1. Domain Name
System
By : Shubham Srivastava

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

3. 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

4. Why DNS ?
 ARPANET utilized a central file HOSTS
 Contains names to IP addresses mapping
 As the system grew
 Speed ???
 Scalability (traffic and load) ???
 Management ???

5. 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

6. 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.

7. The DNS is also…
A globally distributed, scalable, reliable
database
Comprised of three components
 A “name space”
 Servers making that name space available
 Resolvers (clients) which query the servers
about the name space

8. DNS Name Space
root
com in aunetedu
Microsoft applegoogleyahoo
Top Level
Domain
2nd Level
Domain
store
3rd Level
Domain
Generic
ICANN
Country
Specific

9. Lecture 13: 02-22-2005
9
DNS Records
RR 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

10. SOA Record
 SOA – Start of Authority
example.com. 14400 IN SOA ns1.ethii.com. admin.example.com.
2004123001 ; Serial number
86000 ; Refresh rate in seconds
7200 ; Refresh Retry in seconds
3600000 ; Expiry in seconds
600 ; Minimum in seconds (for example: how long a resolver should
consider a negative result for a subdomain to be valid before retrying)

11. DNS Resource Records

12. Types of Name Servers
Authoritative name server – for a host stores that host’s IP address
Primary 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

13. DNS Resolution
 Translation of domain names into IP addresses.
Resolution type
 Recursive Query
 Iterative Query
Types of resolver/DNS Client :
 Full resolver
 Stub resolver

15. Recursive DNS Query
 Goes deep down the hierarchy to resolve
 Puts the burden of resolution on the contacted name server
15

16. 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”
16
Root

17. Full Resolver – DNS Client
DNS 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

18. Stub Resolver – DNS Client
Stub 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
Cache
Q
Q
R
R

19. DNS Zone
root
com in au
eduorg
oxford
Apple
yale
store
engg
Zones
For each DNS domain name included in a zone, the zone becomes
the authoritative source for information about that domain.

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

22. 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.

23. Summary

24. Thanks

25. Questions?