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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd" [
<!ENTITY mdash  "&#x2014;" >

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

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

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        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:
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            <command>-c <replaceable>file</replaceable></command> -
            specifies the configuration file. This is the only mandatory
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            <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
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            file are ignored and "debug" severity and the maximum debuglevel
            (99) are assumed. The flag is convenient, for temporarily
            switching the server into maximum verbosity, e.g. when
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            <command>-p <replaceable>port</replaceable></command> -
            specifies UDP port on which the server will listen. This is only
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            useful during testing, as a DHCPv4 server listening on
            ports other than the standard ones will not be able to
            handle regular DHCPv4 queries.</simpara>
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            <command>-t <replaceable>file</replaceable></command> -
            specifies the configuration file to be tested. Kea-dhcp4
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            will attempt to load it, and will conduct sanity
            checks. Note that certain checks are possible only while
            running the actual server. The actual status is reported
            with exit code (0 = configuration looks ok, 1 = error
            encountered). Kea will print out log messages to standard
            output and error to standard error when testing
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              <command>-v</command> - prints out the Kea version and exits.
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              <command>-V</command> - prints out the Kea extended version with
              additional parameters and exits. The listing includes the versions
              of the libraries dynamically linked to Kea.
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              <command>-W</command> - prints out the Kea configuration report
              and exits. The report is a copy of the
              <filename>config.report</filename> file produced by
              <userinput>./configure</userinput>: it is embedded in the
              executable binary.
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        The <filename>config.report</filename> may also be accessed more
        directly.  The following command may be used to extract this
        information.  The binary <userinput>path</userinput> may be found
        in the install directory or in the <filename>.libs</filename>
        subdirectory in the source tree. For example

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

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

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

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        The server can be stopped using the <command>kill</command> command.
        When running in a console, the server can also be shut down by
        pressing ctrl-c. It detects the key combination and shuts
        down gracefully.
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    <section id="dhcp4-configuration">
      <title>DHCPv4 Server Configuration</title>
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        This section explains how to configure the DHCPv4 server using the
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        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:
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# DHCPv4 configuration starts in this line
"Dhcp4": {

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

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# 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": {
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        "type": "memfile",
        "persist": true,
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        "name": "/var/kea/dhcp4.leases"
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# Finally, we list the subnets from which we will be leasing addresses.
    "subnet4": [
            "subnet": "",
            "pools": [
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                     "pool": " -"
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# DHCPv4 configuration ends with the next line
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} </screen>

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

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

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<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
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<command>DhcpDdns</command>, but we omit them now for clarity. The Dhcp4
configuration starts with the <command>"Dhcp4": {</command> line
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and ends with the corresponding closing brace (in the above example,
the brace after the last comment).  Everything defined between those
lines is considered to be the 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
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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
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reasons to keep multiple copies of the same parameter in your configuration

<para>Moving onto the DHCPv4 configuration elements, the first few elements
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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
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use it for 4000 seconds. (Note that integer numbers are specified as is,
without any quotes around them.) <command>renew-timer</command> and
<command>rebind-timer</command> are values (also in seconds) that
define T1 and T2 timers that govern when the client will begin the renewal and
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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>

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<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:
"interfaces-config": {
    "interfaces": [ "eth0", "eth1" ]

<para>The next couple of lines define the lease database, the place where the server
stores its lease information. This particular example tells the server to use
<command>memfile</command>, which is the simplest (and fastest) database
backend. It uses an in-memory database and stores leases on disk in a CSV
file. This is a very simple configuration. Usually the lease database configuration
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is more extensive and contains additional parameters.  Note that
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
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scope is closed with a closing brace. As more parameters for the Dhcp4 definition
follow, a trailing comma is present.</para>
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<para>Finally, we need to define a list of IPv4 subnets. This is the
most important DHCPv4 configuration structure as the server uses that
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information to process clients' requests. It defines all subnets from
which the server is expected to receive DHCP requests. The subnets are
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specified with the <command>subnet4</command> parameter.  It is a list,
so it starts and ends with square brackets.  Each subnet definition in
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the list has several attributes associated with it, so it is a structure
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>
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<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:
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"subnet4": [
        "pools": [ { "pool":  " -" } ],
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        "subnet": ""
        "pools": [ { "pool": " -" } ],
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        "subnet": ""
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        "pools": [ { "pool": " -" } ],
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        "subnet": ""
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Note that indentation is optional and is used for aesthetic purposes only.
In some cases in may be preferable to use more compact notation.
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<para>After all the parameters have been specified, we have two contexts open:
global and Dhcp4, hence we need two closing curly brackets to close them.
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In a real life configuration file there most likely would be additional
components defined such as Logging or DhcpDdns, so the closing brace would
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be followed by a comma and another object definition.</para>

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  <title>Lease Storage</title>
  <para>All leases issued by the server are stored in the lease database.
  Currently there are four database backends available:  memfile (which is the
  default backend), MySQL, PostgreSQL and Cassandra.</para>
  <title>Memfile - Basic Storage for Leases</title>
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  <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
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  smaller deployments though, the server will store lease information in a CSV file rather
  than a database. As well as requiring less administration, an
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  advantage of using a file for storage is that it
  eliminates a dependency on third-party database software.</para>
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  <para>The configuration of the file backend (Memfile) is controlled through
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  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
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  be used as the storage. The following list gives additional, optional,
  parameters that can be used to configure the Memfile backend.
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      <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><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"

      <simpara><command>lfc-interval</command>: specifies the interval in seconds, at
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      which the server will perform a lease file cleanup (LFC).  This
      removes redundant (historical) information from the lease file
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      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
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      <command>lfc-interval</command> is <userinput>3600</userinput>. A value of 0
      disables the LFC.</simpara>
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  <para>An example configuration of the Memfile backend is presented below:
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"Dhcp4": {
    "lease-database": {
        <userinput>"type": "memfile"</userinput>,
        <userinput>"persist": true</userinput>,
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        <userinput>"name": "/tmp/kea-leases4.csv",</userinput>
        <userinput>"lfc-interval": 1800</userinput>
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    This configuration selects the <filename>/tmp/kea-leases4.csv</filename> as
    the storage for lease information and enables persistence (writing lease updates
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    to this file). It also configures the backend perform the periodic cleanup
    of the lease files, executed every 30 minutes.

  <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
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  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,
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  the server does not update the existing client's lease in the file, as it would
  potentially require rewriting the entire file. Instead, it simply appends the new lease
  information to the end of the file: the previous lease entries for the
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  client are not removed. When the server loads leases from the lease file, e.g.
  at the server startup, it assumes that the latest lease entry for the client
  is the valid one. The previous entries are discarded. This means that the
  server can re-construct the accurate information about the leases even though
  there may be many lease entries for each client. However, storing many entries
  for each client results in bloated lease file and impairs the performance of
  the server's startup and reconfiguration as it needs to process a larger number
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  of lease entries.

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

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  <para>Lease file cleanup is performed by a separate process (in background) to avoid
  a performance impact on the server process. In order to avoid the conflicts
  between two processes both using the same lease files, the LFC process
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  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"/>.
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<section id="database-configuration4">
  <title>Lease Database Configuration</title>
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    <para>Lease database access information must be configured for the DHCPv4 server,
    even if it has already been configured for the DHCPv6 server. The servers
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    store their information independently, so each server can use a separate
    database or both servers can use the same database.</para>

  <para>Lease database configuration is controlled through the Dhcp4/lease-database
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  parameters. The type of the database must be set to "memfile", "mysql", "postgresql" or
  "cql", e.g.
"Dhcp4": { "lease-database": { <userinput>"type": "mysql"</userinput>, ... }, ... }
  Next, the name of the database to hold the leases must be set: this is the
  name used when the database was created
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  (see <xref linkend="mysql-database-create"/>,
  <xref linkend="pgsql-database-create"/> or
  <xref linkend="cql-database-create"/>).
"Dhcp4": { "lease-database": { <userinput>"name": "<replaceable>database-name</replaceable>" </userinput>, ... }, ... }
  If the database is located on a different system to the DHCPv4 server, the
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  database host name must also be specified. (It should be noted that this
  configuration may have a severe impact on server performance.):
"Dhcp4": { "lease-database": { <userinput>"host": <replaceable>remote-host-name</replaceable></userinput>, ... }, ... }
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  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:
"Dhcp4": { "lease-database": { <userinput>"host" : ""</userinput>, ... }, ... }
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  Should the database use a port different than default, it may be
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  specified as well:
"Dhcp4": { "lease-database": { <userinput>"port" : 12345</userinput>, ... }, ... }
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  Should the database be located on a different system, you may need to specify a longer interval
  for the connection timeout:
"Dhcp4": { "lease-database": { <userinput>"connect-timeout" : <replaceable>timeout-in-seconds</replaceable></userinput>, ... }, ... }
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The default value of five seconds should be more than adequate for local connections.
If a timeout is given though, it should be an integer greater than zero.
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    Note that host parameter is used by MySQL and PostgreSQL
    backends. Cassandra has a concept of contact points that could be
    used to contact the cluster, instead of a single IP or
    hostname. It takes a list of comma separated IP addresses. This may be specified as:
"Dhcp4": { "lease-database": { <userinput>"contact-points" : ","</userinput>, ... }, ... }

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

<section id="hosts4-storage">
  <title>Hosts Storage</title>
    <para>Kea is also able to store information about host reservations in the
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    database. The hosts database configuration uses the same syntax as the lease
    database. In fact, a Kea server opens independent connections for each
    purpose, be it lease or hosts information. This arrangement gives the most
    flexibility. Kea can be used to keep leases and host reservations
    separately, but can also point to the same database. Currently the
    supported hosts database types are MySQL and PostgreSQL. The Cassandra
    backend does not support host reservations yet.</para>

    <para>Please note that usage of hosts storage is optional. A user can define
    all host reservations in the configuration file. That is the recommended way
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    if the number of reservations is small. However, when the number of
    reservations grows it's more convenient to use host storage. Please note
    that both storage methods (configuration file and one of the supported databases)
    can be used together. If hosts are defined in both places, the definitions
    from the configuration file are checked first and external storage is checked
    later, if necessary.</para>
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<section id="hosts-database-configuration4">
  <title>DHCPv4 Hosts Database Configuration</title>
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  <para>Hosts database configuration is controlled through the Dhcp4/hosts-database
  parameters. If enabled, the type of the database must be set to "mysql" or
  "postgresql". Other hosts backends may be added in later versions of Kea.
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"Dhcp4": { "hosts-database": { <userinput>"type": "mysql"</userinput>, ... }, ... }
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  Next, the name of the database to hold the reservations must be set: this is the
  name used when the lease database was created (see <xref linkend="supported-databases"/>
  for instructions how to setup the desired database type).
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"Dhcp4": { "hosts-database": { <userinput>"name": "<replaceable>database-name</replaceable>" </userinput>, ... }, ... }
  If the database is located on a different system than the DHCPv4 server, the
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  database host name must also be specified. (Again it should be noted that this
  configuration may have a severe impact on server performance.):
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"Dhcp4": { "hosts-database": { <userinput>"host": <replaceable>remote-host-name</replaceable></userinput>, ... }, ... }
  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:
"Dhcp4": { "hosts-database": { <userinput>"host" : ""</userinput>, ... }, ... }
  Should the database use a port different than default, it may be
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  specified as well:
"Dhcp4": { "hosts-database": { <userinput>"port" : 12345</userinput>, ... }, ... }

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  <para>Finally, the credentials of the account under which the server will
  access the database should be set:
"Dhcp4": { "hosts-database": { <userinput>"user": "<replaceable>user-name</replaceable>"</userinput>,
                               <userinput>"password": "<replaceable>password</replaceable>"</userinput>,
                              ... },
           ... }
  If there is no password to the account, set the password to the empty string
  "". (This is also the default.)</para>
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<section id="read-only-database-configuration4">
<title>Using Read-Only Databases for Host Reservations</title>
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In some deployments the database user whose name is specified in the database backend
configuration may not have write privileges to the database. This is often
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required by the policy within a given network to secure the data from being
unintentionally modified. In many cases administrators have inventory databases
deployed, which contain substantially more information about the hosts than
static reservations assigned to them. The inventory database can be used to create
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a view of a Kea hosts database and such view is often read only.
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Kea host database backends operate with an implicit configuration to both
read from and write to the database. If the database user does not have
write access to the host database, the backend will fail to start and the
server will refuse to start (or reconfigure). However, if access to a read
only host database is required for retrieving reservations for clients
and/or assign specific addresses and options, it is possible to explicitly
configure Kea to start in "read-only" mode. This is controlled by the
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<command>readonly</command> boolean parameter as follows:
"Dhcp4": { "hosts-database": { <userinput>"readonly": true</userinput>, ... }, ... }
Setting this parameter to <userinput>false</userinput> would configure the
database backend to operate in "read-write" mode, which is also a default
configuration if the parameter is not specified.
<note><para>The <command>readonly</command> parameter is currently only supported
for MySQL and PostgreSQL databases.</para></note>

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<section id="dhcp4-interface-configuration">
  <title>Interface Configuration</title>
  <para>The DHCPv4 server has to be configured to listen on specific network
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  interfaces.  The simplest network interface configuration tells the server to
  listen on all available interfaces:
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"Dhcp4": {
    "interfaces-config": {
        "interfaces": [ <userinput>"*"</userinput> ]
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  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:
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"Dhcp4": {
    "interfaces-config": {
        "interfaces": [ <userinput>"eth1", "eth3"</userinput> ]
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  <para>It is possible to use wildcard interface name (asterisk) concurrently
  with explicit interface names:
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"Dhcp4": {
    "interfaces-config": {
        "interfaces": [ <userinput>"eth1", "eth3", "*"</userinput> ]
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>Some deployments of DHCP servers require that the servers listen
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  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:
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"Dhcp4": {
    "interfaces-config": {
        "interfaces": [ <userinput>"eth1/", "eth3/"</userinput> ]

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

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

  <para>Kea supports responding to directly connected clients which don't have
  an address configured. This requires that the server
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  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
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  unicast packets from the relay agents, the
  DHCP server should be configured to utilize IP/UDP datagram sockets instead of raw sockets.
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  The following configuration demonstrates how this can be achieved:

"Dhcp4": {
    "interfaces-config": {
        "interfaces": [ <userinput>"eth1", "eth3"</userinput> ],
        "dhcp-socket-type": "udp"
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  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
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  default value <userinput>raw</userinput> is used.

  <para>Using UDP sockets automatically disables the reception of broadcast
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  packets from directly connected clients. This effectively means that the
  UDP sockets can be used for relayed traffic only. When using the raw sockets,
  both the traffic from the directly connected clients and the relayed traffic
  will be handled. Caution should be taken when configuring the server to open
  multiple raw sockets on the interface with several IPv4 addresses assigned.
  If the directly connected client sends the message to the broadcast address
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  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.

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    <para>Specifying the value <userinput>raw</userinput> as the socket type,
    doesn't guarantee that the raw sockets will be used! The use of raw sockets
    to handle the traffic from the directly connected clients is currently
    supported on Linux and BSD systems only. If the raw sockets are not
    supported on the particular OS, the server will issue a warning and
    fall back to use IP/UDP sockets.</para>
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  <para>In typical environment the DHCP server is expected to send back a
  response on the same network interface on which the query is received. This is
  the default behavior. However, in some deployments it is desired that the
  outbound (response) packets will be sent as regular traffic and the outbound
  interface will be determined by the routing tables. This kind of asymetric
  traffic is uncommon, but valid. Kea now supports a parameter called
  <command>outbound-interface</command> that controls this behavior. It supports
  two values. The first one, <userinput>same-as-inbound</userinput>, tells Kea
  to send back the response on the same inteface the query packet is received. This
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  is the default behavior. The second one, <userinput>use-routing</userinput>
  tells Kea to send regular UDP packets and let the kernel's routing table to
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  determine most appropriate interface. This only works when
  <command>dhcp-socket-type</command> is  set to <userinput>udp</userinput>.
  An example configuration looks as follows:
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"Dhcp4": {
    "interfaces-config": {
        "interfaces": [ "eth1", "eth3" ],
        "dhcp-socket-type": "udp",
        <userinput>"outbound-interface": "use-routing"</userinput>

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

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

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<section id="dhcpinform-unicast-issues">
  <title>Issues with Unicast Responses to DHCPINFORM</title>
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  <para>The use of UDP sockets has certain benefits in deployments
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  where the server receives only relayed traffic; these benefits are
  mentioned in <xref linkend="dhcp4-interface-configuration"/>. From
  the administrator's perspective it is often desirable to
  configure the system's firewall to filter out the unwanted traffic, and
  the use of UDP sockets facilitates this. However, the administrator must
  also be aware of the implications related to filtering certain types
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  of traffic as it may impair the DHCP server's operation.
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  <para>In this section we are focusing on the case when the server
  receives the DHCPINFORM message from the client via a relay. According
  to <ulink url="http://tools.ietf.org/html/rfc2131">RFC 2131</ulink>,
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  the server should unicast the DHCPACK response to the address carried in
  the "ciaddr" field. When the UDP socket is in use, the DHCP server
  relies on the low level functions of an operating system to build the
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  data link, IP and UDP layers of the outgoing message. Typically, the
  OS will first use ARP to obtain the client's link layer address to be
  inserted into the frame's header, if the address is not cached from
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  a previous transaction that the client had with the server.
  When the ARP exchange is successful, the DHCP message can be unicast
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  to the client, using the  obtained address.
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  <para>Some system administrators block ARP messages in their network,
  which causes issues for the server when it responds to the
  DHCPINFORM messages, because the server is unable to send the
  DHCPACK if the preceding ARP communication fails. Since the OS is
  entirely responsible for the ARP communication and then sending
  the DHCP packet over the wire, the DHCP server has no means to
  determine that the ARP exchange failed and the DHCP response message
  was dropped. Thus, the server does not log any error messages when
  the outgoing DHCP response is dropped. At the same time, all hooks
  pertaining to the packet sending operation will be called, even
  though the message never reaches its destination.

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

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  <para>Server administrators willing to support DHCPINFORM
  messages via relays should not block ARP traffic in their
  networks or should use raw sockets instead of UDP sockets.
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<section id="ipv4-subnet-id">
  <title>IPv4 Subnet Identifier</title>
    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
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    be automatically assigned by the configuration mechanism. The identifiers
    are assigned from 1 and are monotonically increased for each subsequent
    subnet: 1, 2, 3 ....
    If there are multiple subnets configured with auto-generated identifiers and
    one of them is removed, the subnet identifiers may be renumbered. For example:
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    if there are four subnets and the third is removed the last subnet will be assigned
    the identifier that the third subnet had before removal. As a result, the leases
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    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
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    future version of Kea.  However, the only remedy for this issue
    at present is to
    manually specify a unique identifier for each subnet.
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        The following configuration will assign the specified subnet
        identifier to the newly configured subnet:

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"Dhcp4": {
    "subnet4": [
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            "subnet": "",
            <userinput>"id": 1024</userinput>,
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    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
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    <!-- @todo: describe whether database needs to be updated after changing
      id -->

<section id="dhcp4-address-config">
  <title>Configuration of IPv4 Address Pools</title>
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    The main role of a DHCPv4 server is address assignment. For this, the server has to
    be configured with at least one subnet and one pool of dynamic addresses for it to manage.
    For example, assume that the server is connected to a network
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    segment that uses the prefix. The Administrator of that network
    has decided that addresses from range to are going to
    be managed by the Dhcp4 server. Such a configuration can be achieved in the
    following way:
"Dhcp4": {
    <userinput>"subnet4": [
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            "subnet": "",
            "pools": [
                { "pool": " -" }
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    Note that <command>subnet</command> is defined as a simple string, but
    the <command>pools</command> parameter is actually a list of pools: for
    this reason, the pool definition is enclosed in square brackets, even
    though only one range of addresses is specified.</para>

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

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    <para>It is possible to define more than one pool in a subnet: continuing
    the previous example, further assume that should be also be
    managed by the server. It could be written as to Alternatively, it can be expressed more simply as Both formats are supported by Dhcp4 and can be mixed in the
    pool list.  For example, one could define the following pools:
"Dhcp4": {
    "subnet4": [
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            "subnet": "",
            <userinput>"pools": [
                { "pool": "" },
                { "pool": "" }
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    White space in pool definitions is ignored, so spaces before and after the hyphen are optional.
    They can be used to improve readability.
    The number of pools is not limited, but for performance reasons it is recommended to
    use as few as possible.
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    The server may be configured to serve more than one subnet:
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"Dhcp4": {
    "subnet4": [
            "subnet": "",
            "pools": [ { "pool": " -" } ],
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            "subnet": "",
            "pools": [ { "pool": " -" } ],
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            "subnet": "",
            "pools": [ { "pool": " -" } ],
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    When configuring a DHCPv4 server using prefix/length notation, please pay
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    attention to the boundary values. When specifying that the server can use
    a given pool, it will also be able to allocate the first (typically network
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    address) and the last (typically broadcast address) address from that pool.
    In the aforementioned example of pool, both and 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.
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    <section id="dhcp4-std-options">
      <title>Standard DHCPv4 Options</title>
        One of the major features of the DHCPv4 server is to provide configuration
        options to clients. Most of the options are sent by the server only if the
        client explicitly requests them using the Parameter Request List option.
        Those that do not require inclusion in the Parameter Request List
        option are commonly used options, e.g. "Domain Server", and options which
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        require special behavior, e.g. "Client FQDN" is returned to the client
        if the client has included this option in its message to the server.

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