Dns Essay Research Paper Domain Name SystemIntroduction

Dns Essay, Research Paper

Domain Name System


A name that identifies one or more IP addresses. For example, the domain name

microsoft.com represents about a dozen IP addresses. Domain names are used inURL’s to

identify particular Web pages. For example, in the URL http://www.hotmail.com/ the

domain name is hotmail.com.

Every domain name has a suffix that indicates which top level domain (TLD) it belongs

to. There are only a limited number of such domains. For example:

? gov – Government agencies

? edu – Educational institutions

? org – Organizations (nonprofit)

? mil – Military

? com – commercial business

? net – Network organizations

? ca – Canada

? th – Thailand

Because the Internet is based on IP addresses, not domain names, every Web Server

requires a Domain Name System server to translate domain names into IP addresses.

The Internet Protocol addresss is a 32- bit integer. If somebody wants to send a

message it is necessary to include the destination address, but people prefer to assign

machines pronounceable, easily remembered names (host names). For this reason the

Domain Name System is used. These logical names also allow independence from

knowing the physical location of a host. A host may be moved to a different network,

while the users continue to use the same logical name. The Domain Name System (DNS)

is a distributed database used by TCP/IP applications to map between hostnames and IP

addresses, and to provide electronic mail routing information. Each site (university

department, campus,company, or department within a company, for example) maintains

its own database of information and runs a server program that other systems across the

Internet can query. The DNS provides the protocol which allows clients and servers to

communicate with each other.

The system accesses the DNS through a resolver. The resolver gets the hostname

and returns the IP address or gets an IP address and looks up a hostname. The resolver

returns the IP address before asking the TCP to open a connection or sending a datagram

using UDP.

DNS Organization

The domain name system uses a hierarchical naming scheme known as domain

names,which is similar to the Unix filesystem tree. The root of the DNS tree is a special

node with a null label. The name of each node (except root) has to be up to 63

characters.The domain name of any node in the tree is the list of labels, starting at that

node, working up to the root, using a period (”dot”) to separate the labels (individual

sections of a name might represent sites or a group, but the domain system simply calls

each section a label ). The difference between the Unix filesystem and the tree of the

DNS is that in the DNS we start on the ground and “go up” till the root. Writing them in

this order makes it possible to compress messages that contain multiple domain names.

Thus, the domain name “tau.ac.il” contains three labels: “tau”, “ac”, and “il”. Any suffix

of a label in a domain name is also called a domain. In the above example the lowest

level domain is “tau.ac.il” (the domain name for the Tel-Aviv University Academic

organization in Israel), the second level domain is “ac.il” (the domain name for Academic

organizations of Israel), and the top level domain (for this name) is “il” (the domain name

for Israel). The node il is the second level node (after root).

Every node in the tree must have a unique domain name, but the same label can be

used at different points in the tree. The top-level domains are divided into three areas:

1. arpa is a special domain used for address-to-name mapping.

2. The seven 3-character domain names ( generic (organizational) domains).

3. The 2-character domains are based on the country codes. These are called the country

(the geographical) domains.

The seven generic domains are as listed above.

The Internet scheme can accomodate a wide variety of organizations, and allows

each group to choose between geographical or organizational naming hierarchies. Most

sites follow the Internet scheme so they can attach their TCP/IP installations to the

connected Internet without changing names. The zone is a subtree of the DNS that is

administered separately. A common zone is a second-level domain, “ac.il” for example.

Thus a lot of second-level domains divide their zone into smaller zones.

Whenever a new system is installed in a zone, the DNS administrator for the zone

allocates a name and an IP address for the new system and enters these into the name

server’s database. A name server is said to have authority forone zone or multiple zones.

Often, server software executes on a dedicated processor, and this computing machine is

called the name Server.

The person responsible for a zone must provide a primary name server for that

zone and one or more secondary name servers. The main difference between a primary

and a secondary is that the primary loads all the information for the zone from disk files,

while the secondaries obtain all the information from the primary. When a secondary

obtains the information from its primary it is called a zone transfer.

The Domain Name System (DNS) translates easily remembered textual names,

such as www.yahoo.com, into numerical IP addresses, such as, allowing

for easier Internet use.

Steps to creating a Domain Name System :

1. Realize that when you sign up with an Internet service provider (ISP), part of your

initial configuration is to specify a DNS server to use. (The ISP will tell you the DNS

address to use and how to configure the DNS information.).

2. Once you’re set up, type a textual URL into the Address or Go To bar of your Web


3. Understand that you can access the Web page only when your ISP’s designated DNS

server finds the numerical IP address associated with this URL and provides you with

information collected from the IP address.

4. Recognize that if you get a DNS error message, it typically means that the DNS server

can’t find an IP address that matches the name you typed. But DNS servers can also have

problems. If you can’t access any Web pages, there might be a DNS problem. If you know

any IP addresses, try them. If a known IP address doesn’t work, the DNS server is down.

Switch to another DNS server, or call your ISP to ask for another DNS address. If the IP

address doesn’t work, call your ISP for more information.


Domain names are assigned based on the type of organization or site requesting the

name. Site types are generally differentiated by the three-letter suffix at the end of the

URL. Some of the more common suffixes are .com for commercial sites, .org for

nonprofit sites, and .gov for sites maintained by the government.

Countries outside the United States generally use different site naming conventions, and

the name of the country that hosts a particular site is usually given by a two-letter suffix.

Examples include .uk for the United Kingdom and .de for Germany.

When a new host is added to the zone, the administrator adds the appropriate

information (name and IP address) to a disk file on the system running the primary. The

primary name server is then notified to reread its configuration files. The secondaries

query the primary on regular basis (normally every 3 hours) and if the primary contains

newer data, the secondary obtains the new data using a zone transfer.

If the name server doesn’t contain the information requested, it must contact

another name server. Not every server, however, knows how to contact every other

server. Instead, every name server must know how to contact the root name servers. The

root servers then know the name and location (i.e. IP address) of each authoritative name

servers for all the second-level domains. There are six root servers in the world and every

primary name server has to know the address of one of root server. In practice, the

organization often collects information from all of their sub-zones into a single server.

The tree shows how a given server can contact other servers only. This tree

doesn’t indicate physical network connection. Servers may be located at arbitrary

locations on the network. Therefore, the tree of servers is a logical conection between

servers, which uses the Internet for communication.

DNS Caching

A fundamental property of the DNS is caching. That is, when a name server

receives information about a mapping, it caches that information. Thus a later query for

the same mapping can use the cached result, and not result in additional queries to other

servers. The DNS uses the caching for optimizing search cost.

How does it work?

Every server has a cache for recently used names as well as a record of where the maping

information for that name was obtained. When a client ask the server to resolve certain

name the server does as follows:

Check if it has authority for the name. If yes, the server doesn’t need caching information.

if not, the server checks its cache whether the name has been resolved recently. if yes, the

server reports the caching information to its clients.

We can examine the cache when the server cashed the information once, but didn’t

change it. Since information about a particular name can change, the server may have

incorrect information in its caching table. The Time to Live (TTL) value is used to decide

when to age information. Whenever an authority responds to a request, it includes a TTL

value in the response which specifies how long it guarantees the binding to remain.

DNS Formatting:

When the user wants to send a message, it invokes an application program and

supplies the name of a machine with which the application must communicate. The

application program must find the machine’s IP address. It passes the domain name to a

local resolver (L.R.) and requests an IP address. The local resolver checks its cache and:

If the L.R. has an answer, it returns the answer.

If the L.R. hasn’t one, it sends the message to the server. The server then returns a similar

message that contains the answer to the questions for which the server has bindings. If the

server can’t answer, it sends responsive information about other servers that the client can


describes one name.


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