DNS- Domain Name Server

Today I am going to talk about the DNS server. I knew some of the you already knew about the DNS. DNS stands for Domain Name Server and is a service to translate hostname into IP address. DNS is a distributed database implemented in a hierarchy of name servers and DNS is an application level protocol for message exchange between clients and servers.

Now question is Why we required DNS service ?

• It is easier to remember a host name than it is to remember an IP address. 
• A name has more meaning to a user than a 4 byte number. 
• Applications such as FTP, HTTP, email, etc., all require the user to input a destination 
• The user generally enters a host name 
• The application takes the host name supplied by the user and forwards it to DNS for translation to an IP address 

Fig 1.1- DNS 

How DNS- Domain Name Server Works ?

• DNS works by exchanging messages between client and server machines. 
• A client application will pass the destination host name to the DNS process (in Unix referred to as the get host by name() routine to get the IP address. 
• The application then sits and waits for the response to return. 

DNS Queries 

Recursive : The client machine sends a request to the local name server, which, if it does not find the address in its database, sends a request to the root name server, which, in turn, will route the query to an intermediate or authoritative name server. Note that the root name server can contain some hostname to IP address mappings. The intermediate name server always knows who the authoritative name server is. 

Iterative : The local server queries the root server. If address not in its database, will have the name/address of an intermediate or authoritative name server and forward that information to the local name server so that it can directly communicate with the intermediate or authoritative name server. This is to prevent the overloading of the root servers that handle millions of requests. 

Operation of DNS

• The DNS data is stored in the database in the form of resource records (RR). The RRs are directly inserted in the DNS messages. 
• The RRs are a 4 tuple that consist of: {name, value, type, TTL}. 

Summary

• DNS provides a mechanism for maintaining the user friendliness of the Internet by hiding some of the operational details. 
• DNS servers have to be created manually. Recently an update protocol was introduced that allows DNS to exchange data for additions and deletions. 

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