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Chapter 25
Domain Name System
25.1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
2525--1 NAME SPACE1 NAME SPACE
TT bb bibi thth i di d tt hihiToTo bebe unambiguous,unambiguous, thethe namesnames assignedassigned toto machinesmachinesmustmust bebe carefullycarefully selectedselected fromfrom aa namename spacespace withwith
l tl t t lt l thth bi dibi di b tb t ththcompletecomplete controlcontrol overover thethe bindingbinding betweenbetween thethe namesnamesandand IPIP addressesaddresses..
Flat Name SpaceTopics discussed in this section:Topics discussed in this section:
Hierarchical Name Space
25.3
2525--2 2 DOMAIN NAME SPACEDOMAIN NAME SPACE
TT hh hi hi lhi hi l d id iToTo havehave aa hierarchicalhierarchical namename space,space, aa domaindomain namenamespacespace waswas designeddesigned.. InIn thisthis designdesign thethe namesnames areared fi dd fi d ii i di d i hi h hhdefineddefined inin anan invertedinverted--treetree structurestructure withwith thethe rootroot atatthethe toptop.. TheThe treetree cancan havehave onlyonly 128128 levelslevels:: levellevel 00( )( ) l ll l 127127(root)(root) toto levellevel 127127..
Label
Topics discussed in this section:Topics discussed in this section:be
Domain NameDomain
25.4
2525--3 3 DISTRIBUTION OF NAME SPACEDISTRIBUTION OF NAME SPACE
TheThe informationinformation containedcontained inin thethe domaindomain namename spacespaceTheThe informationinformation containedcontained inin thethe domaindomain namename spacespacemustmust bebe storedstored.. However,However, itit isis veryvery inefficientinefficient andand alsoalsounreliableunreliable toto havehave justjust oneone computercomputer storestore suchsuch aa hugehugeunreliableunreliable toto havehave justjust oneone computercomputer storestore suchsuch aa hugehugeamountamount ofof informationinformation.. InIn thisthis section,section, wewe discussdiscuss thethedistributiondistribution ofof thethe domaindomain namename spacespacedistributiondistribution ofof thethe domaindomain namename spacespace..
Hierarchy of Name ServersTopics discussed in this section:Topics discussed in this section:
ZoneRoot ServerPrimary and Secondary Servers
25.9
Primary and Secondary Servers
Note
A primary server loads all information from the disk file; the secondary server
loads all information from the primary server.
When the secondary downloadsi f ti f th i it i ll dinformation from the primary, it is called
zone transfer.
25.12
2525--4 4 DNS IN THE INTERNETDNS IN THE INTERNET
DNSDNS isis aa protocolprotocol thatthat cancan bebe usedused inin differentdifferentDNSDNS isis aa protocolprotocol thatthat cancan bebe usedused inin differentdifferentplatformsplatforms.. InIn thethe Internet,Internet, thethe domaindomain namename spacespace(tree)(tree) isis divideddivided intointo threethree differentdifferent sectionssections:: genericgeneric(tree)(tree) isis divideddivided intointo threethree differentdifferent sectionssections:: genericgenericdomains,domains, countrycountry domains,domains, andand thethe inverseinverse domaindomain..
Generic DomainsTopics discussed in this section:Topics discussed in this section:
Country DomainsInverse Domain
25.13
2525--5 5 RESOLUTIONRESOLUTION
MappingMapping aa namename toto anan addressaddress oror anan addressaddress toto aanamename isis calledcalled namename--addressaddress resolutionresolution..
Topics discussed in this sectionTopics discussed in this sectionResolverMapping Names to Addresses
Topics discussed in this section:Topics discussed in this section:
Mapping Names to AddressesMapping Addresses to NamesRecursive ResolutionCaching
25.19
2525--6 6 DNS MESSAGESDNS MESSAGES
DNSDNS hashas twotwo typestypes ofof messagesmessages:: queryquery andand responseresponseDNSDNS hashas twotwo typestypes ofof messagesmessages:: queryquery andand responseresponse..BothBoth typestypes havehave thethe samesame formatformat.. TheThe queryquery messagemessageconsistsconsists ofof aa headerheader andand questionquestion recordsrecords;; thetheconsistsconsists ofof aa headerheader andand questionquestion recordsrecords;; thetheresponseresponse messagemessage consistsconsists ofof aa header,header, questionquestionrecordsrecords answeranswer recordsrecords authoritativeauthoritative recordsrecords andandrecords,records, answeranswer records,records, authoritativeauthoritative records,records, andandadditionaladditional recordsrecords..
HeaderTopics discussed in this section:Topics discussed in this section:
25.22
2525--7 7 TYPES OF RECORDSTYPES OF RECORDS
AA ii S tiS ti 2525 66 tt tt ff ddAsAs wewe sawsaw inin SectionSection 2525..66,, twotwo typestypes ofof recordsrecords areareusedused inin DNSDNS.. TheThe questionquestion recordsrecords areare usedused inin thethe
titi titi ff thth ddquestionquestion sectionsection ofof thethe queryquery andand responseresponse messagesmessages..TheThe resourceresource recordsrecords areare usedused inin thethe answer,answer,
th it tith it ti dd dditi ldditi l i f tii f ti titi ffauthoritative,authoritative, andand additionaladditional informationinformation sectionssections ofofthethe responseresponse messagemessage..
Q ti R dTopics discussed in this section:Topics discussed in this section:Question RecordResource Record
25.25
2525--8 8 REGISTRARSREGISTRARS
HowHow areare newnew domainsdomains addedadded toto DNS?DNS? ThisThis isis donedoneHowHow areare newnew domainsdomains addedadded toto DNS?DNS? ThisThis isis donedonethroughthrough aa registrar,registrar, aa commercialcommercial entityentity accreditedaccredited bybyICANNICANN AA registrarregistrar firstfirst verifiesverifies thatthat thethe requestedrequestedICANNICANN.. AA registrarregistrar firstfirst verifiesverifies thatthat thethe requestedrequesteddomaindomain namename isis uniqueunique andand thenthen entersenters itit intointo thetheDNSDNS databasedatabase AA feefee isis chargedchargedDNSDNS databasedatabase.. AA feefee isis chargedcharged..
25.26
2525--9 9 DYNAMIC DOMAIN NAME DYNAMIC DOMAIN NAME SYSTEM (DDNS)SYSTEM (DDNS)
TheThe DNSDNS mastermaster filefile mustmust bebe updatedupdated dynamicallydynamically..TheThe DynamicDynamic DomainDomain NameName SystemSystem (DDNS)(DDNS)thereforetherefore waswas deviseddevised toto respondrespond toto thisthis needneed.. InInDDNS,DDNS, whenwhen aa bindingbinding betweenbetween aa namename andand ananaddressaddress isis determined,determined, thethe informationinformation isis sent,sent, usuallyusuallybyby DHCPDHCP toto aa primaryprimary DNSDNS serverserver.. TheThe primaryprimary serverserverupdatesupdates thethe zonezone.. TheThe secondarysecondary serversservers areare notifiednotifiedeithereither activelyactively oror passivelypassively..
25.27
2525--10 10 ENCAPSULATIONENCAPSULATION
DNSDNS cancan useuse eithereither UDPUDP oror TCPTCP InIn bothboth casescases thetheDNSDNS cancan useuse eithereither UDPUDP oror TCPTCP.. InIn bothboth casescases thethewellwell--knownknown portport usedused byby thethe serverserver isis portport 5353.. UDPUDP isisusedused whenwhen thethe sizesize ofof thethe responseresponse messagemessage isis lessless thanthanusedused whenwhen thethe sizesize ofof thethe responseresponse messagemessage isis lessless thanthan512512 bytesbytes becausebecause mostmost UDPUDP packagespackages havehave aa 512512--bytebytepacketpacket sizesize limitlimit IfIf thethe sizesize ofof thethe responseresponse messagemessage isispacketpacket sizesize limitlimit.. IfIf thethe sizesize ofof thethe responseresponse messagemessage isismoremore thanthan 512512 bytes,bytes, aa TCPTCP connectionconnection isis usedused..
25.28