DNS (Domain Name System or Service) is a distributed Internet directory service. DNS is used mostly to translate between domain names (www.domainname.com) and IP addresses (123.123.123.123), and to control Internet email delivery. Most Internet services rely on DNS to work, and if DNS fails, web sites cannot be located and email delivery stalls.

DNS has two independent aspects:

1. It specifies the name syntax and rules for delegating authority over names. The basic syntax is:local.group.site

2. It specifies the implementation of a distributed computing system that efficiently maps names to addresses.

In the DNS naming scheme, a decentralized and hierarchical mechanism is used by delegating authority for parts of the namespace and distributing responsibility for mapping names and addresses. The naming scheme of DNS is used to assign network device names globally and it is implemented by geographically distributed set of severs to names to addresses.

In theory, the domain name standard in DNS protocol specifies an abstract hierarchical namespace with arbitrary values for labels. Any group can build an instance of the domain system to choose labels for all parts of its hierarchy. However most users of the DNS protocols follow the hierarchical labels used by the official Internet domain system. Some of the top level domains: COM, EDU, GOV, NET, ORG, BIZ ... plus many country codes.

The distributed scheme of DNS allows efficient and reliable mapping of names to IP addresses. Most names can be mapped locally and a set of servers operating at multiple sites cooperatively solve the mapping problem of a large network. Because of the distributing nature, no single machine failure will prevent the DNS from operating correctly. DNS is general purpose and it is not restricted to network device names.

Protocol Structure

• ID -- Identification field used to correlate queries and responses.
• Q -- identifies the message as a query or response.
• Query -- describes the type of message: 0 Standard query (name to address); 1 Inverse query; 2 Server status request.
• A -- Authoritative Answer. When set to 1, identifies the response as one made by an authoritative name server.
• T -- Truncation. When set to 1, indicates the message has been truncated.
• R -- Set to 1 by the resolve to request recursive service by the name server.
• V -- Signals the availability of recursive service by the name server.
• B -- Reserved for future use. Must be set to 0.
• Rcode -- Response Code, that is set by the name server to identify the status of the query.
• Question count -- Defines the number of entries in the question section.
• Answer count -- Defines the number of resource records in the answer section.
• Authority count -- Defines the number of name server resource records in the authority section.
• Additional count -- Defines the number of resource records in the additional records section.

Related Terms: IP, IPv6, TCP, UDP, IGMP, ICMP, SNMP

Sponsor Source: DNS is defined by IETF (http://www.ietf.org) RFC1034 and updated by 1035, 1101, 1183, 1348, 1876, 1982, 2181, 2308, 2535

Reference: http://www.javvin.com/protocol/rfc1034.pdf: Domain Names C Concept and Facilities