dhcp6-srv.xml 97 KB
Newer Older
1 2 3 4 5 6 7 8 9 10 11 12 13
<?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="dhcp6">
    <title>The DHCPv6 Server</title>

    <section id="dhcp6-start-stop">
      <title>Starting and Stopping the DHCPv6 Server</title>

      <para>
14
      It is recommended that the Kea DHCPv4 server be started and stopped
Jeremy C. Reed's avatar
Jeremy C. Reed committed
15
      using <command>keactrl</command> (described in <xref linkend="keactrl"/>).
16 17
      However, it is also possible to run the server directly: it accepts
      the following command-line switches:
18 19
      </para>

20 21
      <itemizedlist>
          <listitem>
22 23 24 25
            <simpara>
            <command>-c <replaceable>file</replaceable></command> -
            specifies the configuration file. This is the only mandatory
            switch.</simpara>
26 27
          </listitem>
          <listitem>
28
            <simpara>
29
            <command>-d</command> - specifies whether the server
30
            logging should be switched to verbose mode. In verbose mode,
31
            the logging severity and debuglevel specified in the configuration
32 33 34 35
            file are ignored and "debug" severity and the maximum debuglevel
            (99) are assumed. The flag is convenient, for temporarily
            switching the server into maximum verbosity, e.g. when
            debugging.</simpara>
36 37
          </listitem>
          <listitem>
38 39
            <simpara>
            <command>-p <replaceable>port</replaceable></command> -
40 41
            specifies UDP port on which the server will listen. This is only
            useful during testing, as a DHCPv6 server listening on
42 43
            ports other than default DHCPv6 ports will not be able to
            handle regular DHCPv6 queries.</simpara>
44
          </listitem>
Tomek Mrugalski's avatar
Tomek Mrugalski committed
45 46
          <listitem>
            <simpara>
Tomek Mrugalski's avatar
Tomek Mrugalski committed
47
              <command>-v</command> - prints out Kea version and exits.
Tomek Mrugalski's avatar
Tomek Mrugalski committed
48 49 50 51 52
            </simpara>
          </listitem>
          <listitem>
            <simpara>
              <command>-V</command> - prints out Kea extended version with
Tomek Mrugalski's avatar
Tomek Mrugalski committed
53
              additional parameters and exits.
Tomek Mrugalski's avatar
Tomek Mrugalski committed
54 55
            </simpara>
          </listitem>
56 57
      </itemizedlist>

58
      <para>
59 60 61
	    When running in a console, the server can be shut down by
	    pressing ctrl-c. It detects the key combination and shuts
	    down gracefully.
62 63 64
      </para>

      <para>
65
        On start-up, the server will detect available network interfaces
66 67
        and will attempt to open UDP sockets on all interfaces
        mentioned in the configuration file.
68 69 70
      </para>

      <para>
71 72
        Since the DHCPv6 server opens privileged ports, it requires root
        access. Make sure you run this daemon as root.
73 74 75 76 77 78
      </para>

    </section>

    <section id="dhcp6-configuration">
      <title>DHCPv6 Server Configuration</title>
79 80 81 82


<section>
  <title>Introduction</title>
83
      <para>
84
        This section explains how to configure the DHCPv6 server using the
85 86
        Kea configuration backend. (Kea configuration using any other
        backends is outside of scope of this document.) Before DHCPv6
87 88
        is started, its configuration file has to be created. The
        basic configuration looks as follows:
89
<screen>
90
{
91
# DHCPv6 configuration starts on the next line
92 93 94 95 96 97 98 99 100 101 102 103
"Dhcp6": {

# First we set up global values
    "interfaces": [ "eth0" ],
    "renew-timer": 1000,
    "rebind-timer": 2000,
    "preferred-lifetime": 3000,
    "valid-lifetime": 4000,

# Next we specify the type of lease database
    "lease-database": {
        "type": "memfile",
104
        "persist": true,
105 106 107 108 109 110 111 112 113 114 115 116 117 118
        "name": "/var/kea/dhcp6.leases"
    },

# Finally, we list the subnets from which we will be leasing addresses.
    "subnet6": [
        {
            "subnet": "2001:db8:1::/64",
            "pools": [
                 {
                     "pool": "2001:db8:1::1-2001:db8:1::ffff"
                 }
             ]
        }
    ]
119
# DHCPv6 configuration ends with the next line
120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139
}

} </screen>
</para>

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

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

<para>The configuration starts in the first line with the initial
opening curly bracket (or brace). Each configuration consists of
one or more objects. In this specific example, we have only one
object called Dhcp6. This is a simplified configuration, as usually
there will be additional objects, like <command>Logging</command> or
<command>DhcpDns</command>, but we omit them now for clarity. The Dhcp6
140
configuration starts with the <command>"Dhcp6": {</command> line
141 142 143 144
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 Dhcp6 configuration.</para>

145
<para>In the general case, the order in which those parameters appear does not
146
matter. There are two caveats here though. The first one is to remember that
147 148 149
the configuration file must be well formed JSON. That means that 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
150
moving a parameter to or from the last position in a given scope may require
151 152
moving the comma as well. The second caveat is that it is uncommon &mdash; although
legal JSON &mdash; to
153 154 155 156 157 158 159 160 161 162 163 164
repeat the same parameter multiple times. If that happens, the last occurrence of a
given parameter in a given scope is used while all previous instances are
ignored. This is unlikely to cause any confusion as there are no real life
reasons to keep multiple copies of the same parameter in your configuration
file.</para>

<para>Moving onto the DHCPv6 configuration elements,
the line defining <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 line could
look like this:
165
<screen>
166
"interfaces": [ "eth0", "eth1" ],
167 168
</screen>

169
As "<command>interfaces</command>" is not the last parameter in the
170
configuration, a trailing comma is required.</para>
171
<para>A number of other parameters follow. <command>valid-lifetime</command>
172
defines for how long the addresses (leases) given out by the server are valid. If
173
nothing changes, a client that got an address is allowed to use it for 4000
174 175 176 177 178
seconds. (Note that integer numbers are specified as is, without any quotes
around them.) The address will become deprecated in 3000 seconds (clients are
allowed to keep old connections, but can't use this address for creating new
connections). <command>renew-timer</command> and <command>
rebind-timer</command> are values that define T1 and T2 timers that govern when
179
the client will begin the renewal and rebind procedures.</para>
180

181
<para>The next couple of lines define the lease database, the place where the server
182 183
stores its lease information. This particular example tells the server to use
<command>memfile</command>, which is the simplest (and fastest) database
184
backend. It uses an in-memory database and stores leases on disk in a CSV
185 186 187 188 189 190 191 192 193 194 195
file. This is a very simple configuration. Usually, lease database configuration
is more extensive and contains additional parameters.  Note that
<command>lease-database</command>
is an object and opens up a new scope, using an opening brace.
Its parameters (just one in this example -- <command>type</command>)
follow. Had there been more than one, they would be separated by commas. This
scope is closed with a closing brace. As more parameters follow, a trailing
comma is present.</para>

<para>Finally, we need to define a list of IPv6 subnets. This is the
most important DHCPv6 configuration structure as the server uses that
196 197
information to process clients' requests. It defines all subnets from
which the server is expected to receive DHCP requests. The subnets are
198 199
specified with the <command>subnet6</command> parameter.  It is a list,
so it starts and ends with square brackets.  Each subnet definition in
200
the list has several attributes associated with it, so it is a structure
201 202 203 204 205 206 207 208 209 210
and is opened and closed with braces. At minimum, a subnet definition
has to have at least two parameters: <command>subnet</command> (that
defines the whole subnet) and <command>pool</command> (which is a list of
dynamically allocated pools that are governed by the DHCP server).</para>

<para>The example contains a single subnet. Had more than one been defined,
additional elements
in the <command>subnet6</command> parameter would be specified and
separated by commas. For example, to define two subnets, the following
syntax would be used:
211
<screen>
212 213 214 215 216 217 218 219 220 221
"subnet6": [
    {
        "pools": [
        {
            "pool": "2001:db8:1::/112"
        }
        ],
        "subnet": "2001:db8:1::/64"
    },
    {
222 223
        "pools": [ { "pool": "2001:db8:2::1-2001:db8:2::ffff" } ],
        "subnet": "2001:db8:2::/64",
224 225 226
        "interface": "eth0"
    }
]
227
</screen>
228 229
Note that indentation is optional and is used for aesthetic purposes only.
In some cases in may be preferable to use more compact notation.
230
</para>
231

232

233 234
<para>After all parameters are specified, we have two contexts open:
global and Dhcp6, hence we need two closing curly brackets to close them.
235 236
In a real life configuration file there most likely would be additional
components defined such as Logging or DhcpDdns, so the closing brace would
237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256
be followed by a comma and another object definition.</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 and/or
viewers to check whether the syntax is correct. One example of such a
JSON validator is available at <ulink url="http://jsonviewer.stack.hu/"/>.
</para>
</section>

<section>
  <title>Lease Storage</title>
  <para>All leases issued by the server are stored in the lease database.
  Currently there are three database backends available:
  memfile (which is the default backend), MySQL and PostgreSQL.</para>
<section>
  <title>Memfile - Basic Storage for Leases</title>

  <para>The server is able to store lease data in different repositories. Larger
  deployments may elect to store leases in a database. <xref
257
  linkend="database-configuration6"/> describes this option. In typical
258 259 260 261 262 263
  smaller deployments though, the server will use a CSV file rather than a database to
  store lease information. As well as requiring less administration, an
  advantage of using a file for storage is that it
  eliminates a dependency on third-party database software.</para>

  <para>The configuration of the file backend (Memfile) is controlled through
264
  the Dhcp6/lease-database parameters. <!-- @todo: we don't have default
265 266 267
  parameters. Let's comment this out When default parameters are used, the
  Memfile backend will write leases to a disk in the
  [kea-install-dir]/var/kea/kea-leases4.csv. -->
Jeremy C. Reed's avatar
Jeremy C. Reed committed
268
  The following configuration sets the name of the lease file to
269
  <filename>/tmp/kea-leases6.csv</filename>:
Jeremy C. Reed's avatar
Jeremy C. Reed committed
270

271
<screen>
272
"Dhcp6": {
273 274
    "lease-database": {
        <userinput>"type": "memfile"</userinput>,
275
        <userinput>"persist": true</userinput>,
276
        <userinput>"name": "/tmp/kea-leases6.csv"</userinput>
277 278 279 280 281 282 283
    }
    ...
}
</screen>
  </para>

  <para>The "persist" parameter controls whether the leases are written to disk.
284
  It is strongly recommended that this parameter is set to true at all times
285 286 287 288
  during the normal operation of the server. (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.)
289
          </para>
290 291 292 293 294 295 296 297 298 299 300 301 302 303 304
</section>

<section id="database-configuration6">
  <title>Database Configuration</title>

  <note>
    <para>Database access information must be configured for the DHCPv6 server,
    even if it has already been configured for the DHCPv4 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 Dhcp6/lease-database
  parameters. The type of the database must be set to "mysql" or "postgresql",
  e.g.
305
<screen>
306
"Dhcp6": { "lease-database": { <userinput>"type": "mysql"</userinput>, ... }, ... }
307
</screen>
308 309 310
  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"/>).
311
<screen>
312
"Dhcp6": { "lease-database": { <userinput>"name": "<replaceable>database-name</replaceable>" </userinput>, ... }, ... }
313
</screen>
314 315 316
  If the database is located on a different system than the DHCPv6 server, the
  database host name must also be specified (although it should be noted that this
  configuration may have a severe impact on server performance):
317
<screen>
318
"Dhcp6": { "lease-database": { <userinput>"host": <replaceable>remote-host-name</replaceable>"</userinput>, ... }, ... }
319
</screen>
320 321
  The usual state of affairs will be to have the database on the same machine as
  the DHCPv6 server.  In this case, set the value to the empty string:
322
<screen>
323
"Dhcp6": { "lease-database": { <userinput>"host" : ""</userinput>, ... }, ... }
324
</screen>
325 326 327
  </para>
  <para>Finally, the credentials of the account under which the server will
  access the database should be set:
328
<screen>
329
"Dhcp6": { "lease-database": { <userinput>"user": "<replaceable>user-name</replaceable>"</userinput>,
330
                               <userinput>"password": "<replaceable>password</replaceable>"</userinput>,
331 332
                              ... },
           ... }
333
</screen>
334 335 336 337
  If there is no password to the account, set the password to the empty string
  "". (This is also the default.)</para>
</section>
</section>
338

339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358
<section id="dhcp6-interface-selection">
  <title>Interface selection</title>
  <para>The DHCPv6 server has to be configured to listen on specific network
  interfaces.  The simplest network interface configuration tells the server to
  listen on all available interfaces:
  <screen>
"Dhcp6": { <userinput>"interfaces": ["*"]</userinput>, ... }</screen>
  The asterisk plays the role of a wildcard and means "listen on all interfaces".
  However, it is usually a good idea to explicitly specify interface names:
  <screen>
"Dhcp6": { <userinput>"interfaces": [ "eth1", "eth3" ]</userinput>, ... }</screen>
  </para>
  <para>It is possible to use wildcard interface name (asterisk) concurrently
  with explicit interface names:
  <screen>
"Dhcp6": { <userinput>"interfaces": [ "eth1", "eth3", "*" ]</userinput>, ... }</screen>
It is anticipated that this will form of usage only be used where it is desired to
temporarily override a list of interface names and listen on all interfaces.
  </para>
</section>
359 360 361 362

    <section id="ipv6-subnet-id">
      <title>IPv6 Subnet Identifier</title>
      <para>
363
        The subnet identifier is a unique number associated with a particular subnet.
364
        In principle, it is used to associate clients' leases with respective subnets.
365
        When the subnet identifier is not specified for a subnet being configured, it will
366 367 368 369 370 371 372
        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:
373 374 375
       if there are four subnets and the third is removed the last subnet will be assigned
       the identifier that the third subnet had before removal. As a result, the leases
       stored in the lease database for subnet 3 are now associated with
376
       subnet 4, which may have unexpected consequences. In the future it is planned
377
       to implement a mechanism to preserve auto-generated subnet ids upon removal
378
       of one of the subnets. Currently, the only remedy for this issue is to
379
       manually specify a unique subnet identifier for each subnet.
380
      </para>
381

382
      <para>
383
	The following configuration will assign the specified subnet
384 385
	identifier to the newly configured subnet:

386 387 388 389 390 391 392 393 394 395 396 397
	<screen>
"Dhcp6": {
    "subnet6": [
        "subnet": "2001:db8:1::/64",
        <userinput>"id": 1024</userinput>,
        ...
    ]
}
</screen>
	This identifier will not change for this subnet unless the "id" parameter is
	removed or set to 0. The value of 0 forces auto-generation of the subnet
	identifier.
398
      </para>
399 400
      <!-- @todo: describe whether database needs to be updated after changing
      id -->
401 402 403 404 405
    </section>

    <section id="dhcp6-unicast">
      <title>Unicast traffic support</title>
      <para>
406
        When the DHCPv6 server starts, by default it listens to the DHCP traffic
407 408 409 410 411
        sent to multicast address ff02::1:2 on each interface that it is
        configured to listen on (see <xref linkend="dhcp6-interface-selection"/>).
        In some cases it is useful to configure a server to handle incoming
        traffic sent to the global unicast addresses as well. The most common
        reason for that is to have relays send their traffic to the server
412 413 414 415
        directly. To configure the server to listen on a specific unicast address, the
        notation to specify interfaces has been extended.  An interface name can be
        optionally followed by a slash, followed by the global unicast address on which
        the server should listen. This will be done in addition to normal
416 417 418 419 420
        link-local binding + listening on ff02::1:2 address. The sample commands
        listed below show how to listen on 2001:db8::1 (a global address)
        configured on the eth1 interface.
      </para>
      <para>
421 422 423 424 425 426
 <screen>
"Dhcp6": {
    <userinput>"interfaces": [ "eth1/2001:db8::1"],</userinput>
    ...
}</screen>

427
        When this configuration gets committed, the server will start to listen on
428 429 430 431 432 433 434 435 436 437
        eth1 on link-local address, multicast group (ff02::1:2) and 2001:db8::1.
      </para>
      <para>
        It is possible to mix interface names, wildcards and interface name/addresses
        on the Dhcp6/interface list. It is not possible to specify more than one
        unicast address on a given interface.
      </para>
      <para>
        Care should be taken to specify proper unicast addresses. The server will
        attempt to bind to those addresses specified, without any additional checks.
438 439
        This approach is selected on purpose, so the software can be used to
        communicate over uncommon addresses if the administrator so desires.
440 441 442 443 444 445 446 447 448 449 450 451 452 453
      </para>
    </section>

    <section id="dhcp6-address-config">
      <title>Subnet and Address Pool</title>
      <para>
        The essential role of a DHCPv6 server is address assignment. For this,
        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 2001:db8:1::/64
        prefix. The Administrator of that network has decided that addresses from range
        2001:db8:1::1 to 2001:db8:1::ffff are going to be managed by the Dhcp6
        server. Such a configuration can be achieved in the following way:
        <screen>
454 455 456 457 458 459
"Dhcp6": {
    <userinput>"subnet6": [
       {
           "subnet": "2001:db8:1::/64",
           "pools": [
               {
460
                   "pool": "2001:db8:1::1-2001:db8:1::ffff"
461 462 463 464 465 466 467
               }
           ],
           ...
       }
    ]</userinput>
}</screen>

468 469 470 471
        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>
472 473

        <para>Each <command>pool</command> is a structure that contains the
474
        parameters that describe a single pool. Currently there is only one
475 476 477 478
        parameter, <command>pool</command>, which gives the range of addresses
        in the pool. Additional parameters will be added in future releases of
        Kea.</para>

479 480
        <para>It is possible to define more than one pool in a
        subnet: continuing the previous example, further assume that
481
        2001:db8:1:0:5::/80 should also be managed by the server. It could be written as
482 483 484 485 486
        2001:db8:1:0:5:: to 2001:db8:1::5:ffff:ffff:ffff, but typing so many 'f's
        is cumbersome. It can be expressed more simply as 2001:db8:1:0:5::/80. Both
        formats are supported by Dhcp6 and can be mixed in the pool list.
        For example, one could define the following pools:
        <screen>
487 488 489 490 491
"Dhcp6": {
    <userinput>"subnet6": [
    {
        "subnet": "2001:db8:1::/64",
        "pools": [
492 493
            { "pool": "2001:db8:1::1-2001:db8:1::ffff" },
            { "pool": "2001:db8:1:05::/80" }
494 495 496 497 498
        ]</userinput>,
        ...
    }
    ]
}</screen>
499 500 501 502 503 504 505
        The number of pools is not limited, but for performance reasons it is recommended to
        use as few as possible.
      </para>
      <para>
         The server may be configured to serve more than one subnet. To add a second subnet,
         use a command similar to the following:
        <screen>
506 507 508 509 510
"Dhcp6": {
    <userinput>"subnet6": [
    {
        "subnet": "2001:db8:1::/64",
        "pools": [
511
            { "pool": "2001:db8:1::1-2001:db8:1::ffff" }
512 513 514 515 516
        ]
    },
    {
        "subnet": "2001:db8:2::/64",
        "pools": [
517
            { "pool": "2001:db8:2::/64" }
518 519 520 521 522 523
        ]
    },
</userinput>
        ...
    ]
}</screen>
524
        In this example, we allow the server to
525
        dynamically assign all addresses available in the whole subnet. Although
526
        rather wasteful, it is certainly a valid configuration to dedicate the
527
        whole /64 subnet for that purpose. Note that the Kea server does not preallocate
528
        the leases, so there is no danger in using gigantic address pools.
529 530 531
      </para>
      <para>
        When configuring a DHCPv6 server using prefix/length notation, please pay
532 533
        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
534 535 536
        address) address from that pool. For example for pool 2001:db8:2::/64 the
        2001:db8:2:: address may be assigned as well. If you want to avoid this,
        use the "min-max" notation.
537 538 539 540 541 542 543
      </para>
    </section>

    <section>
<!-- @todo: add real meat to the prefix delegation config this is just place holder stuff -->
      <title>Subnet and Prefix Delegation Pools</title>
      <para>
544 545
        Subnets may also be configured to delegate prefixes, as defined in
	<ulink url="http://tools.ietf.org/html/rfc3633">RFC 3633</ulink>.
546 547
        A subnet may have one or more prefix delegation pools.  Each pool has
        a prefixed address, which is specified as a prefix and a prefix length,
548
        as well as a delegated prefix length. <command>delegated-len</command>
549 550 551
	must not be shorter (that is it must be numerically greater or equal)
	than <command>prefix-len</command>.
	If both <command>delegated-len</command>
552 553
	and <command>prefix-len</command> are equal, the server will be able to
	delegate only one prefix. A sample configuration is shown below:
554
      <screen>
555 556 557 558 559 560 561 562 563 564 565 566 567 568 569
"Dhcp6": {
    "subnet6": [
        {
            "subnet": "2001:d8b:1::/64",
            <userinput>"pd-pools": [
                {
                    "prefix": "2001:db8:1::",
                    "prefix-len": 64,
                    "delegated-len": 96
                }
            ]</userinput>
        }
    ],
    ...
}</screen>
570 571 572 573 574 575
      </para>
    </section>

    <section id="dhcp6-std-options">
      <title>Standard DHCPv6 options</title>
      <para>
576 577
        One of the major features of a DHCPv6 server is to provide configuration
        options to clients.  Although there are several options that require
578
        special behavior, most options are sent by the server only if the client
579
        explicitly requests them.  The following example shows how to
580
        configure DNS servers, which is one of the most frequently used
581 582
        options.  Numbers in the first column are added for easier reference and
        will not appear on screen.  Options specified in this way are considered
583 584 585
        global and apply to all configured subnets.

        <screen>
586
"Dhcp6": {
587 588 589 590 591
    "option-data": [
        {
           <userinput>"name": "dns-servers",
           "code": 23,
           "space": "dhcp6",
592
           "csv-format": true,
593 594 595 596 597
           "data": "2001:db8::cafe, 2001:db8::babe"</userinput>
        },
        ...
    ]
}
598 599
</screen>
      </para>
600

601
    <para>
602 603
      The <command>option-data></command> line creates a new entry in
      the option-data table.  This table contains
604
      information on all global options that the server is supposed to configure
605
      in all subnets.  The <command>name</command> line specifies the option name.
606
      (For a complete list
607
      of currently supported names, see <xref
608
      linkend="dhcp6-std-options-list"/>.)  The next line specifies the option code,
609 610 611
      which must match one of the values from that list. The line beginning with
      <command>space</command> specifies the option space, which must always be set
      to "dhcp6" as these are standard DHCPv6 options.  For other name spaces,
612
      including custom option spaces, see <xref
613
      linkend="dhcp6-option-spaces"/>. The next line specifies the format in
614
      which the data will be entered: use of CSV (comma separated values) is
615 616
      recommended. The <command>data</command> line gives the actual value to be sent to
      clients.  Data is specified as normal text, with values separated by
617
      commas if more than one value is allowed.
618 619 620
    </para>

    <para>
621
      Options can also be configured as hexadecimal values.  If "csv-format" is
622
      set to false, the option data must be specified as a string of hexadecimal
623
      numbers.  The
624 625 626
      following commands configure the DNS-SERVERS option for all
      subnets with the following addresses: 2001:db8:1::cafe and
      2001:db8:1::babe.
627
        <screen>
628
"Dhcp6": {
629 630 631 632 633
    "option-data": [
        {
           <userinput>"name": "dns-servers",
           "code": 23,
           "space": "dhcp6",
634
           "csv-format": false,
635 636 637 638 639 640 641 642 643 644
           "data": "2001 0DB8 0001 0000 0000 0000 0000 CAFE
                    2001 0DB8 0001 0000 0000 0000 0000 BABE"</userinput>
        },
        ...
    ]
}
</screen>

       The value for the setting of the "data" element is split across two
       lines in this document for clarity: when entering the command, the
645
       whole string should be entered on the same line.  Care should be taken
646 647
       to use proper encoding when using hexadecimal format as Kea's ability
       to validate data correctness in hexadecimal is limited.
648 649
      </para>

650 651 652 653 654 655 656
      <para>
        Most of the parameters in the "option-data" structure are optional and
        can be omitted in some circumstances as discussed in the
        <xref linkend="dhcp6-option-data-defaults"/>.
      </para>


657 658 659 660 661 662 663 664 665 666 667 668 669 670
    <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
      (Dhcp6/option-data), rather you should set only subnet-specific values
      (Dhcp6/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 2001:db8:1::3.
671 672 673 674 675 676 677 678
<screen>
"Dhcp6": {
    "subnet6": [
        {
            <userinput>"option-data": [
                {
                    "name": "dns-servers",
                    "code": 23,
Jeremy C. Reed's avatar
Jeremy C. Reed committed
679
                    "space": "dhcp6",
680
                    "csv-format": true,
681 682 683 684 685 686 687 688 689 690 691
                    "data": "2001:db8:1::3"
                },
                ...
            ]</userinput>,
            ...
        },
        ...
    ],
    ...
}
</screen>
692 693 694 695 696 697 698 699 700 701 702 703 704
    </para>

    <para>
      The currently supported standard DHCPv6 options are
      listed in <xref linkend="dhcp6-std-options-list"/>.
      The "Name" and "Code"
      are the values that should be used as a name in the option-data
      structures. "Type" designates the format of the data: the meanings of
      the various types is given in <xref linkend="dhcp-types"/>.
    </para>
    <para>
      Some options are designated as arrays, which means that more than one
      value is allowed in such an option. For example the option dns-servers
705
      allows the specification of more than one IPv6 address, allowing
706
      clients to obtain the addresses of multiple DNS servers.
707 708 709 710 711 712 713 714 715 716 717 718 719 720
    </para>

<!-- @todo: describe record types -->

      <para>
        The <xref linkend="dhcp6-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="dhcp6-custom-options"/> in the 'dhcp6' option space. This
        definition should match the option format described in the relevant
721 722
        RFC but the configuration mechanism would allow any option format as it has
        no means to validate the format at the moment.
723 724 725 726 727 728 729 730
      </para>


    <para>
      <table frame="all" id="dhcp6-std-options-list">
        <title>List of standard DHCPv6 options</title>
        <tgroup cols='4'>
        <colspec colname='name'/>
731 732 733
        <colspec colname='code' align='center'/>
        <colspec colname='type' align='center'/>
        <colspec colname='array' align='center'/>
734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795 796 797 798
        <thead>
          <row><entry>Name</entry><entry>Code</entry><entry>Type</entry><entry>Array?</entry></row>
        </thead>
        <tbody>
<!-- Our engine uses those options on its own, admin must not configure them on his own
<row><entry>clientid</entry><entry>1</entry><entry>binary</entry><entry>false</entry></row>
<row><entry>serverid</entry><entry>2</entry><entry>binary</entry><entry>false</entry></row>
<row><entry>ia-na</entry><entry>3</entry><entry>record</entry><entry>false</entry></row>
<row><entry>ia-ta</entry><entry>4</entry><entry>uint32</entry><entry>false</entry></row>
<row><entry>iaaddr</entry><entry>5</entry><entry>record</entry><entry>false</entry></row>
<row><entry>oro</entry><entry>6</entry><entry>uint16</entry><entry>true</entry></row> -->
<row><entry>preference</entry><entry>7</entry><entry>uint8</entry><entry>false</entry></row>

<!-- Our engine uses those options on its own, admin must not configure them on his own
<row><entry>elapsed-time</entry><entry>8</entry><entry>uint16</entry><entry>false</entry></row>
<row><entry>relay-msg</entry><entry>9</entry><entry>binary</entry><entry>false</entry></row>
<row><entry>auth</entry><entry>11</entry><entry>binary</entry><entry>false</entry></row>
<row><entry>unicast</entry><entry>12</entry><entry>ipv6-address</entry><entry>false</entry></row>
<row><entry>status-code</entry><entry>13</entry><entry>record</entry><entry>false</entry></row>
<row><entry>rapid-commit</entry><entry>14</entry><entry>empty</entry><entry>false</entry></row>
<row><entry>user-class</entry><entry>15</entry><entry>binary</entry><entry>false</entry></row>
<row><entry>vendor-class</entry><entry>16</entry><entry>record</entry><entry>false</entry></row>
<row><entry>vendor-opts</entry><entry>17</entry><entry>uint32</entry><entry>false</entry></row>
<row><entry>interface-id</entry><entry>18</entry><entry>binary</entry><entry>false</entry></row>
<row><entry>reconf-msg</entry><entry>19</entry><entry>uint8</entry><entry>false</entry></row>
<row><entry>reconf-accept</entry><entry>20</entry><entry>empty</entry><entry>false</entry></row> -->
<row><entry>sip-server-dns</entry><entry>21</entry><entry>fqdn</entry><entry>true</entry></row>
<row><entry>sip-server-addr</entry><entry>22</entry><entry>ipv6-address</entry><entry>true</entry></row>
<row><entry>dns-servers</entry><entry>23</entry><entry>ipv6-address</entry><entry>true</entry></row>
<row><entry>domain-search</entry><entry>24</entry><entry>fqdn</entry><entry>true</entry></row>
<!-- <row><entry>ia-pd</entry><entry>25</entry><entry>record</entry><entry>false</entry></row> -->
<!-- <row><entry>iaprefix</entry><entry>26</entry><entry>record</entry><entry>false</entry></row> -->
<row><entry>nis-servers</entry><entry>27</entry><entry>ipv6-address</entry><entry>true</entry></row>
<row><entry>nisp-servers</entry><entry>28</entry><entry>ipv6-address</entry><entry>true</entry></row>
<row><entry>nis-domain-name</entry><entry>29</entry><entry>fqdn</entry><entry>true</entry></row>
<row><entry>nisp-domain-name</entry><entry>30</entry><entry>fqdn</entry><entry>true</entry></row>
<row><entry>sntp-servers</entry><entry>31</entry><entry>ipv6-address</entry><entry>true</entry></row>
<row><entry>information-refresh-time</entry><entry>32</entry><entry>uint32</entry><entry>false</entry></row>
<row><entry>bcmcs-server-dns</entry><entry>33</entry><entry>fqdn</entry><entry>true</entry></row>
<row><entry>bcmcs-server-addr</entry><entry>34</entry><entry>ipv6-address</entry><entry>true</entry></row>
<row><entry>geoconf-civic</entry><entry>36</entry><entry>record</entry><entry>false</entry></row>
<row><entry>remote-id</entry><entry>37</entry><entry>record</entry><entry>false</entry></row>
<row><entry>subscriber-id</entry><entry>38</entry><entry>binary</entry><entry>false</entry></row>
<row><entry>client-fqdn</entry><entry>39</entry><entry>record</entry><entry>false</entry></row>
<row><entry>pana-agent</entry><entry>40</entry><entry>ipv6-address</entry><entry>true</entry></row>
<row><entry>new-posix-timezone</entry><entry>41</entry><entry>string</entry><entry>false</entry></row>
<row><entry>new-tzdb-timezone</entry><entry>42</entry><entry>string</entry><entry>false</entry></row>
<row><entry>ero</entry><entry>43</entry><entry>uint16</entry><entry>true</entry></row>
<row><entry>lq-query</entry><entry>44</entry><entry>record</entry><entry>false</entry></row>
<row><entry>client-data</entry><entry>45</entry><entry>empty</entry><entry>false</entry></row>
<row><entry>clt-time</entry><entry>46</entry><entry>uint32</entry><entry>false</entry></row>
<row><entry>lq-relay-data</entry><entry>47</entry><entry>record</entry><entry>false</entry></row>
<row><entry>lq-client-link</entry><entry>48</entry><entry>ipv6-address</entry><entry>true</entry></row>
        </tbody>
        </tgroup>
      </table>
    </para>
    </section>

    <section id="dhcp6-custom-options">
      <title>Custom DHCPv6 options</title>
      <para>It is also possible to define options other than the standard ones.
      Assume that we want to define a new DHCPv6 option called "foo" which will have
      code 100 and will convey a single unsigned 32 bit integer value. We can define
      such an option by using the following commands:
799 800 801 802 803 804 805
<screen>
"Dhcp6": {
    "option-def": [
        {
            <userinput>"name": "foo",
            "code": 100,
            "type": "uint32",
806
            "array": false,
807 808 809 810 811 812 813 814 815 816 817
            "record-types": "",
            "space": "dhcp6",
            "encapsulate": ""</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".
818 819 820 821 822
      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
823 824 825
      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.
826 827 828 829
      </para>
      <para>Once the new option format is defined, its value is set
      in the same way as for a standard option. For example the following
      commands set a global value that applies to all subnets.
830 831 832 833
<screen>
"Dhcp6": {
    "option-data": [
        {
834
            <userinput>"name": "foo",
835 836
            "code": 100,
            "space": "dhcp6",
837
            "csv-format": true,
838 839 840 841 842 843
            "data": "12345"</userinput>
        }, ...
    ],
    ...
}
</screen>
844 845 846 847 848 849 850 851 852 853 854 855
      </para>

      <para>New options can take more complex forms than simple use of
      primitives (uint8, string, ipv6-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 IPv6
      address, followed by an unsigned 16 bit integer, followed by a
      boolean value, followed by a text string. Such an option could
      be defined in the following way:
<screen>
856 857 858 859 860 861 862
"Dhcp6": {
    "option-def": [
        {
            <userinput>"name": "bar",
            "code": 101,
            "space": "dhcp6",
            "type": "record",
863
            "array": false,
864 865 866 867 868 869
            "record-types": "ipv4-address, uint16, boolean, string",
            "encapsulate": ""</userinput>
        }, ...
    ],
    ...
}
870 871 872 873 874 875 876 877
</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>
878 879 880 881 882 883
"Dhcp6": {
    "option-data": [
        {
            <userinput>"name": "bar",
            "space": "dhcp6",
            "code": 101,
884
            "csv-format": true,
885 886 887 888 889
            "data": "2001:db8:1::10, 123, false, Hello World"</userinput>
        }
    ],
    ...
}</screen>
890 891 892 893

      <command>csv-format</command> is set <command>true</command> to indicate
      that the <command>data</command> field comprises a command-separated list
      of values.  The values in the "data" must correspond to the types set in
894 895
      the "record-types" field of the option definition.
      </para>
896

897
      <note>
898
       <para>In the general case, boolean values are specified as <command>true</command> or
899 900 901 902 903
       <command>false</command>, without quotes. Some specific boolean parameters may
       accept also <command>"true"</command>, <command>"false"</command>,
       <command>0</command>, <command>1</command>, <command>"0"</command> and
       <command>"1"</command>. Future Kea versions will accept all those values
       for all boolean parameters.</para>
904
      </note>
905

906 907 908 909 910 911 912 913 914 915 916 917 918 919 920 921
    </section>

    <section id="dhcp6-vendor-opts">
      <title>DHCPv6 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-opts-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 IPv6 address, an unsigned 16 bit integer
      and a string. The "foo" option is conveyed in a vendor specific
      information option. This option comprises a single uint32 value
      that is set to "12345". The sub-option "foo" follows the data
      field holding this value.
      <screen>
922 923 924 925 926 927 928
"Dhcp6": {
    "option-def": [
        {
            <userinput>"name": "foo",
            "code": 1,
            "space": "vendor-encapsulated-options-space",
            "type": "record",
929
            "array": false,
930 931 932 933 934 935 936
            "record-types": "ipv6-address, uint16, string",
            "encapsulates": ""</userinput>
        }
    ],
    ...
}</screen>
     (Note that the option space is set to <command>vendor-opts-space</command>.)
937 938
     Once the option format is defined, the next step is to define actual values
     for that option:
939 940 941 942 943 944 945
<screen>
"Dhcp6": {
    "option-data": [
        {
            <userinput>"name": "foo"
            "space": "vendor-encapsulated-options-space",
            "code": 1,
946
            "csv-format": true,
947 948 949 950 951 952 953 954 955 956 957 958 959 960 961 962
            "data": "2001:db8:1::10, 123, Hello World"</userinput>
        },
        ...
    ],
    ...
}</screen>
    We should also define values for the vendor-opts, that will convey our
    option foo.
<screen>
"Dhcp6": {
    "option-data": [
        ...,
        {
            <userinput>"name": "vendor-encapsulated-options"
            "space": "dhcp6",
            "code": 17,
963
            "csv-format": true,
964
            "data": "12345"</userinput>
965 966 967 968
        }
    ],
    ...
}</screen>
969 970 971 972 973 974 975
      </para>
    </section>

    <section id="dhcp6-option-spaces">
      <title>Nested DHCPv6 options (custom option spaces)</title>
      <para>It is sometimes useful to define completely new option
      spaces.  This is useful if the user wants his new option to
976
      convey sub-options that use a separate numbering scheme, for
977 978 979 980
      example sub-options with codes 1 and 2. Those option codes
      conflict with standard DHCPv6 options, so a separate option
      space must be defined.
      </para>
981 982 983
      <para>Note that it is not required to create a new option space when
      defining sub-options for a standard option because it is
      created by default if the standard option is meant to convey
984 985 986 987 988 989 990
      any sub-options (see <xref linkend="dhcp6-vendor-opts"/>).
      </para>
      <para>
      Assume that we want to have a DHCPv6 option called "container"
      with code 102 that conveys two sub-options with codes 1 and 2.
      First we need to define the new sub-options:
<screen>
991 992 993 994 995 996 997 998
"Dhcp6": {
    "option-def": [
        {
            <userinput>"name": "subopt1",
            "code": 1,
            "space": "isc",
            "type": "ipv6-address".
            "record-types": "",
999
            "array": false,
1000 1001 1002 1003 1004 1005 1006 1007
            "encapsulate ""
        },
        {
            "name": "subopt2",
            "code": 2,
            "space": "isc",
            "type": "string",
            "record-types": "",
1008
            "array": false
1009 1010 1011 1012 1013
            "encapsulate": ""</userinput>
        }
    ],
    ...
}</screen>
1014 1015 1016 1017 1018 1019 1020
    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 DHCPv6 option and specify that it
should include options from the isc option space:
<screen>
1021
"Dhcp6": {
1022 1023 1024 1025 1026 1027 1028
    "option-def": [
        ...,
        {
            <userinput>"name": "container",
            "code": 102,
            "space": "dhcp6",
            "type": "empty",
1029
            "array": false,
1030 1031 1032 1033 1034 1035 1036 1037 1038
            "record-types": "",
            "encapsulate": "isc"</userinput>
        }
    ],
    ...
}</screen>

    The name of the option space in which the sub-options are defined is set in
    the <command>encapsulate</command> field. The <command>type</command> field
1039 1040
    is set to <command>empty</command> which limits this option to only carrying
    data in sub-options.
1041 1042 1043 1044
    </para>
    <para>
    Finally, we can set values for the new options:
<screen>
1045 1046 1047 1048 1049 1050
"Dhcp6": {
    "option-data": [
        {
            <userinput>"name": "subopt1",
            "space": "isc",
            "code": 1,
1051
            "csv-format": true,
1052 1053 1054 1055 1056 1057
            "data": "2001:db8::abcd"</userinput>
        },
        }
            <userinput>"name": "subopt2",
            "space": "isc",
            "code": 2,
1058
            "csv-format": true,
1059 1060 1061 1062 1063 1064
            "data": "Hello world"</userinput>
        },
        {
            <userinput>"name": "container",
            "space": "dhcp6",
            "code": 102,
1065
            "csv-format": true,
1066 1067 1068 1069 1070
            "data": ""</userinput>
        }
    ],
    ...
}
1071
</screen>
1072

1073 1074
    Even though the "container" option does not carry any data except
    sub-options, the "data" field must be explicitly set to an empty value.
1075
    This is required because in the current version of Kea, the default
1076 1077 1078
    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.
1079
    </para>
1080
    
1081
    <para>Note that it is possible to create an option which carries some data
1082 1083 1084 1085 1086
    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
1087 1088
    value, sub-options.) The value specified with the "data" parameter &mdash; which
    should be a valid integer enclosed in quotes, e.g. "123" &mdash; would then be
1089
    assigned to the uint16 field in the "container" option.
1090 1091 1092
    </para>
    </section>

1093
    <section id="dhcp6-option-data-defaults">
1094
      <title>Unspecified parameters for DHCPv6 option configuration</title>
1095
      <para>In many cases it is not required to specify all parameters for
1096
      an option configuration and the default values can be used. However, it is
1097 1098 1099 1100 1101 1102 1103 1104 1105 1106 1107 1108 1109 1110 1111 1112 1113 1114 1115 1116 1117 1118 1119 1120 1121 1122 1123 1124 1125 1126 1127 1128 1129 1130 1131 1132 1133 1134 1135 1136 1137 1138 1139 1140 1141 1142 1143 1144 1145 1146 1147 1148 1149 1150 1151 1152 1153 1154 1155 1156 1157 1158 1159 1160 1161 1162
      important to understand the implications of not specifing some of them
      as it may result in configuration errors. The list below explains
      the behavior of the server when a particular parameter is not explicitly
      specified:

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

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

        <listitem>
          <simpara><command>space</command> - if the option space is unspecified it
          will default to 'dhcp6' which is an option space holding DHCPv6 standard
          options.
          </simpara>
        </listitem>

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

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

    </section>

1163 1164 1165 1166 1167
    <section id="dhcp6-config-subnets">
      <title>IPv6 Subnet Selection</title>
      <para>
        The DHCPv6 server may receive requests from local (connected to the
        same subnet as the server) and remote (connecting via relays) clients.
1168 1169
        As the server may have many subnet configurations defined, it must select
        an appropriate subnet for a given request.
1170 1171
      </para>
      <para>
1172 1173
        The server can not assume which of the configured subnets are local. In IPv4
        it is possible as there is a reasonable expectation that the
1174 1175
        server will have a (global) IPv4 address configured on the interface,
        and can use that information to detect whether a subnet is local or
1176 1177
        not. That assumption is not true in IPv6, the DHCPv6 server must be able
        to operate while only having link-local addresses. Therefore an optional
1178 1179
        &quot;interface&quot; parameter is available within a subnet definition
        to designate that a given subnet is local, i.e. reachable directly over
1180
        the specified interface. For example the server that is intended to serve
1181 1182 1183 1184 1185 1186 1187 1188 1189 1190 1191 1192 1193 1194 1195 1196
        a local subnet over eth0 may be configured as follows:
	<screen>
"Dhcp6": {
    "subnet6": [
        {
            "subnet": "2001:db8:beef::/48",
            "pools": [
                 {
                     "pool": "2001:db8:beef::/48"
                 }
             ],
            <userinput>"interface": "eth0"</userinput>
        }
    ],
    ...
}
1197 1198 1199 1200 1201 1202 1203 1204
</screen>
        </para>
      </section>

      <section id="dhcp6-relays">
        <title>DHCPv6 Relays</title>
        <para>
          A DHCPv6 server with multiple subnets defined must select the
1205
          appropriate subnet when it receives a request from a client.  For clients
1206 1207 1208 1209 1210 1211 1212 1213 1214 1215 1216 1217 1218 1219 1220 1221 1222 1223 1224 1225 1226 1227 1228 1229 1230 1231 1232 1233 1234 1235 1236 1237 1238 1239 1240 1241 1242 1243 1244 1245 1246 1247 1248 1249 1250 1251
          connected via relays, two mechanisms are used:
        </para>
        <para>
          The first uses the linkaddr field in the RELAY_FORW message. The name
          of this field is somewhat misleading in that it does not contain a link-layer
          address: instead, it holds an address (typically a global address) that is
          used to identify a link. The DHCPv6 server checks if the address belongs
          to a defined subnet and, if it does, that subnet is selected for the client's
          request.
        </para>
        <para>
          The second mechanism is based on interface-id options. While forwarding a client's
          message, relays may insert an interface-id option into the message that
          identifies the interface on the relay that received the message. (Some
          relays allow configuration of that parameter, but it is sometimes
          hardcoded and may range from the very simple (e.g. "vlan100") to the very cryptic:
          one example seen on real hardware was "ISAM144|299|ipv6|nt:vp:1:110"). The
          server can use this information to select the appropriate subnet.
          The information is also returned to the relay which then knows the
          interface to use to transmit the response to the client. In order for
          this to work successfully, the relay interface IDs must be unique within
          the network and the server configuration must match those values.
        </para>