1. Chapter 19 DNS (Domain Name System)

2. 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, 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

3. 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( )
3. IP( )

4. 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

5. 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 /16 have domain name suffix khu.ac.kr

6. Distributed, Hierarchical Database
Root DNS Servers
com DNS servers
org DNS servers
edu DNS servers
poly.edu
DNS servers
umass.edu
yahoo.com
amazon.com
pbs.org
Client wants IP for 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

7. Resolution Process Let’s look at the resolution process step by step
Local DNS
What is the IP address of khu.ac.kr?
Client

8. Resolution Process (cont’d)
The Local DNS asks a root name server, m, for khu.ac.kr’s address
m.root-server
Local DNS
What is the IP address of khu.ac.kr?
Client

9. Resolution Process (cont’d)
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
Here’s a list of the kr name servers.
Ask one of them.
Client

10. Resolution Process (cont’d)
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

11. Resolution Process (cont’d)
The kr name sever f refers Local DNS to the ac.kr name servers
m.root-server
Local DNS
f.kr-server
Here’s a list of the ac.kr name servers.
Ask one of them.
Client

12. Resolution Process (cont’d)
The local DNS asks an ac.kr name sever, g.ac-kr for khu.ac.kr’s address
m.root-server
Local DNS
What is the IP address of khu.ac.kr?
f.kr-server
g.ac.kr-server
Client

13. Resolution Process (cont’d)
The g.ac.kr-server responds with khu.ac.kr’s address
m.root-server
Local DNS
Here’s the IP address for khu. ac.kr
( )
f.kr-server
g.ac.kr-server
Client

14. Resolution Process (cont’d)
The local DNS responds to client with khu.ac.kr’s address
m.root-server
Local DNS
Here’s the IP address for khu. ac.kr
( )
f.kr-server
g.ac.kr-server
Client

15. 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

16. Resolution Process (Caching)
The client asks its Local DNS for networking.khu.ac.kr’s 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

17. 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
m.root-server
Local DNS
f.kr-server
What is the IP address of networking.khu.ac.kr?
g.ac.kr-server
Client

18. Resolution Process (Caching)
g.ackr-server responds with networking.khu.ac.kr’s address
m.root-server
Local DNS
Here’s 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

19. Resolution Process (Caching)
g.ackr-server responds with networking.khu.ac.kr’s 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

20. Resolution Process (Caching)
Finally, the Local DNS responds to client with networking.khu.ac.kr’s address
m.root-server
Local DNS
Here’s the IP address for networking.khu.ac.kr
( )
f.kr-server
g.ackr-server
h.khu.ac.kr-server
Client

21. DNS: Root name servers 13 root name servers worldwide
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
a Verisign, Dulles, VA
c Cogent, Herndon, VA (also Los Angeles)
d U Maryland, College Park, MD
g US, DoD, Vienna, VA
h ARL, Aberdeen, MD
j Verisign, (11 locations)
k RIPE, London (also Amsterdam, Frankfurt)
i Autonomica, Stockholm (plus 3 other locations)
m WIDE Tokyo
e NASA, Mt View, CA
f Internet Software C. Palo Alto, CA (and 17 other locations)
13 root name servers worldwide
b USC-ISI Marina del Rey, CA
l ICANN Los Angeles, CA

22. 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