rfc1034.txt

bind 9.3结合mysql数据库

Network Working Group                                     P. Mockapetris
Request for Comments: 1034                                           ISI
Obsoletes: RFCs 882, 883, 973                              November 1987


                 DOMAIN NAMES - CONCEPTS AND FACILITIES



1. STATUS OF THIS MEMO

This RFC is an introduction to the Domain Name System (DNS), and omits
many details which can be found in a companion RFC, "Domain Names -
Implementation and Specification" [RFC-1035].  That RFC assumes that the
reader is familiar with the concepts discussed in this memo.

A subset of DNS functions and data types constitute an official
protocol.  The official protocol includes standard queries and their
responses and most of the Internet class data formats (e.g., host
addresses).

However, the domain system is intentionally extensible.  Researchers are
continuously proposing, implementing and experimenting with new data
types, query types, classes, functions, etc.  Thus while the components
of the official protocol are expected to stay essentially unchanged and
operate as a production service, experimental behavior should always be
expected in extensions beyond the official protocol.  Experimental or
obsolete features are clearly marked in these RFCs, and such information
should be used with caution.

The reader is especially cautioned not to depend on the values which
appear in examples to be current or complete, since their purpose is
primarily pedagogical.  Distribution of this memo is unlimited.

2. INTRODUCTION

This RFC introduces domain style names, their use for Internet mail and
host address support, and the protocols and servers used to implement
domain name facilities.

2.1. The history of domain names

The impetus for the development of the domain system was growth in the
Internet:

   - Host name to address mappings were maintained by the Network
     Information Center (NIC) in a single file (HOSTS.TXT) which
     was FTPed by all hosts [RFC-952, RFC-953].  The total network



Mockapetris                                                     [Page 1]

RFC 1034             Domain Concepts and Facilities        November 1987


     bandwidth consumed in distributing a new version by this
     scheme is proportional to the square of the number of hosts in
     the network, and even when multiple levels of FTP are used,
     the outgoing FTP load on the NIC host is considerable.
     Explosive growth in the number of hosts didn't bode well for
     the future.

   - The network population was also changing in character.  The
     timeshared hosts that made up the original ARPANET were being
     replaced with local networks of workstations.  Local
     organizations were administering their own names and
     addresses, but had to wait for the NIC to change HOSTS.TXT to
     make changes visible to the Internet at large.  Organizations
     also wanted some local structure on the name space.

   - The applications on the Internet were getting more
     sophisticated and creating a need for general purpose name
     service.


The result was several ideas about name spaces and their management
[IEN-116, RFC-799, RFC-819, RFC-830].  The proposals varied, but a
common thread was the idea of a hierarchical name space, with the
hierarchy roughly corresponding to organizational structure, and names
using "."  as the character to mark the boundary between hierarchy
levels.  A design using a distributed database and generalized resources
was described in [RFC-882, RFC-883].  Based on experience with several
implementations, the system evolved into the scheme described in this
memo.

The terms "domain" or "domain name" are used in many contexts beyond the
DNS described here.  Very often, the term domain name is used to refer
to a name with structure indicated by dots, but no relation to the DNS.
This is particularly true in mail addressing [Quarterman 86].

2.2. DNS design goals

The design goals of the DNS influence its structure.  They are:

   - The primary goal is a consistent name space which will be used
     for referring to resources.  In order to avoid the problems
     caused by ad hoc encodings, names should not be required to
     contain network identifiers, addresses, routes, or similar
     information as part of the name.

   - The sheer size of the database and frequency of updates
     suggest that it must be maintained in a distributed manner,
     with local caching to improve performance.  Approaches that



Mockapetris                                                     [Page 2]

RFC 1034             Domain Concepts and Facilities        November 1987


     attempt to collect a consistent copy of the entire database
     will become more and more expensive and difficult, and hence
     should be avoided.  The same principle holds for the structure
     of the name space, and in particular mechanisms for creating
     and deleting names; these should also be distributed.

   - Where there tradeoffs between the cost of acquiring data, the
     speed of updates, and the accuracy of caches, the source of
     the data should control the tradeoff.

   - The costs of implementing such a facility dictate that it be
     generally useful, and not restricted to a single application.
     We should be able to use names to retrieve host addresses,
     mailbox data, and other as yet undetermined information.  All
     data associated with a name is tagged with a type, and queries
     can be limited to a single type.

   - Because we want the name space to be useful in dissimilar
     networks and applications, we provide the ability to use the
     same name space with different protocol families or
     management.  For example, host address formats differ between
     protocols, though all protocols have the notion of address.
     The DNS tags all data with a class as well as the type, so
     that we can allow parallel use of different formats for data
     of type address.

   - We want name server transactions to be independent of the
     communications system that carries them.  Some systems may
     wish to use datagrams for queries and responses, and only
     establish virtual circuits for transactions that need the
     reliability (e.g., database updates, long transactions); other
     systems will use virtual circuits exclusively.

   - The system should be useful across a wide spectrum of host
     capabilities.  Both personal computers and large timeshared
     hosts should be able to use the system, though perhaps in
     different ways.

2.3. Assumptions about usage

The organization of the domain system derives from some assumptions
about the needs and usage patterns of its user community and is designed
to avoid many of the the complicated problems found in general purpose
database systems.

The assumptions are:

   - The size of the total database will initially be proportional



Mockapetris                                                     [Page 3]

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     to the number of hosts using the system, but will eventually
     grow to be proportional to the number of users on those hosts
     as mailboxes and other information are added to the domain
     system.

   - Most of the data in the system will change very slowly (e.g.,
     mailbox bindings, host addresses), but that the system should
     be able to deal with subsets that change more rapidly (on the
     order of seconds or minutes).

   - The administrative boundaries used to distribute
     responsibility for the database will usually correspond to
     organizations that have one or more hosts.  Each organization
     that has responsibility for a particular set of domains will
     provide redundant name servers, either on the organization's
     own hosts or other hosts that the organization arranges to
     use.

   - Clients of the domain system should be able to identify
     trusted name servers they prefer to use before accepting
     referrals to name servers outside of this "trusted" set.

   - Access to information is more critical than instantaneous
     updates or guarantees of consistency.  Hence the update
     process allows updates to percolate out through the users of
     the domain system rather than guaranteeing that all copies are
     simultaneously updated.  When updates are unavailable due to
     network or host failure, the usual course is to believe old
     information while continuing efforts to update it.  The
     general model is that copies are distributed with timeouts for
     refreshing.  The distributor sets the timeout value and the
     recipient of the distribution is responsible for performing
     the refresh.  In special situations, very short intervals can
     be specified, or the owner can prohibit copies.

   - In any system that has a distributed database, a particular
     name server may be presented with a query that can only be
     answered by some other server.  The two general approaches to
     dealing with this problem are "recursive", in which the first
     server pursues the query for the client at another server, and
     "iterative", in which the server refers the client to another
     server and lets the client pursue the query.  Both approaches
     have advantages and disadvantages, but the iterative approach
     is preferred for the datagram style of access.  The domain
     system requires implementation of the iterative approach, but
     allows the recursive approach as an option.





Mockapetris                                                     [Page 4]

RFC 1034             Domain Concepts and Facilities        November 1987


The domain system assumes that all data originates in master files
scattered through the hosts that use the domain system.  These master
files are updated by local system administrators.  Master files are text
files that are read by a local name server, and hence become available
through the name servers to users of the domain system.  The user
programs access name servers through standard programs called resolvers.

The standard format of master files allows them to be exchanged between
hosts (via FTP, mail, or some other mechanism); this facility is useful
when an organization wants a domain, but doesn't want to support a name
server.  The organization can maintain the master files locally using a
text editor, transfer them to a foreign host which runs a name server,
and then arrange with the system administrator of the name server to get
the files loaded.

Each host's name servers and resolvers are configured by a local system
administrator [RFC-1033].  For a name server, this configuration data
includes the identity of local master files and instructions on which
non-local master files are to be loaded from foreign servers.  The name
server uses the master files or copies to load its zones.  For
resolvers, the configuration data identifies the name servers which
should be the primary sources of information.

The domain system defines procedures for accessing the data and for
referrals to other name servers.  The domain system also defines
procedures for caching retrieved data and for periodic refreshing of
data defined by the system administrator.

The system administrators provide:

   - The definition of zone boundaries.

   - Master files of data.

   - Updates to master files.

   - Statements of the refresh policies desired.

The domain system provides:

   - Standard formats for resource data.

   - Standard methods for querying the database.

   - Standard methods for name servers to refresh local data from
     foreign name servers.





Mockapetris                                                     [Page 5]

RFC 1034             Domain Concepts and Facilities        November 1987


2.4. Elements of the 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