825N/A - Copyright (C) 2004-2013 Internet Systems Consortium, Inc. ("ISC") 825N/A - Copyright (C) 2000-2003 Internet Software Consortium. 859N/A - Permission to use, copy, modify, and/or distribute this software for any 825N/A - purpose with or without fee is hereby granted, provided that the above 825N/A - copyright notice and this permission notice appear in all copies. 860N/A - THE SOFTWARE IS PROVIDED "AS IS" AND ISC DISCLAIMS ALL WARRANTIES WITH 860N/A - REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY 825N/A - AND FITNESS. IN NO EVENT SHALL ISC BE LIABLE FOR ANY SPECIAL, DIRECT, 825N/A - INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM 825N/A - LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE 825N/A - OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR 825N/A - PERFORMANCE OF THIS SOFTWARE. 825N/A<
meta http-
equiv="Content-Type" content="text/html; charset=ISO-8859-1">
825N/A<
title>Chapter�1.�Introduction</
title>
825N/A<
meta name="generator" content="DocBook XSL Stylesheets V1.71.1">
825N/A<
link rel="start" href="Bv9ARM.html" title="BIND 9 Administrator Reference Manual">
825N/A<
link rel="up" href="Bv9ARM.html" title="BIND 9 Administrator Reference Manual">
825N/A<
link rel="prev" href="Bv9ARM.html" title="BIND 9 Administrator Reference Manual">
825N/A<
body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
825N/A<
table width="100%" summary="Navigation header">
825N/A<
tr><
th colspan="3" align="center">Chapter�1.�Introduction</
th></
tr>
825N/A<
td width="20%" align="left">
857N/A<
th width="60%" align="center">�</
th>
825N/A<
div class="chapter" lang="en">
825N/A<
div class="titlepage"><
div><
div><
h2 class="title">
825N/A<
p><
b>Table of Contents</
b></
p>
825N/A<
dt><
span class="sect1"><
a href="Bv9ARM.ch01.html#id2564402">Organization of This Document</
a></
span></
dt>
825N/A<
dt><
span class="sect1"><
a href="Bv9ARM.ch01.html#id2564541">Conventions Used in This Document</
a></
span></
dt>
825N/A<
dt><
span class="sect1"><
a href="Bv9ARM.ch01.html#id2564791">The Domain Name System (<
acronym class="acronym">DNS</
acronym>)</
a></
span></
dt>
825N/A<
dt><
span class="sect2"><
a href="Bv9ARM.ch01.html#id2567563">Name Servers in Multiple Roles</
a></
span></
dt>
825N/A The Internet Domain Name System (<
acronym class="acronym">DNS</
acronym>)
825N/A to specify the names of entities in the Internet in a hierarchical
825N/A manner, the rules used for delegating authority over names, and the
825N/A system implementation that actually maps names to Internet
825N/A addresses. <
acronym class="acronym">DNS</
acronym> data is maintained in a
825N/A hierarchical databases.
825N/A<
div class="sect1" lang="en">
825N/A<
div class="titlepage"><
div><
div><
h2 class="title" style="clear: both">
825N/A<
a name="id2564378"></
a>Scope of Document</
h2></
div></
div></
div>
825N/A The Berkeley Internet Name Domain
825N/A (<
acronym class="acronym">BIND</
acronym>) implements a
825N/A domain name server for a number of operating systems. This
825N/A document provides basic information about the installation and
825N/A care of the Internet Systems Consortium (<
acronym class="acronym">ISC</
acronym>)
825N/A <
acronym class="acronym">BIND</
acronym> version 9 software package for
825N/A This version of the manual corresponds to BIND version 9.9.
825N/A<
div class="sect1" lang="en">
825N/A<
div class="titlepage"><
div><
div><
h2 class="title" style="clear: both">
825N/A<
a name="id2564402"></
a>Organization of This Document</
h2></
div></
div></
div>
825N/A In this document, <
span class="emphasis"><
em>Chapter 1</
em></
span> introduces
825N/A the basic <
acronym class="acronym">DNS</
acronym> and <
acronym class="acronym">BIND</
acronym> concepts. <
span class="emphasis"><
em>Chapter 2</
em></
span>
825N/A describes resource requirements for running <
acronym class="acronym">BIND</
acronym> in various
825N/A environments. Information in <
span class="emphasis"><
em>Chapter 3</
em></
span> is
825N/A <
span class="emphasis"><
em>task-oriented</
em></
span> in its presentation and is
825N/A organized functionally, to aid in the process of installing the
825N/A <
acronym class="acronym">BIND</
acronym> 9 software. The task-oriented
825N/A <
span class="emphasis"><
em>Chapter 4</
em></
span>, which contains more advanced
825N/A concepts that the system administrator may need for implementing
825N/A certain options. <
span class="emphasis"><
em>Chapter 5</
em></
span>
825N/A describes the <
acronym class="acronym">BIND</
acronym> 9 lightweight
825N/A resolver. The contents of <
span class="emphasis"><
em>Chapter 6</
em></
span> are
825N/A organized as in a reference manual to aid in the ongoing
825N/A maintenance of the software. <
span class="emphasis"><
em>Chapter 7</
em></
span> addresses
825N/A security considerations, and
825N/A <
span class="emphasis"><
em>Chapter 8</
em></
span> contains troubleshooting help. The
825N/A main body of the document is followed by several
825N/A <
span class="emphasis"><
em>appendices</
em></
span> which contain useful reference
825N/A information, such as a <
span class="emphasis"><
em>bibliography</
em></
span> and
825N/A historic information related to <
acronym class="acronym">BIND</
acronym>
825N/A<
div class="sect1" lang="en">
825N/A<
div class="titlepage"><
div><
div><
h2 class="title" style="clear: both">
825N/A<
a name="id2564541"></
a>Conventions Used in This Document</
h2></
div></
div></
div>
825N/A In this document, we use the following general typographic
825N/A<
div class="informaltable"><
table border="1">
825N/A <
span class="emphasis"><
em>To describe:</
em></
span>
825N/A <
span class="emphasis"><
em>We use the style:</
em></
span>
825N/A a pathname, filename, URL, hostname,
825N/A mailing list name, or new term or concept
825N/A <
code class="filename">Fixed width</
code>
825N/A <
strong class="userinput"><
code>Fixed Width Bold</
code></
strong>
825N/A <
code class="computeroutput">Fixed Width</
code>
825N/A The following conventions are used in descriptions of the
825N/A <
acronym class="acronym">BIND</
acronym> configuration file:</
p>
825N/A<
div class="informaltable"><
table border="1">
825N/A <
span class="emphasis"><
em>To describe:</
em></
span>
825N/A <
span class="emphasis"><
em>We use the style:</
em></
span>
825N/A <
code class="literal">Fixed Width</
code>
825N/A <
code class="varname">Fixed Width</
code>
825N/A [<
span class="optional">Text is enclosed in square brackets</
span>]
825N/A<
div class="sect1" lang="en">
825N/A<
div class="titlepage"><
div><
div><
h2 class="title" style="clear: both">
825N/A<
a name="id2564791"></
a>The Domain Name System (<
acronym class="acronym">DNS</
acronym>)</
h2></
div></
div></
div>
825N/A The purpose of this document is to explain the installation
825N/A and upkeep of the <
acronym class="acronym">BIND</
acronym> (Berkeley Internet
825N/A Name Domain) software package, and we
825N/A begin by reviewing the fundamentals of the Domain Name System
825N/A (<
acronym class="acronym">DNS</
acronym>) as they relate to <
acronym class="acronym">BIND</
acronym>.
825N/A<
div class="sect2" lang="en">
825N/A<
div class="titlepage"><
div><
div><
h3 class="title">
825N/A<
a name="id2564812"></
a>DNS Fundamentals</
h3></
div></
div></
div>
825N/A The Domain Name System (DNS) is a hierarchical, distributed
825N/A database. It stores information for mapping Internet host names to
825N/A addresses and vice versa, mail routing information, and other data
825N/A used by Internet applications.
825N/A Clients look up information in the DNS by calling a
825N/A <
span class="emphasis"><
em>resolver</
em></
span> library, which sends queries to one or
825N/A more <
span class="emphasis"><
em>name servers</
em></
span> and interprets the responses.
825N/A The <
acronym class="acronym">BIND</
acronym> 9 software distribution
825N/A name server, <
span><
strong class="command">named</
strong></
span>, and a resolver
825N/A library, <
span><
strong class="command">liblwres</
strong></
span>. The older
825N/A <
span><
strong class="command">libbind</
strong></
span> resolver library is also available
825N/A from ISC as a separate download.
825N/A<
div class="sect2" lang="en">
825N/A<
div class="titlepage"><
div><
div><
h3 class="title">
825N/A<
a name="id2564846"></
a>Domains and Domain Names</
h3></
div></
div></
div>
825N/A The data stored in the DNS is identified by <
span class="emphasis"><
em>domain names</
em></
span> that are organized as a tree according to
825N/A organizational or administrative boundaries. Each node of the tree,
825N/A called a <
span class="emphasis"><
em>domain</
em></
span>, is given a label. The domain
825N/A node is the concatenation of all the labels on the path from the
825N/A node to the <
span class="emphasis"><
em>root</
em></
span> node. This is represented
825N/A in written form as a string of labels listed from right to left and
825N/A separated by dots. A label need only be unique within its parent
825N/A For example, a domain name for a host at the
825N/A company <
span class="emphasis"><
em>Example, Inc.</
em></
span> could be
825N/A where <
code class="literal">com</
code> is the
825N/A top level domain to which
825N/A <
code class="literal">example</
code> is
825N/A a subdomain of <
code class="literal">com</
code>, and
825N/A <
code class="literal">ourhost</
code> is the
825N/A For administrative purposes, the name space is partitioned into
825N/A areas called <
span class="emphasis"><
em>zones</
em></
span>, each starting at a node and
825N/A extending down to the leaf nodes or to nodes where other zones
825N/A The data for each zone is stored in a <
span class="emphasis"><
em>name server</
em></
span>, which answers queries about the zone using the
825N/A <
span class="emphasis"><
em>DNS protocol</
em></
span>.
825N/A The data associated with each domain name is stored in the
825N/A form of <
span class="emphasis"><
em>resource records</
em></
span> (<
acronym class="acronym">RR</
acronym>s).
825N/A Some of the supported resource record types are described in
825N/A <
a href="Bv9ARM.ch06.html#types_of_resource_records_and_when_to_use_them" title="Types of Resource Records and When to Use Them">the section called “Types of Resource Records and When to Use Them”</
a>.
825N/A For more detailed information about the design of the DNS and
825N/A the DNS protocol, please refer to the standards documents listed in
825N/A <
a href="Bv9ARM.ch09.html#rfcs" title="Request for Comments (RFCs)">the section called “Request for Comments (RFCs)”</
a>.
825N/A<
div class="sect2" lang="en">
825N/A<
div class="titlepage"><
div><
div><
h3 class="title">
825N/A<
a name="id2567184"></
a>Zones</
h3></
div></
div></
div>
825N/A To properly operate a name server, it is important to understand
825N/A the difference between a <
span class="emphasis"><
em>zone</
em></
span>
825N/A and a <
span class="emphasis"><
em>domain</
em></
span>.
825N/A As stated previously, a zone is a point of delegation in
825N/A the <
acronym class="acronym">DNS</
acronym> tree. A zone consists of
825N/A those contiguous parts of the domain
825N/A tree for which a name server has complete information and over which
825N/A it has authority. It contains all domain names from a certain point
825N/A downward in the domain tree except those which are delegated to
825N/A other zones. A delegation point is marked by one or more
825N/A <
span class="emphasis"><
em>NS records</
em></
span> in the
825N/A parent zone, which should be matched by equivalent NS records at
825N/A the root of the delegated zone.
For instance, consider the <
code class="literal">
example.com</
code>
domain which includes names
the <
code class="literal">
example.com</
code> zone includes
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 <
acronym class="acronym">DNS</
acronym>
<
span class="emphasis"><
em>domain</
em></
span>, even if it is
<
span class="emphasis"><
em>terminal</
em></
span>, that is, has no
<
span class="emphasis"><
em>subdomains</
em></
span>. 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 RFCs 1033, 1034 and 1035
gain a complete understanding of this difficult and subtle
Though <
acronym class="acronym">BIND</
acronym> is called a "domain name
it deals primarily in terms of zones. The master and slave
declarations in the <
code class="filename">
named.conf</
code> file
zones, not domains. When you ask some other site if it is willing to
be a slave server for your <
span class="emphasis"><
em>domain</
em></
span>, you are
actually asking for slave service for some collection of zones.
<
div class="sect2" lang="en">
<
div class="titlepage"><
div><
div><
h3 class="title">
<
a name="id2567260"></
a>Authoritative Name Servers</
h3></
div></
div></
div>
Each zone is served by at least
one <
span class="emphasis"><
em>authoritative name server</
em></
span>,
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
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
<
span><
strong class="command">dig</
strong></
span> (<
a href="Bv9ARM.ch03.html#diagnostic_tools" title="Diagnostic Tools">the section called “Diagnostic Tools”</
a>).
<
div class="sect3" lang="en">
<
div class="titlepage"><
div><
div><
h4 class="title">
<
a name="id2567284"></
a>The Primary Master</
h4></
div></
div></
div>
The authoritative server where the master copy of the zone
data is maintained is called the
<
span class="emphasis"><
em>primary master</
em></
span> server, or simply the
<
span class="emphasis"><
em>primary</
em></
span>. 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
<
span class="emphasis"><
em>zone file</
em></
span> or
<
span class="emphasis"><
em>master file</
em></
span>.
In some cases, however, the master file may not be edited
by humans at all, but may instead be the result of
<
span class="emphasis"><
em>dynamic update</
em></
span> operations.
<
div class="sect3" lang="en">
<
div class="titlepage"><
div><
div><
h4 class="title">
<
a name="id2567382"></
a>Slave Servers</
h4></
div></
div></
div>
The other authoritative servers, the <
span class="emphasis"><
em>slave</
em></
span>
servers (also known as <
span class="emphasis"><
em>secondary</
em></
span> servers)
the zone contents from another server using a replication process
known as a <
span class="emphasis"><
em>zone transfer</
em></
span>. Typically the data
transferred directly from the primary master, but it is also
to transfer it from another slave. In other words, a slave server
may itself act as a master to a subordinate slave server.
<
div class="sect3" lang="en">
<
div class="titlepage"><
div><
div><
h4 class="title">
<
a name="id2567403"></
a>Stealth Servers</
h4></
div></
div></
div>
Usually all of the zone's authoritative servers are listed in
NS records in the parent zone. These NS records constitute
a <
span class="emphasis"><
em>delegation</
em></
span> of the zone from the parent.
The authoritative servers are also listed in the zone file itself,
at the <
span class="emphasis"><
em>top level</
em></
span> or <
span class="emphasis"><
em>apex</
em></
span>
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
A <
span class="emphasis"><
em>stealth server</
em></
span> 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
zone to speed up access to the zone's records or to make sure that
zone is available even if all the "official" servers for the zone
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
is behind a firewall and therefore unable to communicate directly
<
div class="sect2" lang="en">
<
div class="titlepage"><
div><
div><
h3 class="title">
<
a name="id2567433"></
a>Caching Name Servers</
h3></
div></
div></
div>
The resolver libraries provided by most operating systems are
<
span class="emphasis"><
em>stub resolvers</
em></
span>, meaning that they are not
performing the full DNS resolution process by themselves by talking
directly to the authoritative servers. Instead, they rely on a
name server to perform the resolution on their behalf. Such a
is called a <
span class="emphasis"><
em>recursive</
em></
span> name server; it performs
<
span class="emphasis"><
em>recursive lookups</
em></
span> 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
<
span class="emphasis"><
em>recursive server</
em></
span> and
<
span class="emphasis"><
em>caching server</
em></
span> 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.
<
div class="sect3" lang="en">
<
div class="titlepage"><
div><
div><
h4 class="title">
<
a name="id2567537"></
a>Forwarding</
h4></
div></
div></
div>
Even a caching name server does not necessarily perform
the complete recursive lookup itself. Instead, it can
<
span class="emphasis"><
em>forward</
em></
span> some or all of the queries
that it cannot satisfy from its cache to another caching name
commonly referred to as a <
span class="emphasis"><
em>forwarder</
em></
span>.
There may be one or more forwarders,
and they are queried in turn until the list is exhausted or an
is found. Forwarders are typically used when you do not
wish all the servers at a given site to interact directly with the
the Internet servers. A typical scenario would involve a number
of internal <
acronym class="acronym">DNS</
acronym> 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 <
acronym class="acronym">DNS</
acronym> servers
on the internal server's behalf.
<
div class="sect2" lang="en">
<
div class="titlepage"><
div><
div><
h3 class="title">
<
a name="id2567563"></
a>Name Servers in Multiple Roles</
h3></
div></
div></
div>
The <
acronym class="acronym">BIND</
acronym> name server can
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
often advantageous to run them on separate server machines.
A server that only provides authoritative name service
(an <
span class="emphasis"><
em>authoritative-only</
em></
span> 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 <
span class="emphasis"><
em>caching-only</
em></
span> server)
does not need to be reachable from the Internet at large and can
be placed inside a firewall.
<
table width="100%" summary="Navigation footer">
<
td width="40%" align="left">
<
td width="20%" align="center">�</
td>
<
td width="40%" align="right">�<
a accesskey="n" href="Bv9ARM.ch02.html">Next</
a>
<
td width="40%" align="left" valign="top">BIND 9 Administrator Reference Manual�</
td>
<
td width="20%" align="center"><
a accesskey="h" href="Bv9ARM.html">Home</
a></
td>
<
td width="40%" align="right" valign="top">�Chapter�2.�<
acronym class="acronym">BIND</
acronym> Resource Requirements</
td>