doc/arm/introduction.rst

..
   Copyright (C) Internet Systems Consortium, Inc. ("ISC")

   This Source Code Form is subject to the terms of the Mozilla Public
   License, v. 2.0. If a copy of the MPL was not distributed with this
   file, You can obtain one at http://mozilla.org/MPL/2.0/.

   See the COPYRIGHT file distributed with this work for additional
   information regarding copyright ownership.

..
   Copyright (C) Internet Systems Consortium, Inc. ("ISC")

   This Source Code Form is subject to the terms of the Mozilla Public
   License, v. 2.0. If a copy of the MPL was not distributed with this
   file, You can obtain one at http://mozilla.org/MPL/2.0/.

   See the COPYRIGHT file distributed with this work for additional
   information regarding copyright ownership.

.. _Introduction:

Introduction
============

The Internet Domain Name System (DNS) consists of the syntax to specify
the names of entities in the Internet in a hierarchical manner, the
rules used for delegating authority over names, and the system
implementation that actually maps names to Internet addresses. DNS data
is maintained in a group of distributed hierarchical databases.

.. _doc_scope:

Scope of Document
-----------------

The Berkeley Internet Name Domain (BIND) implements a domain name server
for a number of operating systems. This document provides basic
information about the installation and care of the Internet Systems
Consortium (ISC) BIND version 9 software package for system
administrators.

This manual covers BIND version |release|.

.. _organization:

Organization of This Document
-----------------------------

In this document, *Chapter 1* introduces the basic DNS and BIND
concepts. *Chapter 2* describes resource requirements for running BIND
in various environments. Information in *Chapter 3* is *task-oriented*
in its presentation and is organized functionally, to aid in the process
of installing the BIND 9 software. The task-oriented section is followed
by *Chapter 4*, which is organized as a reference manual to aid in the ongoing
maintenance of the software. *Chapter 5* contains more advanced concepts that
the system administrator may need for implementing certain options. *Chapter 6*
addresses security considerations, and *Chapter 7* contains troubleshooting help.
The main body of the document is followed by several *appendices* which contain
useful reference information, such as a *bibliography* and historic
information related to BIND and the Domain Name System.

.. _conventions:

Conventions Used in This Document
---------------------------------

In this document, we use the following general typographic conventions:

+-------------------------------------+--------------------------------+
| *To describe:*                      | *We use the style:*            |
+-------------------------------------+--------------------------------+
| a pathname, filename, URL,          | ``Fixed width``                |
| hostname, mailing list name, or new |                                |
| term or concept                     |                                |
+-------------------------------------+--------------------------------+
| literal user input                  | ``Fixed Width Bold``           |
+-------------------------------------+--------------------------------+
| program output                      | ``Fixed Width``                |
+-------------------------------------+--------------------------------+

The following conventions are used in descriptions of the BIND
configuration file:

+-------------------------------------+--------------------------------+
| *To describe:*                      | *We use the style:*            |
+-------------------------------------+--------------------------------+
| keywords                            | ``Fixed Width``                |
+-------------------------------------+--------------------------------+
| variables                           | ``Fixed Width``                |
+-------------------------------------+--------------------------------+
| Optional input                      | [Text is enclosed in square    |
|                                     | brackets]                      |
+-------------------------------------+--------------------------------+

.. _dns_overview:

The Domain Name System (DNS)
----------------------------

The purpose of this document is to explain the installation and upkeep
of the BIND (Berkeley Internet Name Domain) software package, and we
begin by reviewing the fundamentals of the Domain Name System (DNS) as
they relate to BIND.

.. _dns_fundamentals:

DNS Fundamentals
~~~~~~~~~~~~~~~~

The Domain Name System (DNS) is a hierarchical, distributed database. It
stores information for mapping Internet host names to IP addresses and
vice versa, mail routing information, and other data used by Internet
applications.

Clients look up information in the DNS by calling a *resolver* library,
which sends queries to one or more *name servers* and interprets the
responses. The BIND 9 software distribution contains a name server,
``named``, and a set of associated tools.

.. _domain_names:

Domains and Domain Names
~~~~~~~~~~~~~~~~~~~~~~~~

The data stored in the DNS is identified by *domain names* that are
organized as a tree according to organizational or administrative
boundaries. Each node of the tree, called a *domain*, is given a label.
The domain name of the node is the concatenation of all the labels on
the path from the node to the *root* node. This is represented in
written form as a string of labels listed from right to left and
separated by dots. A label need only be unique within its parent domain.

For example, a domain name for a host at the company *Example, Inc.*
could be ``ourhost.example.com``, where ``com`` is the top level domain
to which ``ourhost.example.com`` belongs, ``example`` is a subdomain of
``com``, and ``ourhost`` is the name of the host.

For administrative purposes, the name space is partitioned into areas
called *zones*, each starting at a node and extending down to the leaf
nodes or to nodes where other zones start. The data for each zone is
stored in a *name server*, which answers queries about the zone using
the *DNS protocol*.

The data associated with each domain name is stored in the form of
*resource records* (RRs). Some of the supported resource record types
are described in :ref:`types_of_resource_records_and_when_to_use_them`.

For more detailed information about the design of the DNS and the DNS
protocol, please refer to the standards documents listed in :ref:`rfcs`.

Zones
~~~~~

To properly operate a name server, it is important to understand the
difference between a *zone* and a *domain*.

As stated previously, a zone is a point of delegation in the DNS tree. A
zone consists of those contiguous parts of the domain tree for which a
name server has complete information and over which it has authority. It
contains all domain names from a certain point downward in the domain
tree except those which are delegated to other zones. A delegation point
is marked by one or more *NS records* in the parent zone, which should
be matched by equivalent NS records at the root of the delegated zone.

For instance, consider the ``example.com`` domain which includes names
such as ``host.aaa.example.com`` and ``host.bbb.example.com`` even
though the ``example.com`` zone includes only delegations for the
``aaa.example.com`` and ``bbb.example.com`` zones. A zone can map
exactly to a single domain, but could also include only part of a
domain, the rest of which could be delegated to other name servers.
Every name in the DNS tree is a *domain*, even if it is *terminal*, that
is, has no *subdomains*. Every subdomain is a domain and every domain
except the root is also a subdomain. The terminology is not intuitive
and we suggest that you read :rfc:`1033`, :rfc:`1034` and :rfc:`1035` to gain a complete
understanding of this difficult and subtle topic.

Though BIND is called a "domain name server", it deals primarily in
terms of zones. The master and slave declarations in the ``named.conf``
file specify zones, not domains. When you ask some other site if it is
willing to be a slave server for your *domain*, you are actually asking
for slave service for some collection of zones.

.. _auth_servers:

Authoritative Name Servers
~~~~~~~~~~~~~~~~~~~~~~~~~~

Each zone is served by at least one *authoritative name server*, which
contains the complete data for the zone. To make the DNS tolerant of
server and network failures, most zones have two or more authoritative
servers, on different networks.

Responses from authoritative servers have the "authoritative answer"
(AA) bit set in the response packets. This makes them easy to identify
when debugging DNS configurations using tools like ``dig`` (:ref:`diagnostic_tools`).

.. _primary_master:

The Primary Master
^^^^^^^^^^^^^^^^^^

The authoritative server where the master copy of the zone data is
maintained is called the *primary master* server, or simply the
*primary*. Typically it loads the zone contents from some local file
edited by humans or perhaps generated mechanically from some other local
file which is edited by humans. This file is called the *zone file* or
*master file*.

In some cases, however, the master file may not be edited by humans at
all, but may instead be the result of *dynamic update* operations.

.. _slave_server:

Slave Servers
^^^^^^^^^^^^^

The other authoritative servers, the *slave* servers (also known as
*secondary* servers) load the zone contents from another server using a
replication process known as a *zone transfer*. Typically the data are
transferred directly from the primary master, but it is also possible to
transfer it from another slave. In other words, a slave server may
itself act as a master to a subordinate slave server.

Periodically, the slave server must send a refresh query to determine
whether the zone contents have been updated. This is done by sending a
query for the zone's SOA record and checking whether the SERIAL field
has been updated; if so, a new transfer request is initiated. The timing
of these refresh queries is controlled by the SOA REFRESH and RETRY
fields, but can be overridden with the ``max-refresh-time``,
``min-refresh-time``, ``max-retry-time``, and ``min-retry-time``
options.

If the zone data cannot be updated within the time specified by the SOA
EXPIRE option (up to a hard-coded maximum of 24 weeks) then the slave
zone expires and will no longer respond to queries.

.. _stealth_server:

Stealth Servers
^^^^^^^^^^^^^^^

Usually all of the zone's authoritative servers are listed in NS records
in the parent zone. These NS records constitute a *delegation* of the
zone from the parent. The authoritative servers are also listed in the
zone file itself, at the *top level* or *apex* of the zone. You can list
servers in the zone's top-level NS records that are not in the parent's
NS delegation, but you cannot list servers in the parent's delegation
that are not present at the zone's top level.

A *stealth server* is a server that is authoritative for a zone but is
not listed in that zone's NS records. Stealth servers can be used for
keeping a local copy of a zone to speed up access to the zone's records
or to make sure that the zone is available even if all the "official"
servers for the zone are inaccessible.

A configuration where the primary master server itself is a stealth
server is often referred to as a "hidden primary" configuration. One use
for this configuration is when the primary master is behind a firewall
and therefore unable to communicate directly with the outside world.

.. _cache_servers:

Caching Name Servers
~~~~~~~~~~~~~~~~~~~~

The resolver libraries provided by most operating systems are *stub
resolvers*, meaning that they are not capable of performing the full DNS
resolution process by themselves by talking directly to the
authoritative servers. Instead, they rely on a local name server to
perform the resolution on their behalf. Such a server is called a
*recursive* name server; it performs *recursive lookups* for local
clients.

To improve performance, recursive servers cache the results of the
lookups they perform. Since the processes of recursion and caching are
intimately connected, the terms *recursive server* and *caching server*
are often used synonymously.

The length of time for which a record may be retained in the cache of a
caching name server is controlled by the Time To Live (TTL) field
associated with each resource record.

.. _forwarder:

Forwarding
^^^^^^^^^^

Even a caching name server does not necessarily perform the complete
recursive lookup itself. Instead, it can *forward* some or all of the
queries that it cannot satisfy from its cache to another caching name
server, commonly referred to as a *forwarder*.

There may be one or more forwarders, and they are queried in turn until
the list is exhausted or an answer is found. Forwarders are typically
used when you do not wish all the servers at a given site to interact
directly with the rest of the Internet servers. A typical scenario would
involve a number of internal DNS servers and an Internet firewall.
Servers unable to pass packets through the firewall would forward to the
server that can do it, and that server would query the Internet DNS
servers on the internal server's behalf.

.. _multi_role:

Name Servers in Multiple Roles
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The BIND name server can simultaneously act as a master for some zones,
a slave for other zones, and as a caching (recursive) server for a set
of local clients.

However, since the functions of authoritative name service and
caching/recursive name service are logically separate, it is often
advantageous to run them on separate server machines. A server that only
provides authoritative name service (an *authoritative-only* server) can
run with recursion disabled, improving reliability and security. A
server that is not authoritative for any zones and only provides
recursive service to local clients (a *caching-only* server) does not
need to be reachable from the Internet at large and can be placed inside
a firewall.