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Chapter: Computer Networks : Application Layer

Domain Name System

There are several applications in the application layer of the Internet model that follow the client/server paradigm.

Domain Name System


There are several applications in the application layer of the Internet model that follow the client/server paradigm. The client/server programs can be divided into two categories: those that can be directly used by the user, such as e-mail, and those that support other application programs. The Domain Name System (DNS) is a supporting program that is used by other programs such as e-mail.


1. Need for DNS


To identify an entity, TCP/IP Protocols use the IP address, which uniquely identifies the connection of a host to an internet. In the case of ARPANET, a file named hosts.txt is used to list all hosts and their IP addresses, this work suitable for small network but not for large network due to heavy load and latency. Therefore, people prefer to use names instead of addresses that is, we need a system that can map a name to an address and conversely an address to a name. Thus, preferred system is called as Domain Name system.


2. DNS in the Internet:


DNS is a protocol that can be used in different platforms. In the Internet, the domain name space is divided into three sections are


1.        Generic domains

2.        Country domains and

3.        Inverse domain


1. Generic domain:

There are 14 generic domains, each specifying an organization type. The generic domain defines registered hosts according to their generic behavior. Each node in the tree defines a domain, which is an index to the domain mane space data base.

Looking at the tree, we see that the first level in the generic domain section allows seven possible three-character labels. These labels describe the organization types as shown below

2. Country domains:


Each country domain specifies a country. This section follows the same format as the generic domains but uses two-character country abbreviations in place of three character organizational abbreviations at the first level. Second level labels can be organizational, or they can be more specific, national designations. The following figure 5.3 shows the country domain section the address cs.Keio.ac.jp refers to computer science department of Keio University in Japan. To create a new domain, permission is required of the domain in which it will be included.


For example, if a new university is chartered, say the University of Chennai, it must ask the manager of the edu domain to assign it unc.edu, in order to avoid conflicts and each domain can keep track of all its subdomains


Once a new domain has been created and registered, it can create subdomains, such as cs.unc.edu, without getting permission from anybody higher up the tree.


3. Inverse domain:


The inverse domain finds a domain name for a given IP address. This is called address-to-name resolution. It is used to map an address to a name. This may happen, for example, when a server lists only the IP address of the client. To determine if the client is on the authorized list, it can be send a query to the DNS server and ask for a mapping of address to name in figure 5.4


3. Types of Records:

There are two types of DNS records:


1.        Question records

2.        Resource records


Question Records:


The question records are used in the question section of the query and response messages. It is used by the client to get information from a server.


Resource Records:


Every domain whether it is a single host or a top level domain, can have a set of resource records associated with it. For a single host, the most common resource record is just its IP address, but many other kinds also exist. When a resolver gives a domain name to DNS, what it gets back are the resource records associated with that name. Thus, the primary function of DNS is to map domain names onto resource records. The server database consists of resource records. This record is used in the answer, authoritative and additional information sections of the response message.


4. Domain Name space:


DNS can be pictured as an inverted hierarchical tree structure with one root node at the top and a maximum of 128 levels.



Each node in the tree has a label, which is string with a maximum of 63 characters.


Domain Name:


Each node in the tree has a domain name. A full domain name is a sequence of labels separated by dots (.).


Fully Qualified Domain Name (FQDN):


A FQDN is a domain name consisting of labels beginning with the host and going back through each level to the root node. Exà Challenger.atc.fh.da.Edu


Partially Qualified Domain Name (PQDN):


In PQDN is a domain name that does not include all the levels between the host and the root node. Exà Challenger.


5. Name Server:


In theory atleast, a single name server could contain the entire DNS database and respond to all queries about it. In practice, this server would be so overloaded as to be useless. To avoid problems associated with having only a single source of information, the DNS name space is divided into non-overlapping zones. One possible way to divide the name space, where the zone boundaries are placed within a zone is upto that zones administrator. This decision is made in larger part based on how many name servers are desired. To improve reliability, some servers for a zone can be located outside the zone.


The DNS client, called a resolver, maps a name to an address, or an address to a name. When a resolver has a query about the domain name, it passes the query to one of the local name servers. If the domain being sought falls under the jurisdiction of the name server, such as ai.cs.yale.edu falling under cs.yale.edu, it returns the authoritative resource records. An authoritative record is one that comes from the authority that manages the record and it thus always correct. While, DNS helps in mapping names onto their IP addresses. It does not help locate people, resources, services or objects in general. For locating these things, another directory service has been defined, called LDAP (Light Weight Directory Access protocol).


6. DNS Messages:


These are two types of DNS Messages queries and responses. Both types have the same format. Queries Messages:


The query message consists of a header and question


Response Messages:


The response message consists of a header, question records, answer records, authoritative records and additional records.


7. Header Format:


Both are have the same header format. The header is 12 bytes.


1.        Number of questions records

2.        Number of authoritative records. (All 0s in query message)



1.        Number of answers records (All 0s in query message)

2.        Number of additional records. (All 0s in query message)


The identification subfield is used by the client to match the response with the query.

The flag subfield is a collection of subfields that define the types of the message, the type of answer requested, and the type of desired resolution and so on.


The Number of question records subfield contains the number of queries in the question section of the message

The number of answer records subfield contains the number of answer records in the answer section of the response message. Its value is zero in the query message.


The number of authoritative records subfield contains the number of authoritative records in the authoritative section of a response message. Its value is zero in the query section.


The number of additional records subfield contains the number of additional records in the additional section of a response message. Its value is zero in the query message.

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