1 kyung hee university chapter 19 dns (domain name system)
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11Kyung Hee University
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