DNS D omain N ame S ysterm/Service/Server The Internet's Directory Service 7 th Lecture
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Transcript of DNS D omain N ame S ysterm/Service/Server The Internet's Directory Service 7 th Lecture
© 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.
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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
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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
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• 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.
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• 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.
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• 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