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GT ECE 4110 - Domain Name System (DNS)

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Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Slide 39Slide 40Slide 41Slide 42Slide 43Slide 44Slide 45Slide 46Slide 47Slide 48Chapter 18Domain NameSystem(DNS)Domain Name System (DNS)Need System to map name to an IP address and vice versaWe have used a host file in our Linux laboratory.Not feasible for the entire Internet. Thus, divide huge amount of info and store in parts on many different computers. Host needing info contacts the closest server containing the needed info. This is DNS.Hierarchical Name Space is used. Names are made up of several parts: acme.gatech.eduDomain Name Space: names are defined in an inverted tree structure. Read names from node up to root of tree.Source: TCP/IP Protocol Suite by ForouzanCONTENTSCONTENTS• NAME SPACE• DOMAIN NAME SPACE• DISTRIBUTION OF NAME SPACE• DNS IN THE INTERNET• RESOLUTION• DNS MESSAGES• TYPES OF RECORDS• COMPRESSION• EXAMPLES• DDNS• ENCAPSULATIONNAME SPACE18.118.1DOMAINNAMESPACE18.218.2Figure 18-1Domain name spaceFigure 18-2Domain namesand labelsFigure 18-3FQDN and PQDNFigure 18-4DomainsDISTRIBUTION OFNAME SPACE18.318.3DNS servers are used to distribute the info among many servers. We use a hierarchy of serversjust like the hierarchy of names.What a server has authority for is called a zone. A root server’s zone is the whole tree.We use primary and redundant servers.A primary server loads all information from A primary server loads all information from the disk file; the secondary server loads the disk file; the secondary server loads all information from the the primary server. all information from the the primary server. When the primary downloads information When the primary downloads information from the secondary, it is called from the secondary, it is called zone transfer.zone transfer.DNS IN THE INTERNET18.418.4Figure 18-7DNS in the InternetFigure 18-8Generic domainsFigure 18-9Country domainsFigure 18-10Inverse domainRESOLUTION18.518.5DNS uses a client server architecture. A host needing info contacts a client named a resolver. The resolver client contacts a DNS server.Recursive Resolution:The resolver asks for a recursive answer from a DNS server. The server must respond with the complete answer. If it does not know the answer the server itself asks a parent server in the hierarchy. If the parent does not know, the parent asks a higher level server in the hierarchy. Eventually the resolver will be told the answer by the first DNS server the resolver contacted.Iterative Resolution:If client does not specify a recursive answer, client will get an iterative answer.This means if the first server contacted does not know the answer, the server returns the IP address of what the server thinks is a smarter server.This continues until the answer is found.ResolutionDNS uses either TCP or UDP. Always port 53. UDP is used when messages are less than 512 bytes because many UDP implementations have a 512 byte maximum size limit.If message larger than 512 bytes:If client knows message is larger than 512 it will use a TCP connectionIf client does not know size of message opens a UDP port to server,but if the response is larger than 512, server truncates response and sets the TC bit as a sign to the client to try again using a TCP connection instead.Protocol that transports DNS messagesFigure 18-11Recursive resolutionHere is a typical list of root servers held by a typical name server: ; This file holds the information on root name servers; needed to initialize cache of Internet domain name; servers (e.g. reference this file in the; "cache . <file>" configuration file of BIND domain: name servers).;; This file is made available by InterNIC registration; services under anonymous FTP as; file /domain/named.root; on server FTP.RS.INTERNIC.NET; last update: Aug 22, 1997; related version of root zone: 1997082200;;; formerly NS.INTERNIC.NET;. 3600000 IN NS A.ROOT-SERVERS.NET.A.ROOT-SERVERS.NET. 3600000 A 198.41.0.4;; formerly NS1.ISI.EDU;. 3600000 NS B.ROOT-SERVERS.NET.B.ROOT-SERVERS.NET. 3600000 A 128.9.0.107;; formerly C.PSI.NET;. 3600000 NS C.ROOT-SERVERS.NET.ETC……Source:http://computer.howstuffworks.com/dns5.htmFigure 18-12Iterative resolutionDNSMESSAGES18.618.6Figure 18-13DNS messagesDNS Message FormatsTwo basic types: Query and ResponseIdentification: 2 byte field so client may match response to the question. Client creates number,Server just repeats the number in the requestFlags: QR Query/Response: One bit 0=query 1=responseOpcode: four bits define type of query or response 0=normal 1=inverse, 2=server statusis requestedAA authoritative answer: One bit value of 1 means server responding is authoritative serverTC truncated: One bit if it equals 1 means answer was larger than than 512 bytes and was truncated RD recursion desired: one bit if set to 1 means we want a recursive answerRA recursion available: One bit when set to 1 means a recursive response is available. This isset only in the response messageReserved: three bit field set to 000rCode: Four bit field contains error statusNumber of Question Records: two byte field with number of queries in the question section of the messageNumber of Answer Records: two byte field with number of answers contained in answer section of the messageNumber of Authoritative Records: Two byte field containing the number of authoritative records in theauthoritative records section of a response messageNumber of Additional Records: Two byte field containing the number additional records in the additional section of a response message.Header:Question Section: Section consisting of one or more question records. Exists in both query and responseAnswer Section: Section consisting of one or more answer records. Exists in response only.Authoritative Section: Section consisting of one or more resource records. Exists in response only. This contains the domain name about one or more of the authoritative servers for the query.Additional Info Section: Contains one or more resource records. Exists in response only. Remainder of DNS Message FormatTYPESOFRECORDS18.718.7Types of RecordsTwo Types of Records in DNS•Question Records are found in


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GT ECE 4110 - Domain Name System (DNS)

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