Google Public DNS, which was launched in 2009, is a free DNS service provider. It claims to handle more than 130 billion requests on an average day.
Domain Name System translates the computer's domain name into an IP address that is used TO locate a computer in large networks. For a layman, it is easier to remember domain names like 'www.buzzle.com', rather than remember its IP address.
DNS servers, also known as name servers, store information about these domain names and their corresponding DNS records. DNS records hold information regarding the address, web host, domain registrant, active name servers, and the like. Whenever the request for information regarding a specific IP address/computer is made, the DNS server responds with the relevant information, by fetching the same from its database.
DNS servers are classified on the basis of the functions they perform. Different functions, like forwarding the query, answering it, look-up in the database, etc., can be performed by a single server in different zones. Zone data is important for DNS servers, as it tells the server how to behave and communicate with other servers. DNS servers are categorized as mentioned below.
Zone Master Server
As the name suggests, the zone master server is the authoritative server. It will have a database of all the data associated with a particular zone. This data is available on two servers, namely the primary master server, and the secondary slave server. Whenever particular information is requested, the network/zone computers consult the master server.
Primary Master Server
The primary master server has the master copy of the domain data, and this master data is loaded onto the disk when its operation starts. This is the main server which is referred to when relevant data is needed; changes to the database can be made in this server's zone data. In case there is excessive load on the primary master server, data is shared on the secondary server, and authority is delegated to it by the primary server.
Secondary Slave Server
In case there is a breakdown or failure of the master server, the secondary slave server provides the relevant information. In busy zones, due to heavy traffic, it is advisable to have a separate server with master data. Thus, slave name servers are backup servers for the master servers. If the master server has delegated authority to the secondary server, customers can directly contact these servers, instead of the master servers.
Caching-Only Name Server
The caching-only server stores results of the queries that are made. The next time this information is needed, the server fetches it immediately instead of waiting. These servers are not authorized for any domains, and they only perform queries. When the server is initially set up, its cache is empty. It is only after some period of time that the server fetches data based on client requests. This information is stored in the cache, and reused whenever needed. These servers cannot perform zone transfers.
DNS servers that are authoritative for the root domain are root domain servers. These servers are needed to be used for DNS namespace, i.e., the entire DNS domain structure. They are the first step in translating host names like buzzle.com to IP addresses that are used for communication between network hosts. This process of converting to an IP address is known as resolving. As of February 2013, there are about 13 root name servers specified. The basic role of a root name server is to answer the requests for namespace, and redirect these requests to top level domain (TLD) name servers. These root servers publish the contents of root zone files onto the Internet. The ISPs download copies of the root tables, so as to complete the resolving process locally.
These servers are also known as proxy, client, or remote servers. These servers forward all requests to other DNS servers, and cache the results. They are mostly used in off-site locations, where all the off-site queries reach this server first, and it then forwards the queries to other DNS servers. Such an arrangement reduces the external access of the local servers, and thus, speeds up responses. These servers act as a single point management for the entire server network.
Authoritative DNS servers are handed over the responsibility of assigning domain names and mapping them into IP addresses. These servers delegate authority over the other name servers in the domain, and the queries are answered. These different types of DNS servers are commonly found for the world wide web, email, and active directory. Telnet and SSH use DNS servers for remote access to UNIX systems.