Connected: An Internet Encyclopedia
Elements of DNS
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Elements of DNS
Elements of DNS
The DNS has three major components:
- The DOMAIN NAME SPACE and RESOURCE RECORDS, which are
specifications for a tree structured name space and data
associated with the names. Conceptually, each node and leaf
of the domain name space tree names a set of information, and
query operations are attempts to extract specific types of
information from a particular set. A query names the domain
name of interest and describes the type of resource
information that is desired. For example, the Internet
uses some of its domain names to identify hosts; queries for
address resources return Internet host addresses.
- NAME SERVERS are server programs which hold information about
the domain tree's structure and set information. A name
server may cache structure or set information about any part
of the domain tree, but in general a particular name server
has complete information about a subset of the domain space,
and pointers to other name servers that can be used to lead to
information from any part of the domain tree. Name servers
know the parts of the domain tree for which they have complete
information; a name server is said to be an AUTHORITY for
these parts of the name space. Authoritative information is
organized into units called ZONEs, and these zones can be
automatically distributed to the name servers which provide
redundant service for the data in a zone.
- RESOLVERS are programs that extract information from name
servers in response to client requests. Resolvers must be
able to access at least one name server and use that name
server's information to answer a query directly, or pursue the
query using referrals to other name servers. A resolver will
typically be a system routine that is directly accessible to
user programs; hence no protocol is necessary between the
resolver and the user program.
These three components roughly correspond to the three layers or views
of the domain system:
- From the user's point of view, the domain system is accessed
through a simple procedure or OS call to a local resolver.
The domain space consists of a single tree and the user can
request information from any section of the tree.
- From the resolver's point of view, the domain system is
composed of an unknown number of name servers. Each name
server has one or more pieces of the whole domain tree's data,
but the resolver views each of these databases as essentially
static.
- From a name server's point of view, the domain system consists
of separate sets of local information called zones. The name
server has local copies of some of the zones. The name server
must periodically refresh its zones from master copies in
local files or foreign name servers. The name server must
concurrently process queries that arrive from resolvers.
In the interests of performance, implementations may couple these
functions. For example, a resolver on the same machine as a name server
might share a database consisting of the the zones managed by the name
server and the cache managed by the resolver.
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Connected: An Internet Encyclopedia
Elements of DNS