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