Domain Name Service is a database of host names and addresses distributed across thousands of servers, DNS protocols enable a user to submit a database query to a local server and receive a response that may have been obtained from a remote server. DNS also provides information that is needed to route electronic mail to the destination.

DNS was setup to provide a better method of keeping track of Internet names and addresses. DNS databases provide automated name-to-address translation services. DNS is also a distributed database that is maintained by different organizations across the internet.

BIND (Berkley Internet Domain) package is free software that lets organizations implement their systems to function as name servers. A client program capable of looking up information in the Domain Name System is a standard part of TCP/IP products and is called a resolver. 

On many systems you can use nslookup which is an interactive resolver program that enables a user to communicate directly to a Domain Name Server, sending queries and getting back responses. When using nslookup sometimes the first resolver may not respond due to it being busy, so it goes to the second resolver to output the requested information.

Authoritative and Cached Responses

All data is entered and updated at a primary name server. The data at a primary server is on its own hard disk. Secondary servers download their information from the primary. All of an organization’s name servers-whether primary or secondary-are authoritative for its domain. To cut down on traffic, your local server will cache (save) the answers on its own hard disks for a while. Any user who repeats a query that has been made recently gets the locally cached answer.