Wednesday, May 26, 2010

 The DNS system consists of three components:
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DNS data (called resource records ) servers (called name servers )
Internet protocols for fetching data from the servers

 Top-level domains
 Maps to IP number
 Registration

 Geographic structure
 .or.us
 ac.uk

 Domain names within organizations
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 computer.foobar.com
computer.location.foobar.com
computer.division.location.foobar
computer.subdivision.candy.foobar.com

 No universal standard
 Each organization can choose how to structure names
 How does eastern do it?

 The first label in a domain name is done for humans, not computers
 www is not necessary for a web server, but is common

 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

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

 DNS

 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

 Users tend to look up names of local machines
 Users tend to look up the same domains repeatedly

 Client computer generates a resolve request
 Application calls library routine gethostbyname
 Directs it to the local DNS server
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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

 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.

 Domain name
 Record type
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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

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

 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

 Use NSLOOKUP to find information on domain servers
 http://www.stopspam.org/usenet/mmf/man/nsl ookup.html

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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

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Originally designed to act like office memos
Evolved to today’s sophisticated uses
 Automated responses

 mailbox@computer
 User portion and mail system host
 Email addressing formats
 Left up to sys admins

 ASCII text
 Header
 body

 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

 User email interface
 Transfer program

 Simple Mail Transfer Protocol
 TCP connection
 Runs on port 25
 Server protocol

 Email gateway or email relay
 Forwards email to all recipients of a list

 Post Office Protocol
 Client access

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