Domain Name Server.ppt Computer networks notes

dns notes

1. TCP/IP Protocol Suite 1
Chapter 17
Upon completion you will be able to:
Domain Name
System: DNS
•Understand how the DNS is organized
•Know the domains in the DNS
•Know how a name or address is resolved
•Be familiar with the query and response formats
•Understand the need for DDNS
Objectives

2. TCP/IP Protocol Suite 2
17.1 NAME SPACE
Thenamesassignedtomachinesmustbeuniquebecausetheaddresses
areunique.Anamespacethatmapseachaddresstoauniquenamecan
beorganizedintwoways:flatorhierarchical.
The topics discussed in this section include:
Flat Name Space
Hierarchical Name Space

3. TCP/IP Protocol Suite 3
17.2 DOMAIN NAME SPACE
Thedomainnamespaceishierarchicalindesign.Thenamesaredefined
inaninverted-treestructurewiththerootatthetop.Thetreecanhave
128levels:level0(root)tolevel127.
The topics discussed in this section include:
Label
Domain Name
Domain

4. TCP/IP Protocol Suite 4
Figure 17.1Domain name space

5. TCP/IP Protocol Suite 5
Figure 17.2Domain names and labels

6. TCP/IP Protocol Suite 6
Figure 17.3FQDN and PQDN

7. TCP/IP Protocol Suite 7
Figure 17.4Domains

8. TCP/IP Protocol Suite 8
17.3 DISTRIBUTION OF
NAME SPACE
Theinformationcontainedinthedomainnamespaceisdistributed
amongmanycomputerscalledDNSservers.
The topics discussed in this section include:
Hierarchy of Name Servers
Zone
Root Server
Primary and Secondary Servers

9. TCP/IP Protocol Suite 9
Figure 17.5Hierarchy of name servers

10. TCP/IP Protocol Suite 10
Figure 17.6Zones and domains

11. TCP/IP Protocol Suite 11
A primary server loads all information
from the disk file; the secondary server
loads all information from the primary
server. When the secondary downloads
information from the primary, it is called
zone transfer.
Note:

12. TCP/IP Protocol Suite 12
17.4 DNS IN THE INTERNET
Thedomainnamespace(tree)isdividedintothreedifferentsections:
genericdomains,countrydomains,andtheinversedomain.
The topics discussed in this section include:
Generic Domains
Country Domains
Inverse Domain
Registrar

13. TCP/IP Protocol Suite 13
Figure 17.7DNS used in the Internet

14. TCP/IP Protocol Suite 14
Figure 17.8Generic domains

15. TCP/IP Protocol Suite 15
Table 17.1 Generic domain labels

16. TCP/IP Protocol Suite 16
Table 17.1 Generic domain labels (Continued)

17. TCP/IP Protocol Suite 17
Figure 17.9Country domains

18. TCP/IP Protocol Suite 18
Figure 17.10Inverse domain

19. TCP/IP Protocol Suite 19
17.5 RESOLUTION
Mappinganametoanaddressoranaddresstoanameiscalledname-
addressresolution.
The topics discussed in this section include:
Resolver
Mapping Names to Addresses
Mapping Addresses to Names
Recursive Resolution
Iterative Resolution
Caching

20. TCP/IP Protocol Suite 20
Figure 17.11Recursive resolution

21. TCP/IP Protocol Suite 21
Figure 17.12Iterative resolution

22. TCP/IP Protocol Suite 22
17.6 DNS MESSAGES
TheDNSquerymessageconsistsofaheaderandquestionrecords;the
DNSresponsemessageconsistsofaheader,questionrecords,answer
records,authoritativerecords,andadditionalrecords.
The topics discussed in this section include:
Header

23. TCP/IP Protocol Suite 23
Figure 17.13DNS messages

24. TCP/IP Protocol Suite 24
Figure 17.14Query and response messages

25. TCP/IP Protocol Suite 25
Figure 17.15Header format

26. TCP/IP Protocol Suite 26
Figure 17.16Flags field

27. TCP/IP Protocol Suite 27
Table 17.2 Values of rCode

28. TCP/IP Protocol Suite 28
17.7 TYPES OF RECORDS
TwotypesofrecordsareusedinDNS.Thequestionrecordsareusedin
thequestionsectionofthequeryandresponsemessages.Theresource
recordsareusedintheanswer,authoritative,andadditionalinformation
sectionsoftheresponsemessage.
The topics discussed in this section include:
Question Record
Resource Record

29. TCP/IP Protocol Suite 29
Figure 17.17Question record format

30. TCP/IP Protocol Suite 30
Figure 17.18Query name format

31. TCP/IP Protocol Suite 31
Table 17.3 Types

32. TCP/IP Protocol Suite 32
Table 17.4 Classes

33. TCP/IP Protocol Suite 33
Figure 17.19Resource record format

34. TCP/IP Protocol Suite 34
17.8 COMPRESSION
DNSrequiresthatadomainnamebereplacedbyanoffsetpointerifitis
repeated.DNSdefinesa2-byteoffsetpointerthatpointstoaprevious
occurrenceofthedomainnameorpartofit.

35. TCP/IP Protocol Suite 35
Figure 17.20Format of an offset pointer

36. TCP/IP Protocol Suite 36
Aresolversendsaquerymessagetoalocalservertofindthe
IPaddressforthehost“chal.fhda.edu.”.Wediscussthequery
andresponsemessagesseparately.
Example1
QROpCode AATC RD RA ReservedrCode
00000 00 1 0 000 0000
Figure17.21showsthequerymessagesentbytheresolver.Thefirst2bytes
showtheidentifier(1333).Itisusedasasequencenumberandrelatesa
responsetoaquery.Becausearesolvermayevensendmanyqueriestothe
sameserver,theidentifierhelpstosortresponsesthatarriveoutoforder.
Thenextbytescontaintheflagswiththevalueof0x0100inhexadecimal.
Inbinaryitis0000000100000000,butitismoremeaningfultodivideitinto
thefieldsasshownbelow:

37. TCP/IP Protocol Suite 37
Figure 17.21Example 1: Query message

38. TCP/IP Protocol Suite 38
Example1 (Continued)
QROpCodeAATCRDRAReservedrCode
1 0000 0 01 1 000 0000
TheQRbitdefinesthemessageasaquery.TheOpCodeis0000,which
definesastandardquery.Therecursiondesired(RD)bitisset.(Referback
toFigure17.16fortheflagsfielddescriptions.)Themessagecontainsonly
onequestionrecord.Thedomainnameis4chal4fhda3edu0.Thenext2
bytesdefinethequerytypeasanIPaddress;thelast2bytesdefinetheclass
astheInternet.
Figure17.22showstheresponseoftheserver.Theresponseissimilartothe
queryexceptthattheflagsaredifferentandthenumberofanswerrecords
isone.Theflagsvalueis0x8180inhexadecimal.Inbinaryitis
1000000110000000,butagainwedivideitintofieldsasshownbelow:

39. TCP/IP Protocol Suite 39
Example1 (Continued)
TheQRbitdefinesthemessageasaresponse.TheOpCodeis0000,which
definesastandardresponse.Therecursionavailable(RA)andRDbitsare
set.Themessagecontainsonequestionrecordandoneanswerrecord.The
questionrecordisrepeatedfromthequerymessage.Theanswerrecordhas
avalueof0xC00C(splitintwolines),whichpointstothequestionrecord
insteadofrepeatingthedomainname.Thenextfielddefinesthedomain
type(address).Thefieldafterthatdefinestheclass(Internet).Thefieldwith
thevalue12,000istheTTL(12,000s).Thenextfieldisthelengthofthe
resourcedata,whichisanIPaddress(153.18.8.105).

40. TCP/IP Protocol Suite 40
Figure 17.22Example 1: Response message

41. TCP/IP Protocol Suite 41
AnFTPserverhasreceivedapacketfromanFTPclientwith
IPaddress153.2.7.9.TheFTPserverwantstoverifythatthe
FTPclientisanauthorizedclient.TheFTPservercanconsult
afilecontainingthelistofauthorizedclients.However,thefile
consistsonlyofdomainnames.TheFTPserverhasonlytheIP
addressoftherequestingclient,whichwasthesourceIP
addressinthereceivedIPdatagram.TheFTPserverasksthe
resolver(DNSclient)tosendaninversequerytoaDNSserver
toaskforthenameoftheFTPclient.Wediscussthequeryand
responsemessagesseparately.
Example2

42. TCP/IP Protocol Suite 42
Example2 (Continued)
QROpCodeAA TC RD RA ReservedrCode
0 0001 0 0 1 0 000 0000
Figure17.23showsthequerymessagesentfromtheresolvertotheserver.
Thefirst2bytesshowtheidentifier(0x1200).Theflagsvalueis0x0900in
hexadecimal.Inbinaryitis0000100100000000,andwedivideitintofields
asshownbelow:
TheOpCodeis0001,whichdefinesaninversequery.Themessagecontains
onlyonequestionrecord.Thedomainnameis19171231537in-addr4arpa.
Thenext2bytesdefinethequerytypeasPTR,andthelast2bytesdefine
theclassastheInternet.

43. TCP/IP Protocol Suite 43
Figure 17.23Example 2: Inverse query message

44. TCP/IP Protocol Suite 44
Example2 (Continued)
QROpCodeAATCRDRAReservedrCode
1 0001 1 01 1 000 0000
Figure17.24showstheresponse.Theflagsvalueis0x8D80in
hexadecimal.Inbinaryitis1000110110000000,andwedivideitintofields
asshownbelow:

45. TCP/IP Protocol Suite 45
Figure 17.24Example 2: Inverse response message

46. TCP/IP Protocol Suite 46
InUNIXandWindows,thenslookuputilitycanbeusedto
retrieveaddress/namemapping.Thefollowingshowshowwe
canretrieveanaddresswhenthedomainnameisgiven.
Example3
$ nslookup fhda.edu
Name: fhda.edu
Address: 153.18.8.1
Thenslookuputilitycanalsobeusedtoretrievethedomain
namewhentheaddressisgivenasshownbelow:
$ nslookup 153.18.8.1
1.8.18.153.in-addr.arpa name = tiptoe.fhda.edu.

47. TCP/IP Protocol Suite 47
17.9 DDNS
TheDynamicDomainNameSystem(DDNS)updatestheDNSmaster
filedynamically.

48. TCP/IP Protocol Suite 48
17.10 ENCAPSULATION
DNSusesUDPasthetransportprotocolwhenthesizeoftheresponse
messageislessthan512bytes.Ifthesizeoftheresponsemessageis
morethan512bytes,aTCPconnectionisused.

49. TCP/IP Protocol Suite 49
DNS can use the services of UDP or
TCP using the well-known port 53.
Note: