DNS - T

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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

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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

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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

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LOAD DISTRIBUTION

Ndtv.com – Breaking News

Replicated of hostname at Different Servers & Different IP Address

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STRUCTURE OF DNS

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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

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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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DNS MESSAGE FORMAT

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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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THANKYOU

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