DNS : The internet’s directory service

PROBLEM FACED…???
 Identify human - names & id
 Similarly host identified – host name
 Host name like www.yahoo.co.in , www.karunya.edu

 Fully alphanumeric characters – little information – location of host
 Difficult to process by router
 Host are identified by IP address

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SERVICES PROVIDED BY DNS
 Prefer only alphanumeric characters only
 Routers Support –IP Address
 To solve Translate hostname to IP address

 done by DNS
 distributed database implemented – hierarchy of DNS Servers
 Application layer – host to query the database
 It runs UDP Port & Port 53
 Commonly used with Http , ftp, Smtp

3

HOW ACTUALLY WORKS…?
 https://www.karunya.edu/results through Brower
 First send Web server ,before it have to fetch IP Address of Host
name

 Client Side DNS application
 For that browser extract host name from URL & Send along with
DNS Application
 DNS Application send to hostname to DNS Server & Receive
Reply of IP Address
 Browser receives IP Address from DNS ,initiate Connection with
TCP to the HTTP Server -- Send Web server –fetch Web pages

4

HOST ALIASING

 Suppose rest1.relay.west-coast.enter-prise.com –canonical
hostname
 Aliases are enterpriase.com , www.enterprise.com

 Aliases are easier than original host name

5

LOAD DISTRIBUTION

 Ndtv.com – Breaking News
 Replicated of hostname at Different Servers & Different IP Address

6

 To scale - large of no servers in hierarchical & distributed among
world
 Single Server Can’t do all mapping technique
 Three Classes of DNS Servers
(1) Root DNS Servers
(2) Top level Domain DNS Servers

(3) Authoritative DNS Server

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ROOT DNS SERVERS
 13 root DNS Servers
 Mostly –North America
 Security & Reliability purpose act as replicated servers

TOP LEVEL DOMAIN SERVER
 .com, .org, .au, .edu, .gov
 Country domains - .in, .uk , .us,.pak
 Company Network Solution maintain – com top level domain
 Company Educause maintains – edu top level domain

9

AUTHORITATIVE DNS SERVER
 Organization host their host name with IP Address – Details are
stored
 Each organization can choose their own authoritative server

 For storing details – cost based

LOCAL DNS SERVER --- NAME SERVER
 Identify near local DNS Server via ISP
 ISP host with IP address of local DNS Server

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DNS CACHING

 Request arrives with Host name for IP address ,Both – Stored in
local memory –DNS Server
 Same Request arrives – provides Desired IP Address
 Cache in formation –discarded – period of time

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RESOURCE RECORD
(Name,Value,Type,TTL)

 TTL-time to live of resource record
Type-A , Name-host name, Value – IP Address
Type-NS, Name-Domain,Value –Host name of authoritative
DNS Server,know how to obtain IP Address

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 12 bytes in header part followed by four variable length fields
 Identification is set by client and returned by server. It match the
request and response
 QR is a 1-bit field: 0 means the message is a query, 1 means it's a
response.

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 opcode is a 4-bit field. The normal value is 0 (a standard query). Other
values are 1 (an inverse query) and 2(server status request).
 AA is a 1-bit flag that means "authoritative answer.“
 TC is a 1-bit field that means "truncated.“With UDP this means the
total size of the reply exceeded 512 bytes, and only the first 512 bytes
of the reply was returned.

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 RD is a 1-bit field that means "recursion desired." This bit can be set
in a query and is then returned in the response. This flag tells the
name server to handle the query itself, called a recursive query. If
the bit is not set, and the requested name server doesn't have an
authoritative answer
 RA is a 1-bit field that means "recursion available." This bit is set to
1 in the response if the server supports recursion.
 There is a 3-bit field that must be 0.

 rcode is a 4-bit field with the return code. The common values are 0
(no error) and 3 (name error).

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DNS : The internet’s directory service

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DNS : The internet’s directory service