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Domain Name System

This material is prepared for the students attending "Computer Networks" to help them better understand the domain name system.

Kyungran KANG

on 25 April 2014

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Transcript of Domain Name System

Domain Name System
DNS is a
Distributed Database
implemented in hierarchy of many name servers
DNS is an
Application-layer Protocol
that allows a host to query the distributed database
DNS Resource Records
DNS Resolution Procedure
Iterative query
- Contacted server replies with name of server to contact
- “I don’t know this name, but ask this server”

Recursive query
- A DNS server queries other servers until it gets the information, or until the name query fails
DNS Protocol
Query message
Reply messages
Both with same message format

RR format: (name, value, type, ttl)
(Name, Value, Type, TTL)

If Type =A,
is a hostname and
is the IP address for the hostname

e.g. (www.ajou.ac.kr,, A)
(Name, Value, Type, TTL)
If Type=MX, the
is the canonical name of a mail server that has an alias hostname Name.

(gmail.com, mail.google.com, MX)
(gmail.com, googlemail.l.google.com, MX)
Root DNS Servers
It is contacted by local name server that can not resolve name.
There are 13 root name servers distributed over the world and they are accessed as a single server.

Distributed, Hierarchical Database
DNS uses a large number of servers organized in a hierarchical fashion and distributed around the world.
Root DNS servers
Top-level domain (TLD) servers
Authoritative DNS servers
Local name server
Top Level DNS Servers
gTLD (generic TLD) - .edu, .com, .biz, .museum, .org, ...
ccTLD (country code TLD) - .kr, .uk, .us, .kp, ...

A TLD server is responsible for its serving TLD
e.g. Network Solutions maintains servers for .com TLD
Educause for .edu TLD
KISA for .kr TLD
Authoritative DNS Server
It provides publicly accessible DNS records that map the names of the hosts in its organization to IP addresses.
The organization can have its own authoritative DNS server to hold DNS records.
- e.g.
ajou.ac.kr : madang.ajou.ac.kr
The organization can pay to have the records in an DNS server of some service provider.
- e.g.
deyen.co.kr : ns.inetpia.info
Local DNS server
It does not strictly belong to DNS hierarchy
When host makes DNS query, query is sent to its local DNS server
- Therefore, it is called “default name server”

It has local cache of recent name-to-address translation pairs but may be out of date!
It acts as proxy, forwards query into hierarchy.
Full transcript