00000002.htm

一份很好的linux入门资料

&nbsp;&nbsp;&nbsp;out&nbsp;then&nbsp;that&nbsp;such&nbsp;a&nbsp;host&nbsp;was&nbsp;supposed&nbsp;to&nbsp;(i.e.,&nbsp;the&nbsp;information&nbsp;was&nbsp;<BR>
&nbsp;&nbsp;&nbsp;in&nbsp;the&nbsp;root&nbsp;servers)&nbsp;run&nbsp;secondary&nbsp;for&nbsp;some&nbsp;domain&nbsp;(or&nbsp;reverse&nbsp;(in-&nbsp;<BR>
&nbsp;&nbsp;&nbsp;addr.arpa))&nbsp;domain,&nbsp;without&nbsp;that&nbsp;host's&nbsp;nameserver&nbsp;manager&nbsp;having&nbsp;<BR>
&nbsp;&nbsp;&nbsp;been&nbsp;asked&nbsp;or&nbsp;even&nbsp;been&nbsp;told&nbsp;so!&nbsp;<BR>
&nbsp;<BR>
&nbsp;&nbsp;&nbsp;Newer&nbsp;BIND&nbsp;versions&nbsp;(4.9&nbsp;and&nbsp;later)&nbsp;solved&nbsp;this&nbsp;problem.&nbsp;&nbsp;At&nbsp;the&nbsp;same&nbsp;<BR>
&nbsp;&nbsp;&nbsp;time&nbsp;though&nbsp;the&nbsp;fix&nbsp;has&nbsp;the&nbsp;disadvantage&nbsp;that&nbsp;it's&nbsp;far&nbsp;less&nbsp;easy&nbsp;to&nbsp;<BR>
&nbsp;&nbsp;&nbsp;spot&nbsp;this&nbsp;problem.&nbsp;<BR>
&nbsp;<BR>
&nbsp;&nbsp;&nbsp;Practice&nbsp;has&nbsp;shown&nbsp;that&nbsp;most&nbsp;domain&nbsp;registrars&nbsp;accept&nbsp;registrations&nbsp;<BR>
&nbsp;&nbsp;&nbsp;of&nbsp;nameservers&nbsp;without&nbsp;checking&nbsp;if&nbsp;primary&nbsp;(!)&nbsp;and&nbsp;secondary&nbsp;servers&nbsp;<BR>
&nbsp;&nbsp;&nbsp;have&nbsp;been&nbsp;set&nbsp;up,&nbsp;informed,&nbsp;or&nbsp;even&nbsp;asked.&nbsp;&nbsp;It&nbsp;should&nbsp;also&nbsp;be&nbsp;noted&nbsp;<BR>
&nbsp;&nbsp;&nbsp;that&nbsp;a&nbsp;combination&nbsp;of&nbsp;long-lasting&nbsp;unreachability&nbsp;of&nbsp;primary&nbsp;<BR>
&nbsp;&nbsp;&nbsp;nameservers,&nbsp;(therefore)&nbsp;expiration&nbsp;of&nbsp;zone&nbsp;information,&nbsp;plus&nbsp;static&nbsp;<BR>
&nbsp;&nbsp;&nbsp;IP&nbsp;routing,&nbsp;can&nbsp;lead&nbsp;to&nbsp;massive&nbsp;network&nbsp;traffic&nbsp;that&nbsp;can&nbsp;fill&nbsp;up&nbsp;<BR>
&nbsp;&nbsp;&nbsp;lines&nbsp;completely.&nbsp;<BR>
&nbsp;<BR>
4.&nbsp;&quot;MX&nbsp;records&nbsp;surprise&quot;&nbsp;<BR>
&nbsp;<BR>
&nbsp;&nbsp;&nbsp;In&nbsp;a&nbsp;sense&nbsp;similar&nbsp;to&nbsp;point&nbsp;3.&nbsp;Sometimes&nbsp;nameserver&nbsp;managers&nbsp;enter&nbsp;MX&nbsp;<BR>
&nbsp;&nbsp;&nbsp;records&nbsp;in&nbsp;their&nbsp;zone&nbsp;files&nbsp;that&nbsp;point&nbsp;to&nbsp;external&nbsp;hosts,&nbsp;without&nbsp;<BR>
&nbsp;&nbsp;&nbsp;first&nbsp;asking&nbsp;or&nbsp;even&nbsp;informing&nbsp;the&nbsp;systems&nbsp;managers&nbsp;of&nbsp;those&nbsp;external&nbsp;<BR>
&nbsp;&nbsp;&nbsp;hosts.&nbsp;&nbsp;This&nbsp;has&nbsp;to&nbsp;be&nbsp;fought&nbsp;out&nbsp;between&nbsp;the&nbsp;nameserver&nbsp;manager&nbsp;and&nbsp;<BR>
&nbsp;&nbsp;&nbsp;the&nbsp;systems&nbsp;managers&nbsp;involved.&nbsp;Only&nbsp;as&nbsp;a&nbsp;last&nbsp;resort,&nbsp;if&nbsp;really&nbsp;<BR>
&nbsp;&nbsp;&nbsp;nothing&nbsp;helps&nbsp;to&nbsp;get&nbsp;the&nbsp;offending&nbsp;records&nbsp;removed,&nbsp;can&nbsp;the&nbsp;systems&nbsp;<BR>
&nbsp;&nbsp;&nbsp;manager&nbsp;turn&nbsp;to&nbsp;the&nbsp;naming&nbsp;authority&nbsp;of&nbsp;the&nbsp;domain&nbsp;above&nbsp;the&nbsp;<BR>
&nbsp;&nbsp;&nbsp;offending&nbsp;domain&nbsp;to&nbsp;get&nbsp;the&nbsp;problem&nbsp;sorted&nbsp;out.&nbsp;<BR>
&nbsp;<BR>
5.&nbsp;&quot;Name&nbsp;extension&nbsp;surprise&quot;&nbsp;<BR>
&nbsp;<BR>
&nbsp;&nbsp;&nbsp;Sometimes&nbsp;one&nbsp;encounters&nbsp;weird&nbsp;names,&nbsp;which&nbsp;appear&nbsp;to&nbsp;be&nbsp;an&nbsp;external&nbsp;<BR>
&nbsp;&nbsp;&nbsp;name&nbsp;extended&nbsp;with&nbsp;a&nbsp;local&nbsp;domain.&nbsp;This&nbsp;is&nbsp;caused&nbsp;by&nbsp;forgetting&nbsp;to&nbsp;<BR>
&nbsp;&nbsp;&nbsp;terminate&nbsp;a&nbsp;name&nbsp;with&nbsp;a&nbsp;dot:&nbsp;names&nbsp;in&nbsp;zone&nbsp;files&nbsp;that&nbsp;don't&nbsp;end&nbsp;with&nbsp;<BR>
&nbsp;&nbsp;&nbsp;a&nbsp;dot&nbsp;are&nbsp;always&nbsp;expanded&nbsp;with&nbsp;the&nbsp;name&nbsp;of&nbsp;the&nbsp;current&nbsp;zone&nbsp;(the&nbsp;<BR>
&nbsp;&nbsp;&nbsp;domain&nbsp;that&nbsp;the&nbsp;zone&nbsp;file&nbsp;stands&nbsp;for&nbsp;or&nbsp;the&nbsp;last&nbsp;$ORIGIN).&nbsp;<BR>
&nbsp;<BR>
&nbsp;&nbsp;&nbsp;Example:&nbsp;zone&nbsp;file&nbsp;for&nbsp;foo.xx:&nbsp;<BR>
&nbsp;<BR>
&nbsp;&nbsp;&nbsp;pqr&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;MX&nbsp;100&nbsp;&nbsp;relay.yy.&nbsp;<BR>
&nbsp;&nbsp;&nbsp;xyz&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;MX&nbsp;100&nbsp;&nbsp;relay.yy&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(no&nbsp;trailing&nbsp;dot!)&nbsp;<BR>
&nbsp;<BR>
&nbsp;<BR>
&nbsp;<BR>
Beertema&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;&nbsp;&nbsp;&nbsp;[Page&nbsp;3]&nbsp;<BR>
&nbsp;<BR>
RFC&nbsp;1537&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Common&nbsp;DNS&nbsp;Data&nbsp;File&nbsp;Configuration&nbsp;Errors&nbsp;&nbsp;&nbsp;&nbsp;October&nbsp;1993&nbsp;<BR>
&nbsp;<BR>
&nbsp;<BR>
&nbsp;&nbsp;&nbsp;When&nbsp;fully&nbsp;written&nbsp;out&nbsp;this&nbsp;stands&nbsp;for:&nbsp;<BR>
&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;pqr.foo.xx.&nbsp;&nbsp;MX&nbsp;100&nbsp;&nbsp;relay.yy.&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;xyz.foo.xx.&nbsp;&nbsp;MX&nbsp;100&nbsp;&nbsp;relay.yy.foo.xx.&nbsp;&nbsp;&nbsp;(name&nbsp;extension!)&nbsp;<BR>
&nbsp;<BR>
6.&nbsp;Missing&nbsp;secondary&nbsp;servers&nbsp;<BR>
&nbsp;<BR>
&nbsp;&nbsp;&nbsp;It&nbsp;is&nbsp;required&nbsp;that&nbsp;there&nbsp;be&nbsp;a&nbsp;least&nbsp;2&nbsp;nameservers&nbsp;for&nbsp;a&nbsp;domain.&nbsp;For&nbsp;<BR>
&nbsp;&nbsp;&nbsp;obvious&nbsp;reasons&nbsp;the&nbsp;nameservers&nbsp;for&nbsp;top&nbsp;level&nbsp;domains&nbsp;need&nbsp;to&nbsp;be&nbsp;very&nbsp;<BR>
&nbsp;&nbsp;&nbsp;well&nbsp;reachable&nbsp;from&nbsp;all&nbsp;over&nbsp;the&nbsp;Internet.&nbsp;This&nbsp;implies&nbsp;that&nbsp;there&nbsp;<BR>
&nbsp;&nbsp;&nbsp;must&nbsp;be&nbsp;more&nbsp;than&nbsp;just&nbsp;2&nbsp;of&nbsp;them;&nbsp;besides,&nbsp;most&nbsp;of&nbsp;the&nbsp;(secondary)&nbsp;<BR>
&nbsp;&nbsp;&nbsp;servers&nbsp;should&nbsp;be&nbsp;placed&nbsp;at&nbsp;&quot;strategic&quot;&nbsp;locations,&nbsp;e.g.,&nbsp;close&nbsp;to&nbsp;a&nbsp;<BR>
&nbsp;&nbsp;&nbsp;point&nbsp;where&nbsp;international&nbsp;and/or&nbsp;intercontinental&nbsp;lines&nbsp;come&nbsp;<BR>
&nbsp;&nbsp;&nbsp;together.&nbsp;&nbsp;To&nbsp;keep&nbsp;things&nbsp;manageable,&nbsp;there&nbsp;shouldn't&nbsp;be&nbsp;too&nbsp;many&nbsp;<BR>
&nbsp;&nbsp;&nbsp;servers&nbsp;for&nbsp;a&nbsp;domain&nbsp;either.&nbsp;<BR>
&nbsp;<BR>
&nbsp;&nbsp;&nbsp;Important&nbsp;aspects&nbsp;in&nbsp;selecting&nbsp;the&nbsp;location&nbsp;of&nbsp;primary&nbsp;and&nbsp;secondary&nbsp;<BR>
&nbsp;&nbsp;&nbsp;servers&nbsp;are&nbsp;reliability&nbsp;(network,&nbsp;host)&nbsp;and&nbsp;expedient&nbsp;contacts:&nbsp;in&nbsp;<BR>
&nbsp;&nbsp;&nbsp;case&nbsp;of&nbsp;problems,&nbsp;changes/fixes&nbsp;must&nbsp;be&nbsp;carried&nbsp;out&nbsp;quickly.&nbsp;&nbsp;It&nbsp;<BR>
&nbsp;&nbsp;&nbsp;should&nbsp;be&nbsp;considered&nbsp;logical&nbsp;that&nbsp;primary&nbsp;servers&nbsp;for&nbsp;European&nbsp;top&nbsp;<BR>
&nbsp;&nbsp;&nbsp;level&nbsp;domains&nbsp;should&nbsp;run&nbsp;on&nbsp;a&nbsp;host&nbsp;in&nbsp;Europe,&nbsp;preferably&nbsp;(if&nbsp;<BR>
&nbsp;&nbsp;&nbsp;possible)&nbsp;in&nbsp;the&nbsp;country&nbsp;itself.&nbsp;For&nbsp;each&nbsp;top&nbsp;level&nbsp;domain&nbsp;there&nbsp;<BR>
&nbsp;&nbsp;&nbsp;should&nbsp;be&nbsp;2&nbsp;secondary&nbsp;servers&nbsp;in&nbsp;Europe&nbsp;and&nbsp;2&nbsp;in&nbsp;the&nbsp;USA,&nbsp;but&nbsp;there&nbsp;<BR>
&nbsp;&nbsp;&nbsp;may&nbsp;of&nbsp;course&nbsp;be&nbsp;more&nbsp;on&nbsp;either&nbsp;side.&nbsp;&nbsp;An&nbsp;excessive&nbsp;number&nbsp;of&nbsp;<BR>
&nbsp;&nbsp;&nbsp;nameservers&nbsp;is&nbsp;not&nbsp;a&nbsp;good&nbsp;idea&nbsp;though;&nbsp;a&nbsp;recommended&nbsp;maximum&nbsp;is&nbsp;7&nbsp;<BR>
&nbsp;&nbsp;&nbsp;nameservers.&nbsp;&nbsp;In&nbsp;Europe,&nbsp;EUnet&nbsp;has&nbsp;offered&nbsp;to&nbsp;run&nbsp;secondary&nbsp;server&nbsp;<BR>
&nbsp;&nbsp;&nbsp;for&nbsp;each&nbsp;European&nbsp;top&nbsp;level&nbsp;domain.&nbsp;<BR>
&nbsp;<BR>
7.&nbsp;Wildcard&nbsp;MX&nbsp;records&nbsp;<BR>
&nbsp;<BR>
&nbsp;&nbsp;&nbsp;Wildcard&nbsp;MX&nbsp;records&nbsp;should&nbsp;be&nbsp;avoided&nbsp;where&nbsp;possible.&nbsp;They&nbsp;often&nbsp;<BR>
&nbsp;&nbsp;&nbsp;cause&nbsp;confusion&nbsp;and&nbsp;errors:&nbsp;especially&nbsp;beginning&nbsp;nameserver&nbsp;managers&nbsp;<BR>
&nbsp;&nbsp;&nbsp;tend&nbsp;to&nbsp;overlook&nbsp;the&nbsp;fact&nbsp;that&nbsp;a&nbsp;host/domain&nbsp;listed&nbsp;with&nbsp;ANY&nbsp;type&nbsp;of&nbsp;<BR>
&nbsp;&nbsp;&nbsp;record&nbsp;in&nbsp;a&nbsp;zone&nbsp;file&nbsp;is&nbsp;NOT&nbsp;covered&nbsp;by&nbsp;an&nbsp;overall&nbsp;wildcard&nbsp;MX&nbsp;record&nbsp;<BR>
&nbsp;&nbsp;&nbsp;in&nbsp;that&nbsp;zone;&nbsp;this&nbsp;goes&nbsp;not&nbsp;only&nbsp;for&nbsp;simple&nbsp;domain/host&nbsp;names,&nbsp;but&nbsp;<BR>
&nbsp;&nbsp;&nbsp;also&nbsp;for&nbsp;names&nbsp;that&nbsp;cover&nbsp;one&nbsp;or&nbsp;more&nbsp;domains.&nbsp;Take&nbsp;the&nbsp;following&nbsp;<BR>
&nbsp;&nbsp;&nbsp;example&nbsp;in&nbsp;zone&nbsp;foo.bar:&nbsp;<BR>
&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;*&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;MX&nbsp;100&nbsp;&nbsp;mailhost&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;pqr&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;MX&nbsp;100&nbsp;&nbsp;mailhost&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;abc.def&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;MX&nbsp;100&nbsp;&nbsp;mailhost&nbsp;<BR>
&nbsp;<BR>
&nbsp;&nbsp;&nbsp;This&nbsp;makes&nbsp;pqr.foo.bar,&nbsp;def.foo.bar&nbsp;and&nbsp;abd.def.foo.bar&nbsp;valid&nbsp;<BR>
&nbsp;&nbsp;&nbsp;domains,&nbsp;but&nbsp;the&nbsp;wildcard&nbsp;MX&nbsp;record&nbsp;covers&nbsp;NONE&nbsp;of&nbsp;them,&nbsp;nor&nbsp;anything&nbsp;<BR>
&nbsp;&nbsp;&nbsp;below&nbsp;them.&nbsp;&nbsp;To&nbsp;cover&nbsp;everything&nbsp;by&nbsp;MX&nbsp;records,&nbsp;the&nbsp;required&nbsp;entries&nbsp;<BR>
&nbsp;&nbsp;&nbsp;are:&nbsp;<BR>
&nbsp;<BR>
&nbsp;<BR>
&nbsp;<BR>
&nbsp;<BR>
&nbsp;<BR>
Beertema&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;&nbsp;&nbsp;&nbsp;[Page&nbsp;4]&nbsp;<BR>
&nbsp;<BR>
RFC&nbsp;1537&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Common&nbsp;DNS&nbsp;Data&nbsp;File&nbsp;Configuration&nbsp;Errors&nbsp;&nbsp;&nbsp;&nbsp;October&nbsp;1993&nbsp;<BR>
&nbsp;<BR>
&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;*&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;MX&nbsp;100&nbsp;&nbsp;mailhost&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;pqr&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;MX&nbsp;100&nbsp;&nbsp;mailhost&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;*.pqr&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;MX&nbsp;100&nbsp;&nbsp;mailhost&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;abc.def&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;MX&nbsp;100&nbsp;&nbsp;mailhost&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;*.def&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;MX&nbsp;100&nbsp;&nbsp;mailhost&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;*.abc.def&nbsp;&nbsp;&nbsp;&nbsp;MX&nbsp;100&nbsp;&nbsp;mailhost&nbsp;<BR>
&nbsp;<BR>
&nbsp;&nbsp;&nbsp;An&nbsp;overall&nbsp;wildcard&nbsp;MX&nbsp;record&nbsp;is&nbsp;almost&nbsp;never&nbsp;useful.&nbsp;<BR>
&nbsp;<BR>
&nbsp;&nbsp;&nbsp;In&nbsp;particular&nbsp;the&nbsp;zone&nbsp;file&nbsp;of&nbsp;a&nbsp;top&nbsp;level&nbsp;domain&nbsp;should&nbsp;NEVER&nbsp;<BR>
&nbsp;&nbsp;&nbsp;contain&nbsp;only&nbsp;an&nbsp;overall&nbsp;wildcard&nbsp;MX&nbsp;record&nbsp;(*.XX).&nbsp;The&nbsp;effect&nbsp;of&nbsp;such&nbsp;<BR>
&nbsp;&nbsp;&nbsp;a&nbsp;wildcard&nbsp;MX&nbsp;record&nbsp;can&nbsp;be&nbsp;that&nbsp;mail&nbsp;is&nbsp;unnecessarily&nbsp;sent&nbsp;across&nbsp;<BR>
&nbsp;&nbsp;&nbsp;possibly&nbsp;expensive&nbsp;links,&nbsp;only&nbsp;to&nbsp;fail&nbsp;at&nbsp;the&nbsp;destination&nbsp;or&nbsp;gateway&nbsp;<BR>
&nbsp;&nbsp;&nbsp;that&nbsp;the&nbsp;record&nbsp;points&nbsp;to.&nbsp;Top&nbsp;level&nbsp;domain&nbsp;zone&nbsp;files&nbsp;should&nbsp;<BR>
&nbsp;&nbsp;&nbsp;explicitly&nbsp;list&nbsp;at&nbsp;least&nbsp;all&nbsp;the&nbsp;officially&nbsp;registered&nbsp;primary&nbsp;<BR>
&nbsp;&nbsp;&nbsp;subdomains.&nbsp;<BR>
&nbsp;<BR>
&nbsp;&nbsp;&nbsp;Whereas&nbsp;overall&nbsp;wildcard&nbsp;MX&nbsp;records&nbsp;should&nbsp;be&nbsp;avoided,&nbsp;wildcard&nbsp;MX&nbsp;<BR>
&nbsp;&nbsp;&nbsp;records&nbsp;are&nbsp;acceptable&nbsp;as&nbsp;an&nbsp;explicit&nbsp;part&nbsp;of&nbsp;subdomain&nbsp;entries,&nbsp;<BR>
&nbsp;&nbsp;&nbsp;provided&nbsp;they&nbsp;are&nbsp;allowed&nbsp;under&nbsp;a&nbsp;given&nbsp;subdomain&nbsp;(to&nbsp;be&nbsp;determined&nbsp;<BR>
&nbsp;&nbsp;&nbsp;by&nbsp;the&nbsp;naming&nbsp;authority&nbsp;for&nbsp;that&nbsp;domain).&nbsp;<BR>
&nbsp;<BR>
&nbsp;&nbsp;&nbsp;Example:&nbsp;<BR>
&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;foo.xx.&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;MX&nbsp;100&nbsp;&nbsp;gateway.xx.&nbsp;<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;MX&nbsp;200&nbsp;&nbsp;fallback.yy.&nbsp;<BR>