Domain Name System: DNS

TA: Awad A Younis Class: CS457

Fall 2014

2

� Before DNS existence: • Users of ARPAnet used a hosts file called HOSTS.TXT

• Using this file, the conversion of the name to an IP Address was achieved by

� Downloading a large file contains all hosts and their IPs from a central

location

� Mapping between names and IPs Addresses

• As the internet grows this solution becomes impractical

• Domain Name Service (DNS) was created in 1983

� DNS: • Converts domain names to IP Addresses

� DNS is a distributed database

� No one computer is used to maintain a complete database of all of the domains

• Why DNS: routers and switches only understand the IP address

3

• The top node, symbolized by a single dot, is known as the root.

• Below the root, you have the top-level domains like:

� com, net, org, mil, gov, edu, etc.

• How do you access your email at mail.example.org?

4

• There are two components to DNS:

� Resolver: resides on client computers (Bingo!)

� Name server: answers questions about DN.

• It is listed in  the  resolver’s  configuration  file

It may not know how to access this site

5

How can you bypass the DNS?

6

� Example: • Suppose your computer wants to find the IP address of:

network-surveys.cr.yp.to.

• It contacts a series of DNS servers around the Internet.

• There are several DNS servers with information about network-surveys.cr.yp.to.

• A central root server (located at Internet HQ in Virginia) has the following data

in a file on disk:

.:198.41.0.4 : root server's IP address

&to:198.6.1.82 : .to  server’s IP address

• Your computer also has 198.41.0.4 in a file on disk.

7

� Example:

1. Your computer sends its question to the root server, and receives a response

from the root server's data:

network-surveys.cr.yp.to?

| Your | --------------------------> |198.41.0.4 |

|computer| <--------------- |root server|

&to:198.6.1.82

• The response &to:198.6.1.82 is a delegation.

• It says ``For information about .to, ask the DNS server at IP address 198.6.1.82'‘

8

� Example:

• The DNS server at 198.6.1.82 has the following data in a file on disk: .to:198.6.1.82 &yp.to:131.193.178.160 2. Your computer sends its question to that DNS server, and receives a response: network-surveys.cr.yp.to? | Your | --------------------------> |198.6.1.82| |computer| <------------------------ |.to server | &yp.to:131.193.178.160 • The response &yp.to:131.193.178.160 is another delegation.

• It says ``For information about .yp.to, ask the DNS server at IP address

131.193.178.160''

9

� Example: • The DNS server at 131.193.178.160 has the following data in a file on disk:

.yp.to:131.193.178.160

=network-surveys.cr.yp.to: 131.193.178.100

• Your computer sends its question to that DNS server, and receives a response:

network-surveys.cr.yp.to?

| Your | ------------------------------------------> |131.193.178.160|

|computer| <------------------------------------------ | .yp.to server |

=network-surveys.cr.yp.to: 131.193.178.100

• The response =network-surveys.cr.yp.to: 131.193.178.100, final answer

10

� DIG: Domain Information Groper 1. Header Section:

2. Question Section: It shows what you asked dig to do

11

� DIG: Domain Information Groper 3. Answer Section: It shows the answer for the query asked

4. Authority Section: It show who given the answer

12

� DIG: Domain Information Groper

5. Additional Section: It show if any additional info that DNS server provided

� Project 3: • Client • MyReslover

� Makefile � Query.h � Response.h � Header.h

• Check up for: – www.cnn.commmm – DNS server time out – DNS server not found – No TCP/IP for big answer just stick with whatever UDP can handle

13

Thank You

14