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




 
RFC 1034             Domain Concepts and Facilities        November 1987


     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.

3. DOMAIN NAME SPACE and RESOURCE RECORDS

3.1. Name space specifications and terminology

The domain name space is a tree structure.  Each node and leaf on the
tree corresponds to a resource set (which may be empty).  The domain
system makes no distinctions between the uses of the interior nodes and
leaves, and this memo uses the term "node" to refer to both.

Each node has a label, which is zero to 63 octets in length.  Brother
nodes may not have the same label, although the same label can be used
for nodes which are not brothers.  One label is reserved, and that is
the null (i.e., zero length) label used for the root.

The domain name of a node is the list of the labels on the path from the
node to the root of the tree.  By convention, the labels that compose a
domain name are printed or read left to right, from the most specific
(lowest, farthest from the root) to the least specific (highest, closest
to the root).

Internally, programs that manipulate domain names should represent them
as sequences of labels, where each label is a length octet followed by
an octet string.  Because all domain names end at the root, which has a
null string for a label, these internal representations can use a length
byte of zero to terminate a domain name.

By convention, domain names can be stored with arbitrary case, but
domain name comparisons for all present domain functions are done in a
case-insensitive manner, assuming an ASCII character set, and a high
order zero bit.  This means that you are free to create a node with
label "A" or a node with label "a", but not both as brothers; you could
refer to either using "a" or "A".  When you receive a domain name or




 
RFC 1034             Domain Concepts and Facilities        November 1987


label, you should preserve its case.  The rationale for this choice is
that we may someday need to add full binary domain names for new
services; existing services would not be changed.

When a user needs to type a domain name, the length of each label is
omitted and the labels are separated by dots (".").  Since a complete
domain name ends with the root label, this leads to a printed form which
ends in a dot.  We use this property to distinguish between: