00000004.htm

一份很好的linux入门资料

&nbsp;<BR>
By&nbsp;convention,&nbsp;domain&nbsp;names&nbsp;can&nbsp;be&nbsp;stored&nbsp;with&nbsp;arbitrary&nbsp;case,&nbsp;but&nbsp;<BR>
domain&nbsp;name&nbsp;comparisons&nbsp;for&nbsp;all&nbsp;present&nbsp;domain&nbsp;functions&nbsp;are&nbsp;done&nbsp;in&nbsp;a&nbsp;<BR>
case-insensitive&nbsp;manner,&nbsp;assuming&nbsp;an&nbsp;ASCII&nbsp;character&nbsp;set,&nbsp;and&nbsp;a&nbsp;high&nbsp;<BR>
order&nbsp;zero&nbsp;bit.&nbsp;&nbsp;This&nbsp;means&nbsp;that&nbsp;you&nbsp;are&nbsp;free&nbsp;to&nbsp;create&nbsp;a&nbsp;node&nbsp;with&nbsp;<BR>
label&nbsp;&quot;A&quot;&nbsp;or&nbsp;a&nbsp;node&nbsp;with&nbsp;label&nbsp;&quot;a&quot;,&nbsp;but&nbsp;not&nbsp;both&nbsp;as&nbsp;brothers;&nbsp;you&nbsp;could&nbsp;<BR>
refer&nbsp;to&nbsp;either&nbsp;using&nbsp;&quot;a&quot;&nbsp;or&nbsp;&quot;A&quot;.&nbsp;&nbsp;When&nbsp;you&nbsp;receive&nbsp;a&nbsp;domain&nbsp;name&nbsp;or&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;7]&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>
label,&nbsp;you&nbsp;should&nbsp;preserve&nbsp;its&nbsp;case.&nbsp;&nbsp;The&nbsp;rationale&nbsp;for&nbsp;this&nbsp;choice&nbsp;is&nbsp;<BR>
that&nbsp;we&nbsp;may&nbsp;someday&nbsp;need&nbsp;to&nbsp;add&nbsp;full&nbsp;binary&nbsp;domain&nbsp;names&nbsp;for&nbsp;new&nbsp;<BR>
services;&nbsp;existing&nbsp;services&nbsp;would&nbsp;not&nbsp;be&nbsp;changed.&nbsp;<BR>
&nbsp;<BR>
When&nbsp;a&nbsp;user&nbsp;needs&nbsp;to&nbsp;type&nbsp;a&nbsp;domain&nbsp;name,&nbsp;the&nbsp;length&nbsp;of&nbsp;each&nbsp;label&nbsp;is&nbsp;<BR>
omitted&nbsp;and&nbsp;the&nbsp;labels&nbsp;are&nbsp;separated&nbsp;by&nbsp;dots&nbsp;(&quot;.&quot;).&nbsp;&nbsp;Since&nbsp;a&nbsp;complete&nbsp;<BR>
domain&nbsp;name&nbsp;ends&nbsp;with&nbsp;the&nbsp;root&nbsp;label,&nbsp;this&nbsp;leads&nbsp;to&nbsp;a&nbsp;printed&nbsp;form&nbsp;which&nbsp;<BR>
ends&nbsp;in&nbsp;a&nbsp;dot.&nbsp;&nbsp;We&nbsp;use&nbsp;this&nbsp;property&nbsp;to&nbsp;distinguish&nbsp;between:&nbsp;<BR>
&nbsp;<BR>
&nbsp;&nbsp;&nbsp;-&nbsp;a&nbsp;character&nbsp;string&nbsp;which&nbsp;represents&nbsp;a&nbsp;complete&nbsp;domain&nbsp;name&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(often&nbsp;called&nbsp;&quot;absolute&quot;).&nbsp;&nbsp;For&nbsp;example,&nbsp;&quot;poneria.ISI.EDU.&quot;&nbsp;<BR>
&nbsp;<BR>
&nbsp;&nbsp;&nbsp;-&nbsp;a&nbsp;character&nbsp;string&nbsp;that&nbsp;represents&nbsp;the&nbsp;starting&nbsp;labels&nbsp;of&nbsp;a&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;domain&nbsp;name&nbsp;which&nbsp;is&nbsp;incomplete,&nbsp;and&nbsp;should&nbsp;be&nbsp;completed&nbsp;by&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;local&nbsp;software&nbsp;using&nbsp;knowledge&nbsp;of&nbsp;the&nbsp;local&nbsp;domain&nbsp;(often&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;called&nbsp;&quot;relative&quot;).&nbsp;&nbsp;For&nbsp;example,&nbsp;&quot;poneria&quot;&nbsp;used&nbsp;in&nbsp;the&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;ISI.EDU&nbsp;domain.&nbsp;<BR>
&nbsp;<BR>
Relative&nbsp;names&nbsp;are&nbsp;either&nbsp;taken&nbsp;relative&nbsp;to&nbsp;a&nbsp;well&nbsp;known&nbsp;origin,&nbsp;or&nbsp;to&nbsp;a&nbsp;<BR>
list&nbsp;of&nbsp;domains&nbsp;used&nbsp;as&nbsp;a&nbsp;search&nbsp;list.&nbsp;&nbsp;Relative&nbsp;names&nbsp;appear&nbsp;mostly&nbsp;at&nbsp;<BR>
the&nbsp;user&nbsp;interface,&nbsp;where&nbsp;their&nbsp;interpretation&nbsp;varies&nbsp;from&nbsp;<BR>
implementation&nbsp;to&nbsp;implementation,&nbsp;and&nbsp;in&nbsp;master&nbsp;files,&nbsp;where&nbsp;they&nbsp;are&nbsp;<BR>
relative&nbsp;to&nbsp;a&nbsp;single&nbsp;origin&nbsp;domain&nbsp;name.&nbsp;&nbsp;The&nbsp;most&nbsp;common&nbsp;interpretation&nbsp;<BR>
uses&nbsp;the&nbsp;root&nbsp;&quot;.&quot;&nbsp;as&nbsp;either&nbsp;the&nbsp;single&nbsp;origin&nbsp;or&nbsp;as&nbsp;one&nbsp;of&nbsp;the&nbsp;members&nbsp;<BR>
of&nbsp;the&nbsp;search&nbsp;list,&nbsp;so&nbsp;a&nbsp;multi-label&nbsp;relative&nbsp;name&nbsp;is&nbsp;often&nbsp;one&nbsp;where&nbsp;<BR>
the&nbsp;trailing&nbsp;dot&nbsp;has&nbsp;been&nbsp;omitted&nbsp;to&nbsp;save&nbsp;typing.&nbsp;<BR>
&nbsp;<BR>
To&nbsp;simplify&nbsp;implementations,&nbsp;the&nbsp;total&nbsp;number&nbsp;of&nbsp;octets&nbsp;that&nbsp;represent&nbsp;a&nbsp;<BR>
domain&nbsp;name&nbsp;(i.e.,&nbsp;the&nbsp;sum&nbsp;of&nbsp;all&nbsp;label&nbsp;octets&nbsp;and&nbsp;label&nbsp;lengths)&nbsp;is&nbsp;<BR>
limited&nbsp;to&nbsp;255.&nbsp;<BR>
&nbsp;<BR>
A&nbsp;domain&nbsp;is&nbsp;identified&nbsp;by&nbsp;a&nbsp;domain&nbsp;name,&nbsp;and&nbsp;consists&nbsp;of&nbsp;that&nbsp;part&nbsp;of&nbsp;<BR>
the&nbsp;domain&nbsp;name&nbsp;space&nbsp;that&nbsp;is&nbsp;at&nbsp;or&nbsp;below&nbsp;the&nbsp;domain&nbsp;name&nbsp;which&nbsp;<BR>
specifies&nbsp;the&nbsp;domain.&nbsp;&nbsp;A&nbsp;domain&nbsp;is&nbsp;a&nbsp;subdomain&nbsp;of&nbsp;another&nbsp;domain&nbsp;if&nbsp;it&nbsp;<BR>
is&nbsp;contained&nbsp;within&nbsp;that&nbsp;domain.&nbsp;&nbsp;This&nbsp;relationship&nbsp;can&nbsp;be&nbsp;tested&nbsp;by&nbsp;<BR>
seeing&nbsp;if&nbsp;the&nbsp;subdomain's&nbsp;name&nbsp;ends&nbsp;with&nbsp;the&nbsp;containing&nbsp;domain's&nbsp;name.&nbsp;<BR>
For&nbsp;example,&nbsp;A.B.C.D&nbsp;is&nbsp;a&nbsp;subdomain&nbsp;of&nbsp;B.C.D,&nbsp;C.D,&nbsp;D,&nbsp;and&nbsp;&quot;&nbsp;&quot;.&nbsp;<BR>
&nbsp;<BR>
3.2.&nbsp;Administrative&nbsp;guidelines&nbsp;on&nbsp;use&nbsp;<BR>
&nbsp;<BR>
As&nbsp;a&nbsp;matter&nbsp;of&nbsp;policy,&nbsp;the&nbsp;DNS&nbsp;technical&nbsp;specifications&nbsp;do&nbsp;not&nbsp;mandate&nbsp;a&nbsp;<BR>
particular&nbsp;tree&nbsp;structure&nbsp;or&nbsp;rules&nbsp;for&nbsp;selecting&nbsp;labels;&nbsp;its&nbsp;goal&nbsp;is&nbsp;to&nbsp;<BR>
be&nbsp;as&nbsp;general&nbsp;as&nbsp;possible,&nbsp;so&nbsp;that&nbsp;it&nbsp;can&nbsp;be&nbsp;used&nbsp;to&nbsp;build&nbsp;arbitrary&nbsp;<BR>
applications.&nbsp;&nbsp;In&nbsp;particular,&nbsp;the&nbsp;system&nbsp;was&nbsp;designed&nbsp;so&nbsp;that&nbsp;the&nbsp;name&nbsp;<BR>
space&nbsp;did&nbsp;not&nbsp;have&nbsp;to&nbsp;be&nbsp;organized&nbsp;along&nbsp;the&nbsp;lines&nbsp;of&nbsp;network&nbsp;<BR>
boundaries,&nbsp;name&nbsp;servers,&nbsp;etc.&nbsp;&nbsp;The&nbsp;rationale&nbsp;for&nbsp;this&nbsp;is&nbsp;not&nbsp;that&nbsp;the&nbsp;<BR>
name&nbsp;space&nbsp;should&nbsp;have&nbsp;no&nbsp;implied&nbsp;semantics,&nbsp;but&nbsp;rather&nbsp;that&nbsp;the&nbsp;choice&nbsp;<BR>
of&nbsp;implied&nbsp;semantics&nbsp;should&nbsp;be&nbsp;left&nbsp;open&nbsp;to&nbsp;be&nbsp;used&nbsp;for&nbsp;the&nbsp;problem&nbsp;at&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;8]&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>
hand,&nbsp;and&nbsp;that&nbsp;different&nbsp;parts&nbsp;of&nbsp;the&nbsp;tree&nbsp;can&nbsp;have&nbsp;different&nbsp;implied&nbsp;<BR>
semantics.&nbsp;&nbsp;For&nbsp;example,&nbsp;the&nbsp;IN-ADDR.ARPA&nbsp;domain&nbsp;is&nbsp;organized&nbsp;and&nbsp;<BR>
distributed&nbsp;by&nbsp;network&nbsp;and&nbsp;host&nbsp;address&nbsp;because&nbsp;its&nbsp;role&nbsp;is&nbsp;to&nbsp;translate&nbsp;<BR>
from&nbsp;network&nbsp;or&nbsp;host&nbsp;numbers&nbsp;to&nbsp;names;&nbsp;NetBIOS&nbsp;domains&nbsp;[RFC-1001,&nbsp;RFC-&nbsp;<BR>
1002]&nbsp;are&nbsp;flat&nbsp;because&nbsp;that&nbsp;is&nbsp;appropriate&nbsp;for&nbsp;that&nbsp;application.&nbsp;<BR>
&nbsp;<BR>
However,&nbsp;there&nbsp;are&nbsp;some&nbsp;guidelines&nbsp;that&nbsp;apply&nbsp;to&nbsp;the&nbsp;&quot;normal&quot;&nbsp;parts&nbsp;of&nbsp;<BR>
the&nbsp;name&nbsp;space&nbsp;used&nbsp;for&nbsp;hosts,&nbsp;mailboxes,&nbsp;etc.,&nbsp;that&nbsp;will&nbsp;make&nbsp;the&nbsp;name&nbsp;<BR>
space&nbsp;more&nbsp;uniform,&nbsp;provide&nbsp;for&nbsp;growth,&nbsp;and&nbsp;minimize&nbsp;problems&nbsp;as&nbsp;<BR>
software&nbsp;is&nbsp;converted&nbsp;from&nbsp;the&nbsp;older&nbsp;host&nbsp;table.&nbsp;&nbsp;The&nbsp;political&nbsp;<BR>
decisions&nbsp;about&nbsp;the&nbsp;top&nbsp;levels&nbsp;of&nbsp;the&nbsp;tree&nbsp;originated&nbsp;in&nbsp;RFC-920.&nbsp;<BR>
Current&nbsp;policy&nbsp;for&nbsp;the&nbsp;top&nbsp;levels&nbsp;is&nbsp;discussed&nbsp;in&nbsp;[RFC-1032].&nbsp;&nbsp;MILNET&nbsp;<BR>
conversion&nbsp;issues&nbsp;are&nbsp;covered&nbsp;in&nbsp;[RFC-1031].&nbsp;<BR>
&nbsp;<BR>
Lower&nbsp;domains&nbsp;which&nbsp;will&nbsp;eventually&nbsp;be&nbsp;broken&nbsp;into&nbsp;multiple&nbsp;zones&nbsp;should&nbsp;<BR>
provide&nbsp;branching&nbsp;at&nbsp;the&nbsp;top&nbsp;of&nbsp;the&nbsp;domain&nbsp;so&nbsp;that&nbsp;the&nbsp;eventual&nbsp;<BR>
decomposition&nbsp;can&nbsp;be&nbsp;done&nbsp;without&nbsp;renaming.&nbsp;&nbsp;Node&nbsp;labels&nbsp;which&nbsp;use&nbsp;<BR>
special&nbsp;characters,&nbsp;leading&nbsp;digits,&nbsp;etc.,&nbsp;are&nbsp;likely&nbsp;to&nbsp;break&nbsp;older&nbsp;<BR>
software&nbsp;which&nbsp;depends&nbsp;on&nbsp;more&nbsp;restrictive&nbsp;choices.&nbsp;<BR>
&nbsp;<BR>
3.3.&nbsp;Technical&nbsp;guidelines&nbsp;on&nbsp;use&nbsp;<BR>
&nbsp;<BR>
Before&nbsp;the&nbsp;DNS&nbsp;can&nbsp;be&nbsp;used&nbsp;to&nbsp;hold&nbsp;naming&nbsp;information&nbsp;for&nbsp;some&nbsp;kind&nbsp;of&nbsp;<BR>
object,&nbsp;two&nbsp;needs&nbsp;must&nbsp;be&nbsp;met:&nbsp;<BR>
&nbsp;<BR>
&nbsp;&nbsp;&nbsp;-&nbsp;A&nbsp;convention&nbsp;for&nbsp;mapping&nbsp;between&nbsp;object&nbsp;names&nbsp;and&nbsp;domain&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;names.&nbsp;&nbsp;This&nbsp;describes&nbsp;how&nbsp;information&nbsp;about&nbsp;an&nbsp;object&nbsp;is&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;accessed.&nbsp;<BR>
&nbsp;<BR>
&nbsp;&nbsp;&nbsp;-&nbsp;RR&nbsp;types&nbsp;and&nbsp;data&nbsp;formats&nbsp;for&nbsp;describing&nbsp;the&nbsp;object.&nbsp;<BR>
&nbsp;<BR>
These&nbsp;rules&nbsp;can&nbsp;be&nbsp;quite&nbsp;simple&nbsp;or&nbsp;fairly&nbsp;complex.&nbsp;&nbsp;Very&nbsp;often,&nbsp;the&nbsp;<BR>
designer&nbsp;must&nbsp;take&nbsp;into&nbsp;account&nbsp;existing&nbsp;formats&nbsp;and&nbsp;plan&nbsp;for&nbsp;upward&nbsp;<BR>
compatibility&nbsp;for&nbsp;existing&nbsp;usage.&nbsp;&nbsp;Multiple&nbsp;mappings&nbsp;or&nbsp;levels&nbsp;of&nbsp;<BR>
mapping&nbsp;may&nbsp;be&nbsp;required.&nbsp;<BR>
&nbsp;<BR>
For&nbsp;hosts,&nbsp;the&nbsp;mapping&nbsp;depends&nbsp;on&nbsp;the&nbsp;existing&nbsp;syntax&nbsp;for&nbsp;host&nbsp;names&nbsp;<BR>
which&nbsp;is&nbsp;a&nbsp;subset&nbsp;of&nbsp;the&nbsp;usual&nbsp;text&nbsp;representation&nbsp;for&nbsp;domain&nbsp;names,&nbsp;<BR>
together&nbsp;with&nbsp;RR&nbsp;formats&nbsp;for&nbsp;describing&nbsp;host&nbsp;addresses,&nbsp;etc.&nbsp;&nbsp;Because&nbsp;we&nbsp;<BR>
need&nbsp;a&nbsp;reliable&nbsp;inverse&nbsp;mapping&nbsp;from&nbsp;address&nbsp;to&nbsp;host&nbsp;name,&nbsp;a&nbsp;special&nbsp;<BR>
mapping&nbsp;for&nbsp;addresses&nbsp;into&nbsp;the&nbsp;IN-ADDR.ARPA&nbsp;domain&nbsp;is&nbsp;also&nbsp;defined.&nbsp;<BR>
&nbsp;<BR>
For&nbsp;mailboxes,&nbsp;the&nbsp;mapping&nbsp;is&nbsp;slightly&nbsp;more&nbsp;complex.&nbsp;&nbsp;The&nbsp;usual&nbsp;mail&nbsp;<BR>
address&nbsp;&lt;local-part&gt;@&lt;mail-domain&gt;&nbsp;is&nbsp;mapped&nbsp;into&nbsp;a&nbsp;domain&nbsp;name&nbsp;by&nbsp;<BR>
converting&nbsp;&lt;local-part&gt;&nbsp;into&nbsp;a&nbsp;single&nbsp;label&nbsp;(regardles&nbsp;of&nbsp;dots&nbsp;it&nbsp;<BR>
contains),&nbsp;converting&nbsp;&lt;mail-domain&gt;&nbsp;into&nbsp;a&nbsp;domain&nbsp;name&nbsp;using&nbsp;the&nbsp;usual&nbsp;<BR>
text&nbsp;format&nbsp;for&nbsp;domain&nbsp;names&nbsp;(dots&nbsp;denote&nbsp;label&nbsp;breaks),&nbsp;and&nbsp;<BR>
concatenating&nbsp;the&nbsp;two&nbsp;to&nbsp;form&nbsp;a&nbsp;single&nbsp;domain&nbsp;name.&nbsp;&nbsp;Thus&nbsp;the&nbsp;mailbox&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;9]&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>
<A HREF="mailto:[e-mail]">[e-mail]</A>&nbsp;is&nbsp;represented&nbsp;as&nbsp;a&nbsp;domain&nbsp;name&nbsp;by&nbsp;<BR>
HOSTMASTER.SRI-NIC.ARPA.&nbsp;&nbsp;An&nbsp;appreciation&nbsp;for&nbsp;the&nbsp;reasons&nbsp;behind&nbsp;this&nbsp;<BR>