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DNS: Components and Operations in Web Networking

Explore the Domain Name System (DNS) and its key components, such as domain name space, name servers, resolvers, and zones. Learn about DNS design goals, domain structure, delegation, and message formats.

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DNS: Components and Operations in Web Networking

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  1. DNS—Domain Name System RFC: 1034, 1035 學生: 朱家彥 陶宛琳 指導教授: 梁德昭 老師

  2. Introduction —Why DNS • ARPAnet – Administrators typically emailed their changes to the NIC, and periodically ftped to SRI-NIC and grabbed the current HOSTS.TXT. • Problems with Host.txt • Traffic and load • Name collisions • Consistency

  3. Introduction — What is DNS • DNS is used primarily to map between hostnames and IP addresses

  4. Introduction — DNS Design Goal • Consistency • Can be maintained in a distributed manner(decentralize) • Source of data should control the tradeoff • Not restricted to a single application • Can be used with different protocol families • Independent from communication systems • Be useful across a wide spectrum of host capabilities

  5. DNS — Major Components • DOMAIN NAME SPACE AND RESOURCE RECORDS • NAME SERVERS • RESOLVERS

  6. DOMAIN NAME SPACE • Tree structure (inverted tree ) • Each node and leaf on the tree corresponds to a resource set and has a label • Length of every label is limited to 63 octets • Brother nodes may not have the same label • The null label ( 0 octet ) is reserved for the root • Depth of of the tree structure is limited to 127 levels

  7. DOMAIN NAME—1 • The domain name of a node in the domain name space is the list of labels on the path from the node to the list • Domain names are printed or read left of right, ie.from the most specific (lowest, farthest from the root) to the least specific (highest, closest to the root). Example: www.tku.edu.tw

  8. DOMAIN NAME — 2 • Upper case, lower case • How users use domain names • Absolute(FQDN-Fully qualified domain name) • Relative(PQDN-Partial qualified domain name) • The total number of octets that represent a domain name is limited to 255

  9. DOMAIN • A domain is a sub tree of the domain name space • A domain is identified by a domain name. And this domain name is the same as the very top node in the domain. • A domain is a subdomain of another domain if it is contained with in that domain.

  10. Delegation • As mentioned before, a main goal of DNS is to decentralize administration. This is achieved through Delegation. • Delegation is done by giving the administration power of an subdomain to another organization

  11. NAME SERVER AND ZONE — Name Server • The programs that store information about the domain name space are called name servers. • Name servers generally have complete information of some part of the domain name space, called a zone • The name server is said to have authority of the zone • Name servers can be authoritative to multiple zones • The principal activity of name servers is to answer standard queries

  12. NAME SERVER AND ZONE — Zone • Every zone has at least one node for which it is authoritative, and all of the nodes in a particular zone are connected • The name of the node in the zone that is closer than any other nodes in the zone, is the name of the zone

  13. NAME SERVER AND ZONE — Types of name servers • Primary Name Server • A primary name server gets the information of the zone it authorizes from files on local • Secondary Name Server • Gets zone information from the primary name server of the zone • Periodically contact to the primary name server for latest information. If updates has been made in the primary name server. Then zone transfer have to be taken place

  14. Caching • To store data for future reference • Speed up query

  15. 假設先前已經查詢過 eecs.berkeley.edu這個位址

  16. Resolver • Programs that interface user programs to domain name servers. • The resolver is located on the same machine as the program that requests the resolver’s services, but it may need to consult name servers on other hosts. • Eliminate name server load. • Interpret response. (RRs or error)

  17. “” name server Recursive “ ” tw name server jp Name server tw edu name server co edu tku name server tku Resolver im name server 163.13.200.193 im Userprogram QNAME=im2.im.tku.edu.tw. QTYPE=A

  18. “” name server Non-Recursive(interative) “ ” tw name server local name server jp tw edu name server co edu Resolver tku name server tku im name server 163.13.200.193 im Userprogram QNAME=im2.im.tku.edu.tw. QTYPE=A

  19. DNS Message Format 0 15 16 31 Flag Header Section Flag 16 bits

  20. Resource Record • A domain name identifies a node. Each node has a set of resource information, composed of separate resource records(RRs). 1. Owner: domain name where the RR pertain. 2. Type:

  21. Resource Record 3. CLASS: identify a protocol family. 4. TTL: time interval to live, 32 bit integer. 0 => not be cached. 5. RDATA: describe the resource data, string.

  22. Resource Record(example) @ IN SOA im.tku.edu.tw. root.mail.im.tku.edu.tw. ( 1999081006 ; Serial 86400 ; Refresh - 1 days 1800 ; Retry 1728000 ; Expire - 20 days 259200 ) ; Minimum TTL - 3 days IN NS mail.im.tku.edu.tw. IN MX 0 mail.im.tku.edu.tw. IN MX 2 mis.im.tku.edu.tw. mail IN A 163.13.200.222 herbert IN A 163.13.200.222 wwwbbs IN A 163.13.200.37 www IN CNAME sun 40 IN PTR bbs.im.tku.edu.tw. 32 IN PTR mis.im.tku.edu.tw.

  23. Configuration related files • BIND 8 • /etc/named.conf • /etc/namedb $ ls named.hosts named.local named.rev root.cache

  24. named.conf options { directory “/etc/namedb”; }; zone “.” in { type hint; file “root.cache”; }; zone “im.tku.edu.tw” in { type master; file “named.hosts”; }; zone “200.13.163.in-addr.arpa” in { type master; file “named.rev”; }; zone “0.0.127.in-addr.arpa” in { type master; file “named.local”; };

  25. named.hosts @ IN SOA im1.im.tku.edu.tw. root.im1.im.tku.edu.tw. ( 1999110319 ; Serial 43200 ; Refresh 3600 ; Retry 3600000 ; Expire 2419200) ; Time to live ; Define the nameservers and the mail servers. IN MX 1 mail.im.tku.edu.tw. IN MX 2 mis.im.tku.edu.tw. IN NS im1.im.tku.edu.tw. im1 IN A 163.13.200.222 wwwbbs IN A 163.13.200.37 backup IN CNAME ftp

  26. named.rev @ IN SOA im1.im.tku.edu.tw. root.im1.im.tku.edu.tw. ( 1999110319 ; Serial 43200 ; Refresh 3600 ; Retry 3600000 ; Expire 2419200 ) ; Minimum IN NS im1.im.tku.edu.tw. IN NS tkgis.tku.edu.tw. ; ; Define address-to-host mappings is this zone (163.13.200) 40 IN PTR bbs.im.tku.edu.tw. 39 IN PTR ftp.im.tku.edu.tw. 37 IN PTR wwwbbs.im.tku.edu.tw.

  27. named.local @ IN SOA im1.im.tku.edu.tw. root.im1.im.tku.edu.tw. ( 95071603 ; Serial 43200 ; Refresh 3600 ; Retry 3600000 ; Expire 2419200 ) ; Minimum IN NS im1.im.tku.edu.tw. 1 IN PTR localhost. ;

  28. Zone Transfer 1. Secondary servers refresh authoritative data from Primary servers. 2. Periodically check SERIAL number. Primary Secondary Check serial Zone Transfer Serial: 98112301 Serial: 98112301 Serial: 98112203

  29. Tool -----nslookup • Domain name->IP address • IP address-> Domain name • ex: %nslookup Default Server: mail.im.tku.edu.tw Address: 163.13.200.222 > bbs.im.tku.edu.tw <---enter Name: bbs.im.tku.edu.tw Address: 163.13.200.40

  30. Tool -----nslookup %nslookup Default Server: mail.im.tku.edu.tw Address: 163.13.200.222 > server xxx.xxx.xxx.xxx > ls im.tku.edu.tw > im.dns > set type=NS > set type=MX

  31. Reference • RFC: 1034, 1035 • TCP/IP illustrated, Volume 1 中譯本

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