| Dns Essay, Research Paper
Domain Name System
Introduction
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 216.34.122.81, 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
browser.
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.
Tips:
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
contact.
describes one name.
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