified by the "qualifying-suffix" parameter.  The default
      value is "example.com".  To alter its value simply set it to the desired
      string:
      </para>
<screen>
&gt; <userinput>config set Dhcp4/dhcp-ddns/generated-prefix "our.net"</userinput>
&gt; <userinput>config commit</userinput>
</screen>
      </section>
      <para>
      When generating a name, b10-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 lease address is 172.16.1.10 and
      assuming default values for generated-prefix and qualifying-suffix, the
      generated FQDN would be:
      </para>
      <para>
        myhost-172-16-1-10.example.com.
      </para>
    </section>

  </section> <!-- end of configuring b10-dhcp4 server section with many subsections -->

    <section id="dhcp4-serverid">
      <title>Server Identifier in DHCPv4</title>
      <para>
        The DHCPv4 protocol uses a "server identifier" for clients to be able
        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-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 siaddr field
      in the DHCPv4 packet for that purpose. That specific field can be configured
      using next-server directive. It is possible to define it in global scope or
      for a given subnet only. If both are defined, subnet value takes precedence.
      The value in subnet can be set to 0.0.0.0, which means that next-server should
      not be sent. It may also be set to empty string, which means the same as if
      it was not defined at all - use global value.
      </para>

<screen>
&gt; <userinput>config add Dhcp4/next-server</userinput>
&gt; <userinput>config set Dhcp4/next-server "192.0.2.123"</userinput>
&gt; <userinput>config commit</userinput>
<userinput></userinput>
&gt; <userinput>config add Dhcp4/subnet[0]/next-server</userinput>
&gt; <userinput>config set Dhcp4/subnet[0]/next-server "192.0.2.234"</userinput>
&gt; <userinput>config commit</userinput>
</screen>

    </section>

    <section id="dhcp4-echo-client-id">
      <title>Echoing client-id (RFC6842)</title>
      <para>Original DHCPv4 spec (RFC2131) 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 RFC6842 for details. That
      behavior has changed with the publication of RFC6842 which
      updated RFC2131. That update now states that the server must
      send client-id if client sent it. That is the default behaviour
      that Kea offers. However, in some cases older devices that do
      not support RFC6842 may refuse to accept responses that include
      client-id option. To enable backward compatibility, an optional
      configuration parameter has been introduced. To configure it,
      use the following commands:</para>

<screen>
&gt; <userinput>config add Dhcp4/echo-client-id</userinput>
&gt; <userinput>config set Dhcp4/echo-client-id False</userinput>
&gt; <userinput>config commit</userinput>
</screen>

    </section>

    <section id="dhcp4-subnet-selection">
      <title>How DHCPv4 server selects subnet for a 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 of 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 192.0.2.3
        IPv4 address, the server will only process messages received through
        this interface from the 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 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 subnet is selected for the client,
        depending on the classes that the client belongs to.</para>
      </note>
    </section>

    <section id="dhcp4-relay-override">
      <title>Using 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 that
        the server will assign addresses from. In such typical case, the
        server is able to use IPv4 address inserted by the relay (in GIADDR
        field of the DHCPv4 packet) to select appropriate subnet.
      </para>
      <para>
        However, that is not always the case. In certain uncommon, but
        valid deployments, the relay address may not match the subnet. This
        usually means that there is more than one subnet allocated for a given
        link. 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 case, the DHCPv4 server needs
        additional information (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 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>
&gt; <userinput>config add Dhcp4/subnet4</userinput>
&gt; <userinput>config set Dhcp4/subnet4[0]/subnet "192.0.2.0/24"</userinput>
&gt; <userinput>config set Dhcp4/subnet4[0]/pool [ "192.0.2.10 - 192.0.2.20" ]</userinput>
&gt; <userinput>config set Dhcp4/subnet4[0]/relay/ip-address "10.0.0.1"</userinput>
&gt; <userinput>config commit</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 an uses address
          10.1.1.1. The following configuration can serve that configuration:
        <screen>
&gt; <userinput>config add Dhcp4/subnet4</userinput>
&gt; <userinput>config set Dhcp4/subnet4[0]/subnet "10.1.1.0/24"</userinput>
&gt; <userinput>config set Dhcp4/subnet4[0]/pool [ "10.1.1.2 - 10.1.1.20" ]</userinput>
&gt; <userinput>config set Dhcp4/subnet4[0]/client-class "docsis3.0"</userinput>
&gt; <userinput>config set Dhcp4/subnet4[0]/relay/ip-address "10.1.1.1"</userinput>
&gt; <userinput>config add Dhcp4/subnet4</userinput>
&gt; <userinput>config set Dhcp4/subnet4[1]/subnet "192.0.2.0/24"</userinput>
&gt; <userinput>config set Dhcp4/subnet4[1]/pool [ "192.0.2.10 - 192.0.2.20" ]</userinput>
&gt; <userinput>config set Dhcp4/subnet4[1]/relay/ip-address "10.1.1.1"</userinput>
&gt; <userinput>config commit</userinput></screen>
      </para>
      </section>

    <section id="dhcp4-std">
      <title>Supported Standards</title>
      <para>The following standards and draft standards are currently
      supported:</para>
      <itemizedlist>
          <listitem>
            <simpara><ulink url="http://tools.ietf.org/html/rfc2131">RFC 2131</ulink>: Supported messages are DISCOVER, OFFER,
            REQUEST, RELEASE, ACK, and NAK.</simpara>
          </listitem>
          <listitem>
            <simpara><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><ulink url="http://tools.ietf.org/html/rfc3046">RFC 3046</ulink>:
            Relay Agent Information option is supported.</simpara>
          </listitem>
          <listitem>
            <simpara><ulink url="http://tools.ietf.org/html/rfc3925">RFC 3925</ulink>:
            Vendor-Identifying Vendor Class and Vendor-Identifying Vendor-Specific
            Information option are supported.</simpara>
          </listitem>
          <listitem>
            <simpara><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.</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>
              BOOTP (<ulink url="http://tools.ietf.org/html/rfc951">RFC 951</ulink>)
              is not supported.
            </simpara>
          </listitem>
          <listitem>
            <simpara>Raw sockets operation is working on Linux
            only. See <xref linkend="iface-detect"/> for details.</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.
            </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>