24. dns domain name system address 1. name server domain name ip address ftp.cs.mit.eduxx.xx.xx.xx...

24. DNS

Domain Name System

http://www.voa.gov IP address

1. Name serverdomain name IP address

ftp.cs.mit.edu xx.xx.xx.xx

24.2 Mapping Domain Names To Address

map

resolve

• Name server has：– domain name database which contains domain

names and its IP addresses for all computers in the domain

– information about locations of some name servers, such as root name server

• Distributed processing:

a set of servers operating at multiple sites solve the mapping cooperatively

ftp.cs.mit.edu

root

cn jp edu com

edu MIT cmcom

ustcpku nju

…...

...

….. ...

…...

…...

…...

...

us ...

edu com

• Links among name servers– a single server is responsible for all computers

that have a given suffix

nju name server --- *. nju.edu.cn

edu name server --- *. edu.cn

– each server knows the location of a root server – each server knows the locations of servers of s

ubparts of the hierarchy

nju DNS DB

nju resolve

www.nju.edn.cn 202.119.32.6 A

cs.nju.edn.cn 202.119.36.5 A localhost 127.0.0.1 A…… …… …??...

name IP address type

netra.nju.edu.cn 202.119.32.8 MX

nameserver IP1

name server IP address

names.edu.cn IP i

…… ...

edu.cn DNS DB

edu.cn resolve

name server IP address

nameserver IP1names.tsinghua.edu.cn IP2names.pku.edu.cn IP3names.nju.edu.cn IP4

…… …...

name IP address type

www.edu.cn IP I A…… …… …...

Client (user machine) :• must know at least one name server, usually a local

name server --- set “DNS sever”• has a name resolver --- gethostbyname( )

2. Domain name resolution

name resolver name server

ftp.cs.mit.edu

IP address

54SC

Basic idea:

Given a name to resolve, the root can choose the correct server for that name because the root knows which server resolves each domain.

ftp.cs.mit.edu

www.pku.edu.cn

lxn@cis.uoguelph.ca

Two-step name resolution：• When a domain name server receives a query, it

checks its DNS database. If it finds the name, it returns the result

• If the name server can not resolve the name, the server contacts other domain name servers

root

cn edu

edu MIT

njuwww.mit.eduIterative resolution

Recursive resolution

Two ways to use DNS:• Iterative resolution：

by contacting name servers one at a time• Recursive resolution:

by asking the name server system to perform a complete translation (resolution)

– name resolver nju DNS server

– nju DNS server root DNS server

– nju DNS server edu DNS server

www.mit.edu

gethostbyname( )nju DNS server

S54

C S54

Root DNS server

www.mit.edu

www.mit.edu

edu’ DNS IP addr.

www.mit.edu

mit.edu’ DNS IP addr.

www.mit.edu

Resolving a name：

edu’ DNS IP addr.

root

cn edu

edu MIT

njuwww.mit.edu

Recursive resolution?

root

cn edu

edu MIT

nju

Why not?

Iterative resolution

root

cn edu

edu MIT

nju

• Caching– each server maintains a cache of recently used

names as well as a record of where the mapping information for that name was obtained . Each name has a timer

– host downloads DNS database from a local domain name server, maintains its own cache.

• Replicating root server

13 servers

3. Optimization for domain name resolution