rfc1034.html

this gives details of the network programming

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



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     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



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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:

   - a character string which represents a complete domain name
     (often called "absolute").  For example, "poneria.ISI.EDU."

   - a character string that represents the starting labels of a
     domain name which is incomplete, and should be completed by
     local software using knowledge of the local domain (often
     called "relative").  For example, "poneria" used in the
     ISI.EDU domain.

Relative names are either taken relative to a well known origin, or to a
list of domains used as a search list.  Relative names appear mostly at
the user interface, where their interpretation varies from
implementation to implementation, and in master files, where they are
relative to a single origin domain name.  The most common interpretation
uses the root "." as either the single origin or as one of the members
of the search list, so a multi-label relative name is often one where
the trailing dot has been omitted to save typing.

To simplify implementations, the total number of octets that represent a
domain name (i.e., the sum of all label octets and label lengths) is
limited to 255.

A domain is identified by a domain name, and consists of that part of
the domain name space that is at or below the domain name which
specifies the domain.  A domain is a subdomain of another domain if it
is contained within that domain.  This relationship can be tested by
seeing if the subdomain's name ends with the containing domain's name.
For example, A.B.C.D is a subdomain of B.C.D, C.D, D, and " ".

3.2. Administrative guidelines on use

As a matter of policy, the DNS technical specifications do not mandate a
particular tree structure or rules for selecting labels; its goal is to
be as general as possible, so that it can be used to build arbitrary
applications.  In particular, the system was designed so that the name
space did not have to be organized along the lines of network
boundaries, name servers, etc.  The rationale for this is not that the
name space should have no implied semantics, but rather that the choice
of implied semantics should be left open to be used for the problem at



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hand, and that different parts of the tree can have different implied
semantics.  For example, the IN-ADDR.ARPA domain is organized and
distributed by network and host address because its role is to translate
from network or host numbers to names; NetBIOS domains [<A href="../../../../rfc.net/rfc1001.html">RFC-1001</A>, RFC-
1002] are flat because that is appropriate for that application.

However, there are some guidelines that apply to the "normal" parts of
the name space used for hosts, mailboxes, etc., that will make the name
space more uniform, provide for growth, and minimize problems as
software is converted from the older host table.  The political
decisions about the top levels of the tree originated in <A href="../../../../rfc.net/rfc920.html">RFC-920</A>.
Current policy for the top levels is discussed in [<A href="../../../../rfc.net/rfc1032.html">RFC-1032</A>].  MILNET
conversion issues are covered in [<A href="../../../../rfc.net/rfc1031.html">RFC-1031</A>].

Lower domains which will eventually be broken into multiple zones should
provide branching at the top of the domain so that the eventual
decomposition can be done without renaming.  Node labels which use
special characters, leading digits, etc., are likely to break older
software which depends on more restrictive choices.

3.3. Technical guidelines on use

Before the DNS can be used to hold naming information for some kind of
object, two needs must be met:

   - A convention for mapping between object names and domain
     names.  This describes how information about an object is
     accessed.

   - RR types and data formats for describing the object.

These rules can be quite simple or fairly complex.  Very often, the
designer must take into account existing formats and plan for upward
compatibility for existing usage.  Multiple mappings or levels of
mapping may be required.

For hosts, the mapping depends on the existing syntax for host names
which is a subset of the usual text representation for domain names,
together with RR formats for describing host addresses, etc.  Because we
need a reliable inverse mapping from address to host name, a special
mapping for addresses into the IN-ADDR.ARPA domain is also defined.

For mailboxes, the mapping is slightly more complex.  The usual mail
address &lt;local-part&gt;@&lt;mail-domain&gt; is mapped into a domain name by
converting &lt;local-part&gt; into a single label (regardles of dots it
contains), converting &lt;mail-domain&gt; into a domain name using the usual
text format for domain names (dots denote label breaks), and
concatenating the two to form a single domain name.  Thus the mailbox



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[e-mail] is represented as a domain name by
HOSTMASTER.SRI-NIC.ARPA.  An appreciation for the reasons behind this
design also must take into account the scheme for mail exchanges [RFC-
974].

The typical user is not concerned with defining these rules, but should
understand that they usually are the result of numerous compromises
between desires for upward compatibility with old usage, interactions
between different object definitions, and the inevitable urge to add new
features when defining the rules.  The way the DNS is used to support
some object is often more crucial than the restrictions inherent in the
DNS.

3.4. Example name space

The following figure shows a part of the current domain name space, and
is used in many examples in this RFC.  Note that the tree is a very
small subset of the actual name space.

                                   |
                                   |
             +---------------------+------------------+
             |                     |                  |
            MIL                   EDU                ARPA
             |                     |                  |
             |                     |                  |
       +-----+-----+               |     +------+-----+-----+
       |     |     |               |     |      |           |
      BRL  NOSC  DARPA             |  IN-ADDR  SRI-NIC     ACC
                                   |
       +--------+------------------+---------------+--------+
       |        |                  |               |        |
      UCI      MIT                 |              UDEL     YALE
                |                 ISI
                |                  |
            +---+---+              |
            |       |              |
           LCS  ACHILLES  +--+-----+-----+--------+
            |             |  |     |     |        |
            XX            A  C   VAXA  VENERA Mockapetris

In this example, the root domain has three immediate subdomains: MIL,
EDU, and ARPA.  The LCS.MIT.EDU domain has one immediate subdomain named
XX.LCS.MIT.EDU.  All of the leaves are also domains.

3.5. Preferred name syntax

The DNS specifications attempt to be as general as possible in the rules



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for constructing domain names.  The idea is that the name of any
existing object can be expressed as a domain name with minimal changes.
However, when assigning a domain name for an object, the prudent user
will select a name which satisfies both the rules of the domain system
and any existing rules for the object, whether these rules are published
or implied by existing programs.

For example, when naming a mail domain, the user should satisfy both the
rules of this memo and those in <A href="../../../../rfc.net/rfc822.html">RFC-822</A>.  When creating a new host name,
the old rules for HOSTS.TXT should be followed.  This avoids problems
when old software is converted to use domain names.

The following syntax will result in fewer problems with many
applications that use domain names (e.g., mail, TELNET).