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Transcript of May 23
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CS335 Networking &
Network Administration
Tuesday, November 25, 2008
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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
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Domain Name System
Top-level domains Maps to IP number Registration
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DNS
Geographic structure .or.us ac.uk
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DNS
Domain names within organizations computer.foobar.com computer.location.foobar.com computer.division.location.foobar computer.subdivision.candy.foobar.com
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Organization DNS
No universal standard Each organization can choose how to
structure names How does eastern do it?
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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
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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
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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
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Server hierarchy
DNS
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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
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Locality of reference principle
Users tend to look up names of local machines
Users tend to look up the same domains repeatedly
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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
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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.
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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
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Abbreviations
Ex. Mail refers to mail.lagrande.k12.or.us Simplifies typing in full paths Put in a DNS record instead
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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
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NSLOOKUP
Use NSLOOKUP to find information on domain servers
http://www.stopspam.org/usenet/mmf/man/nslookup.html
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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
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Electronic mail
Originally designed to act like office memos Evolved to today’s sophisticated uses Automated responses
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Email addresses
mailbox@computer User portion and mail system host Email addressing formats
Left up to sys admins
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Email message format
ASCII text Header body
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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
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Mail transfer
User email interface Transfer program
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SMTP
Simple Mail Transfer Protocol TCP connection Runs on port 25 Server protocol
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Mail Gateways
Email gateway or email relay Forwards email to all recipients of a list
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POP
Post Office Protocol Client access
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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