A name server is a computer application that implements a

network service In computer networking, a network service is an application running at the network layer and above, that provides data storage, manipulation, presentation, communication or other capability which is often implemented using a client–server or pe ...

for providing responses to queries against a

directory service In computing, a directory service or name service maps the names of network resources to their respective network addresses. It is a shared information infrastructure for locating, managing, administering and organizing everyday items and network ...

. It translates an often humanly meaningful, text-based identifier to a system-internal, often numeric identification or addressing component. This service is performed by the server in response to a service protocol request. An example of a name server is the server component of the

Domain Name System The Domain Name System (DNS) is a hierarchical and distributed name service that provides a naming system for computers, services, and other resources on the Internet or other Internet Protocol (IP) networks. It associates various information ...

(DNS), one of the two principal

namespace In computing, a namespace is a set of signs (''names'') that are used to identify and refer to objects of various kinds. A namespace ensures that all of a given set of objects have unique names so that they can be easily identified. Namespaces ...

s of the

Internet The Internet (or internet) is the Global network, global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a internetworking, network of networks ...

. The most important function of DNS servers is the translation (resolution) of human-memorable

domain name In the Internet, a domain name is a string that identifies a realm of administrative autonomy, authority, or control. Domain names are often used to identify services provided through the Internet, such as websites, email services, and more. ...

s and hostnames into the corresponding numeric

Internet Protocol The Internet Protocol (IP) is the network layer communications protocol in the Internet protocol suite for relaying datagrams across network boundaries. Its routing function enables internetworking, and essentially establishes the Internet. IP ...

(IP) addresses, the second principal name space of the Internet which is used to identify and locate computer systems and resources on the Internet.

Domain name server

The Internet maintains two principal

s: the domain name hierarchyRFC 1034, ''Domain Names—Concepts and Facilities'', P. Mockapetris, The Internet Society (November 1987) and the IP address system.RFC 781, ''Internet Protocol—DARPA Internet Program Protocol Specification'', Information Sciences Institute, J. Postel (Ed.), The Internet Society (September 1981) The Domain Name System maintains the domain namespace and provides translation services between these two namespaces. Internet name servers implement the Domain Name System.RFC 1035, ''Domain Names — Implementation and Specification'', P. Mockapetris, The Internet Society (November 1987) The top hierarchy of the Domain Name System is served by the root name servers maintained by delegation by the

Internet Corporation for Assigned Names and Numbers The Internet Corporation for Assigned Names and Numbers (ICANN ) is a global multistakeholder group and nonprofit organization headquartered in the United States responsible for coordinating the maintenance and procedures of several dat ...

(ICANN). DNS servers, which are located all over the world, translate domain names into IP addresses, giving them control over which server a user may access via a given domain. Below the root, Internet resources are organized into a hierarchy of domains, administered by the respective registrars and domain name holders. A DNS name server is a server that stores the DNS records, such as address (A, AAAA) records, name server (NS) records, and mail exchanger (MX) records for a domain name (see also List of DNS record types) and responds with answers to queries against its database.

Types of name servers

Name servers are usually either ''authoritative'' or ''recursive'', as described below. Although not the usual practice today, name servers can be both authoritative ''and'' recursive, if they are configured to give authoritative answers to queries in some zones, while acting as a caching name server for all other zones.

Authoritative name server

An authoritative name server is a name server that is responsible for giving

answers Answer commonly refers to a response to a question. Answer may also refer to: Music * Answer, an element of a fugue Albums * ''Answer'' (Angela Aki album), 2009 * ''Answer'' (Supercar album), 2004 * ''Answers'' (album), 1994 * '' The Answers ...

in response to questions asked about names in a zone. An authoritative-only name server returns answers only to queries about domain names for which it is responsible (as specifically configured by its administrator). An authoritative name server can either be a ''primary'' server or a ''secondary'' server. A primary server for a zone is the server that stores the definitive versions of all records in that zone. It is identified in the start-of-authority (SOA) resource record. A secondary server for a zone uses an automatic updating mechanism to maintain an identical copy of the primary server's database for a zone. Examples of such mechanisms include DNS zone transfers and file transfer protocols. DNS provides a mechanism whereby the primary for a zone can notify all the known secondaries for that zone when the contents of the zone have changed. The contents of a zone are either manually configured by an administrator, or managed using Dynamic DNS. Every domain name appears in a zone served by one or more authoritative name servers. The

fully qualified domain name A fully qualified domain name (FQDN), sometimes also called an absolute domain name, is a domain name that specifies its exact location in the tree hierarchy of the Domain Name System (DNS). It specifies all domain levels, including the top-level ...

s of the authoritative name servers of a zone are listed in the NS records of that zone. If the server for a zone is not also authoritative for its parent zone, the server for the parent zone must be configured with a

delegation Delegation is the process of distributing and entrusting work to another person.Schermerhorn, J., Davidson, P., Poole, D., Woods, P., Simon, A., & McBarron, E. (2017). ''Management'' (6th ed., pp. 282–286). Brisbane: John Wiley & Sons Australia. ...

for the zone. When a domain is registered with a domain name registrar, the zone administrator provides the list of name servers (typically at least two, for redundancy) that are authoritative for the zone that contains the domain. The registrar provides the names of these servers to the domain registry for the

top-level domain A top-level domain (TLD) is one of the domain name, domains at the highest level in the hierarchical Domain Name System of the Internet after the root domain. The top-level domain names are installed in the DNS root zone, root zone of the nam ...

containing the zone. The domain registry in turn configures the authoritative name servers for that top-level domain with delegations for each server for the zone. If the fully qualified domain name of any name server for a zone appears within that zone, the zone administrator provides IP addresses for that name server, which are installed in the parent zone as glue records; otherwise, the delegation consists of the list of NS records for that zone.

Authoritative answer

A name server indicates that its response is authoritative by setting the ''Authoritative Answer'' (''AA'') bit in the response to a query on a name for which it is authoritative. Name servers providing answers for which they are not authoritative (for example, name servers for parent zones) do not set the ''AA'' bit.

Recursive Resolver

A ''Recursive Resolver'' (sometimes called a Recursive Name Server) is a DNS name server that accepts recursive queries (defined below) from clients (who are using a stub resolver), and then resolves those queries, either from a cache of prior results, or by asking one or more authoritative servers.

Recursive query

If a name server cannot answer a query because it does not contain an entry for the host in its DNS cache, it may recursively query name servers higher up in the hierarchy. This is known as a ''recursive query'' or ''recursive lookup''. A server providing recursive queries is known as a ''recursive name server'' or ''recursive DNS'', sometimes abbreviated as recdns. In principle, authoritative name servers suffice for the operation of the Internet. However, with only authoritative name-servers operating, every DNS query must start with recursive queries at the root zone of the Domain Name System and each user system must implement resolver software capable of recursive operation.

Caching name server

Caching name servers (''DNS caches'') are usually recursive resolvers that store DNS query results for a period of time determined in the configuration (time-to-live) of each domain-name record. DNS caches improve the efficiency of the DNS by reducing DNS traffic across the Internet, and by reducing load on authoritative name-servers, particularly root name-servers. Because they can answer questions more quickly, they also increase the performance of end-user applications that use the DNS. Caching name servers are often also recursive name servers—they perform every step necessary to answer any DNS query they receive. To do this the name server queries each authoritative name-server in turn, starting from the DNS root zone. It continues until it reaches the authoritative server for the zone that contains the queried domain name. That server provides the answer to the question, or definitively says it can't be answered, and the ''caching resolver'' then returns this response to the client that asked the question. The authority, resolving and caching functions can all be present in a DNS server implementation, but this is not required: a DNS server can implement any one of these functions alone, without implementing the others.

Internet service providers An Internet service provider (ISP) is an organization that provides a myriad of services related to accessing, using, managing, or participating in the Internet. ISPs can be organized in various forms, such as commercial, community-owned, non ...

typically provide caching resolvers for their customers. In addition, many home-networking routers implement caching resolvers to improve efficiency in the local network. Some systems utilize nscd, which stands for the "name service caching daemon"., though this is typically not used for caching DNS names.

References

External links

Free and Public DNS Servers
, article on ''about.com'' by Tim Fisher, retrieved on 2015-02-08 21-59 UTC
DNS & BIND Resources
{{Webarchive, url=https://web.archive.org/web/20060902123229/http://www.bind9.net/ , date=2006-09-02 Internet Protocol based network software Domain Name System de:Domain Name System#Nameserver