Chapter 25 Domain Name System - Kasetsart Universityplw/dccn/presentation/ch25.pdf · 2525--9 9 DYNAMIC DOMAIN NAME DYNAMIC DOMAIN NAME SYSTEM (DDNS) TheThe DNS DNS mastemaster file

Chapter 25

Domain Name System

25.1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Figure 25.1 Example of using the DNS service

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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

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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

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Figure 25.2 Domain name space

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Figure 25.3 Domain names and labels

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Figure 25.4 FQDN and PQDN

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Figure 25.5 Domains

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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

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Primary and Secondary Servers

Figure 25.6 Hierarchy of name servers

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Figure 25.7 Zones and domains

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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.

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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

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Figure 25.8 DNS IN THE INTERNET

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Figure 25.9 Generic domains

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Table 25.1 Generic domain labels

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Figure 25.10 Country domains

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Figure 25.11 Inverse domain

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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

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Figure 25.12 Recursive resolution

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Figure 25.13 Iterative resolution

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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:

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Figure 25.14 Query and response messages

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Figure 25.15 Header format

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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

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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..

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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..

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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..

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Note

DNS can use the services of UDP or TCP i th ll k t 53using the well-known port 53.

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