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a82212bd36e1074408974b466798b9966bbaf49bvboxsync<tr><th colspan="3" align="center">Chapter�1.�Introduction</th></tr>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync<td width="20%" align="right">�<a accesskey="n" href="Bv9ARM.ch02.html">Next</a>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync<a name="Bv9ARM.ch01"></a>Chapter�1.�Introduction</h1></div></div></div>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync<dt><span class="section"><a href="Bv9ARM.ch01.html#doc_scope">Scope of Document</a></span></dt>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync<dt><span class="section"><a href="Bv9ARM.ch01.html#organization">Organization of This Document</a></span></dt>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync<dt><span class="section"><a href="Bv9ARM.ch01.html#conventions">Conventions Used in This Document</a></span></dt>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync<dt><span class="section"><a href="Bv9ARM.ch01.html#dns_overview">The Domain Name System (<acronym class="acronym">DNS</acronym>)</a></span></dt>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync<dt><span class="section"><a href="Bv9ARM.ch01.html#dns_fundamentals">DNS Fundamentals</a></span></dt>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync<dt><span class="section"><a href="Bv9ARM.ch01.html#domain_names">Domains and Domain Names</a></span></dt>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync<dt><span class="section"><a href="Bv9ARM.ch01.html#zones">Zones</a></span></dt>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync<dt><span class="section"><a href="Bv9ARM.ch01.html#auth_servers">Authoritative Name Servers</a></span></dt>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync<dt><span class="section"><a href="Bv9ARM.ch01.html#cache_servers">Caching Name Servers</a></span></dt>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync<dt><span class="section"><a href="Bv9ARM.ch01.html#multi_role">Name Servers in Multiple Roles</a></span></dt>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync The Internet Domain Name System (<acronym class="acronym">DNS</acronym>)
a82212bd36e1074408974b466798b9966bbaf49bvboxsync consists of the syntax
a82212bd36e1074408974b466798b9966bbaf49bvboxsync to specify the names of entities in the Internet in a hierarchical
a82212bd36e1074408974b466798b9966bbaf49bvboxsync manner, the rules used for delegating authority over names, and the
a82212bd36e1074408974b466798b9966bbaf49bvboxsync system implementation that actually maps names to Internet
a82212bd36e1074408974b466798b9966bbaf49bvboxsync addresses. <acronym class="acronym">DNS</acronym> data is maintained in a
a82212bd36e1074408974b466798b9966bbaf49bvboxsync group of distributed
a82212bd36e1074408974b466798b9966bbaf49bvboxsync hierarchical databases.
a82212bd36e1074408974b466798b9966bbaf49bvboxsync<div class="titlepage"><div><div><h2 class="title" style="clear: both">
a82212bd36e1074408974b466798b9966bbaf49bvboxsync<a name="doc_scope"></a>Scope of Document</h2></div></div></div>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync The Berkeley Internet Name Domain
a82212bd36e1074408974b466798b9966bbaf49bvboxsync (<acronym class="acronym">BIND</acronym>) implements a
a82212bd36e1074408974b466798b9966bbaf49bvboxsync domain name server for a number of operating systems. This
a82212bd36e1074408974b466798b9966bbaf49bvboxsync document provides basic information about the installation and
a82212bd36e1074408974b466798b9966bbaf49bvboxsync care of the Internet Systems Consortium (<acronym class="acronym">ISC</acronym>)
a82212bd36e1074408974b466798b9966bbaf49bvboxsync <acronym class="acronym">BIND</acronym> version 9 software package for
a82212bd36e1074408974b466798b9966bbaf49bvboxsync system administrators.
a82212bd36e1074408974b466798b9966bbaf49bvboxsync<p>This version of the manual corresponds to BIND version 9.11.</p>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync<div class="titlepage"><div><div><h2 class="title" style="clear: both">
a82212bd36e1074408974b466798b9966bbaf49bvboxsync<a name="organization"></a>Organization of This Document</h2></div></div></div>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync In this document, <span class="emphasis"><em>Chapter 1</em></span> introduces
a82212bd36e1074408974b466798b9966bbaf49bvboxsync the basic <acronym class="acronym">DNS</acronym> and <acronym class="acronym">BIND</acronym> concepts. <span class="emphasis"><em>Chapter 2</em></span>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync describes resource requirements for running <acronym class="acronym">BIND</acronym> in various
a82212bd36e1074408974b466798b9966bbaf49bvboxsync environments. Information in <span class="emphasis"><em>Chapter 3</em></span> is
a82212bd36e1074408974b466798b9966bbaf49bvboxsync <span class="emphasis"><em>task-oriented</em></span> in its presentation and is
a82212bd36e1074408974b466798b9966bbaf49bvboxsync organized functionally, to aid in the process of installing the
a82212bd36e1074408974b466798b9966bbaf49bvboxsync <acronym class="acronym">BIND</acronym> 9 software. The task-oriented
a82212bd36e1074408974b466798b9966bbaf49bvboxsync section is followed by
a82212bd36e1074408974b466798b9966bbaf49bvboxsync <span class="emphasis"><em>Chapter 4</em></span>, which contains more advanced
a82212bd36e1074408974b466798b9966bbaf49bvboxsync concepts that the system administrator may need for implementing
a82212bd36e1074408974b466798b9966bbaf49bvboxsync certain options. <span class="emphasis"><em>Chapter 5</em></span>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync describes the <acronym class="acronym">BIND</acronym> 9 lightweight
a82212bd36e1074408974b466798b9966bbaf49bvboxsync resolver. The contents of <span class="emphasis"><em>Chapter 6</em></span> are
a82212bd36e1074408974b466798b9966bbaf49bvboxsync organized as in a reference manual to aid in the ongoing
a82212bd36e1074408974b466798b9966bbaf49bvboxsync maintenance of the software. <span class="emphasis"><em>Chapter 7</em></span> addresses
a82212bd36e1074408974b466798b9966bbaf49bvboxsync security considerations, and
a82212bd36e1074408974b466798b9966bbaf49bvboxsync <span class="emphasis"><em>Chapter 8</em></span> contains troubleshooting help. The
a82212bd36e1074408974b466798b9966bbaf49bvboxsync main body of the document is followed by several
a82212bd36e1074408974b466798b9966bbaf49bvboxsync <span class="emphasis"><em>appendices</em></span> which contain useful reference
a82212bd36e1074408974b466798b9966bbaf49bvboxsync information, such as a <span class="emphasis"><em>bibliography</em></span> and
a82212bd36e1074408974b466798b9966bbaf49bvboxsync historic information related to <acronym class="acronym">BIND</acronym>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync and the Domain Name
a82212bd36e1074408974b466798b9966bbaf49bvboxsync<div class="titlepage"><div><div><h2 class="title" style="clear: both">
a82212bd36e1074408974b466798b9966bbaf49bvboxsync<a name="conventions"></a>Conventions Used in This Document</h2></div></div></div>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync In this document, we use the following general typographic
a82212bd36e1074408974b466798b9966bbaf49bvboxsync conventions:
a82212bd36e1074408974b466798b9966bbaf49bvboxsync <span class="emphasis"><em>To describe:</em></span>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync <span class="emphasis"><em>We use the style:</em></span>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync a pathname, filename, URL, hostname,
a82212bd36e1074408974b466798b9966bbaf49bvboxsync mailing list name, or new term or concept
a82212bd36e1074408974b466798b9966bbaf49bvboxsync literal user
a82212bd36e1074408974b466798b9966bbaf49bvboxsync <strong class="userinput"><code>Fixed Width Bold</code></strong>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync program output
a82212bd36e1074408974b466798b9966bbaf49bvboxsync The following conventions are used in descriptions of the
a82212bd36e1074408974b466798b9966bbaf49bvboxsync <acronym class="acronym">BIND</acronym> configuration file:</p>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync <span class="emphasis"><em>To describe:</em></span>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync <span class="emphasis"><em>We use the style:</em></span>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync Optional input
a82212bd36e1074408974b466798b9966bbaf49bvboxsync [<span class="optional">Text is enclosed in square brackets</span>]
a82212bd36e1074408974b466798b9966bbaf49bvboxsync<div class="titlepage"><div><div><h2 class="title" style="clear: both">
a82212bd36e1074408974b466798b9966bbaf49bvboxsync<a name="dns_overview"></a>The Domain Name System (<acronym class="acronym">DNS</acronym>)</h2></div></div></div>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync The purpose of this document is to explain the installation
a82212bd36e1074408974b466798b9966bbaf49bvboxsync and upkeep of the <acronym class="acronym">BIND</acronym> (Berkeley Internet
a82212bd36e1074408974b466798b9966bbaf49bvboxsync Name Domain) software package, and we
a82212bd36e1074408974b466798b9966bbaf49bvboxsync begin by reviewing the fundamentals of the Domain Name System
a82212bd36e1074408974b466798b9966bbaf49bvboxsync (<acronym class="acronym">DNS</acronym>) as they relate to <acronym class="acronym">BIND</acronym>.
a82212bd36e1074408974b466798b9966bbaf49bvboxsync<a name="dns_fundamentals"></a>DNS Fundamentals</h3></div></div></div>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync The Domain Name System (DNS) is a hierarchical, distributed
a82212bd36e1074408974b466798b9966bbaf49bvboxsync database. It stores information for mapping Internet host names to
a82212bd36e1074408974b466798b9966bbaf49bvboxsync addresses and vice versa, mail routing information, and other data
a82212bd36e1074408974b466798b9966bbaf49bvboxsync used by Internet applications.
a82212bd36e1074408974b466798b9966bbaf49bvboxsync Clients look up information in the DNS by calling a
a82212bd36e1074408974b466798b9966bbaf49bvboxsync <span class="emphasis"><em>resolver</em></span> library, which sends queries to one or
a82212bd36e1074408974b466798b9966bbaf49bvboxsync more <span class="emphasis"><em>name servers</em></span> and interprets the responses.
a82212bd36e1074408974b466798b9966bbaf49bvboxsync The <acronym class="acronym">BIND</acronym> 9 software distribution
a82212bd36e1074408974b466798b9966bbaf49bvboxsync name server, <span class="command"><strong>named</strong></span>, and a resolver
a82212bd36e1074408974b466798b9966bbaf49bvboxsync library, <span class="command"><strong>liblwres</strong></span>. The older
a82212bd36e1074408974b466798b9966bbaf49bvboxsync <span class="command"><strong>libbind</strong></span> resolver library is also available
a82212bd36e1074408974b466798b9966bbaf49bvboxsync from ISC as a separate download.
a82212bd36e1074408974b466798b9966bbaf49bvboxsync<a name="domain_names"></a>Domains and Domain Names</h3></div></div></div>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync 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
a82212bd36e1074408974b466798b9966bbaf49bvboxsync organizational or administrative boundaries. Each node of the tree,
a82212bd36e1074408974b466798b9966bbaf49bvboxsync called a <span class="emphasis"><em>domain</em></span>, is given a label. The domain
a82212bd36e1074408974b466798b9966bbaf49bvboxsync name of the
a82212bd36e1074408974b466798b9966bbaf49bvboxsync node is the concatenation of all the labels on the path from the
a82212bd36e1074408974b466798b9966bbaf49bvboxsync node to the <span class="emphasis"><em>root</em></span> node. This is represented
a82212bd36e1074408974b466798b9966bbaf49bvboxsync in written form as a string of labels listed from right to left and
a82212bd36e1074408974b466798b9966bbaf49bvboxsync separated by dots. A label need only be unique within its parent
a82212bd36e1074408974b466798b9966bbaf49bvboxsync For example, a domain name for a host at the
a82212bd36e1074408974b466798b9966bbaf49bvboxsync company <span class="emphasis"><em>Example, Inc.</em></span> could be
a82212bd36e1074408974b466798b9966bbaf49bvboxsync top level domain to which
a82212bd36e1074408974b466798b9966bbaf49bvboxsync <code class="literal">ourhost.example.com</code> belongs,
a82212bd36e1074408974b466798b9966bbaf49bvboxsync a subdomain of <code class="literal">com</code>, and
a82212bd36e1074408974b466798b9966bbaf49bvboxsync name of the host.
a82212bd36e1074408974b466798b9966bbaf49bvboxsync For administrative purposes, the name space is partitioned into
a82212bd36e1074408974b466798b9966bbaf49bvboxsync areas called <span class="emphasis"><em>zones</em></span>, each starting at a node and
a82212bd36e1074408974b466798b9966bbaf49bvboxsync extending down to the leaf nodes or to nodes where other zones
a82212bd36e1074408974b466798b9966bbaf49bvboxsync 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
a82212bd36e1074408974b466798b9966bbaf49bvboxsync <span class="emphasis"><em>DNS protocol</em></span>.
a82212bd36e1074408974b466798b9966bbaf49bvboxsync The data associated with each domain name is stored in the
a82212bd36e1074408974b466798b9966bbaf49bvboxsync form of <span class="emphasis"><em>resource records</em></span> (<acronym class="acronym">RR</acronym>s).
a82212bd36e1074408974b466798b9966bbaf49bvboxsync Some of the supported resource record types are described in
a82212bd36e1074408974b466798b9966bbaf49bvboxsync <a class="xref" 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>.
a82212bd36e1074408974b466798b9966bbaf49bvboxsync For more detailed information about the design of the DNS and
a82212bd36e1074408974b466798b9966bbaf49bvboxsync the DNS protocol, please refer to the standards documents listed in
a82212bd36e1074408974b466798b9966bbaf49bvboxsync <a class="xref" href="Bv9ARM.ch11.html#rfcs" title="Request for Comments (RFCs)">the section called “Request for Comments (RFCs)”</a>.
a82212bd36e1074408974b466798b9966bbaf49bvboxsync To properly operate a name server, it is important to understand
a82212bd36e1074408974b466798b9966bbaf49bvboxsync the difference between a <span class="emphasis"><em>zone</em></span>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync and a <span class="emphasis"><em>domain</em></span>.
a82212bd36e1074408974b466798b9966bbaf49bvboxsync As stated previously, a zone is a point of delegation in
a82212bd36e1074408974b466798b9966bbaf49bvboxsync the <acronym class="acronym">DNS</acronym> tree. A zone consists of
a82212bd36e1074408974b466798b9966bbaf49bvboxsync those contiguous parts of the domain
a82212bd36e1074408974b466798b9966bbaf49bvboxsync tree for which a name server has complete information and over which
a82212bd36e1074408974b466798b9966bbaf49bvboxsync it has authority. It contains all domain names from a certain point
a82212bd36e1074408974b466798b9966bbaf49bvboxsync downward in the domain tree except those which are delegated to
a82212bd36e1074408974b466798b9966bbaf49bvboxsync other zones. A delegation point is marked by one or more
a82212bd36e1074408974b466798b9966bbaf49bvboxsync <span class="emphasis"><em>NS records</em></span> in the
a82212bd36e1074408974b466798b9966bbaf49bvboxsync parent zone, which should be matched by equivalent NS records at
a82212bd36e1074408974b466798b9966bbaf49bvboxsync the root of the delegated zone.
a82212bd36e1074408974b466798b9966bbaf49bvboxsync For instance, consider the <code class="literal">example.com</code>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync domain which includes names
a82212bd36e1074408974b466798b9966bbaf49bvboxsync such as <code class="literal">host.aaa.example.com</code> and
a82212bd36e1074408974b466798b9966bbaf49bvboxsync <code class="literal">host.bbb.example.com</code> even though
a82212bd36e1074408974b466798b9966bbaf49bvboxsync the <code class="literal">example.com</code> zone includes
a82212bd36e1074408974b466798b9966bbaf49bvboxsync only delegations for the <code class="literal">aaa.example.com</code> and
a82212bd36e1074408974b466798b9966bbaf49bvboxsync <code class="literal">bbb.example.com</code> zones. A zone can
a82212bd36e1074408974b466798b9966bbaf49bvboxsync exactly to a single domain, but could also include only part of a
a82212bd36e1074408974b466798b9966bbaf49bvboxsync domain, the rest of which could be delegated to other
a82212bd36e1074408974b466798b9966bbaf49bvboxsync name servers. Every name in the <acronym class="acronym">DNS</acronym>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync <span class="emphasis"><em>domain</em></span>, even if it is
a82212bd36e1074408974b466798b9966bbaf49bvboxsync <span class="emphasis"><em>terminal</em></span>, that is, has no
a82212bd36e1074408974b466798b9966bbaf49bvboxsync <span class="emphasis"><em>subdomains</em></span>. Every subdomain is a domain and
a82212bd36e1074408974b466798b9966bbaf49bvboxsync every domain except the root is also a subdomain. The terminology is
a82212bd36e1074408974b466798b9966bbaf49bvboxsync not intuitive and we suggest that you read RFCs 1033, 1034 and 1035
a82212bd36e1074408974b466798b9966bbaf49bvboxsync gain a complete understanding of this difficult and subtle
a82212bd36e1074408974b466798b9966bbaf49bvboxsync Though <acronym class="acronym">BIND</acronym> is called a "domain name
a82212bd36e1074408974b466798b9966bbaf49bvboxsync it deals primarily in terms of zones. The master and slave
a82212bd36e1074408974b466798b9966bbaf49bvboxsync declarations in the <code class="filename">named.conf</code> file
a82212bd36e1074408974b466798b9966bbaf49bvboxsync zones, not domains. When you ask some other site if it is willing to
a82212bd36e1074408974b466798b9966bbaf49bvboxsync be a slave server for your <span class="emphasis"><em>domain</em></span>, you are
a82212bd36e1074408974b466798b9966bbaf49bvboxsync actually asking for slave service for some collection of zones.
a82212bd36e1074408974b466798b9966bbaf49bvboxsync<a name="auth_servers"></a>Authoritative Name Servers</h3></div></div></div>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync Each zone is served by at least
a82212bd36e1074408974b466798b9966bbaf49bvboxsync one <span class="emphasis"><em>authoritative name server</em></span>,
a82212bd36e1074408974b466798b9966bbaf49bvboxsync which contains the complete data for the zone.
a82212bd36e1074408974b466798b9966bbaf49bvboxsync To make the DNS tolerant of server and network failures,
a82212bd36e1074408974b466798b9966bbaf49bvboxsync most zones have two or more authoritative servers, on
a82212bd36e1074408974b466798b9966bbaf49bvboxsync different networks.
a82212bd36e1074408974b466798b9966bbaf49bvboxsync Responses from authoritative servers have the "authoritative
a82212bd36e1074408974b466798b9966bbaf49bvboxsync answer" (AA) bit set in the response packets. This makes them
a82212bd36e1074408974b466798b9966bbaf49bvboxsync easy to identify when debugging DNS configurations using tools like
a82212bd36e1074408974b466798b9966bbaf49bvboxsync <span class="command"><strong>dig</strong></span> (<a class="xref" href="Bv9ARM.ch03.html#diagnostic_tools" title="Diagnostic Tools">the section called “Diagnostic Tools”</a>).
a82212bd36e1074408974b466798b9966bbaf49bvboxsync<a name="primary_master"></a>The Primary Master</h4></div></div></div>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync The authoritative server where the master copy of the zone
a82212bd36e1074408974b466798b9966bbaf49bvboxsync data is maintained is called the
a82212bd36e1074408974b466798b9966bbaf49bvboxsync <span class="emphasis"><em>primary master</em></span> server, or simply the
a82212bd36e1074408974b466798b9966bbaf49bvboxsync <span class="emphasis"><em>primary</em></span>. Typically it loads the zone
a82212bd36e1074408974b466798b9966bbaf49bvboxsync contents from some local file edited by humans or perhaps
a82212bd36e1074408974b466798b9966bbaf49bvboxsync generated mechanically from some other local file which is
a82212bd36e1074408974b466798b9966bbaf49bvboxsync edited by humans. This file is called the
a82212bd36e1074408974b466798b9966bbaf49bvboxsync <span class="emphasis"><em>zone file</em></span> or
a82212bd36e1074408974b466798b9966bbaf49bvboxsync <span class="emphasis"><em>master file</em></span>.
a82212bd36e1074408974b466798b9966bbaf49bvboxsync In some cases, however, the master file may not be edited
a82212bd36e1074408974b466798b9966bbaf49bvboxsync by humans at all, but may instead be the result of
a82212bd36e1074408974b466798b9966bbaf49bvboxsync <span class="emphasis"><em>dynamic update</em></span> operations.
a82212bd36e1074408974b466798b9966bbaf49bvboxsync<a name="slave_server"></a>Slave Servers</h4></div></div></div>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync The other authoritative servers, the <span class="emphasis"><em>slave</em></span>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync servers (also known as <span class="emphasis"><em>secondary</em></span> servers)
a82212bd36e1074408974b466798b9966bbaf49bvboxsync the zone contents from another server using a replication process
a82212bd36e1074408974b466798b9966bbaf49bvboxsync known as a <span class="emphasis"><em>zone transfer</em></span>. Typically the data
a82212bd36e1074408974b466798b9966bbaf49bvboxsync transferred directly from the primary master, but it is also
a82212bd36e1074408974b466798b9966bbaf49bvboxsync to transfer it from another slave. In other words, a slave server
a82212bd36e1074408974b466798b9966bbaf49bvboxsync may itself act as a master to a subordinate slave server.
a82212bd36e1074408974b466798b9966bbaf49bvboxsync<a name="stealth_server"></a>Stealth Servers</h4></div></div></div>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync Usually all of the zone's authoritative servers are listed in
a82212bd36e1074408974b466798b9966bbaf49bvboxsync NS records in the parent zone. These NS records constitute
a82212bd36e1074408974b466798b9966bbaf49bvboxsync a <span class="emphasis"><em>delegation</em></span> of the zone from the parent.
a82212bd36e1074408974b466798b9966bbaf49bvboxsync The authoritative servers are also listed in the zone file itself,
a82212bd36e1074408974b466798b9966bbaf49bvboxsync at the <span class="emphasis"><em>top level</em></span> or <span class="emphasis"><em>apex</em></span>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync of the zone. You can list servers in the zone's top-level NS
a82212bd36e1074408974b466798b9966bbaf49bvboxsync records that are not in the parent's NS delegation, but you cannot
a82212bd36e1074408974b466798b9966bbaf49bvboxsync list servers in the parent's delegation that are not present at
a82212bd36e1074408974b466798b9966bbaf49bvboxsync the zone's top level.
a82212bd36e1074408974b466798b9966bbaf49bvboxsync A <span class="emphasis"><em>stealth server</em></span> is a server that is
a82212bd36e1074408974b466798b9966bbaf49bvboxsync authoritative for a zone but is not listed in that zone's NS
a82212bd36e1074408974b466798b9966bbaf49bvboxsync records. Stealth servers can be used for keeping a local copy of
a82212bd36e1074408974b466798b9966bbaf49bvboxsync zone to speed up access to the zone's records or to make sure that
a82212bd36e1074408974b466798b9966bbaf49bvboxsync zone is available even if all the "official" servers for the zone
a82212bd36e1074408974b466798b9966bbaf49bvboxsync inaccessible.
a82212bd36e1074408974b466798b9966bbaf49bvboxsync A configuration where the primary master server itself is a
a82212bd36e1074408974b466798b9966bbaf49bvboxsync stealth server is often referred to as a "hidden primary"
a82212bd36e1074408974b466798b9966bbaf49bvboxsync configuration. One use for this configuration is when the primary
a82212bd36e1074408974b466798b9966bbaf49bvboxsync is behind a firewall and therefore unable to communicate directly
a82212bd36e1074408974b466798b9966bbaf49bvboxsync with the outside world.
a82212bd36e1074408974b466798b9966bbaf49bvboxsync<a name="cache_servers"></a>Caching Name Servers</h3></div></div></div>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync The resolver libraries provided by most operating systems are
a82212bd36e1074408974b466798b9966bbaf49bvboxsync <span class="emphasis"><em>stub resolvers</em></span>, meaning that they are not
a82212bd36e1074408974b466798b9966bbaf49bvboxsync performing the full DNS resolution process by themselves by talking
a82212bd36e1074408974b466798b9966bbaf49bvboxsync directly to the authoritative servers. Instead, they rely on a
a82212bd36e1074408974b466798b9966bbaf49bvboxsync name server to perform the resolution on their behalf. Such a
a82212bd36e1074408974b466798b9966bbaf49bvboxsync is called a <span class="emphasis"><em>recursive</em></span> name server; it performs
a82212bd36e1074408974b466798b9966bbaf49bvboxsync <span class="emphasis"><em>recursive lookups</em></span> for local clients.
a82212bd36e1074408974b466798b9966bbaf49bvboxsync To improve performance, recursive servers cache the results of
a82212bd36e1074408974b466798b9966bbaf49bvboxsync the lookups they perform. Since the processes of recursion and
a82212bd36e1074408974b466798b9966bbaf49bvboxsync caching are intimately connected, the terms
a82212bd36e1074408974b466798b9966bbaf49bvboxsync <span class="emphasis"><em>recursive server</em></span> and
a82212bd36e1074408974b466798b9966bbaf49bvboxsync <span class="emphasis"><em>caching server</em></span> are often used synonymously.
a82212bd36e1074408974b466798b9966bbaf49bvboxsync The length of time for which a record may be retained in
a82212bd36e1074408974b466798b9966bbaf49bvboxsync the cache of a caching name server is controlled by the
a82212bd36e1074408974b466798b9966bbaf49bvboxsync Time To Live (TTL) field associated with each resource record.
a82212bd36e1074408974b466798b9966bbaf49bvboxsync<a name="forwarder"></a>Forwarding</h4></div></div></div>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync Even a caching name server does not necessarily perform
a82212bd36e1074408974b466798b9966bbaf49bvboxsync the complete recursive lookup itself. Instead, it can
a82212bd36e1074408974b466798b9966bbaf49bvboxsync <span class="emphasis"><em>forward</em></span> some or all of the queries
a82212bd36e1074408974b466798b9966bbaf49bvboxsync that it cannot satisfy from its cache to another caching name
a82212bd36e1074408974b466798b9966bbaf49bvboxsync commonly referred to as a <span class="emphasis"><em>forwarder</em></span>.
a82212bd36e1074408974b466798b9966bbaf49bvboxsync There may be one or more forwarders,
a82212bd36e1074408974b466798b9966bbaf49bvboxsync and they are queried in turn until the list is exhausted or an
a82212bd36e1074408974b466798b9966bbaf49bvboxsync is found. Forwarders are typically used when you do not
a82212bd36e1074408974b466798b9966bbaf49bvboxsync wish all the servers at a given site to interact directly with the
a82212bd36e1074408974b466798b9966bbaf49bvboxsync the Internet servers. A typical scenario would involve a number
a82212bd36e1074408974b466798b9966bbaf49bvboxsync of internal <acronym class="acronym">DNS</acronym> servers and an
a82212bd36e1074408974b466798b9966bbaf49bvboxsync Internet firewall. Servers unable
a82212bd36e1074408974b466798b9966bbaf49bvboxsync to pass packets through the firewall would forward to the server
a82212bd36e1074408974b466798b9966bbaf49bvboxsync that can do it, and that server would query the Internet <acronym class="acronym">DNS</acronym> servers
a82212bd36e1074408974b466798b9966bbaf49bvboxsync on the internal server's behalf.
a82212bd36e1074408974b466798b9966bbaf49bvboxsync<a name="multi_role"></a>Name Servers in Multiple Roles</h3></div></div></div>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync The <acronym class="acronym">BIND</acronym> name server can
a82212bd36e1074408974b466798b9966bbaf49bvboxsync simultaneously act as
a82212bd36e1074408974b466798b9966bbaf49bvboxsync a master for some zones, a slave for other zones, and as a caching
a82212bd36e1074408974b466798b9966bbaf49bvboxsync (recursive) server for a set of local clients.
a82212bd36e1074408974b466798b9966bbaf49bvboxsync However, since the functions of authoritative name service
a82212bd36e1074408974b466798b9966bbaf49bvboxsync and caching/recursive name service are logically separate, it is
a82212bd36e1074408974b466798b9966bbaf49bvboxsync often advantageous to run them on separate server machines.
a82212bd36e1074408974b466798b9966bbaf49bvboxsync A server that only provides authoritative name service
a82212bd36e1074408974b466798b9966bbaf49bvboxsync (an <span class="emphasis"><em>authoritative-only</em></span> server) can run with
a82212bd36e1074408974b466798b9966bbaf49bvboxsync recursion disabled, improving reliability and security.
a82212bd36e1074408974b466798b9966bbaf49bvboxsync A server that is not authoritative for any zones and only provides
a82212bd36e1074408974b466798b9966bbaf49bvboxsync recursive service to local
a82212bd36e1074408974b466798b9966bbaf49bvboxsync clients (a <span class="emphasis"><em>caching-only</em></span> server)
a82212bd36e1074408974b466798b9966bbaf49bvboxsync does not need to be reachable from the Internet at large and can
a82212bd36e1074408974b466798b9966bbaf49bvboxsync be placed inside a firewall.
a82212bd36e1074408974b466798b9966bbaf49bvboxsync<td width="40%" align="right">�<a accesskey="n" href="Bv9ARM.ch02.html">Next</a>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync<td width="40%" align="left" valign="top">BIND 9 Administrator Reference Manual�</td>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync<td width="20%" align="center"><a accesskey="h" href="Bv9ARM.html">Home</a></td>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync<td width="40%" align="right" valign="top">�Chapter�2.�<acronym class="acronym">BIND</acronym> Resource Requirements</td>
a82212bd36e1074408974b466798b9966bbaf49bvboxsync<p xmlns:db="http://docbook.org/ns/docbook" style="text-align: center;">BIND 9.11.0b3</p>