计算机网络讲义——DNS课件

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1、DNS:DomainNameSystemPeople:manyidentifiers:SSN,name,Passport#Internethosts,routers:IPaddress(32bit)-usedforaddressingdatagrams“name”,e.g.,gaia.cs.umass.edu-usedbyhumansQ:mapbetweenIPaddressesandname?DomainNameSystem:distributeddatabaseimplementedinhierarchyofmanynameserv

2、ersapplication-layerprotocolhost,routers,nameserverstocommunicatetoresolvenames(address/nametranslation)note:coreInternetfunctionimplementedasapplication-layerprotocolcomplexityatnetwork’s“edge”12:ApplicationLayerDNSnameserversnoserverhasallname-to-IPaddressmappingslocal

3、nameservers:eachISP,companyhaslocal(default)nameserverhostDNSqueryfirstgoestolocalnameserverauthoritativenameserver:forahost:storesthathost’sIPaddress,namecanperformname/addresstranslationforthathost’snameWhynotcentralizeDNS?singlepointoffailuretrafficvolumedistantcentra

4、lizeddatabasemaintenancedoesn’tscale!22:ApplicationLayerDNS:Rootnameserverscontactedbylocalnameserverthatcannotresolvenamerootnameserver:contactsauthoritativenameserverifnamemappingnotknowngetsmappingreturnsmappingtolocalnameserver~dozenrootnameserversworldwide32:Applica

5、tionLayerSimpleDNSexamplehostsurf.eurecom.frwantsIPaddressofgaia.cs.umass.edu1.ContactsitslocalDNSserver,dns.eurecom.fr2.dns.eurecom.frcontactsrootnameserver,ifnecessary3.rootnameservercontactsauthoritativenameserver,dns.umass.edu,ifnecessaryrequestinghostsurf.eurecom.fr

6、gaia.cs.umass.edurootnameserverauthorititivenameserverdns.umass.edulocalnameserverdns.eurecom.fr12345642:ApplicationLayerDNSexampleRootnameserver:maynotknowauthoratiativenameservermayknowintermediatenameserver:whotocontacttofindauthoritativenameserverrequestinghostsurf.e

7、urecom.frgaia.cs.umass.edurootnameserverlocalnameserverdns.eurecom.fr123456authoritativenameserverdns.cs.umass.eduintermediatenameserverdns.umass.edu7852:ApplicationLayerDNS:iteratedqueriesrecursivequery:putsburdenofnameresolutiononcontactednameserverheavyload?iteratedqu

8、ery:contactedserverreplieswithnameofservertocontact“Idon’tknowthisname,butaskthisserver”requestinghosts