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CS335 Networking & Network Administration

CS335 Networking & Network Administration. Wednesday, May 26, 2010. Domain Name System DNS. The DNS system consists of three components: DNS data (called resource records ) servers (called name servers ) Internet protocols for fetching data from the servers. Domain Name System.

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CS335 Networking & Network Administration

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  1. CS335 Networking &Network Administration Wednesday, May 26, 2010

  2. Domain Name System DNS • The DNS system consists of three components: • DNS data (called resource records) • servers (called name servers) • Internet protocols for fetching data from the servers

  3. Domain Name System • Top-level domains • Maps to IP number • Registration

  4. DNS • Geographic structure • .or.us • ac.uk

  5. DNS • Domain names within organizations • computer.foobar.com • computer.location.foobar.com • computer.division.location.foobar • computer.subdivision.candy.foobar.com

  6. Organization DNS • No universal standard • Each organization can choose how to structure names • How does eastern do it?

  7. www or ftp • The first label in a domain name is done for humans, not computers • www is not necessary for a web server, but is common

  8. DNS client-server model • Provides autonomy for organizations • Can assign names as they see fit within their suffix without informing a central authority • Entire system operates as a large, distributed database • Each server contains information that links it to other domain name servers • When an application needs to translate a name to an IP address the app becomes a client of the naming system

  9. DNS server hierarchy • Root server at the top • Is the authority for the top-level domain • Doesn’t contain all possible domain names, but contains information to reach other servers

  10. Server hierarchy • DNS

  11. Server architecture • Single server is simplest • Depends on system size • Large organizations might need more than one to handle requests at high speed • Administration is done by humans • Each group can make changes as necessary without centralized coordination

  12. Locality of reference principle • Users tend to look up names of local machines • Users tend to look up the same domains repeatedly

  13. How it works • Client computer generates a resolve request • Application calls library routine gethostbyname • Directs it to the local DNS server • If it is not an authority for that domain • Then to the ISP’s DNS server • Then up the tree to the root server if necessary • Waits for an authoritative answer

  14. Authoritative servers • The billions of resource records in the DNS are split into millions of files called zones. Zones are kept on authoritative servers distributed all over the Internet, which answer queries based on the resource records stored in the zones they have copies of. Caching servers ask other servers for information and cache any replies. Most name servers are authoritative for some zones and perform a caching function for all other DNS information. Large name servers are often authoritative for tens of thousands of zones, but most name servers are authoritative for just a few zones.

  15. Types of DNS entries • Domain name • Record type • Value • Type A – address type FTP, ping, WWW • MX – Mail eXchanger used by email • Aliases using CNAME • Lets www.foobar.com point to hobbes.foobar.com • Allows companies to move WWW servers without changing names or addresses or lets one server answer to www.foobar.com and ftp.foobar.com with domain records

  16. Abbreviations • Ex. Mail refers to mail.lagrande.k12.or.us • Simplifies typing in full paths • Put in a DNS record instead

  17. DNS resources • http://www.ripe.net/ripe/docs/ripe-192.html • http://www.dns.net/dnsrd/docs/whatis.html • http://www.dns.net/dnsrd/rfc/ • http://web.syr.edu/~djmolta/ist452/ch_07.ppt • Find out what you can about the ARPANET and how it originally resolved IP addresses

  18. NSLOOKUP • Use NSLOOKUP to find information on domain servers • http://www.stopspam.org/usenet/mmf/man/nslookup.html

  19. Master DNS example • ORIGIN lgdsd. • $TTL 86400 • ; <name_of_this_server.> <your_e-mail_address.> • @ IN SOA ns1.lgdsd. hostmaster.lgdsd. ( • 2004073000 ; serial number • 28800 ; refresh 8 hours • 7200 ; retry 2 hour • 604800 ; expire 7 days • 86400 ; def. ttl 1 day • ) • ; <Primary DNS> • IN NS ns1.lgdsd. • ; <Secondary DNS> • IN NS ns2.lgdsd. • ; Aliases • www IN CNAME lgdsd. • mrtg IN CNAME ns2.lgdsd. • xserve IN CNAME ns1.lgdsd. • viruswall IN CNAME ns2.lgdsd. • ;Fixed IPs • lgdsd. IN A 10.10.6.8 ; Mac www server • mail IN A 10.10.6.2 ; Novell GroupWise POA • ns1 IN A 10.10.7.2 ; MAC OSX Server • ns2 IN A 10.10.6.47 ; Linux Redhat 8.0 MRTG Server • opaclhs IN A 10.10.32.2 ; LaGrande High School Follett • opaclms IN A 10.10.16.2 ; Middle School Follett • opacce IN A 10.10.32.2 ;Central Elementary Follett • opacge IN A 10.10.64.3 ; Greenwood elementary Follett • opacice IN A 10.10.80.3 ; Island City Follett • opacwe IN A 10.10.48.3 ; Willow Elementary Follett • iv IN A 10.10.96.3 ; Infinite Vision Server • we4300 IN A 10.10.48.2 ; Willow Novell Server • ice4300 IN A 10.10.80.2 ; Island City Novell Server • ge4300 IN A 10.10.64.2 ; Greenwood Novell server • do4200 IN A 10.10.96.2 ; DO Novell Server • fs4400 IN A 10.10.6.5 ; Student File Server • ce4300 IN A 10.10.6.4 ; Central Novell Server • lms4300 IN A 10.10.7.5 ; LMS Novell Server • lhs6300 IN A 10.10.6.7 ; LHS novell Server

  20. Electronic mail • Originally designed to act like office memos • Evolved to today’s sophisticated uses • Automated responses

  21. Email addresses • mailbox@computer • User portion and mail system host • Email addressing formats • Left up to sys admins

  22. Email message format • ASCII text • Header • body

  23. MIME • Multipurpose Internet Mail Extensions • Original email system designed for text only • To transfer binary data or graphics data needed to be encoded, sent, decoded • MIME is a set of standards for encoding data allowing for new encodings to be invented at any time • MIME includes information so receiving app can decode message

  24. Mail transfer • User email interface • Transfer program

  25. SMTP • Simple Mail Transfer Protocol • TCP connection • Runs on port 25 • Server protocol

  26. Mail Gateways • Email gateway or email relay • Forwards email to all recipients of a list

  27. POP • Post Office Protocol • Client access

  28. SMTP and POP links • SMTP • http://www.faqs.org/rfcs/rfc821.html • http://www.freesoft.org/CIE/Topics/94.htm • POP • http://www.faqs.org/rfcs/rfc1939.html • http://www.networksorcery.com/enp/protocol/pop.htm

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