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<TITLE>BBS水木清华站∶精华区</TITLE></HEAD><BODY><CENTER><H1>BBS水木清华站∶精华区</H1></CENTER>Network&nbsp;Working&nbsp;Group&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;P.&nbsp;Mockapetris&nbsp;<BR>Request&nbsp;for&nbsp;Comments:&nbsp;1034&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;ISI&nbsp;<BR>Obsoletes:&nbsp;RFCs&nbsp;882,&nbsp;883,&nbsp;973&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;November&nbsp;1987&nbsp;<BR>&nbsp;<BR>&nbsp;<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;DOMAIN&nbsp;NAMES&nbsp;-&nbsp;CONCEPTS&nbsp;AND&nbsp;FACILITIES&nbsp;<BR>&nbsp;<BR>&nbsp;<BR>&nbsp;<BR>1.&nbsp;STATUS&nbsp;OF&nbsp;THIS&nbsp;MEMO&nbsp;<BR>&nbsp;<BR>This&nbsp;RFC&nbsp;is&nbsp;an&nbsp;introduction&nbsp;to&nbsp;the&nbsp;Domain&nbsp;Name&nbsp;System&nbsp;(DNS),&nbsp;and&nbsp;omits&nbsp;<BR>many&nbsp;details&nbsp;which&nbsp;can&nbsp;be&nbsp;found&nbsp;in&nbsp;a&nbsp;companion&nbsp;RFC,&nbsp;&quot;Domain&nbsp;Names&nbsp;-&nbsp;<BR>Implementation&nbsp;and&nbsp;Specification&quot;&nbsp;[RFC-1035].&nbsp;&nbsp;That&nbsp;RFC&nbsp;assumes&nbsp;that&nbsp;the&nbsp;<BR>reader&nbsp;is&nbsp;familiar&nbsp;with&nbsp;the&nbsp;concepts&nbsp;discussed&nbsp;in&nbsp;this&nbsp;memo.&nbsp;<BR>&nbsp;<BR>A&nbsp;subset&nbsp;of&nbsp;DNS&nbsp;functions&nbsp;and&nbsp;data&nbsp;types&nbsp;constitute&nbsp;an&nbsp;official&nbsp;<BR>protocol.&nbsp;&nbsp;The&nbsp;official&nbsp;protocol&nbsp;includes&nbsp;standard&nbsp;queries&nbsp;and&nbsp;their&nbsp;<BR>responses&nbsp;and&nbsp;most&nbsp;of&nbsp;the&nbsp;Internet&nbsp;class&nbsp;data&nbsp;formats&nbsp;(e.g.,&nbsp;host&nbsp;<BR>addresses).&nbsp;<BR>&nbsp;<BR>However,&nbsp;the&nbsp;domain&nbsp;system&nbsp;is&nbsp;intentionally&nbsp;extensible.&nbsp;&nbsp;Researchers&nbsp;are&nbsp;<BR>continuously&nbsp;proposing,&nbsp;implementing&nbsp;and&nbsp;experimenting&nbsp;with&nbsp;new&nbsp;data&nbsp;<BR>types,&nbsp;query&nbsp;types,&nbsp;classes,&nbsp;functions,&nbsp;etc.&nbsp;&nbsp;Thus&nbsp;while&nbsp;the&nbsp;components&nbsp;<BR>of&nbsp;the&nbsp;official&nbsp;protocol&nbsp;are&nbsp;expected&nbsp;to&nbsp;stay&nbsp;essentially&nbsp;unchanged&nbsp;and&nbsp;<BR>operate&nbsp;as&nbsp;a&nbsp;production&nbsp;service,&nbsp;experimental&nbsp;behavior&nbsp;should&nbsp;always&nbsp;be&nbsp;<BR>expected&nbsp;in&nbsp;extensions&nbsp;beyond&nbsp;the&nbsp;official&nbsp;protocol.&nbsp;&nbsp;Experimental&nbsp;or&nbsp;<BR>obsolete&nbsp;features&nbsp;are&nbsp;clearly&nbsp;marked&nbsp;in&nbsp;these&nbsp;RFCs,&nbsp;and&nbsp;such&nbsp;information&nbsp;<BR>should&nbsp;be&nbsp;used&nbsp;with&nbsp;caution.&nbsp;<BR>&nbsp;<BR>The&nbsp;reader&nbsp;is&nbsp;especially&nbsp;cautioned&nbsp;not&nbsp;to&nbsp;depend&nbsp;on&nbsp;the&nbsp;values&nbsp;which&nbsp;<BR>appear&nbsp;in&nbsp;examples&nbsp;to&nbsp;be&nbsp;current&nbsp;or&nbsp;complete,&nbsp;since&nbsp;their&nbsp;purpose&nbsp;is&nbsp;<BR>primarily&nbsp;pedagogical.&nbsp;&nbsp;Distribution&nbsp;of&nbsp;this&nbsp;memo&nbsp;is&nbsp;unlimited.&nbsp;<BR>&nbsp;<BR>2.&nbsp;INTRODUCTION&nbsp;<BR>&nbsp;<BR>This&nbsp;RFC&nbsp;introduces&nbsp;domain&nbsp;style&nbsp;names,&nbsp;their&nbsp;use&nbsp;for&nbsp;Internet&nbsp;mail&nbsp;and&nbsp;<BR>host&nbsp;address&nbsp;support,&nbsp;and&nbsp;the&nbsp;protocols&nbsp;and&nbsp;servers&nbsp;used&nbsp;to&nbsp;implement&nbsp;<BR>domain&nbsp;name&nbsp;facilities.&nbsp;<BR>&nbsp;<BR>2.1.&nbsp;The&nbsp;history&nbsp;of&nbsp;domain&nbsp;names&nbsp;<BR>&nbsp;<BR>The&nbsp;impetus&nbsp;for&nbsp;the&nbsp;development&nbsp;of&nbsp;the&nbsp;domain&nbsp;system&nbsp;was&nbsp;growth&nbsp;in&nbsp;the&nbsp;<BR>Internet:&nbsp;<BR>&nbsp;<BR>&nbsp;&nbsp;&nbsp;-&nbsp;Host&nbsp;name&nbsp;to&nbsp;address&nbsp;mappings&nbsp;were&nbsp;maintained&nbsp;by&nbsp;the&nbsp;Network&nbsp;<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Information&nbsp;Center&nbsp;(NIC)&nbsp;in&nbsp;a&nbsp;single&nbsp;file&nbsp;(HOSTS.TXT)&nbsp;which&nbsp;<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;was&nbsp;FTPed&nbsp;by&nbsp;all&nbsp;hosts&nbsp;[RFC-952,&nbsp;RFC-953].&nbsp;&nbsp;The&nbsp;total&nbsp;network&nbsp;<BR>&nbsp;<BR>&nbsp;<BR>&nbsp;<BR>Mockapetris&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;[Page&nbsp;1]&nbsp;<BR>&nbsp;<BR>RFC&nbsp;1034&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Domain&nbsp;Concepts&nbsp;and&nbsp;Facilities&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;November&nbsp;1987&nbsp;<BR>&nbsp;<BR>&nbsp;<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;bandwidth&nbsp;consumed&nbsp;in&nbsp;distributing&nbsp;a&nbsp;new&nbsp;version&nbsp;by&nbsp;this&nbsp;<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;scheme&nbsp;is&nbsp;proportional&nbsp;to&nbsp;the&nbsp;square&nbsp;of&nbsp;the&nbsp;number&nbsp;of&nbsp;hosts&nbsp;in&nbsp;<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;the&nbsp;network,&nbsp;and&nbsp;even&nbsp;when&nbsp;multiple&nbsp;levels&nbsp;of&nbsp;FTP&nbsp;are&nbsp;used,&nbsp;<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;the&nbsp;outgoing&nbsp;FTP&nbsp;load&nbsp;on&nbsp;the&nbsp;NIC&nbsp;host&nbsp;is&nbsp;considerable.&nbsp;<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Explosive&nbsp;growth&nbsp;in&nbsp;the&nbsp;number&nbsp;of&nbsp;hosts&nbsp;didn't&nbsp;bode&nbsp;well&nbsp;for&nbsp;<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;the&nbsp;future.&nbsp;<BR>&nbsp;<BR>&nbsp;&nbsp;&nbsp;-&nbsp;The&nbsp;network&nbsp;population&nbsp;was&nbsp;also&nbsp;changing&nbsp;in&nbsp;character.&nbsp;&nbsp;The&nbsp;<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;timeshared&nbsp;hosts&nbsp;that&nbsp;made&nbsp;up&nbsp;the&nbsp;original&nbsp;ARPANET&nbsp;were&nbsp;being&nbsp;<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;replaced&nbsp;with&nbsp;local&nbsp;networks&nbsp;of&nbsp;workstations.&nbsp;&nbsp;Local&nbsp;<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;organizations&nbsp;were&nbsp;administering&nbsp;their&nbsp;own&nbsp;names&nbsp;and&nbsp;<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;addresses,&nbsp;but&nbsp;had&nbsp;to&nbsp;wait&nbsp;for&nbsp;the&nbsp;NIC&nbsp;to&nbsp;change&nbsp;HOSTS.TXT&nbsp;to&nbsp;<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;make&nbsp;changes&nbsp;visible&nbsp;to&nbsp;the&nbsp;Internet&nbsp;at&nbsp;large.&nbsp;&nbsp;Organizations&nbsp;<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;also&nbsp;wanted&nbsp;some&nbsp;local&nbsp;structure&nbsp;on&nbsp;the&nbsp;name&nbsp;space.&nbsp;<BR>&nbsp;<BR>&nbsp;&nbsp;&nbsp;-&nbsp;The&nbsp;applications&nbsp;on&nbsp;the&nbsp;Internet&nbsp;were&nbsp;getting&nbsp;more&nbsp;<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;sophisticated&nbsp;and&nbsp;creating&nbsp;a&nbsp;need&nbsp;for&nbsp;general&nbsp;purpose&nbsp;name&nbsp;<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;service.&nbsp;<BR>&nbsp;<BR>&nbsp;<BR>The&nbsp;result&nbsp;was&nbsp;several&nbsp;ideas&nbsp;about&nbsp;name&nbsp;spaces&nbsp;and&nbsp;their&nbsp;management&nbsp;<BR>[IEN-116,&nbsp;RFC-799,&nbsp;RFC-819,&nbsp;RFC-830].&nbsp;&nbsp;The&nbsp;proposals&nbsp;varied,&nbsp;but&nbsp;a&nbsp;<BR>common&nbsp;thread&nbsp;was&nbsp;the&nbsp;idea&nbsp;of&nbsp;a&nbsp;hierarchical&nbsp;name&nbsp;space,&nbsp;with&nbsp;the&nbsp;<BR>hierarchy&nbsp;roughly&nbsp;corresponding&nbsp;to&nbsp;organizational&nbsp;structure,&nbsp;and&nbsp;names&nbsp;<BR>using&nbsp;&quot;.&quot;&nbsp;&nbsp;as&nbsp;the&nbsp;character&nbsp;to&nbsp;mark&nbsp;the&nbsp;boundary&nbsp;between&nbsp;hierarchy&nbsp;<BR>levels.&nbsp;&nbsp;A&nbsp;design&nbsp;using&nbsp;a&nbsp;distributed&nbsp;database&nbsp;and&nbsp;generalized&nbsp;resources&nbsp;<BR>was&nbsp;described&nbsp;in&nbsp;[RFC-882,&nbsp;RFC-883].&nbsp;&nbsp;Based&nbsp;on&nbsp;experience&nbsp;with&nbsp;several&nbsp;<BR>implementations,&nbsp;the&nbsp;system&nbsp;evolved&nbsp;into&nbsp;the&nbsp;scheme&nbsp;described&nbsp;in&nbsp;this&nbsp;<BR>memo.&nbsp;<BR>&nbsp;<BR>The&nbsp;terms&nbsp;&quot;domain&quot;&nbsp;or&nbsp;&quot;domain&nbsp;name&quot;&nbsp;are&nbsp;used&nbsp;in&nbsp;many&nbsp;contexts&nbsp;beyond&nbsp;the&nbsp;<BR>DNS&nbsp;described&nbsp;here.&nbsp;&nbsp;Very&nbsp;often,&nbsp;the&nbsp;term&nbsp;domain&nbsp;name&nbsp;is&nbsp;used&nbsp;to&nbsp;refer&nbsp;<BR>to&nbsp;a&nbsp;name&nbsp;with&nbsp;structure&nbsp;indicated&nbsp;by&nbsp;dots,&nbsp;but&nbsp;no&nbsp;relation&nbsp;to&nbsp;the&nbsp;DNS.&nbsp;<BR>This&nbsp;is&nbsp;particularly&nbsp;true&nbsp;in&nbsp;mail&nbsp;addressing&nbsp;[Quarterman&nbsp;86].&nbsp;<BR>&nbsp;<BR>2.2.&nbsp;DNS&nbsp;design&nbsp;goals&nbsp;<BR>&nbsp;<BR>The&nbsp;design&nbsp;goals&nbsp;of&nbsp;the&nbsp;DNS&nbsp;influence&nbsp;its&nbsp;structure.&nbsp;&nbsp;They&nbsp;are:&nbsp;<BR>&nbsp;<BR>&nbsp;&nbsp;&nbsp;-&nbsp;The&nbsp;primary&nbsp;goal&nbsp;is&nbsp;a&nbsp;consistent&nbsp;name&nbsp;space&nbsp;which&nbsp;will&nbsp;be&nbsp;used&nbsp;<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;for&nbsp;referring&nbsp;to&nbsp;resources.&nbsp;&nbsp;In&nbsp;order&nbsp;to&nbsp;avoid&nbsp;the&nbsp;problems&nbsp;<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;caused&nbsp;by&nbsp;ad&nbsp;hoc&nbsp;encodings,&nbsp;names&nbsp;should&nbsp;not&nbsp;be&nbsp;required&nbsp;to&nbsp;<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;contain&nbsp;network&nbsp;identifiers,&nbsp;addresses,&nbsp;routes,&nbsp;or&nbsp;similar&nbsp;<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;information&nbsp;as&nbsp;part&nbsp;of&nbsp;the&nbsp;name.&nbsp;<BR>&nbsp;<BR>&nbsp;&nbsp;&nbsp;-&nbsp;The&nbsp;sheer&nbsp;size&nbsp;of&nbsp;the&nbsp;database&nbsp;and&nbsp;frequency&nbsp;of&nbsp;updates&nbsp;<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;suggest&nbsp;that&nbsp;it&nbsp;must&nbsp;be&nbsp;maintained&nbsp;in&nbsp;a&nbsp;distributed&nbsp;manner,&nbsp;<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;with&nbsp;local&nbsp;caching&nbsp;to&nbsp;improve&nbsp;performance.&nbsp;&nbsp;Approaches&nbsp;that&nbsp;<BR>&nbsp;<BR>&nbsp;<BR>&nbsp;<BR>Mockapetris&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;[Page&nbsp;2]&nbsp;<BR>&nbsp;<BR>RFC&nbsp;1034&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Domain&nbsp;Concepts&nbsp;and&nbsp;Facilities&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;November&nbsp;1987&nbsp;<BR>&nbsp;<BR>&nbsp;<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;attempt&nbsp;to&nbsp;collect&nbsp;a&nbsp;consistent&nbsp;copy&nbsp;of&nbsp;the&nbsp;entire&nbsp;database&nbsp;<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;will&nbsp;become&nbsp;more&nbsp;and&nbsp;more&nbsp;expensive&nbsp;and&nbsp;difficult,&nbsp;and&nbsp;hence&nbsp;<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;should&nbsp;be&nbsp;avoided.&nbsp;&nbsp;The&nbsp;same&nbsp;principle&nbsp;holds&nbsp;for&nbsp;the&nbsp;structure&nbsp;<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;of&nbsp;the&nbsp;name&nbsp;space,&nbsp;and&nbsp;in&nbsp;particular&nbsp;mechanisms&nbsp;for&nbsp;creating&nbsp;<BR>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;and&nbsp;deleting&nbsp;names;&nbsp;these&nbsp;should&nbsp;also&nbsp;be&nbsp;distributed.&nbsp;<BR>&nbsp;<BR>&nbsp;&nbsp;&nbsp;-&nbsp;Where&nbsp;there&nbsp;tradeoffs&nbsp;between&nbsp;the&nbsp;cost&nbsp;of&nbsp;acquiring&nbsp;data,&nbsp;the&nbsp;<BR>
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