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Domain Name Servers

(DNS)

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Section 1: Overview Of DNS

• DNS is a name resolution protocol. It converts host names to IP addresses and vice-versa.

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DNS Structure

• DNS is a a hierarchical database (tree) much like windows file system

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• On the internet the first branches coming out of the tree are the “top-level domains” . Examples are .gov being the government domain , and .com, a commercial domain.

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• Under the “top-level domains” there are other branches containing more domains, like aol.com and cisco.com.

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You can see the tree in its natural environment.

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DNS Host Names

• Host names are based on the tree structure of the DNS.

• Individual node names come first, followed by domain, and so on.

• Example: ganesan.calstatela.edu• Ganesan is in the calstatela domain,

which is in the edu domain, which is under the root domain of the internet.

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DNS Delegation

• Each domain has it’s own name server.

• Name servers give host name information to all hosts and sub-domains within their domains.

• The microsoft.com domain, for example has a name server which stores all of this information.

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• However, authority for a subdomain, such as nerds.microsoft.com, can be passed to a name server that has authority for that subdomain.

• When a name resolution request comes to the microsoft.com name server, it just passes the request off to the nerds.microsoft.com name server.

• Through this we can see that DNS is truly distributed across the Internet, with each domain maintaining only the information that is pertinent to that domain.

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Name Resolution• As a result of DNS being distributed

across domains, when a name server receives a request for name resolution for a host that is outside of its own domain, it may not have the address information for that host.

• Because DNS is hierarchical, it does not need that information, the name server just needs to know how to access the root name server.

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• The name server forwards the name resolution request to the root name server, which then delegates the request to the appropriate domain beneath it, and this process continues until a name server which has address information for the host is reached, and the information is retrieved.

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Caching

• In order to reduce network traffic, which is quite important on the net, DNS’ use caching. (Like when you go the bank to

get your money)

• Whenever a name server receives address information for another host or domain, it stores that information for a specified period of time.

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• That way, if another name resolution request for that host or domain is received, the name server has the address information ready, and does not need to send another inquiry across the Internet.

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• The length of time address information is stored on the name server is determined by the Time-To-Live (TTL) value entered in the domain Start of Authority (SOA) resource record.

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Configuring DNS server• Name Servers store info about their domains in several

different “resource records”. Each resource record stores different info about their domain and host of the domain.

• Resource Records are usually text files stored on the domain name server.

• Some domain name browsers let you edit the records through a gui.

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• For more information on Domain Name Servers, go to www.yahoo.com and type in DNS.

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• Or you can go here:• http://www.howstuffworks.com/dns.htm• http://www.dns.net/dnsrd/• http://www.ee.usm.maine.edu/guides/dns/dns.html

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