The Domain Name System (DNS) is basically a large database which resides on various computers

1. www.oeclib.in
Submitted By:
Odisha Electronics Control Library
Seminar
On
DNS

2. Content
 INTRODUCTION
 DNS HISTORY
 WHAT IS DNS?
 DNS COMPONENTS
o Name Space:
o Resolvers:
o Name Servers:
 WHY WE NEED OF DNS?
 CONCLUSION
 REFERENCES

3. Introduction
 The Domain Name System (DNS) is basically a large
database which resides on various computers and it
contains the names and IP addresses of various hosts on
the internet and various domains.
 The Domain Name System is used to provide
information to the Domain Name Service to use when
queries are made.

4. DNS History
 ARPANET utilized a central file HOSTS.TXT
 Contains names to addresses mapping
 Maintained by SRI’s NIC (Stanford-Research-Institute: Network-
Information-Center)
 Administrators email changes to NIC
 NIC updates HOSTS.TXT periodically
 Administrators FTP (download) HOSTS.TXT

5. DNS History Cont…
 As the system grew, HOSTS.TXT had problems with:
 Scalability (traffic and load)
 Name collisions
 Consistency
 In 1984, Paul Mockapetris released the first version
(RFCs 882 and 883, superseded by 1034 and 1035 …)

6. What is DNS ?
 The “Domain Name System”
 What Internet users use to reference anything by name on
the Internet
 The mechanism by which Internet software translates
names to attributes such as addresses

7. What is DNS ?
 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. Why we need DNS?
 DNS as a Database
 Global Distribution
 Loose Coherency
 Scalability
 Reliability
 Dynamicity

9. DNS as a Database
 Keys to the database are “domain names”
 www.foo.com, 18.in-addr.arpa, 6.4.e164.arpa
 Over 100,000,000 domain names stored
 Each domain name contains one or more attributes
 Known as “resource records”
 Each attribute individually retrievable

10. Global Distribution
 Data is maintained locally, but retrievable globally
 No single computer has all DNS data
 DNS lookups can be performed by any device
 Remote DNS data is locally cachable to improve
performance

11. Loose Coherency
 The database is always internally consistent
 Each version of a subset of the database (a zone) has a serial
number
 The serial number is incremented on each database change
 Changes to the master copy of the database are
replicated according to timing set by the zone
administrator
 Cached data expires according to timeout set by zone
administrator

12. Scalability
 No limit to the size of the database
 One server has over 20,000,000 names
 Not a particularly good idea
 No limit to the number of queries
 24,000 queries per second handled easily
 Queries distributed among masters, slaves, and caches

13. Reliability
 Data is replicated
 Data from master is copied to multiple slaves
 Clients can query
 Master server
 Any of the copies at slave servers
 Clients will typically query local caches
 DNS protocols can use either UDP or TCP
 If UDP, DNS protocol handles retransmission,
sequencing, etc.

14. Dynamicity
 Database can be updated dynamically
 Add/delete/modify of any record
 Modification of the master database triggers replication
 Only master can be dynamically updated
 Creates a single point of failure

15. DNS Components
There are 3 components:
 Name Space:
Specifications for a structured name space and data
associated with the names
 Resolvers:
Client programs that extract information from Name
Servers.
 Name Servers:
Server programs which hold information about the
structure and the names.

16. Name Space
16
 Flat Name Space
In a flat name space, a name is assigned to an address. A name in
this space is a sequence of characters without structure.
 Hierarchical Name Space
In a hierarchical name space, each name is made of several parts.
The first part can define the organization, the second part can
define the name, the third part can define departments, and so on.

17. Resolvers
17
A Resolver maps a name to an address and vice
versa.
Query
Response
Resolver Name Server

18. Iterative Resolution
18
iterative response (referral)
“I don't know. Try a3.nstld.com.”
2
a3.nstl
d.com
3
client
iterative request
“What is the IP address of
www.google.com?”
udel
server
1
iterative response (referral)
“I don't know. Try a.root-servers.net.”
4
a.root
server
5
iterative response (referral)
“I don't know. Try a.gtld-servers.net.”
6
a.gtld-
server
7
iterative response (referral)
“I don't know. Try ns1.google.com.”
8
ns1.goo
gle.com
9
iterative response
“The IP address of www.google.com
is 216.239.37.99.”10

19. Recursive Resolution
19
client
recursive request
“What is the IP address of
www.google.com?”
udel
serve
r
1
edu
server
2
root
server
3 com
server
4
google
server
5
recursive response
“The IP address of www.google.com is
216.239.37.99.”
6
10
7
8
9

20. 20
Master
serverZone transfer
Zone
data
file
From
disk
Authoritative Data
(primary master and
slave zones)
Agent
(looks up queries
on behalf of resolvers)
Cache Data
(responses from
other name servers)
Name Server Process
Name Server
Architecture:

21. 21
Resolver
Query
Response
Authoritative Data
(primary master and
slave zones)
Agent
(looks up queries
on behalf of resolvers)
Cache Data
(responses from
other name servers)
Name Server Process
Authoritative Data:
Name Server (cont’d)

22. 22
Arbitrary
name
server
Response
Resolver
Query
Query
Authoritative Data
(primary master and
slave zones)
Agent
(looks up queries
on behalf of resolvers)
Cache Data
(responses from
other name servers)
Name Server Process
Response
Using Other Name Servers:
Name Server (cont’d)

23. Cached Data :
23
Query
Response
Authoritative Data
(primary master and
slave zones)
Agent
(looks up queries
on behalf of resolvers)
Cache Data
(responses from
other name servers)
Name Server Process
Resolver
Name Server (cont’d)

24. Reference
www.google.com
www.wikipedia.com
www.oeclib.in

25. THANK YOU!