Chapter 17 DNS (Domain Name System)

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Chapter 17 DNS (Domain Name System)
Mi-Jung Choi Dept. of Computer Science and
Engineering mjchoi_at_postech.ac.kr
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Introduction

• 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, postech.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 ?? (DNS?? DOWN !!!)

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Resolver 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

Resolver
DNS Server
Client
1. Host name(khu.ac.kr)
2. Host name(khu.ac.kr)
4. IP(163.180.96.52)
3. IP(163.180.96.52)
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Design 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 (Contd)

• 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 Database

• 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 Process

• Lets look at the resolution process step by step

Local DNS
What is the IP address of khu.ac.kr?
Client
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Resolution Process (contd)

• The Local DNS asks a root name server, m, for
  khu.ac.krs address

m.root-server
Local DNS
What is the IP address of khu.ac.kr?
Client
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Resolution Process (contd)

• The root server m refers local DNS to the kr name
  servers
• This type of response is called a referral

m.root-server
Local DNS
Heres a list of the kr name servers. Ask one of
them.
Client
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Resolution Process (contd)

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

m.root-server
Local DNS
f.kr-server
What is the IP address of khu.ac.kr?
Client
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Resolution Process (contd)

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

m.root-server
Local DNS
f.kr-server
Heres a list of the ac.kr name servers. Ask one
of them.
Client
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Resolution Process (contd)

• The local DNS asks an ac.kr name sever, g.ac-kr
  for khu.ac.krs address

m.root-server
Local DNS
What is the IP address of khu.ac.kr?
f.kr-server
g.ac.kr-server
Client
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Resolution Process (contd)

• The g.ackr-server responds with khu.ac.krs
  address

m.root-server
Local DNS
Heres the IP address for khu.ac.kr (163.180.96.5
2)
f.kr-server
g.ac.kr-server
Client
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Resolution Process (contd)

• The local DNS responds to client with khu.ac.krs
  address

m.root-server
Local DNS
Heres the IP address for khu.
ac.kr (163.180.96.52)
f.kr-server
g.ac.kr-server
Client
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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
• Lets look at the resolution process again

Client
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Resolution Process (Caching)

• The client asks its Local DNS for
  networking.khu.ac.krs address

m.root-server
Local DNS
What is the IP address of networking.khu.ac.kr?
f.kr-server
g.ac.kr-server
h.khu.ac.kr-server
Client
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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.krs address

m.root-server
Local DNS
f.kr-server
What is the IP address of networking.khu.ac.kr?
g.ac.kr-server
Client
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Resolution Process (Caching)

• g.ackr-server responds with networking.khu.ac.krs
  address

m.root-server
Local DNS
Heres a list of the khu.ac.kr name servers. Ask
one of them.
f.kr-server
g.ac.kr-server
h.khu.ac.kr-server
Client
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Resolution Process (Caching)

• g.ackr-server responds with networking.khu.ac.krs
  address

m.root-server
Local DNS
f.kr-server
What is the IP address of networking.khu.ac.kr?
g.ac.kr-server
h.khu.ac.kr-server
Client
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Resolution Process (Caching)

• Finally, the Local DNS responds to client with
  networking.khu.ac.krs address

m.root-server
Local DNS
Heres the IP address for networking.khu.ac.kr (1
63.180.116.228)
f.kr-server
g.ac.kr-server
h.khu.ac.kr-server
Client
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DNS 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
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Objectives

• Be able to explain process-to-process
  communication
• Know the format of a UDP user datagram
• Be able to calculate a UDP checksum
• Understand the operation of UDP
• Know when it is appropriate to use UDP
• Understand the modules in a UDP package

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Examples

• Example 5 After a few seconds, a user datagram
  arrives for port 52,222. The input module checks
  the table and cannot find the entry for this
  destination. The user datagram is dropped and a
  request is made to ICMP to send an unreachable
  port message to the source.
• Example 6 After a few seconds, a process needs
  to send a user datagram. It delivers the data to
  the output module which adds the UDP header and
  sends it