DNS (Domain Name System)

Presented By: Satya Prakash SharmaMCA: III rd sem.

Your Comput

er

Website to

access

bla bla bla bla bla

We are here

We want to reach

here

Main DNS part

Domain Name SystemDNS translates the host names to

an IP Address.Its like a directory lookup service

that provides a mapping between the name of a host on the internet and its numerical address.

Total Four elements comprise the DNS and they are:

Domain Name Space

DNS Database

Name Server

Resolvers

Domain Name Space

DNS Database

Name Server

Resolvers

Domain Name SpaceA (32bit) IP address helps in uniquely

recognizing a particular website.Remembering names rather than IP

address.◦google.com◦yahoo.com◦ in.com◦mit.edu◦bbc.co.uk

continued…Here is a portion of the domain

naming tree...root

ukauusorgnetgovedumilcom

acm

ieee

shore

treas

mitappl

eibm

lcsaiinforaleigh

itso

..

continued…When we move down the internet domain

tree, we will get a leaf node that identify specific hosts on the internet.

These hosts are assigned internet addresses.e.g., the main host at MIT, with a domain

name of mit.edu, has four IP address: 18.7.21.77, 18.7.21.69, 18.7.21.70, 18.7.21.110

The subordinate domain lcs.mit.edu had the IP address 18.26.0.36.

Domain Name Space

DNS Database

Name Server

Resolvers

DNS Database DNS is based on a hierarchical database containing

resource records (RRs) that include the name, IP address, and other information about hosts.

Format for DNS Resource Record is as below:

Domain Name

Type

Time to Live

Rdata

Rdata field length

Class

Bit 0 1632

continued… Domain Name: The domain name in a RR must correspond to

the human readable form. Type: Identifies the type of resource in this RR.

◦ A: a host address.

◦ CNAME: Canonical name. Specifies an alias name for a host and maps this to the canonical (true) name.

◦ MINFO: Mailbox or mail list information. Maps a mailbox or mail list name to a host name.

Class: Identifies the protocol family. Time to Live: This field specifies the time interval that the

resource record may be cached before the source of the information should again be consulted.

Rdata Field Length: Length of the Rdata field in octets. Rdata: The Rdata is a 32-bit IP address, and for the CNAME type,

the Rdata is a domain name.

continued…

UserResponse

User Program

Name Resolver

Name server

Cache

Database

Foreign name server

Cache

Database

User query

User System

Cache

Internet

Query

Que

ry

resp

onse

response

A user program requests an IP address for a domain name.

A resolver module in the local host or local ISP formulates a query for a local name server in the same domain as the resolver.

The local name server checks to see if the name is in its local database or cache.

When a response is received at the local name server, it stores the name/address to its local cache and will maintain this entry for the amount of time specified in time to live field.

The user program will get either the IP address or error message.

Domain Name Space

DNS Database

Name Server

Resolvers

Name ServerWhy it is not

centralized ?o Single point of failure

o Traffic volume

o Maintenance

o No server has all name-to-IP address mappings

Local name servers:• Each ISP, company has local name

server.• Host DNS query first goes to local

name server.

Authoritative name server:

Can perform name/address translation for that host’s name.

DNS: Root name server Contacted by local name server that cannot resolve

name Root name server:

◦ Contacts authoritative name server if name mapping is not known

◦ Gets mapping◦ Returns mapping to local name server

nslookupnslookup is an interactive

resolver that allows the user to communicate directly with a DNS server.

nslookup is available on both Unix and windows workstation.

Domain Name Space

DNS Database

Name Server

Resolvers

Resolver:The client called the resolver is

transparent to the user and is called by an application to resolve names into real IP addresses or vice-versa.

A full resolver a program distinct from the user program which forwards all queries to a name server for processing.

DNS Messages:0 8 16 21 24 28

31Identifier QR opcode AA TC RD RA reserved RCODE

o Identifier: Assigned by the program that generates any kind of query. The same identifier is used in any response, enabling the sender to match queries and responses.

o QR: Indicates whether this message is a query of response. If query = 0, response = 1.o Opcode: Indicates whether this is a standard query , an inverse query (address to name).o AA(authoritative answer): valid in a response, and indicates whether the responding

name server is an authority for the domain name.o TC(Truncated): indicates whether the response message was truncated due to length

greater then permitted on the transmission channel, then the response switches from UDP to TCP.

o RD(Recursion Desired): directs the server to pursue the query recursively.o RA(Recursion Available): set or cleared in a response to denote whether recursive

query support is available in the name server.o RCODE(Response Code): no error, format error(server unable to interpret query), server

failure, name error(domain name does not exist).

THANKS