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Dns

  1. 1. Chapter 25 Domain Name System 25.1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
  2. 2. Figure 25.1 Example of using the DNS service 25.2
  3. 3. 25 25- -1 NAME SPACE 1 NAME SPACE T T b b bi bi th th i d i d t t hi hi To To be be unambiguous, unambiguous, the the names names assigned assigned to to machines machines must must be be carefully carefully selected selected from from a a name name space space with with l t l t t l t l th th bi di bi di b t b t th th complete complete control control over over the the binding binding between between the the names names and and IP IP addresses addresses. . Flat Name Space Topics discussed in this section: Topics discussed in this section: Hierarchical Name Space 25.3
  4. 4. 25 25- -2 2 DOMAIN NAME SPACE DOMAIN NAME SPACE T T h h hi hi l hi hi l d i d i To To have have a a hierarchical hierarchical name name space, space, a a domain domain name name space space was was designed designed. . In In this this design design the the names names are are d fi d d fi d i i i d i d i h i h h h defined defined in in an an inverted inverted- -tree tree structure structure with with the the root root at at the the top top. . The The tree tree can can have have only only 128 128 levels levels: : level level 0 0 ( ) ( ) l l l l 127 127 (root) (root) to to level level 127 127. . Label Topics discussed in this section: Topics discussed in this section: be Domain Name Domain 25.4
  5. 5. Figure 25.2 Domain name space 25.5
  6. 6. Figure 25.3 Domain names and labels 25.6
  7. 7. Figure 25.4 FQDN and PQDN 25.7
  8. 8. Figure 25.5 Domains 25.8
  9. 9. 25 25- -3 3 DISTRIBUTION OF NAME SPACE DISTRIBUTION OF NAME SPACE The The information information contained contained in in the the domain domain name name space space The The information information contained contained in in the the domain domain name name space space must must be be stored stored. . However, However, it it is is very very inefficient inefficient and and also also unreliable unreliable to to have have just just one one computer computer store store such such a a huge huge unreliable unreliable to to have have just just one one computer computer store store such such a a huge huge amount amount of of information information. . In In this this section, section, we we discuss discuss the the distribution distribution of of the the domain domain name name space space distribution distribution of of the the domain domain name name space space. . Hierarchy of Name Servers Topics discussed in this section: Topics discussed in this section: Zone Root Server Primary and Secondary Servers 25.9 Primary and Secondary Servers
  10. 10. Figure 25.6 Hierarchy of name servers 25.10
  11. 11. Figure 25.7 Zones and domains 25.11
  12. 12. Note A primary server loads all information from the disk file; the secondary server loads all information from the primary server. When the secondary downloads i f ti f th i it i ll d information from the primary, it is called zone transfer. 25.12
  13. 13. 25 25- -4 4 DNS IN THE INTERNET DNS IN THE INTERNET DNS DNS is is a a protocol protocol that that can can be be used used in in different different DNS DNS is is a a protocol protocol that that can can be be used used in in different different platforms platforms. . In In the the Internet, Internet, the the domain domain name name space space (tree) (tree) is is divided divided into into three three different different sections sections: : generic generic (tree) (tree) is is divided divided into into three three different different sections sections: : generic generic domains, domains, country country domains, domains, and and the the inverse inverse domain domain. . Generic Domains Topics discussed in this section: Topics discussed in this section: Country Domains Inverse Domain 25.13
  14. 14. Figure 25.8 DNS IN THE INTERNET 25.14
  15. 15. Figure 25.9 Generic domains 25.15
  16. 16. Table 25.1 Generic domain labels 25.16
  17. 17. Figure 25.10 Country domains 25.17
  18. 18. Figure 25.11 Inverse domain 25.18
  19. 19. 25 25- -5 5 RESOLUTION RESOLUTION Mapping Mapping a a name name to to an an address address or or an an address address to to a a name name is is called called name name- -address address resolution resolution. . Topics discussed in this section Topics discussed in this section Resolver Mapping Names to Addresses Topics discussed in this section: Topics discussed in this section: Mapping Names to Addresses Mapping Addresses to Names Recursive Resolution Caching 25.19
  20. 20. Figure 25.12 Recursive resolution 25.20
  21. 21. Figure 25.13 Iterative resolution 25.21
  22. 22. 25 25- -6 6 DNS MESSAGES DNS MESSAGES DNS DNS has has two two types types of of messages messages: : query query and and response response DNS DNS has has two two types types of of messages messages: : query query and and response response. . Both Both types types have have the the same same format format. . The The query query message message consists consists of of a a header header and and question question records records; ; the the consists consists of of a a header header and and question question records records; ; the the response response message message consists consists of of a a header, header, question question records records answer answer records records authoritative authoritative records records and and records, records, answer answer records, records, authoritative authoritative records, records, and and additional additional records records. . Header Topics discussed in this section: Topics discussed in this section: 25.22
  23. 23. Figure 25.14 Query and response messages 25.23
  24. 24. Figure 25.15 Header format 25.24
  25. 25. 25 25- -7 7 TYPES OF RECORDS TYPES OF RECORDS A A i i S ti S ti 25 25 6 6 t t t t f f d d As As we we saw saw in in Section Section 25 25. .6 6, , two two types types of of records records are are used used in in DNS DNS. . The The question question records records are are used used in in the the ti ti ti ti f f th th d d question question section section of of the the query query and and response response messages messages. . The The resource resource records records are are used used in in the the answer, answer, th it ti th it ti d d dditi l dditi l i f ti i f ti ti ti f f authoritative, authoritative, and and additional additional information information sections sections of of the the response response message message. . Q ti R d Topics discussed in this section: Topics discussed in this section: Question Record Resource Record 25.25
  26. 26. 25 25- -8 8 REGISTRARS REGISTRARS How How are are new new domains domains added added to to DNS? DNS? This This is is done done How How are are new new domains domains added added to to DNS? DNS? This This is is done done through through a a registrar, registrar, a a commercial commercial entity entity accredited accredited by by ICANN ICANN A A registrar registrar first first verifies verifies that that the the requested requested ICANN ICANN. . A A registrar registrar first first verifies verifies that that the the requested requested domain domain name name is is unique unique and and then then enters enters it it into into the the DNS DNS database database A A fee fee is is charged charged DNS DNS database database. . A A fee fee is is charged charged. . 25.26
  27. 27. 25 25- -9 9 DYNAMIC DOMAIN NAME DYNAMIC DOMAIN NAME SYSTEM (DDNS) SYSTEM (DDNS) The The DNS DNS master master file file must must be be updated updated dynamically dynamically. . The The Dynamic Dynamic Domain Domain Name Name System System (DDNS) (DDNS) therefore therefore was was devised devised to to respond respond to to this this need need. . In In DDNS, DDNS, when when a a binding binding between between a a name name and and an an address address is is determined, determined, the the information information is is sent, sent, usually usually by by DHCP DHCP to to a a primary primary DNS DNS server server. . The The primary primary server server updates updates the the zone zone. . The The secondary secondary servers servers are are notified notified either either actively actively or or passively passively. . 25.27
  28. 28. 25 25- -10 10 ENCAPSULATION ENCAPSULATION DNS DNS can can use use either either UDP UDP or or TCP TCP In In both both cases cases the the DNS DNS can can use use either either UDP UDP or or TCP TCP. . In In both both cases cases the the well well- -known known port port used used by by the the server server is is port port 53 53. . UDP UDP is is used used when when the the size size of of the the response response message message is is less less than than used used when when the the size size of of the the response response message message is is less less than than 512 512 bytes bytes because because most most UDP UDP packages packages have have a a 512 512- -byte byte packet packet size size limit limit If If the the size size of of the the response response message message is is packet packet size size limit limit. . If If the the size size of of the the response response message message is is more more than than 512 512 bytes, bytes, a a TCP TCP connection connection is is used used. . 25.28
  29. 29. Note DNS can use the services of UDP or TCP i th ll k t 53 using the well-known port 53. 25.29

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