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    31. 

  31. <tr><th colspan="3" align="center">Chapter 1. Introduction</th></tr>
    32. 

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  41. </div>
    42. 

  42. <div class="chapter" lang="en">
    43. 

  43. <div class="titlepage"><div><div><h2 class="title">
    44. 

  44. <a name="Bv9ARM.ch01"></a>Chapter 1. Introduction</h2></div></div></div>
    45. 

  45. <div class="toc">
    46. 

  46. <p><b>Table of Contents</b></p>
    47. 

  47. <dl>
    48. 

  48. <dt><span class="sect1"><a href="Bv9ARM.ch01.html#id2564375">Scope of Document</a></span></dt>
    49. 

  49. <dt><span class="sect1"><a href="Bv9ARM.ch01.html#id2564398">Organization of This Document</a></span></dt>
    50. 

  50. <dt><span class="sect1"><a href="Bv9ARM.ch01.html#id2564538">Conventions Used in This Document</a></span></dt>
    51. 

  51. <dt><span class="sect1"><a href="Bv9ARM.ch01.html#id2564720">The Domain Name System (<acronym class="acronym">DNS</acronym>)</a></span></dt>
    52. 

  52. <dd><dl>
    53. 

  53. <dt><span class="sect2"><a href="Bv9ARM.ch01.html#id2564741">DNS Fundamentals</a></span></dt>
    54. 

  54. <dt><span class="sect2"><a href="Bv9ARM.ch01.html#id2564775">Domains and Domain Names</a></span></dt>
    55. 

  55. <dt><span class="sect2"><a href="Bv9ARM.ch01.html#id2567180">Zones</a></span></dt>
    56. 

  56. <dt><span class="sect2"><a href="Bv9ARM.ch01.html#id2567257">Authoritative Name Servers</a></span></dt>
    57. 

  57. <dt><span class="sect2"><a href="Bv9ARM.ch01.html#id2567430">Caching Name Servers</a></span></dt>
    58. 

  58. <dt><span class="sect2"><a href="Bv9ARM.ch01.html#id2567560">Name Servers in Multiple Roles</a></span></dt>
    59. 

  59. </dl></dd>
    60. 

  60. </dl>
    61. 

  61. </div>
    62. 

  62. <p>
    63. 

  63.       The Internet Domain Name System (<acronym class="acronym">DNS</acronym>)
    64. 

  64.       consists of the syntax
    65. 

  65.       to specify the names of entities in the Internet in a hierarchical
    66. 

  66.       manner, the rules used for delegating authority over names, and the
    67. 

  67.       system implementation that actually maps names to Internet
    68. 

  68.       addresses.  <acronym class="acronym">DNS</acronym> data is maintained in a
    69. 

  69.       group of distributed
    70. 

  70.       hierarchical databases.
    71. 

  71.     </p>
    72. 

  72. <div class="sect1" lang="en">
    73. 

  73. <div class="titlepage"><div><div><h2 class="title" style="clear: both">
    74. 

  74. <a name="id2564375"></a>Scope of Document</h2></div></div></div>
    75. 

  75. <p>
    76. 

  76.         The Berkeley Internet Name Domain
    77. 

  77.         (<acronym class="acronym">BIND</acronym>) implements a
    78. 

  78.         domain name server for a number of operating systems. This
    79. 

  79.         document provides basic information about the installation and
    80. 

  80.         care of the Internet Systems Consortium (<acronym class="acronym">ISC</acronym>)
    81. 

  81.         <acronym class="acronym">BIND</acronym> version 9 software package for
    82. 

  82.         system administrators.
    83. 

  83.       </p>
    84. 

  84. <p>
    85. 

  85.         This version of the manual corresponds to BIND version 9.8.
    86. 

  86.       </p>
    87. 

  87. </div>
    88. 

  88. <div class="sect1" lang="en">
    89. 

  89. <div class="titlepage"><div><div><h2 class="title" style="clear: both">
    90. 

  90. <a name="id2564398"></a>Organization of This Document</h2></div></div></div>
    91. 

  91. <p>
    92. 

  92.         In this document, <span class="emphasis"><em>Chapter 1</em></span> introduces
    93. 

  93.         the basic <acronym class="acronym">DNS</acronym> and <acronym class="acronym">BIND</acronym> concepts. <span class="emphasis"><em>Chapter 2</em></span>
    94. 

  94.         describes resource requirements for running <acronym class="acronym">BIND</acronym> in various
    95. 

  95.         environments. Information in <span class="emphasis"><em>Chapter 3</em></span> is
    96. 

  96.         <span class="emphasis"><em>task-oriented</em></span> in its presentation and is
    97. 

  97.         organized functionally, to aid in the process of installing the
    98. 

  98.         <acronym class="acronym">BIND</acronym> 9 software. The task-oriented
    99. 

  99.         section is followed by
    100. 

  100.         <span class="emphasis"><em>Chapter 4</em></span>, which contains more advanced
    101. 

  101.         concepts that the system administrator may need for implementing
    102. 

  102.         certain options. <span class="emphasis"><em>Chapter 5</em></span>
    103. 

  103.         describes the <acronym class="acronym">BIND</acronym> 9 lightweight
    104. 

  104.         resolver.  The contents of <span class="emphasis"><em>Chapter 6</em></span> are
    105. 

  105.         organized as in a reference manual to aid in the ongoing
    106. 

  106.         maintenance of the software. <span class="emphasis"><em>Chapter 7</em></span> addresses
    107. 

  107.         security considerations, and
    108. 

  108.         <span class="emphasis"><em>Chapter 8</em></span> contains troubleshooting help. The
    109. 

  109.         main body of the document is followed by several
    110. 

  110.         <span class="emphasis"><em>appendices</em></span> which contain useful reference
    111. 

  111.         information, such as a <span class="emphasis"><em>bibliography</em></span> and
    112. 

  112.         historic information related to <acronym class="acronym">BIND</acronym>
    113. 

  113.         and the Domain Name
    114. 

  114.         System.
    115. 

  115.       </p>
    116. 

  116. </div>
    117. 

  117. <div class="sect1" lang="en">
    118. 

  118. <div class="titlepage"><div><div><h2 class="title" style="clear: both">
    119. 

  119. <a name="id2564538"></a>Conventions Used in This Document</h2></div></div></div>
    120. 

  120. <p>
    121. 

  121.         In this document, we use the following general typographic
    122. 

  122.         conventions:
    123. 

  123.       </p>
    124. 

  124. <div class="informaltable"><table border="1">
    125. 

  125. <colgroup>
    126. 

  126. <col>
    127. 

  127. <col>
    128. 

  128. </colgroup>
    129. 

  129. <tbody>
    130. 

  130. <tr>
    131. 

  131. <td>
    132. 

  132.                 <p>
    133. 

  133.                   <span class="emphasis"><em>To describe:</em></span>
    134. 

  134.                 </p>
    135. 

  135.               </td>
    136. 

  136. <td>
    137. 

  137.                 <p>
    138. 

  138.                   <span class="emphasis"><em>We use the style:</em></span>
    139. 

  139.                 </p>
    140. 

  140.               </td>
    141. 

  141. </tr>
    142. 

  142. <tr>
    143. 

  143. <td>
    144. 

  144.                 <p>
    145. 

  145.                   a pathname, filename, URL, hostname,
    146. 

  146.                   mailing list name, or new term or concept
    147. 

  147.                 </p>
    148. 

  148.               </td>
    149. 

  149. <td>
    150. 

  150.                 <p>
    151. 

  151.                   <code class="filename">Fixed width</code>
    152. 

  152.                 </p>
    153. 

  153.               </td>
    154. 

  154. </tr>
    155. 

  155. <tr>
    156. 

  156. <td>
    157. 

  157.                 <p>
    158. 

  158.                   literal user
    159. 

  159.                   input
    160. 

  160.                 </p>
    161. 

  161.               </td>
    162. 

  162. <td>
    163. 

  163.                 <p>
    164. 

  164.                   <strong class="userinput"><code>Fixed Width Bold</code></strong>
    165. 

  165.                 </p>
    166. 

  166.               </td>
    167. 

  167. </tr>
    168. 

  168. <tr>
    169. 

  169. <td>
    170. 

  170.                 <p>
    171. 

  171.                   program output
    172. 

  172.                 </p>
    173. 

  173.               </td>
    174. 

  174. <td>
    175. 

  175.                 <p>
    176. 

  176.                   <code class="computeroutput">Fixed Width</code>
    177. 

  177.                 </p>
    178. 

  178.               </td>
    179. 

  179. </tr>
    180. 

  180. </tbody>
    181. 

  181. </table></div>
    182. 

  182. <p>
    183. 

  183.         The following conventions are used in descriptions of the
    184. 

  184.         <acronym class="acronym">BIND</acronym> configuration file:</p>
    185. 

  185. <div class="informaltable"><table border="1">
    186. 

  186. <colgroup>
    187. 

  187. <col>
    188. 

  188. <col>
    189. 

  189. </colgroup>
    190. 

  190. <tbody>
    191. 

  191. <tr>
    192. 

  192. <td>
    193. 

  193.                   <p>
    194. 

  194.                     <span class="emphasis"><em>To describe:</em></span>
    195. 

  195.                   </p>
    196. 

  196.                 </td>
    197. 

  197. <td>
    198. 

  198.                   <p>
    199. 

  199.                     <span class="emphasis"><em>We use the style:</em></span>
    200. 

  200.                   </p>
    201. 

  201.                 </td>
    202. 

  202. </tr>
    203. 

  203. <tr>
    204. 

  204. <td>
    205. 

  205.                   <p>
    206. 

  206.                     keywords
    207. 

  207.                   </p>
    208. 

  208.                 </td>
    209. 

  209. <td>
    210. 

  210.                   <p>
    211. 

  211.                     <code class="literal">Fixed Width</code>
    212. 

  212.                   </p>
    213. 

  213.                 </td>
    214. 

  214. </tr>
    215. 

  215. <tr>
    216. 

  216. <td>
    217. 

  217.                   <p>
    218. 

  218.                     variables
    219. 

  219.                   </p>
    220. 

  220.                 </td>
    221. 

  221. <td>
    222. 

  222.                   <p>
    223. 

  223.                     <code class="varname">Fixed Width</code>
    224. 

  224.                   </p>
    225. 

  225.                 </td>
    226. 

  226. </tr>
    227. 

  227. <tr>
    228. 

  228. <td>
    229. 

  229.                   <p>
    230. 

  230.                     Optional input
    231. 

  231.                   </p>
    232. 

  232.                 </td>
    233. 

  233. <td>
    234. 

  234.                   <p>
    235. 

  235.                     [<span class="optional">Text is enclosed in square brackets</span>]
    236. 

  236.                   </p>
    237. 

  237.                 </td>
    238. 

  238. </tr>
    239. 

  239. </tbody>
    240. 

  240. </table></div>
    241. 

  241. <p>
    242. 

  242.       </p>
    243. 

  243. </div>
    244. 

  244. <div class="sect1" lang="en">
    245. 

  245. <div class="titlepage"><div><div><h2 class="title" style="clear: both">
    246. 

  246. <a name="id2564720"></a>The Domain Name System (<acronym class="acronym">DNS</acronym>)</h2></div></div></div>
    247. 

  247. <p>
    248. 

  248.         The purpose of this document is to explain the installation
    249. 

  249.         and upkeep of the <acronym class="acronym">BIND</acronym> (Berkeley Internet
    250. 

  250.         Name Domain) software package, and we
    251. 

  251.         begin by reviewing the fundamentals of the Domain Name System
    252. 

  252.         (<acronym class="acronym">DNS</acronym>) as they relate to <acronym class="acronym">BIND</acronym>.
    253. 

  253.       </p>
    254. 

  254. <div class="sect2" lang="en">
    255. 

  255. <div class="titlepage"><div><div><h3 class="title">
    256. 

  256. <a name="id2564741"></a>DNS Fundamentals</h3></div></div></div>
    257. 

  257. <p>
    258. 

  258.           The Domain Name System (DNS) is a hierarchical, distributed
    259. 

  259.           database.  It stores information for mapping Internet host names to
    260. 

  260.           IP
    261. 

  261.           addresses and vice versa, mail routing information, and other data
    262. 

  262.           used by Internet applications.
    263. 

  263.         </p>
    264. 

  264. <p>
    265. 

  265.           Clients look up information in the DNS by calling a
    266. 

  266.           <span class="emphasis"><em>resolver</em></span> library, which sends queries to one or
    267. 

  267.           more <span class="emphasis"><em>name servers</em></span> and interprets the responses.
    268. 

  268.           The <acronym class="acronym">BIND</acronym> 9 software distribution
    269. 

  269.           contains a
    270. 

  270.           name server, <span><strong class="command">named</strong></span>, and a resolver
    271. 

  271.           library, <span><strong class="command">liblwres</strong></span>.  The older
    272. 

  272.           <span><strong class="command">libbind</strong></span> resolver library is also available
    273. 

  273.           from ISC as a separate download.
    274. 

  274.         </p>
    275. 

  275. </div>
    276. 

  276. <div class="sect2" lang="en">
    277. 

  277. <div class="titlepage"><div><div><h3 class="title">
    278. 

  278. <a name="id2564775"></a>Domains and Domain Names</h3></div></div></div>
    279. 

  279. <p>
    280. 

  280.           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
    281. 

  281.           organizational or administrative boundaries. Each node of the tree,
    282. 

  282.           called a <span class="emphasis"><em>domain</em></span>, is given a label. The domain
    283. 

  283.           name of the
    284. 

  284.           node is the concatenation of all the labels on the path from the
    285. 

  285.           node to the <span class="emphasis"><em>root</em></span> node.  This is represented
    286. 

  286.           in written form as a string of labels listed from right to left and
    287. 

  287.           separated by dots. A label need only be unique within its parent
    288. 

  288.           domain.
    289. 

  289.         </p>
    290. 

  290. <p>
    291. 

  291.           For example, a domain name for a host at the
    292. 

  292.           company <span class="emphasis"><em>Example, Inc.</em></span> could be
    293. 

  293.           <code class="literal">ourhost.example.com</code>,
    294. 

  294.           where <code class="literal">com</code> is the
    295. 

  295.           top level domain to which
    296. 

  296.           <code class="literal">ourhost.example.com</code> belongs,
    297. 

  297.           <code class="literal">example</code> is
    298. 

  298.           a subdomain of <code class="literal">com</code>, and
    299. 

  299.           <code class="literal">ourhost</code> is the
    300. 

  300.           name of the host.
    301. 

  301.         </p>
    302. 

  302. <p>
    303. 

  303.           For administrative purposes, the name space is partitioned into
    304. 

  304.           areas called <span class="emphasis"><em>zones</em></span>, each starting at a node and
    305. 

  305.           extending down to the leaf nodes or to nodes where other zones
    306. 

  306.           start.
    307. 

  307.           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
    308. 

  308.           <span class="emphasis"><em>DNS protocol</em></span>.
    309. 

  309.         </p>
    310. 

  310. <p>
    311. 

  311.           The data associated with each domain name is stored in the
    312. 

  312.           form of <span class="emphasis"><em>resource records</em></span> (<acronym class="acronym">RR</acronym>s).
    313. 

  313.           Some of the supported resource record types are described in
    314. 

  314.           <a 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 &#8220;Types of Resource Records and When to Use Them&#8221;</a>.
    315. 

  315.         </p>
    316. 

  316. <p>
    317. 

  317.           For more detailed information about the design of the DNS and
    318. 

  318.           the DNS protocol, please refer to the standards documents listed in
    319. 

  319.           <a href="Bv9ARM.ch09.html#rfcs" title="Request for Comments (RFCs)">the section called &#8220;Request for Comments (RFCs)&#8221;</a>.
    320. 

  320.         </p>
    321. 

  321. </div>
    322. 

  322. <div class="sect2" lang="en">
    323. 

  323. <div class="titlepage"><div><div><h3 class="title">
    324. 

  324. <a name="id2567180"></a>Zones</h3></div></div></div>
    325. 

  325. <p>
    326. 

  326.           To properly operate a name server, it is important to understand
    327. 

  327.           the difference between a <span class="emphasis"><em>zone</em></span>
    328. 

  328.           and a <span class="emphasis"><em>domain</em></span>.
    329. 

  329.         </p>
    330. 

  330. <p>
    331. 

  331.           As stated previously, a zone is a point of delegation in
    332. 

  332.           the <acronym class="acronym">DNS</acronym> tree. A zone consists of
    333. 

  333.           those contiguous parts of the domain
    334. 

  334.           tree for which a name server has complete information and over which
    335. 

  335.           it has authority. It contains all domain names from a certain point
    336. 

  336.           downward in the domain tree except those which are delegated to
    337. 

  337.           other zones. A delegation point is marked by one or more
    338. 

  338.           <span class="emphasis"><em>NS records</em></span> in the
    339. 

  339.           parent zone, which should be matched by equivalent NS records at
    340. 

  340.           the root of the delegated zone.
    341. 

  341.         </p>
    342. 

  342. <p>
    343. 

  343.           For instance, consider the <code class="literal">example.com</code>
    344. 

  344.           domain which includes names
    345. 

  345.           such as <code class="literal">host.aaa.example.com</code> and
    346. 

  346.           <code class="literal">host.bbb.example.com</code> even though
    347. 

  347.           the <code class="literal">example.com</code> zone includes
    348. 

  348.           only delegations for the <code class="literal">aaa.example.com</code> and
    349. 

  349.           <code class="literal">bbb.example.com</code> zones.  A zone can
    350. 

  350.           map
    351. 

  351.           exactly to a single domain, but could also include only part of a
    352. 

  352.           domain, the rest of which could be delegated to other
    353. 

  353.           name servers. Every name in the <acronym class="acronym">DNS</acronym>
    354. 

  354.           tree is a
    355. 

  355.           <span class="emphasis"><em>domain</em></span>, even if it is
    356. 

  356.           <span class="emphasis"><em>terminal</em></span>, that is, has no
    357. 

  357.           <span class="emphasis"><em>subdomains</em></span>.  Every subdomain is a domain and
    358. 

  358.           every domain except the root is also a subdomain. The terminology is
    359. 

  359.           not intuitive and we suggest that you read RFCs 1033, 1034 and 1035
    360. 

  360.           to
    361. 

  361.           gain a complete understanding of this difficult and subtle
    362. 

  362.           topic.
    363. 

  363.         </p>
    364. 

  364. <p>
    365. 

  365.           Though <acronym class="acronym">BIND</acronym> is called a "domain name
    366. 

  366.           server",
    367. 

  367.           it deals primarily in terms of zones. The master and slave
    368. 

  368.           declarations in the <code class="filename">named.conf</code> file
    369. 

  369.           specify
    370. 

  370.           zones, not domains. When you ask some other site if it is willing to
    371. 

  371.           be a slave server for your <span class="emphasis"><em>domain</em></span>, you are
    372. 

  372.           actually asking for slave service for some collection of zones.
    373. 

  373.         </p>
    374. 

  374. </div>
    375. 

  375. <div class="sect2" lang="en">
    376. 

  376. <div class="titlepage"><div><div><h3 class="title">
    377. 

  377. <a name="id2567257"></a>Authoritative Name Servers</h3></div></div></div>
    378. 

  378. <p>
    379. 

  379.           Each zone is served by at least
    380. 

  380.           one <span class="emphasis"><em>authoritative name server</em></span>,
    381. 

  381.           which contains the complete data for the zone.
    382. 

  382.           To make the DNS tolerant of server and network failures,
    383. 

  383.           most zones have two or more authoritative servers, on
    384. 

  384.           different networks.
    385. 

  385.         </p>
    386. 

  386. <p>
    387. 

  387.           Responses from authoritative servers have the "authoritative
    388. 

  388.           answer" (AA) bit set in the response packets.  This makes them
    389. 

  389.           easy to identify when debugging DNS configurations using tools like
    390. 

  390.           <span><strong class="command">dig</strong></span> (<a href="Bv9ARM.ch03.html#diagnostic_tools" title="Diagnostic Tools">the section called &#8220;Diagnostic Tools&#8221;</a>).
    391. 

  391.         </p>
    392. 

  392. <div class="sect3" lang="en">
    393. 

  393. <div class="titlepage"><div><div><h4 class="title">
    394. 

  394. <a name="id2567281"></a>The Primary Master</h4></div></div></div>
    395. 

  395. <p>
    396. 

  396.             The authoritative server where the master copy of the zone
    397. 

  397.             data is maintained is called the
    398. 

  398.             <span class="emphasis"><em>primary master</em></span> server, or simply the
    399. 

  399.             <span class="emphasis"><em>primary</em></span>.  Typically it loads the zone
    400. 

  400.             contents from some local file edited by humans or perhaps
    401. 

  401.             generated mechanically from some other local file which is
    402. 

  402.             edited by humans.  This file is called the
    403. 

  403.             <span class="emphasis"><em>zone file</em></span> or
    404. 

  404.             <span class="emphasis"><em>master file</em></span>.
    405. 

  405.           </p>
    406. 

  406. <p>
    407. 

  407.             In some cases, however, the master file may not be edited
    408. 

  408.             by humans at all, but may instead be the result of
    409. 

  409.             <span class="emphasis"><em>dynamic update</em></span> operations.
    410. 

  410.           </p>
    411. 

  411. </div>
    412. 

  412. <div class="sect3" lang="en">
    413. 

  413. <div class="titlepage"><div><div><h4 class="title">
    414. 

  414. <a name="id2567379"></a>Slave Servers</h4></div></div></div>
    415. 

  415. <p>
    416. 

  416.             The other authoritative servers, the <span class="emphasis"><em>slave</em></span>
    417. 

  417.             servers (also known as <span class="emphasis"><em>secondary</em></span> servers)
    418. 

  418.             load
    419. 

  419.             the zone contents from another server using a replication process
    420. 

  420.             known as a <span class="emphasis"><em>zone transfer</em></span>.  Typically the data
    421. 

  421.             are
    422. 

  422.             transferred directly from the primary master, but it is also
    423. 

  423.             possible
    424. 

  424.             to transfer it from another slave.  In other words, a slave server
    425. 

  425.             may itself act as a master to a subordinate slave server.
    426. 

  426.           </p>
    427. 

  427. </div>
    428. 

  428. <div class="sect3" lang="en">
    429. 

  429. <div class="titlepage"><div><div><h4 class="title">
    430. 

  430. <a name="id2567400"></a>Stealth Servers</h4></div></div></div>
    431. 

  431. <p>
    432. 

  432.             Usually all of the zone's authoritative servers are listed in
    433. 

  433.             NS records in the parent zone.  These NS records constitute
    434. 

  434.             a <span class="emphasis"><em>delegation</em></span> of the zone from the parent.
    435. 

  435.             The authoritative servers are also listed in the zone file itself,
    436. 

  436.             at the <span class="emphasis"><em>top level</em></span> or <span class="emphasis"><em>apex</em></span>
    437. 

  437.             of the zone.  You can list servers in the zone's top-level NS
    438. 

  438.             records that are not in the parent's NS delegation, but you cannot
    439. 

  439.             list servers in the parent's delegation that are not present at
    440. 

  440.             the zone's top level.
    441. 

  441.           </p>
    442. 

  442. <p>
    443. 

  443.             A <span class="emphasis"><em>stealth server</em></span> is a server that is
    444. 

  444.             authoritative for a zone but is not listed in that zone's NS
    445. 

  445.             records.  Stealth servers can be used for keeping a local copy of
    446. 

  446.             a
    447. 

  447.             zone to speed up access to the zone's records or to make sure that
    448. 

  448.             the
    449. 

  449.             zone is available even if all the "official" servers for the zone
    450. 

  450.             are
    451. 

  451.             inaccessible.
    452. 

  452.           </p>
    453. 

  453. <p>
    454. 

  454.             A configuration where the primary master server itself is a
    455. 

  455.             stealth server is often referred to as a "hidden primary"
    456. 

  456.             configuration.  One use for this configuration is when the primary
    457. 

  457.             master
    458. 

  458.             is behind a firewall and therefore unable to communicate directly
    459. 

  459.             with the outside world.
    460. 

  460.           </p>
    461. 

  461. </div>
    462. 

  462. </div>
    463. 

  463. <div class="sect2" lang="en">
    464. 

  464. <div class="titlepage"><div><div><h3 class="title">
    465. 

  465. <a name="id2567430"></a>Caching Name Servers</h3></div></div></div>
    466. 

  466. <p>
    467. 

  467.           The resolver libraries provided by most operating systems are
    468. 

  468.           <span class="emphasis"><em>stub resolvers</em></span>, meaning that they are not
    469. 

  469.           capable of
    470. 

  470.           performing the full DNS resolution process by themselves by talking
    471. 

  471.           directly to the authoritative servers.  Instead, they rely on a
    472. 

  472.           local
    473. 

  473.           name server to perform the resolution on their behalf.  Such a
    474. 

  474.           server
    475. 

  475.           is called a <span class="emphasis"><em>recursive</em></span> name server; it performs
    476. 

  476.           <span class="emphasis"><em>recursive lookups</em></span> for local clients.
    477. 

  477.         </p>
    478. 

  478. <p>
    479. 

  479.           To improve performance, recursive servers cache the results of
    480. 

  480.           the lookups they perform.  Since the processes of recursion and
    481. 

  481.           caching are intimately connected, the terms
    482. 

  482.           <span class="emphasis"><em>recursive server</em></span> and
    483. 

  483.           <span class="emphasis"><em>caching server</em></span> are often used synonymously.
    484. 

  484.         </p>
    485. 

  485. <p>
    486. 

  486.           The length of time for which a record may be retained in
    487. 

  487.           the cache of a caching name server is controlled by the
    488. 

  488.           Time To Live (TTL) field associated with each resource record.
    489. 

  489.         </p>
    490. 

  490. <div class="sect3" lang="en">
    491. 

  491. <div class="titlepage"><div><div><h4 class="title">
    492. 

  492. <a name="id2567533"></a>Forwarding</h4></div></div></div>
    493. 

  493. <p>
    494. 

  494.             Even a caching name server does not necessarily perform
    495. 

  495.             the complete recursive lookup itself.  Instead, it can
    496. 

  496.             <span class="emphasis"><em>forward</em></span> some or all of the queries
    497. 

  497.             that it cannot satisfy from its cache to another caching name
    498. 

  498.             server,
    499. 

  499.             commonly referred to as a <span class="emphasis"><em>forwarder</em></span>.
    500. 

  500.           </p>
    501. 

  501. <p>
    502. 

  502.             There may be one or more forwarders,
    503. 

  503.             and they are queried in turn until the list is exhausted or an
    504. 

  504.             answer
    505. 

  505.             is found. Forwarders are typically used when you do not
    506. 

  506.             wish all the servers at a given site to interact directly with the
    507. 

  507.             rest of
    508. 

  508.             the Internet servers. A typical scenario would involve a number
    509. 

  509.             of internal <acronym class="acronym">DNS</acronym> servers and an
    510. 

  510.             Internet firewall. Servers unable
    511. 

  511.             to pass packets through the firewall would forward to the server
    512. 

  512.             that can do it, and that server would query the Internet <acronym class="acronym">DNS</acronym> servers
    513. 

  513.             on the internal server's behalf.
    514. 

  514.           </p>
    515. 

  515. </div>
    516. 

  516. </div>
    517. 

  517. <div class="sect2" lang="en">
    518. 

  518. <div class="titlepage"><div><div><h3 class="title">
    519. 

  519. <a name="id2567560"></a>Name Servers in Multiple Roles</h3></div></div></div>
    520. 

  520. <p>
    521. 

  521.           The <acronym class="acronym">BIND</acronym> name server can
    522. 

  522.           simultaneously act as
    523. 

  523.           a master for some zones, a slave for other zones, and as a caching
    524. 

  524.           (recursive) server for a set of local clients.
    525. 

  525.         </p>
    526. 

  526. <p>
    527. 

  527.           However, since the functions of authoritative name service
    528. 

  528.           and caching/recursive name service are logically separate, it is
    529. 

  529.           often advantageous to run them on separate server machines.
    530. 

  530. 

  531.           A server that only provides authoritative name service
    532. 

  532.           (an <span class="emphasis"><em>authoritative-only</em></span> server) can run with
    533. 

  533.           recursion disabled, improving reliability and security.
    534. 

  534. 

  535.           A server that is not authoritative for any zones and only provides
    536. 

  536.           recursive service to local
    537. 

  537.           clients (a <span class="emphasis"><em>caching-only</em></span> server)
    538. 

  538.           does not need to be reachable from the Internet at large and can
    539. 

  539.           be placed inside a firewall.
    540. 

  540.         </p>
    541. 

  541. </div>
    542. 

  542. </div>
    543. 

  543. </div>
    544. 

  544. <div class="navfooter">
    545. 

  545. <hr>
    546. 

  546. <table width="100%" summary="Navigation footer">
    547. 

  547. <tr>
    548. 

  548. <td width="40%" align="left">
    549. 

  549. <a accesskey="p" href="Bv9ARM.html">Prev</a> </td>
    550. 

  550. <td width="20%" align="center"> </td>
    551. 

  551. <td width="40%" align="right"> <a accesskey="n" href="Bv9ARM.ch02.html">Next</a>
    552. 

  552. </td>
    553. 

  553. </tr>
    554. 

  554. <tr>
    555. 

  555. <td width="40%" align="left" valign="top">BIND 9 Administrator Reference Manual </td>
    556. 

  556. <td width="20%" align="center"><a accesskey="h" href="Bv9ARM.html">Home</a></td>
    557. 

  557. <td width="40%" align="right" valign="top"> Chapter 2. <acronym class="acronym">BIND</acronym> Resource Requirements</td>
    558. 

  558. </tr>
    559. 

  559. </table>
    560. 

  560. </div>
    561. 

  561. </body>
    562. 

  562. </html>
    563.