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Presented By:Gaurav BansalCSE-A3
www.powerpointpresentationon.blogspot.com
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The Domain Name SystemOverview
What is DNS?? DNS HIERARCHY TYPES OF NAME SERVERS How to Access a Web Page?? TYPES OF DNS QUERIES DNS CACHING Why not Centralize DNS??
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The domain name systemis usually used totranslate a host name into an IP address.
Domain names comprise a hierarchy so thatnames are unique, yet easy to remember.
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An addressis how you get to an endpointo Often hierarchical, which helps with scaling
950 Charter Street, Redwood City CA, 94063 +1.650.381.6003 204.152.187.11
A nameis how an endpoint is referencedo Often with no structurally significant hierarchy
David, Tokyo, itu.int,google.com. Names are more people-friendly.
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Devices on the telephone network all have anumbero People have a hard time remembering numbers,
buto The network needs the numbers to connect endpointso So a directory provides association of names people
know with the numbers where they can be reached Computers on the Internet all have a number
o
The DNS takes names people can relate to andconverts them into the numbers computers need tointeract.
This analogy has a crucial flaw: The DNS is not adirectory service.o There is no way to search the data.
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root zone
gTLDs & ccTLDs
2lds
3lds
4lds
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DNS vs File System
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Naming a Domain Naming a Directory
yahoo.com.au.C:\windows\system32\drivers\
StartHere
StartHere
A . is used asseparator
A \ is used asseparator
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ROOT NAME SERVERS TOP-LEVEL DOMAIN(TLD) NAME SERVERS AUTHORITATIVE NAME SERVERS LOCAL NAME SERVERS
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contacted by local name server that can not resolve name root name server:
o contacts authoritative name server if name mapping not
knowno gets mappingo returns mapping to local name server
13 root name serversworldwide
b USC-ISI Marina del Rey, CAl ICANN Los Angeles, CA
e NASA Mt View, CAf Internet Software C. Palo Alto,CA (and 36 other locations)
i Autonomica, Stockholm (plus 28other locations)
k RIPE London (also 16 other locations)
m WIDE Tokyo (also Seoul,Paris, SF)
a Verisign, Dulles, VAc Cogent, Herndon, VA (also LA)d U Maryland College Park, MDg US DoD Vienna, VAh ARL Aberdeen, MD
j Verisign, ( 21 locations)
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Top-level domain (TLD) servers: responsible forcom, org, net, edu, etc, and all top-level countrydomains uk, fr, ca, jp.o Network solutions maintains servers for com TLDo Educause for edu TLD
Authoritative DNS servers: organizations DNSservers, providing authoritative hostname to IP
mappings for organizations servers (e.g., Web andmail).o Can be maintained by organization or service
provider
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Each ISP (residential ISP, company, university) hasone.oAlso called default name server
When a host makes a DNS query, query is sent toits local DNS servero Acts as a proxy, forwards query into hierarchy.o Reduces lookup latency for commonly searched
hostnames
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When You type http://www.google.com into your webbrowser and hit enter.
What happens now?
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Step 1: Your PC sends a resolutionrequest to its configured DNS Server,typically at your ISP.
Tell me the Address ofwww.google.com
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Step 2: Your ISPs recursive name
server starts by asking one of the rootservers predefined in its hints file.
Tell me the Address ofwww.google.com
I dont know the address but I knowwhos authoritative for the com
domain ask them
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Step 3: Your ISPs recursive name
server then asks one of the com nameservers as directed.
Tell me the Address ofwww.google.com
I dont know the address but I knowwhos authoritative for the
google.com domain ask them
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Step 4: Your ISPs recursive name
server then asks one of thegoogle.com name servers as directed.
Tell me the Address ofwww.google.com
The Address of www.google.com is216.239.53.99
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Step 5: ISP DNS server then send theanswer back to your PC. The DNSserver will remember the answer for a
period of time.
The Address of www.google.com is216.239.53.99
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Step 6: Your PC can then make theactual HTTP request to the web server.
Here it is!
Send me the www.google.com.au
web page
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ALL-IN-ONE
The actual webrequest
DNS
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There are two types of queries:o Recursive querieso Iterative (non-recursive) queries
The type of query is determined by a bit in the DNSquery
Recursive query: When the name server of a host cannotresolve a query, the server issues a query to resolve the
query Iterative queries: When the name server of a host cannot
resolve a query, it sends a referral to another server tothe resolver
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In a recursive query, the resolverexpects the response from thename server
If the server cannot supply theanswer, it will send the query to
the closest known authoritativename server (here: In the worstcase, the closest known server isthe root server)
The root sever sends a referral tothe edu server. Querying thisserver yields a referral to theserver of virginia.edu
and so on
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In an iterative query, the nameserver sends a closest knownauthoritative name server areferral to the root server.
This involves more work for the
resolver
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Caching can substantially reduce overheado The top-level Domain servers very rarely changeo Popular sites (e.g., www.google.com) visited often
Once (any) name server learns mapping, it caches
mapping
o cache entries timeout (disappear) after some timeo TLD servers typically cached in local name servers
Thus root name servers not often visited
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single point of failure traffic volume distant centralized database maintenance
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http://en.wikipedia.org/wiki/Category:Domain_name_system
http://www.livinginternet.com/i/iw_dns.htm http://www.centr.org Domain Names - Concepts and Facilities, P.
Mockapetris Role of the Domain Name System (DNS)-O'Reilly
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