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.

この RFC ではドメインスタイルの名前、 インターネットメイルやホストアドレスとしての使い方、 そして、ドメイン名機能を実装するためのプロトコルとサーバについて 紹介する。

2.1. ドメイン名の歴史 The history of domain names

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

DNS 開発のきっかけはインターネットの成長だった:

   - 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
     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

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

これらの結果は名前空間とその管理に関する数々のアイデアとなった。 [IEN-116, RFC-799, RFC-819, RFC-830]。 提案には違いもあったが、共通点もあった。 階層的な名前空間を使うというアイデア、ほぼ組織に対応した階層構造、 階層の区切りに'.'の文字を使った名前にすること。 分散データベースと一般の資源を使う設計が[RFC-882, RFC-883] になった。 実装経験に基づいたあと、このメモで記述された形にシステム(訳注 DNS)は発展した。

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].

用語「ドメイン」や「ドメイン名」はここで記述された DNS 以外にも 多くの文脈で使われる。 「ドメイン名」は DNS と無関係に点で区切られた構造の名前によく使われる。 このことは特にメイルアドレスあてはまる[Quarterman 86]。

2.2. DNS の設計の目標 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
     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. 利用の前提 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.

DNS の構成は ユーザコミュニティの要求と利用パターンについていくつかの仮定をしており、 汎用データベースシステムに見られるような複雑な問題の多くを回避するように 設計されている。

The assumptions are:


   - The size of the total database will initially be proportional
     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

   - 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

   - 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.

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.

DNS ではすべてのデータは DNS を使うホスト上のマスターファイル群 に分散された起源をもつものとする。

これらのマスターファイルは各ローカル DNS 管理者によって更新される。 マスターファイルはローカルネームサーバが読むテキストファイルであり、 このネームサーバを通してDNS の利用者に利用可能となる。 ユーザープログラムはリゾルバと呼ばれる標準プログラムを通して ネームサーバにアクセスする。

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.

マスターファイルの標準形式により、マスターファイルがホスト間で交換 可能となる(FTP、メイル、その他の方法); この方法はドメインは欲しいがネームサーバを運用をしたくない組織に有用だ。 組織はテキストエディタを使ってローカルにマスターファイルを管理でき、 それをネームサーバを動かしている他のホストに転送し、 ネームサーバのシステム管理者がファイルをロードできるよう整えられる。

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.

各ホストのネームサーバとリゾルバはローカルシステム管理者が設定する[RFC-1033]。 各ネームサーバの設定データにはローカルマスターファイルの識別子と ローカルでないマスターファイルを外部のサーバから読込む指令が含まれている。

ネームサーバーはマスターファイルやコピーを読んで、ゾーン情報を設定する。 リゾルバの設定データには情報源であるネームサーバを指定する。

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.

DNS はデータへのアクセス手段と他のネームサーバを参照する手順を定義する。 また、DNS は受けとったデータをキャッシュする手順、 システム管理者により定義されている周期的なデータ更新の手順を定義する。

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:

DNS が提供するもの:

   - Standard formats for resource data.

   - Standard methods for querying the database.

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

2.4. DNSの要素 Elements of the DNS

The DNS has three major components:


     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:

上記の3つの要素はおおむね DNS の3つのレイヤあるいは視点に対応している:

   - 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

   - 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.

性能面の都合で、これらの機能が結合された実装になっていることもある。 例えば、ネームサーバと同じマシン上にあるリゾルバは ネームサーバの管理するゾーンと リゾルバの管理するキャッシュとからなるデータベースを共有するかもしれない。

2002-10-24 訳 前野年紀