[四川大学]计算机网络通信2-2

412-268-8000 ext.123 412-268-8000 ext.654
Network Programming
Applications/Servers
Web Port 80 Mail Port 25
Extension Telephone No Central Number Exchange Area Code
2.5 DNS: Domain Name System
People: many identifiers:
SSN, name, Passport #
Domain Name System:
distributed database implemented
Internet hosts, routers:
Well-known (port 0-1023) Dynamic or private (port 1024-65535)
NTP daemon
port 123
Web seቤተ መጻሕፍቲ ባይዱver
port 80
Servers/daemons usually use well-
TCP/UDP IP Ethernet Adapter
1
8
5
6
authoritative name server
requesting host
surf.eurecom.fr
2: Application Layer 6
DNS: caching and updating records
authoritative name server:
for a host: stores that host‟s IP
address, name can perform name/address translation for that host‟s name
2: Application Layer
2: Application Layer
3
Simple DNS example
host surf.eurecom.fr wants IP address of 1. Contacts its local DNS server, dns.eurecom.fr 2. dns.eurecom.fr contacts root name server, if necessary 3. root name server contacts authoritative name server, , if necessary
RFC 2136
/html.charters/dnsind-charter.html
2: Application Layer
7
DNS records
DNS: distributed db storing resource records (RR)
RR format: (name, value, type,ttl)
2: Application Layer
1
• DNS name servers
Why not centralize DNS? single point of failure traffic volume distant centralized database maintenance doesn‟t scale!
local name server
dns.eurecom.fr
intermediate name server
1
8
4
5
authoritative name server
requesting host
surf.eurecom.fr
Type=A name is hostname value is IP address
Type=NS
name is domain (e.g. )
value is IP address of
Type=CNAME name is an alias name for some “cannonical” (the real) name value is cannonical name Type=MX value is hostname of mailserver associated with name
2: Application Layer
10
Socket programming
Goal: learn how to build client/server application that communicate using sockets
Socket API
introduced in BSD4.1 UNIX, 1981
no server has all name-to-IP
address mappings local name servers:
each ISP, company has local
(default) name server host DNS query first goes to local name server
IP address (32 bit) - used
for addressing datagrams “name”, e.g., - used by humans
Q: map between IP addresses and name ?
in hierarchy of many name servers application-layer protocol host, routers, name servers to communicate to resolve names (address/name translation) note: core Internet function implemented as applicationlayer protocol complexity at network‟s “edge”
msg header
identification: 16 bit # for
query, reply to query uses same # flags: query or reply recursion desired recursion available reply is authoritative
2: Application Layer 8
authoritative name server for this domain
DNS protocol, messages
DNS protocol : query and reply messages, both with same message format
resolution on contacted name server heavy load?
iterated query:
contacted server replies
intermediate name server
with name of server to contact “I don‟t know this name, but ask this server”
known ports
Any client can identify the server/service HTTP = 80, FTP = 21, Telnet = 23, ... /etc/service defines well-known ports
Clients usually use dynamic ports Assigned by the kernel at run time
Socket API
TCP/UDP IP Ethernet Adapter
2: Application Layer
kernel space
IP
Ethernet Adapter
hardware
12
Network Addressing Analogy
Telephone Call
Professors at CMU
once (any) name server learns mapping, it caches
mapping cache entries timeout (disappear) after some time update/notify mechanisms under design by IETF
explicitly created, used, released by apps
client/server paradigm two types of transport service via socket
API: unreliable datagram reliable, byte stream-oriented
2
root name server
4 5 3
local name server
dns.eurecom.fr
authoritative name server
1
6
requesting host
surf.eurecom.fr

2: Application Layer
4
DNS example
Root name server:
may not know
root name server
2 7 3
6
authoritative name server may know intermediate name server: who to contact to find authoritative name server