© 2010 Computer Science Faculty, Kabul University

DNS Domain Name Systerm/Service/Server

The Internet's Directory Service

7TH LECTURE

16, May, 2010 Baseer Ahmad Baheer

© 2010 Computer Science Faculty, Kabul University

• Human beings can be identified in many ways:

• Name

• SSN

• Driver’s license numbers

© 2010 Computer Science Faculty, Kabul University

• Internet hosts can be identified by:

• Hostname

• Appreciated by humans.

• Host’s location mohe.gov.af• Difficult to process by routers.

• IP Address

• Hierarchical structure.

• Routers use this IP address to route datagram towards its

destination.

© 2010 Computer Science Faculty, Kabul University

Services Provided by DNS

• DNS is:

• A distributed database implemented in a hierarchy of

name servers

• An application-layer protocol that allows hosts and

name servers to communicate in order to provide the

translation service. (Over UDP with 53 port number)

© 2010 Computer Science Faculty, Kabul University

• Translating hostnames to their underlying IP addresses.

• Host aliasing:

• Canonical hostname.

• Hostname: relay1.west-coast.enterprise.com

• Two alises name: www.enterprise.com and enterprise.com

• Mail server aliasing

• Load Distribution

© 2010 Computer Science Faculty, Kabul University

How DNS Works?

© 2010 Computer Science Faculty, Kabul University

© 2010 Computer Science Faculty, Kabul University

Why not centerlize DNS?

• Single point of failure

• Traffic volume

• Distant centralized database

• Maintenance

© 2010 Computer Science Faculty, Kabul University

• 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

• Root name servers

• Authoritative name server:

• For a host: stores that host’s IP address, name

• Can perform name/address translation for that host’s name

© 2010 Computer Science Faculty, Kabul University

© 2010 Computer Science Faculty, Kabul University

© 2010 Computer Science Faculty, Kabul University

DNS Records

• The name servers that together implement the

DNS distributed database, store Resource

Records (RR) for the hostname to IP address

mappings.

© 2010 Computer Science Faculty, Kabul University

• A resource record is a four-tuple that contains

the following fields:

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• The meaning of Name and Value depend on

Type:

• If Type=A, then Name is a hostname and Value is the

IP address for the hostname. Thus, a Type A record

provides the standard hostname to IP address

mapping. As an example, (relay1.bar.foo.com,

145.37.93.126, A) is a Type A record.

© 2010 Computer Science Faculty, Kabul University

• If Type=NS, then Name is a domain (such as foo.com)

and Value is the hostname of a server that knows how

to obtain the IP addresses for hosts in the domain.

This record is used to route DNS queries further along

in the query chain. As an example, (foo.com,

dns.foo.com, NS) is a Type NS record.

© 2010 Computer Science Faculty, Kabul University

• If Type=CNAME, then Value is a canonical hostname

for the alias hostname Name. This record can provide

querying hosts the canonical name for a hostname. As

an example, (foo.com, relay1.bar.foo.com, CNAME) is

a CNAME record.

© 2010 Computer Science Faculty, Kabul University

• If Type=MX, then Value is a hostname of a mail server

that has an alias hostname Name. As an example,

(foo.com. mail.bar.foo.com, MX) is an MX record. MX

records allow the hostnames of mail servers to have

simple aliases.

© 2010 Computer Science Faculty, Kabul University

© 2010 Computer Science Faculty, Kabul University

References