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Copyright (C) Internet Systems Consortium, Inc. ("ISC")
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This Source Code Form is subject to the terms of the Mozilla Public
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License, v. 2.0. If a copy of the MPL was not distributed with this
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file, You can obtain one at http://mozilla.org/MPL/2.0/.
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See the COPYRIGHT file distributed with this work for additional
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information regarding copyright ownership.
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..
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Copyright (C) Internet Systems Consortium, Inc. ("ISC")
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This Source Code Form is subject to the terms of the Mozilla Public
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License, v. 2.0. If a copy of the MPL was not distributed with this
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file, You can obtain one at http://mozilla.org/MPL/2.0/.
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See the COPYRIGHT file distributed with this work for additional
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information regarding copyright ownership.
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.. _Introduction:
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Introduction
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============
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The Internet Domain Name System (DNS) consists of the syntax to specify
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the names of entities in the Internet in a hierarchical manner, the
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rules used for delegating authority over names, and the system
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implementation that actually maps names to Internet addresses. DNS data
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is maintained in a group of distributed hierarchical databases.
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.. _doc_scope:
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Scope of Document
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-----------------
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The Berkeley Internet Name Domain (BIND) implements a domain name server
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for a number of operating systems. This document provides basic
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information about the installation and care of the Internet Systems
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Consortium (ISC) BIND version 9 software package for system
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administrators.
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.. _organization:
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Organization of This Document
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-----------------------------
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In this document, *Chapter 1* introduces the basic DNS and BIND
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concepts. *Chapter 2* describes resource requirements for running BIND
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in various environments. Information in *Chapter 3* is *task-oriented*
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in its presentation and is organized functionally, to aid in the process
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of installing the BIND 9 software. The task-oriented section is followed
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by *Chapter 4*, which is organized as a reference manual to aid in the ongoing
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maintenance of the software. *Chapter 5* contains more advanced concepts that
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the system administrator may need for implementing certain options. *Chapter 6*
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addresses security considerations, and *Chapter 7* contains troubleshooting help.
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The main body of the document is followed by several *appendices* which contain
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useful reference information, such as a *bibliography* and historic
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information related to BIND and the Domain Name System.
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.. _conventions:
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Conventions Used in This Document
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---------------------------------
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In this document, we use the following general typographic conventions:
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+-------------------------------------+--------------------------------+
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| *To describe:* | *We use the style:* |
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+-------------------------------------+--------------------------------+
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| a pathname, filename, URL, | ``Fixed width`` |
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| hostname, mailing list name, or new | |
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| term or concept | |
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+-------------------------------------+--------------------------------+
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| literal user input | ``Fixed Width Bold`` |
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+-------------------------------------+--------------------------------+
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| program output | ``Fixed Width`` |
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+-------------------------------------+--------------------------------+
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The following conventions are used in descriptions of the BIND
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configuration file:
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+-------------------------------------+--------------------------------+
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| *To describe:* | *We use the style:* |
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+-------------------------------------+--------------------------------+
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| keywords | ``Fixed Width`` |
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+-------------------------------------+--------------------------------+
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| variables | ``Fixed Width`` |
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+-------------------------------------+--------------------------------+
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| Optional input | [Text is enclosed in square |
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| | brackets] |
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+-------------------------------------+--------------------------------+
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.. _dns_overview:
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The Domain Name System (DNS)
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----------------------------
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The purpose of this document is to explain the installation and upkeep
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of the BIND (Berkeley Internet Name Domain) software package, and we
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begin by reviewing the fundamentals of the Domain Name System (DNS) as
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they relate to BIND.
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.. _dns_fundamentals:
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DNS Fundamentals
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~~~~~~~~~~~~~~~~
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The Domain Name System (DNS) is a hierarchical, distributed database. It
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stores information for mapping Internet host names to IP addresses and
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vice versa, mail routing information, and other data used by Internet
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applications.
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Clients look up information in the DNS by calling a *resolver* library,
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which sends queries to one or more *name servers* and interprets the
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responses. The BIND 9 software distribution contains a name server,
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``named``, and a set of associated tools.
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.. _domain_names:
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Domains and Domain Names
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~~~~~~~~~~~~~~~~~~~~~~~~
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The data stored in the DNS is identified by *domain names* that are
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organized as a tree according to organizational or administrative
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boundaries. Each node of the tree, called a *domain*, is given a label.
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The domain name of the node is the concatenation of all the labels on
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the path from the node to the *root* node. This is represented in
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written form as a string of labels listed from right to left and
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separated by dots. A label need only be unique within its parent domain.
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For example, a domain name for a host at the company *Example, Inc.*
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could be ``ourhost.example.com``, where ``com`` is the top level domain
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to which ``ourhost.example.com`` belongs, ``example`` is a subdomain of
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``com``, and ``ourhost`` is the name of the host.
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For administrative purposes, the name space is partitioned into areas
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called *zones*, each starting at a node and extending down to the leaf
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nodes or to nodes where other zones start. The data for each zone is
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stored in a *name server*, which answers queries about the zone using
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the *DNS protocol*.
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The data associated with each domain name is stored in the form of
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*resource records* (RRs). Some of the supported resource record types
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are described in :ref:`types_of_resource_records_and_when_to_use_them`.
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For more detailed information about the design of the DNS and the DNS
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protocol, please refer to the standards documents listed in :ref:`rfcs`.
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Zones
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~~~~~
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To properly operate a name server, it is important to understand the
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difference between a *zone* and a *domain*.
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As stated previously, a zone is a point of delegation in the DNS tree. A
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zone consists of those contiguous parts of the domain tree for which a
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name server has complete information and over which it has authority. It
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contains all domain names from a certain point downward in the domain
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tree except those which are delegated to other zones. A delegation point
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is marked by one or more *NS records* in the parent zone, which should
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be matched by equivalent NS records at the root of the delegated zone.
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For instance, consider the ``example.com`` domain which includes names
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such as ``host.aaa.example.com`` and ``host.bbb.example.com`` even
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though the ``example.com`` zone includes only delegations for the
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``aaa.example.com`` and ``bbb.example.com`` zones. A zone can map
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exactly to a single domain, but could also include only part of a
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domain, the rest of which could be delegated to other name servers.
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Every name in the DNS tree is a *domain*, even if it is *terminal*, that
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is, has no *subdomains*. Every subdomain is a domain and every domain
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except the root is also a subdomain. The terminology is not intuitive
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and we suggest that you read :rfc:`1033`, :rfc:`1034` and :rfc:`1035` to gain a complete
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understanding of this difficult and subtle topic.
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Though BIND is called a "domain name server", it deals primarily in
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terms of zones. The master and slave declarations in the ``named.conf``
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file specify zones, not domains. When you ask some other site if it is
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willing to be a slave server for your *domain*, you are actually asking
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for slave service for some collection of zones.
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.. _auth_servers:
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Authoritative Name Servers
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~~~~~~~~~~~~~~~~~~~~~~~~~~
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Each zone is served by at least one *authoritative name server*, which
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contains the complete data for the zone. To make the DNS tolerant of
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server and network failures, most zones have two or more authoritative
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servers, on different networks.
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Responses from authoritative servers have the "authoritative answer"
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(AA) bit set in the response packets. This makes them easy to identify
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when debugging DNS configurations using tools like ``dig`` (:ref:`diagnostic_tools`).
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.. _primary_master:
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The Primary Master
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^^^^^^^^^^^^^^^^^^
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The authoritative server where the master copy of the zone data is
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maintained is called the *primary master* server, or simply the
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*primary*. Typically it loads the zone contents from some local file
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edited by humans or perhaps generated mechanically from some other local
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file which is edited by humans. This file is called the *zone file* or
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*master file*.
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In some cases, however, the master file may not be edited by humans at
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all, but may instead be the result of *dynamic update* operations.
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.. _slave_server:
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Slave Servers
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^^^^^^^^^^^^^
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The other authoritative servers, the *slave* servers (also known as
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*secondary* servers) load the zone contents from another server using a
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replication process known as a *zone transfer*. Typically the data are
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transferred directly from the primary master, but it is also possible to
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transfer it from another slave. In other words, a slave server may
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itself act as a master to a subordinate slave server.
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Periodically, the slave server must send a refresh query to determine
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whether the zone contents have been updated. This is done by sending a
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query for the zone's SOA record and checking whether the SERIAL field
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has been updated; if so, a new transfer request is initiated. The timing
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of these refresh queries is controlled by the SOA REFRESH and RETRY
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fields, but can be overridden with the ``max-refresh-time``,
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``min-refresh-time``, ``max-retry-time``, and ``min-retry-time``
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options.
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If the zone data cannot be updated within the time specified by the SOA
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EXPIRE option (up to a hard-coded maximum of 24 weeks) then the slave
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zone expires and will no longer respond to queries.
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.. _stealth_server:
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Stealth Servers
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^^^^^^^^^^^^^^^
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Usually all of the zone's authoritative servers are listed in NS records
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in the parent zone. These NS records constitute a *delegation* of the
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zone from the parent. The authoritative servers are also listed in the
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zone file itself, at the *top level* or *apex* of the zone. You can list
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servers in the zone's top-level NS records that are not in the parent's
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NS delegation, but you cannot list servers in the parent's delegation
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that are not present at the zone's top level.
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A *stealth server* is a server that is authoritative for a zone but is
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not listed in that zone's NS records. Stealth servers can be used for
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keeping a local copy of a zone to speed up access to the zone's records
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or to make sure that the zone is available even if all the "official"
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servers for the zone are inaccessible.
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A configuration where the primary master server itself is a stealth
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server is often referred to as a "hidden primary" configuration. One use
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for this configuration is when the primary master is behind a firewall
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and therefore unable to communicate directly with the outside world.
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.. _cache_servers:
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Caching Name Servers
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~~~~~~~~~~~~~~~~~~~~
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The resolver libraries provided by most operating systems are *stub
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resolvers*, meaning that they are not capable of performing the full DNS
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resolution process by themselves by talking directly to the
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authoritative servers. Instead, they rely on a local name server to
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perform the resolution on their behalf. Such a server is called a
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*recursive* name server; it performs *recursive lookups* for local
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clients.
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To improve performance, recursive servers cache the results of the
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lookups they perform. Since the processes of recursion and caching are
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intimately connected, the terms *recursive server* and *caching server*
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are often used synonymously.
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The length of time for which a record may be retained in the cache of a
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caching name server is controlled by the Time To Live (TTL) field
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associated with each resource record.
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.. _forwarder:
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Forwarding
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^^^^^^^^^^
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Even a caching name server does not necessarily perform the complete
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recursive lookup itself. Instead, it can *forward* some or all of the
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queries that it cannot satisfy from its cache to another caching name
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server, commonly referred to as a *forwarder*.
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There may be one or more forwarders, and they are queried in turn until
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the list is exhausted or an answer is found. Forwarders are typically
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used when you do not wish all the servers at a given site to interact
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directly with the rest of the Internet servers. A typical scenario would
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involve a number of internal DNS servers and an Internet firewall.
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Servers unable to pass packets through the firewall would forward to the
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server that can do it, and that server would query the Internet DNS
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servers on the internal server's behalf.
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.. _multi_role:
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Name Servers in Multiple Roles
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The BIND name server can simultaneously act as a master for some zones,
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a slave for other zones, and as a caching (recursive) server for a set
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of local clients.
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However, since the functions of authoritative name service and
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caching/recursive name service are logically separate, it is often
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advantageous to run them on separate server machines. A server that only
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provides authoritative name service (an *authoritative-only* server) can
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run with recursion disabled, improving reliability and security. A
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server that is not authoritative for any zones and only provides
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recursive service to local clients (a *caching-only* server) does not
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need to be reachable from the Internet at large and can be placed inside
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a firewall.
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