1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
|
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<!--
- Copyright (C) 2000-2019 Internet Systems Consortium, Inc. ("ISC")
-
- This Source Code Form is subject to the terms of the Mozilla Public
- License, v. 2.0. If a copy of the MPL was not distributed with this
- file, You can obtain one at http://mozilla.org/MPL/2.0/.
-->
<html lang="en">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>Chapter�1.�Introduction</title>
<meta name="generator" content="DocBook XSL Stylesheets V1.78.1">
<link rel="home" href="Bv9ARM.html" title="BIND 9 Administrator Reference Manual">
<link rel="up" href="Bv9ARM.html" title="BIND 9 Administrator Reference Manual">
<link rel="prev" href="Bv9ARM.html" title="BIND 9 Administrator Reference Manual">
<link rel="next" href="Bv9ARM.ch02.html" title="Chapter�2.�BIND Resource Requirements">
</head>
<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
<div class="navheader">
<table width="100%" summary="Navigation header">
<tr><th colspan="3" align="center">Chapter�1.�Introduction</th></tr>
<tr>
<td width="20%" align="left">
<a accesskey="p" href="Bv9ARM.html">Prev</a>�</td>
<th width="60%" align="center">�</th>
<td width="20%" align="right">�<a accesskey="n" href="Bv9ARM.ch02.html">Next</a>
</td>
</tr>
</table>
<hr>
</div>
<div class="chapter">
<div class="titlepage"><div><div><h1 class="title">
<a name="Bv9ARM.ch01"></a>Chapter�1.�Introduction</h1></div></div></div>
<div class="toc">
<p><b>Table of Contents</b></p>
<dl class="toc">
<dt><span class="section"><a href="Bv9ARM.ch01.html#doc_scope">Scope of Document</a></span></dt>
<dt><span class="section"><a href="Bv9ARM.ch01.html#organization">Organization of This Document</a></span></dt>
<dt><span class="section"><a href="Bv9ARM.ch01.html#conventions">Conventions Used in This Document</a></span></dt>
<dt><span class="section"><a href="Bv9ARM.ch01.html#dns_overview">The Domain Name System (<acronym class="acronym">DNS</acronym>)</a></span></dt>
<dd><dl>
<dt><span class="section"><a href="Bv9ARM.ch01.html#dns_fundamentals">DNS Fundamentals</a></span></dt>
<dt><span class="section"><a href="Bv9ARM.ch01.html#domain_names">Domains and Domain Names</a></span></dt>
<dt><span class="section"><a href="Bv9ARM.ch01.html#zones">Zones</a></span></dt>
<dt><span class="section"><a href="Bv9ARM.ch01.html#auth_servers">Authoritative Name Servers</a></span></dt>
<dt><span class="section"><a href="Bv9ARM.ch01.html#cache_servers">Caching Name Servers</a></span></dt>
<dt><span class="section"><a href="Bv9ARM.ch01.html#multi_role">Name Servers in Multiple Roles</a></span></dt>
</dl></dd>
</dl>
</div>
<p>
The Internet Domain Name System (<acronym class="acronym">DNS</acronym>)
consists of the syntax
to specify the names of entities in the Internet in a hierarchical
manner, the rules used for delegating authority over names, and the
system implementation that actually maps names to Internet
addresses. <acronym class="acronym">DNS</acronym> data is maintained in a
group of distributed
hierarchical databases.
</p>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="doc_scope"></a>Scope of Document</h2></div></div></div>
<p>
The Berkeley Internet Name Domain
(<acronym class="acronym">BIND</acronym>) implements a
domain name server for a number of operating systems. This
document provides basic information about the installation and
care of the Internet Systems Consortium (<acronym class="acronym">ISC</acronym>)
<acronym class="acronym">BIND</acronym> version 9 software package for
system administrators.
</p>
<p>This version of the manual corresponds to BIND version 9.11.</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="organization"></a>Organization of This Document</h2></div></div></div>
<p>
In this document, <span class="emphasis"><em>Chapter 1</em></span> introduces
the basic <acronym class="acronym">DNS</acronym> and <acronym class="acronym">BIND</acronym> concepts. <span class="emphasis"><em>Chapter 2</em></span>
describes resource requirements for running <acronym class="acronym">BIND</acronym> in various
environments. Information in <span class="emphasis"><em>Chapter 3</em></span> is
<span class="emphasis"><em>task-oriented</em></span> in its presentation and is
organized functionally, to aid in the process of installing the
<acronym class="acronym">BIND</acronym> 9 software. The task-oriented
section is followed by
<span class="emphasis"><em>Chapter 4</em></span>, which contains more advanced
concepts that the system administrator may need for implementing
certain options. <span class="emphasis"><em>Chapter 5</em></span>
describes the <acronym class="acronym">BIND</acronym> 9 lightweight
resolver. The contents of <span class="emphasis"><em>Chapter 6</em></span> are
organized as in a reference manual to aid in the ongoing
maintenance of the software. <span class="emphasis"><em>Chapter 7</em></span> addresses
security considerations, and
<span class="emphasis"><em>Chapter 8</em></span> contains troubleshooting help. The
main body of the document is followed by several
<span class="emphasis"><em>appendices</em></span> which contain useful reference
information, such as a <span class="emphasis"><em>bibliography</em></span> and
historic information related to <acronym class="acronym">BIND</acronym>
and the Domain Name
System.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="conventions"></a>Conventions Used in This Document</h2></div></div></div>
<p>
In this document, we use the following general typographic
conventions:
</p>
<div class="informaltable">
<table border="1">
<colgroup>
<col width="3.000in" class="1">
<col width="2.625in" class="2">
</colgroup>
<tbody>
<tr>
<td>
<p>
<span class="emphasis"><em>To describe:</em></span>
</p>
</td>
<td>
<p>
<span class="emphasis"><em>We use the style:</em></span>
</p>
</td>
</tr>
<tr>
<td>
<p>
a pathname, filename, URL, hostname,
mailing list name, or new term or concept
</p>
</td>
<td>
<p>
<code class="filename">Fixed width</code>
</p>
</td>
</tr>
<tr>
<td>
<p>
literal user
input
</p>
</td>
<td>
<p>
<strong class="userinput"><code>Fixed Width Bold</code></strong>
</p>
</td>
</tr>
<tr>
<td>
<p>
program output
</p>
</td>
<td>
<p>
<code class="computeroutput">Fixed Width</code>
</p>
</td>
</tr>
</tbody>
</table>
</div>
<p>
The following conventions are used in descriptions of the
<acronym class="acronym">BIND</acronym> configuration file:</p>
<div class="informaltable">
<table border="1">
<colgroup>
<col width="3.000in" class="1">
<col width="2.625in" class="2">
</colgroup>
<tbody>
<tr>
<td>
<p>
<span class="emphasis"><em>To describe:</em></span>
</p>
</td>
<td>
<p>
<span class="emphasis"><em>We use the style:</em></span>
</p>
</td>
</tr>
<tr>
<td>
<p>
keywords
</p>
</td>
<td>
<p>
<code class="literal">Fixed Width</code>
</p>
</td>
</tr>
<tr>
<td>
<p>
variables
</p>
</td>
<td>
<p>
<code class="varname">Fixed Width</code>
</p>
</td>
</tr>
<tr>
<td>
<p>
Optional input
</p>
</td>
<td>
<p>
[<span class="optional">Text is enclosed in square brackets</span>]
</p>
</td>
</tr>
</tbody>
</table>
</div>
<p>
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="dns_overview"></a>The Domain Name System (<acronym class="acronym">DNS</acronym>)</h2></div></div></div>
<p>
The purpose of this document is to explain the installation
and upkeep of the <acronym class="acronym">BIND</acronym> (Berkeley Internet
Name Domain) software package, and we
begin by reviewing the fundamentals of the Domain Name System
(<acronym class="acronym">DNS</acronym>) as they relate to <acronym class="acronym">BIND</acronym>.
</p>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="dns_fundamentals"></a>DNS Fundamentals</h3></div></div></div>
<p>
The Domain Name System (DNS) is a hierarchical, distributed
database. It stores information for mapping Internet host names to
IP
addresses and vice versa, mail routing information, and other data
used by Internet applications.
</p>
<p>
Clients look up information in the DNS by calling a
<span class="emphasis"><em>resolver</em></span> library, which sends queries to one or
more <span class="emphasis"><em>name servers</em></span> and interprets the responses.
The <acronym class="acronym">BIND</acronym> 9 software distribution
contains a name server, <span class="command"><strong>named</strong></span>, and a
resolver library, <span class="command"><strong>liblwres</strong></span>.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="domain_names"></a>Domains and Domain Names</h3></div></div></div>
<p>
The data stored in the DNS is identified by <span class="emphasis"><em>domain names</em></span> that are organized as a tree according to
organizational or administrative boundaries. Each node of the tree,
called a <span class="emphasis"><em>domain</em></span>, is given a label. The domain
name of the
node is the concatenation of all the labels on the path from the
node to the <span class="emphasis"><em>root</em></span> node. This is represented
in written form as a string of labels listed from right to left and
separated by dots. A label need only be unique within its parent
domain.
</p>
<p>
For example, a domain name for a host at the
company <span class="emphasis"><em>Example, Inc.</em></span> could be
<code class="literal">ourhost.example.com</code>,
where <code class="literal">com</code> is the
top level domain to which
<code class="literal">ourhost.example.com</code> belongs,
<code class="literal">example</code> is
a subdomain of <code class="literal">com</code>, and
<code class="literal">ourhost</code> is the
name of the host.
</p>
<p>
For administrative purposes, the name space is partitioned into
areas called <span class="emphasis"><em>zones</em></span>, each starting at a node and
extending down to the leaf nodes or to nodes where other zones
start.
The data for each zone is stored in a <span class="emphasis"><em>name server</em></span>, which answers queries about the zone using the
<span class="emphasis"><em>DNS protocol</em></span>.
</p>
<p>
The data associated with each domain name is stored in the
form of <span class="emphasis"><em>resource records</em></span> (<acronym class="acronym">RR</acronym>s).
Some of the supported resource record types are described in
<a class="xref" href="Bv9ARM.ch06.html#types_of_resource_records_and_when_to_use_them" title="Types of Resource Records and When to Use Them">the section called “Types of Resource Records and When to Use Them”</a>.
</p>
<p>
For more detailed information about the design of the DNS and
the DNS protocol, please refer to the standards documents listed in
<a class="xref" href="Bv9ARM.ch11.html#rfcs" title="Request for Comments (RFCs)">the section called “Request for Comments (RFCs)”</a>.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="zones"></a>Zones</h3></div></div></div>
<p>
To properly operate a name server, it is important to understand
the difference between a <span class="emphasis"><em>zone</em></span>
and a <span class="emphasis"><em>domain</em></span>.
</p>
<p>
As stated previously, a zone is a point of delegation in
the <acronym class="acronym">DNS</acronym> tree. A zone consists of
those contiguous parts of the domain
tree for which a name server has complete information and over which
it has authority. It contains all domain names from a certain point
downward in the domain tree except those which are delegated to
other zones. A delegation point is marked by one or more
<span class="emphasis"><em>NS records</em></span> in the
parent zone, which should be matched by equivalent NS records at
the root of the delegated zone.
</p>
<p>
For instance, consider the <code class="literal">example.com</code>
domain which includes names
such as <code class="literal">host.aaa.example.com</code> and
<code class="literal">host.bbb.example.com</code> even though
the <code class="literal">example.com</code> zone includes
only delegations for the <code class="literal">aaa.example.com</code> and
<code class="literal">bbb.example.com</code> zones. A zone can
map
exactly to a single domain, but could also include only part of a
domain, the rest of which could be delegated to other
name servers. Every name in the <acronym class="acronym">DNS</acronym>
tree is a
<span class="emphasis"><em>domain</em></span>, even if it is
<span class="emphasis"><em>terminal</em></span>, that is, has no
<span class="emphasis"><em>subdomains</em></span>. Every subdomain is a domain and
every domain except the root is also a subdomain. The terminology is
not intuitive and we suggest that you read RFCs 1033, 1034 and 1035
to
gain a complete understanding of this difficult and subtle
topic.
</p>
<p>
Though <acronym class="acronym">BIND</acronym> is called a "domain name
server",
it deals primarily in terms of zones. The master and slave
declarations in the <code class="filename">named.conf</code> file
specify
zones, not domains. When you ask some other site if it is willing to
be a slave server for your <span class="emphasis"><em>domain</em></span>, you are
actually asking for slave service for some collection of zones.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="auth_servers"></a>Authoritative Name Servers</h3></div></div></div>
<p>
Each zone is served by at least
one <span class="emphasis"><em>authoritative name server</em></span>,
which contains the complete data for the zone.
To make the DNS tolerant of server and network failures,
most zones have two or more authoritative servers, on
different networks.
</p>
<p>
Responses from authoritative servers have the "authoritative
answer" (AA) bit set in the response packets. This makes them
easy to identify when debugging DNS configurations using tools like
<span class="command"><strong>dig</strong></span> (<a class="xref" href="Bv9ARM.ch03.html#diagnostic_tools" title="Diagnostic Tools">the section called “Diagnostic Tools”</a>).
</p>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="primary_master"></a>The Primary Master</h4></div></div></div>
<p>
The authoritative server where the master copy of the zone
data is maintained is called the
<span class="emphasis"><em>primary master</em></span> server, or simply the
<span class="emphasis"><em>primary</em></span>. Typically it loads the zone
contents from some local file edited by humans or perhaps
generated mechanically from some other local file which is
edited by humans. This file is called the
<span class="emphasis"><em>zone file</em></span> or
<span class="emphasis"><em>master file</em></span>.
</p>
<p>
In some cases, however, the master file may not be edited
by humans at all, but may instead be the result of
<span class="emphasis"><em>dynamic update</em></span> operations.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="slave_server"></a>Slave Servers</h4></div></div></div>
<p>
The other authoritative servers, the <span class="emphasis"><em>slave</em></span>
servers (also known as <span class="emphasis"><em>secondary</em></span> servers)
load the zone contents from another server using a replication
process known as a <span class="emphasis"><em>zone transfer</em></span>.
Typically the data are transferred directly from the primary
master, but it is also possible to transfer it from another
slave. In other words, a slave server may itself act as a
master to a subordinate slave server.
</p>
<p>
Periodically, the slave server must send a refresh query to
determine whether the zone contents have been updated. This
is done by sending a query for the zone's SOA record and
checking whether the SERIAL field has been updated; if so,
a new transfer request is initiated. The timing of these
refresh queries is controlled by the SOA REFRESH and RETRY
fields, but can be overrridden with the
<span class="command"><strong>max-refresh-time</strong></span>,
<span class="command"><strong>min-refresh-time</strong></span>,
<span class="command"><strong>max-retry-time</strong></span>, and
<span class="command"><strong>min-retry-time</strong></span> options.
</p>
<p>
If the zone data cannot be updated within the time specified
by the SOA EXPIRE option (up to a hard-coded maximum of
24 weeks) then the slave zone expires and will no longer
respond to queries.
</p>
</div>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="stealth_server"></a>Stealth Servers</h4></div></div></div>
<p>
Usually all of the zone's authoritative servers are listed in
NS records in the parent zone. These NS records constitute
a <span class="emphasis"><em>delegation</em></span> of the zone from the parent.
The authoritative servers are also listed in the zone file itself,
at the <span class="emphasis"><em>top level</em></span> or <span class="emphasis"><em>apex</em></span>
of the zone. You can list servers in the zone's top-level NS
records that are not in the parent's NS delegation, but you cannot
list servers in the parent's delegation that are not present at
the zone's top level.
</p>
<p>
A <span class="emphasis"><em>stealth server</em></span> is a server that is
authoritative for a zone but is not listed in that zone's NS
records. Stealth servers can be used for keeping a local copy of
a
zone to speed up access to the zone's records or to make sure that
the
zone is available even if all the "official" servers for the zone
are
inaccessible.
</p>
<p>
A configuration where the primary master server itself is a
stealth server is often referred to as a "hidden primary"
configuration. One use for this configuration is when the primary
master
is behind a firewall and therefore unable to communicate directly
with the outside world.
</p>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="cache_servers"></a>Caching Name Servers</h3></div></div></div>
<p>
The resolver libraries provided by most operating systems are
<span class="emphasis"><em>stub resolvers</em></span>, meaning that they are not
capable of
performing the full DNS resolution process by themselves by talking
directly to the authoritative servers. Instead, they rely on a
local
name server to perform the resolution on their behalf. Such a
server
is called a <span class="emphasis"><em>recursive</em></span> name server; it performs
<span class="emphasis"><em>recursive lookups</em></span> for local clients.
</p>
<p>
To improve performance, recursive servers cache the results of
the lookups they perform. Since the processes of recursion and
caching are intimately connected, the terms
<span class="emphasis"><em>recursive server</em></span> and
<span class="emphasis"><em>caching server</em></span> are often used synonymously.
</p>
<p>
The length of time for which a record may be retained in
the cache of a caching name server is controlled by the
Time To Live (TTL) field associated with each resource record.
</p>
<div class="section">
<div class="titlepage"><div><div><h4 class="title">
<a name="forwarder"></a>Forwarding</h4></div></div></div>
<p>
Even a caching name server does not necessarily perform
the complete recursive lookup itself. Instead, it can
<span class="emphasis"><em>forward</em></span> some or all of the queries
that it cannot satisfy from its cache to another caching name
server,
commonly referred to as a <span class="emphasis"><em>forwarder</em></span>.
</p>
<p>
There may be one or more forwarders,
and they are queried in turn until the list is exhausted or an
answer
is found. Forwarders are typically used when you do not
wish all the servers at a given site to interact directly with the
rest of
the Internet servers. A typical scenario would involve a number
of internal <acronym class="acronym">DNS</acronym> servers and an
Internet firewall. Servers unable
to pass packets through the firewall would forward to the server
that can do it, and that server would query the Internet <acronym class="acronym">DNS</acronym> servers
on the internal server's behalf.
</p>
</div>
</div>
<div class="section">
<div class="titlepage"><div><div><h3 class="title">
<a name="multi_role"></a>Name Servers in Multiple Roles</h3></div></div></div>
<p>
The <acronym class="acronym">BIND</acronym> name server can
simultaneously act as
a master for some zones, a slave for other zones, and as a caching
(recursive) server for a set of local clients.
</p>
<p>
However, since the functions of authoritative name service
and caching/recursive name service are logically separate, it is
often advantageous to run them on separate server machines.
A server that only provides authoritative name service
(an <span class="emphasis"><em>authoritative-only</em></span> server) can run with
recursion disabled, improving reliability and security.
A server that is not authoritative for any zones and only provides
recursive service to local
clients (a <span class="emphasis"><em>caching-only</em></span> server)
does not need to be reachable from the Internet at large and can
be placed inside a firewall.
</p>
</div>
</div>
</div>
<div class="navfooter">
<hr>
<table width="100%" summary="Navigation footer">
<tr>
<td width="40%" align="left">
<a accesskey="p" href="Bv9ARM.html">Prev</a>�</td>
<td width="20%" align="center">�</td>
<td width="40%" align="right">�<a accesskey="n" href="Bv9ARM.ch02.html">Next</a>
</td>
</tr>
<tr>
<td width="40%" align="left" valign="top">BIND 9 Administrator Reference Manual�</td>
<td width="20%" align="center"><a accesskey="h" href="Bv9ARM.html">Home</a></td>
<td width="40%" align="right" valign="top">�Chapter�2.�<acronym class="acronym">BIND</acronym> Resource Requirements</td>
</tr>
</table>
</div>
<p xmlns:db="http://docbook.org/ns/docbook" style="text-align: center;">BIND 9.11.5-P4 (Extended Support Version)</p>
</body>
</html>
|
