Domain Name SystemBy : Shubham Srivastava

Agenda What is DNS Why DNS DNS Overview Resource Records Types of Name Servers Authoritative Name Servers DNS Resolution DNS Zone Zone transfer

What is DNS ?

Domain Name Servers (DNS) are the Internet's equivalent of a phone book.

The mechanism by which Internet software translates names to attributes such as addresses

Why DNS ? ARPANET utilized a central file HOSTS

Contains names to IP addresses mapping

As the system grew Speed ??? Scalability (traffic and load) ??? Management ???

From history of Name Servers we saw how three needs emerged:

The Internet Domain Name System elegantly solves all these problems at the single stroke of a pen (well actually the whole of RFC 1034 to be precise).

The need for a hierarchy of names

Spread operational load

Delegate the administration of our Name servers

DNS Overview

Translates human friendly hostname to IP addresses.

Uses a tree name structure.

The essence of DNS is invention of a hierarchical, domain-based naming scheme and a distributed database system for implementing this naming scheme.

The DNS is also…A globally distributed, scalable,

reliable databaseComprised of three components

A “name space” Servers making that name space

available Resolvers (clients) which query the

servers about the name space

DNS Name Space

root

com in aunetedu

Microsoft applegoogleyahoo

Top Level Domain

2nd Level Domain

store3rd Level Domain

GenericICANN

Country Specific

Lecture 13: 02-22-2005

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DNS RecordsRR format: (Name, TTL, Class, Type,Value)

• DB contains tuples called resource records (RRs)• Classes = Internet (IN)• Each class defines value associated with type

FOR IN class:• Type=CNAME

• name is an alias name for some “canonical” (the real) name

• value is canonical name• Type=MX

• value is hostname of mailserver associated with name

• Type=A (AAAA for IPv6)• name is hostname• value is IPv4 address

• Type=NS• name is domain (e.g. foo.com)• value is name of authoritative

name server for this domain

SOA Record SOA – Start of Authorityexample.com. 14400 IN SOA ns1.ethii.com. admin.example.com.

2004123001 ; Serial number

86000 ; Refresh rate in seconds7200 ; Refresh Retry in seconds3600000 ; Expiry in seconds600 ; Minimum in seconds (for example: how long a resolver should consider a negative result for a subdomain to be valid before retrying)

DNS Resource Records

Types of Name ServersAuthoritative name server – for a host stores that host’s IP addressPrimary name server - Each Zone will have a primary name server and more secondary name servers

Secondary name server – Secondary servers retrieve information from primary servers.

Caching name server - ISPs

DNS Resolution Translation of domain names into IP addresses.Resolution type

Recursive Query Iterative Query

Types of resolver/DNS Client : Full resolver Stub resolver

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Recursive DNS Query Goes deep down the hierarchy to resolve Puts the burden of resolution on the contacted name

server

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Iterative DNS query Contact server replies with the name of the next authority in

the hierarchy “I don’t know this name, but this other server might”

Root

Full Resolver – DNS ClientDNS Client is called a resolver.Full resolver is a program distinct from the user program, which forwards all queries to name server for processing.

User Program Name ServerFull

Resolver

Database

Cache

Cache

Q Q

R R

Stub Resolver – DNS ClientStub resolver is a routine linked with the user program which forwards queries to DNS sever.

User Program

Name Server

Stub Resolver

Foreign Name Server

Database

CacheQ

Q

R

R

DNS Zone

root

com in aueduorg

oxford

Apple

yale

storeengg

Zones

For each DNS domain name included in a zone, the zone becomes the authoritative source for information about that domain.

DNS Zone File Zone file will contain the entire details for that domain

Zone Transfers One of the many mechanisms available for administrators to

replicate DNS databases across a set of DNS servers.

Zone transfer comes in two flavors, full (AXFR - RFC 1034, RFC 5936) and incremental (IXFR - RFC 1995).

Uses TCP for transfer zone files.

client requesting a zone transfer may be a slave server/secondary server, requesting data from a master server/primary server.

Zone transfer is entirely client-initiated.

Summary

Thanks

Questions?