1 kyung hee university chapter 19 dns (domain name system)

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Chapter 19Chapter 19DNS (Domain Name DNS (Domain Name

System)System)

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IntroductionIntroduction People prefer to use easy to remember names instead

of IP addresses

Domain names are alphanumeric names for IP addresses

e.g., kr.yahoo.com, khu.ac.kr, ietf.org

The domain name system (DNS) is an Internet-wide distributed database that translates between domain names and IP addresses

How important is DNS?

Imagine what happens when the local DNS server is down.

KT 1.25 Big Accident (DNS Server DOWN !!!): year 2003

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Resolver and Name ServerResolver and Name Server

An application program on a host accesses the domain

system through a DNS client, called the Resolver

Resolver contacts DNS Server, called name server

DNS server returns IP address to resolver which

passes the IP address to application

1. Host name(khu.ac.kr) 2. Host name(khu.ac.kr)

3. IP(163.180.96.52)4. IP(163.180.96.52)

Client Resolver DNS Server

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Design principle of DNSDesign principle of DNS

The naming system on which DNS is based is a

hierarchical and logical tree structure called the domain

namespace.

An organization obtains authority for parts of the name

space, and can add additional layers of the hierarchy

Names of hosts can be assigned without regard of

location on a link layer network, IP network or

autonomous system

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Design principle of DNS (Cont’d)Design principle of DNS (Cont’d)

In practice, allocation of the domain names generally

follows the allocation of IP address, e.g.,

All hosts with network prefix 163.180/16 have domain name suffix khu.ac.kr

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Distributed, Hierarchical DatabaseDistributed, Hierarchical Database

Root DNS Servers

com DNS servers org DNS servers edu DNS servers

poly.eduDNS servers

umass.eduDNS servers

yahoo.comDNS servers

amazon.comDNS servers

pbs.orgDNS servers

Client wants IP for www.amazon.com; At first, client queries a root server to find com DNS server

Then, client queries com DNS server to get amazon.com DNS server

Thirdly, Client queries amazon.com DNS server to get IP address for www.amazon.com

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Resolution ProcessResolution Process

Let’s look at the resolution process step by step

Client

Local DNS

What is the IP address of khu.ac.kr?

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Resolution Process (cont’d)Resolution Process (cont’d)

The Local DNS asks a root name server, m, for

khu.ac.kr’s address

Client

Local DNS

What is the IP address of khu.ac.kr?

m.root-server

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Resolution Process (cont’d)Resolution Process (cont’d)

The root server m refers local DNS to the kr name servers

This type of response is called a “referral”

Client

Local DNS

Here’s a list of the kr name servers.Ask one of them.

m.root-server

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Resolution Process (cont’d)Resolution Process (cont’d)

The local DNS asks a kr name server, f, for khu.ac.kr

Client

Local DNSm.root-server

f.kr-server

What is the IP address of khu.ac.kr?

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Resolution Process (cont’d)Resolution Process (cont’d)

The kr name sever f refers Local DNS to the ac.kr name servers

Client

Local DNSm.root-server

f.kr-server

Here’s a list of the ac.kr name servers.Ask one of them.

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Resolution Process (cont’d)Resolution Process (cont’d)

The local DNS asks an ac.kr name sever, g.ac-kr for khu.ac.kr’s

address

Client

Local DNSm.root-server

f.kr-server

What is the IP address of khu.ac.kr?

g.ac.kr-server

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Resolution Process (cont’d)Resolution Process (cont’d)

The g.ac.kr-server responds with khu.ac.kr’s address

Client

Local DNSm.root-server

f.kr-server

g.ac.kr-server

Here’s the IP address for khu. ac.kr(163.180.96.52)

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Resolution Process (cont’d)Resolution Process (cont’d)

The local DNS responds to client with khu.ac.kr’s address

Client

Local DNSm.root-server

f.kr-server

g.ac.kr-server

Here’s the IP address for khu. ac.kr(163.180.96.52)

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Resolution Process (Caching)Resolution Process (Caching)

After the previous query, the Local DNS now knows:

The names and IP addresses of the kr name servers

The names and IP addresses of the ac.kr name servers

The IP address of khu.ac.kr

Let’s look at the resolution process again

Client

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Resolution Process (Caching)Resolution Process (Caching)

The client asks its Local DNS for networking.khu.ac.kr’s address

Client

Local DNSm.root-server

f.kr-server

g.ac.kr-server

What is the IP address of networking.khu.ac.kr?

h.khu.ac.kr-server

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Resolution Process (Caching)Resolution Process (Caching)

Local DNS has cached an NS record indicating g.ac.kr-server is an

ac.kr name server, so it asks it for networking.khu.ac.kr’s address

Client

Local DNSm.root-server

f.kr-server

g.ac.kr-server

What is the IP address of networking.khu.ac.kr?

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Resolution Process (Caching)Resolution Process (Caching)

g.ackr-server responds with networking.khu.ac.kr’s address

Client

Local DNSm.root-server

f.kr-server

g.ac.kr-server

Here’s a list of the khu.ac.kr name servers.Ask one of them.

h.khu.ac.kr-server

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Resolution Process (Caching)Resolution Process (Caching)

g.ackr-server responds with networking.khu.ac.kr’s address

Client

Local DNSm.root-server

f.kr-server

g.ac.kr-server

h.khu.ac.kr-server

What is the IP address of networking.khu.ac.kr?

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Resolution Process (Caching)Resolution Process (Caching)

Finally, the Local DNS responds to client with

networking.khu.ac.kr’s address

Client

Local DNSm.root-server

f.kr-server

g.ackr-server

Here’s the IP address for networking.khu.ac.kr(163.180.116.228)

h.khu.ac.kr-server

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DNS: Root name serversDNS: Root name servers

contacted by local name server that can not resolve name

root name server:

contacts authoritative name server if name mapping is not known

gets mapping

returns mapping to local name server

13 root name servers worldwide

b USC-ISI Marina del Rey, CAl ICANN Los Angeles, CA

e NASA, Mt View, CAf Internet Software C. Palo Alto, CA (and 17 other locations)

i Autonomica, Stockholm (plus 3 other locations)

k RIPE, London (also Amsterdam, Frankfurt)

m WIDE Tokyo

a Verisign, Dulles, VAc Cogent, Herndon, VA (also Los Angeles)d U Maryland, College Park, MDg US, DoD, Vienna, VAh ARL, Aberdeen, MDj Verisign, (11 locations)

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Summary

The domain name system (DNS) is an Internet-wide distributed

database that translates between domain names and IP addresses

An application program on a host accesses the domain system

through a DNS client, called the Resolver

Resolver contacts DNS Server, called name server

DNS server returns IP address to resolver which passes the IP

address to application

The naming system on which DNS is based is a hierarchical and

logical tree structure called the domain namespace.

There are 13 root name servers worldwide