Domain Name System DNS 1

8/3/2019 Domain Name System DNS 1

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Presented By:Gaurav BansalCSE-A3

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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.


I dont know the address but I knowwhos authoritative for the com

domain ask them


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server then asks one of the com nameservers as directed.


I dont know the address but I knowwhos authoritative for the

google.com domain ask them


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server then asks one of thegoogle.com name servers as directed.


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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Documents

Domain Name System DNS 1