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5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync<title>Chapter�1.�Introduction</title>
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5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync<div class="chapter" lang="en">
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync<div class="titlepage"><div><div><h2 class="title">
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync<a name="Bv9ARM.ch01"></a>Chapter�1.�Introduction</h2></div></div></div>
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync<div class="toc">
1d77cfa1c746208a4673388517471b4bfebb572bvboxsync<p><b>Table of Contents</b></p>
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync<dl>
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync<dt><span class="sect1"><a href="Bv9ARM.ch01.html#id2563509">Scope of Document</a></span></dt>
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync<dt><span class="sect1"><a href="Bv9ARM.ch01.html#id2563533">Organization of This Document</a></span></dt>
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync<dt><span class="sect1"><a href="Bv9ARM.ch01.html#id2564629">Conventions Used in This Document</a></span></dt>
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync<dt><span class="sect1"><a href="Bv9ARM.ch01.html#id2564810">The Domain Name System (<acronym class="acronym">DNS</acronym>)</a></span></dt>
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync<dd><dl>
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync<dt><span class="sect2"><a href="Bv9ARM.ch01.html#id2564832">DNS Fundamentals</a></span></dt>
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync<dt><span class="sect2"><a href="Bv9ARM.ch01.html#id2564934">Domains and Domain Names</a></span></dt>
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync<dt><span class="sect2"><a href="Bv9ARM.ch01.html#id2567271">Zones</a></span></dt>
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync<dt><span class="sect2"><a href="Bv9ARM.ch01.html#id2567348">Authoritative Name Servers</a></span></dt>
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync<dt><span class="sect2"><a href="Bv9ARM.ch01.html#id2567589">Caching Name Servers</a></span></dt>
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync<dt><span class="sect2"><a href="Bv9ARM.ch01.html#id2567651">Name Servers in Multiple Roles</a></span></dt>
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync</dl></dd>
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync</dl>
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync</div>
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync<p>
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync The Internet Domain Name System (<acronym class="acronym">DNS</acronym>)
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync consists of the syntax
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync to specify the names of entities in the Internet in a hierarchical
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync manner, the rules used for delegating authority over names, and the
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync system implementation that actually maps names to Internet
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync addresses. <acronym class="acronym">DNS</acronym> data is maintained in a
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync group of distributed
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync hierarchical databases.
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync </p>
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync<div class="sect1" lang="en">
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync<div class="titlepage"><div><div><h2 class="title" style="clear: both">
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync<a name="id2563509"></a>Scope of Document</h2></div></div></div>
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync<p>
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync The Berkeley Internet Name Domain
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync (<acronym class="acronym">BIND</acronym>) implements a
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync domain name server for a number of operating systems. This
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync document provides basic information about the installation and
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync care of the Internet Systems Consortium (<acronym class="acronym">ISC</acronym>)
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync <acronym class="acronym">BIND</acronym> version 9 software package for
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync system administrators.
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync </p>
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync<p>This version of the manual corresponds to BIND version 9.11.</p>
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync</div>
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync<div class="sect1" lang="en">
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync<div class="titlepage"><div><div><h2 class="title" style="clear: both">
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync<a name="id2563533"></a>Organization of This Document</h2></div></div></div>
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync<p>
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync In this document, <span class="emphasis"><em>Chapter 1</em></span> introduces
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync the basic <acronym class="acronym">DNS</acronym> and <acronym class="acronym">BIND</acronym> concepts. <span class="emphasis"><em>Chapter 2</em></span>
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync describes resource requirements for running <acronym class="acronym">BIND</acronym> in various
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync environments. Information in <span class="emphasis"><em>Chapter 3</em></span> is
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync <span class="emphasis"><em>task-oriented</em></span> in its presentation and is
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync organized functionally, to aid in the process of installing the
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync <acronym class="acronym">BIND</acronym> 9 software. The task-oriented
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync section is followed by
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync <span class="emphasis"><em>Chapter 4</em></span>, which contains more advanced
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync concepts that the system administrator may need for implementing
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync certain options. <span class="emphasis"><em>Chapter 5</em></span>
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync describes the <acronym class="acronym">BIND</acronym> 9 lightweight
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync resolver. The contents of <span class="emphasis"><em>Chapter 6</em></span> are
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync organized as in a reference manual to aid in the ongoing
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync maintenance of the software. <span class="emphasis"><em>Chapter 7</em></span> addresses
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync security considerations, and
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync <span class="emphasis"><em>Chapter 8</em></span> contains troubleshooting help. The
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync main body of the document is followed by several
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync <span class="emphasis"><em>appendices</em></span> which contain useful reference
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync information, such as a <span class="emphasis"><em>bibliography</em></span> and
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync historic information related to <acronym class="acronym">BIND</acronym>
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync and the Domain Name
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync System.
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync </p>
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync</div>
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync<div class="sect1" lang="en">
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync<div class="titlepage"><div><div><h2 class="title" style="clear: both">
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync<a name="id2564629"></a>Conventions Used in This Document</h2></div></div></div>
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync<p>
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync In this document, we use the following general typographic
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync conventions:
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync </p>
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync<div class="informaltable"><table border="1">
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync<colgroup>
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync<col>
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync<col>
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync</colgroup>
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync<tbody>
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync<tr>
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync<td>
5c92c0cfa499692af1cf0da3562e21da0e95bf9fvboxsync <p>
<span class="emphasis"><em>To describe:</em></span>
</p>
</td>
<td>
<p>
<span class="emphasis"><em>We use the style:</em></span>
</p>
</td>
</tr>
<tr>
<td>
<p>
a pathname, filename, URL, hostname,
mailing list name, or new term or concept
</p>
</td>
<td>
<p>
<code class="filename">Fixed width</code>
</p>
</td>
</tr>
<tr>
<td>
<p>
literal user
input
</p>
</td>
<td>
<p>
<strong class="userinput"><code>Fixed Width Bold</code></strong>
</p>
</td>
</tr>
<tr>
<td>
<p>
program output
</p>
</td>
<td>
<p>
<code class="computeroutput">Fixed Width</code>
</p>
</td>
</tr>
</tbody>
</table></div>
<p>
The following conventions are used in descriptions of the
<acronym class="acronym">BIND</acronym> configuration file:</p>
<div class="informaltable"><table border="1">
<colgroup>
<col>
<col>
</colgroup>
<tbody>
<tr>
<td>
<p>
<span class="emphasis"><em>To describe:</em></span>
</p>
</td>
<td>
<p>
<span class="emphasis"><em>We use the style:</em></span>
</p>
</td>
</tr>
<tr>
<td>
<p>
keywords
</p>
</td>
<td>
<p>
<code class="literal">Fixed Width</code>
</p>
</td>
</tr>
<tr>
<td>
<p>
variables
</p>
</td>
<td>
<p>
<code class="varname">Fixed Width</code>
</p>
</td>
</tr>
<tr>
<td>
<p>
Optional input
</p>
</td>
<td>
<p>
[<span class="optional">Text is enclosed in square brackets</span>]
</p>
</td>
</tr>
</tbody>
</table></div>
<p>
</p>
</div>
<div class="sect1" lang="en">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="id2564810"></a>The Domain Name System (<acronym class="acronym">DNS</acronym>)</h2></div></div></div>
<p>
The purpose of this document is to explain the installation
and upkeep of the <acronym class="acronym">BIND</acronym> (Berkeley Internet
Name Domain) software package, and we
begin by reviewing the fundamentals of the Domain Name System
(<acronym class="acronym">DNS</acronym>) as they relate to <acronym class="acronym">BIND</acronym>.
</p>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="id2564832"></a>DNS Fundamentals</h3></div></div></div>
<p>
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.
</p>
<p>
Clients look up information in the DNS by calling a
<span class="emphasis"><em>resolver</em></span> library, which sends queries to one or
more <span class="emphasis"><em>name servers</em></span> and interprets the responses.
The <acronym class="acronym">BIND</acronym> 9 software distribution
contains a
name server, <span><strong class="command">named</strong></span>, and a resolver
library, <span><strong class="command">liblwres</strong></span>. The older
<span><strong class="command">libbind</strong></span> resolver library is also available
from ISC as a separate download.
</p>
</div>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="id2564934"></a>Domains and Domain Names</h3></div></div></div>
<p>
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
organizational or administrative boundaries. Each node of the tree,
called a <span class="emphasis"><em>domain</em></span>, 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 <span class="emphasis"><em>root</em></span> 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.
</p>
<p>
For example, a domain name for a host at the
company <span class="emphasis"><em>Example, Inc.</em></span> could be
<code class="literal">ourhost.example.com</code>,
where <code class="literal">com</code> is the
top level domain to which
<code class="literal">ourhost.example.com</code> belongs,
<code class="literal">example</code> is
a subdomain of <code class="literal">com</code>, and
<code class="literal">ourhost</code> is the
name of the host.
</p>
<p>
For administrative purposes, the name space is partitioned into
areas called <span class="emphasis"><em>zones</em></span>, 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 <span class="emphasis"><em>name server</em></span>, which answers queries about the zone using the
<span class="emphasis"><em>DNS protocol</em></span>.
</p>
<p>
The data associated with each domain name is stored in the
form of <span class="emphasis"><em>resource records</em></span> (<acronym class="acronym">RR</acronym>s).
Some of the supported resource record types are described in
<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 &#8220;Types of Resource Records and When to Use Them&#8221;</a>.
</p>
<p>
For more detailed information about the design of the DNS and
the DNS protocol, please refer to the standards documents listed in
<a href="Bv9ARM.ch09.html#rfcs" title="Request for Comments (RFCs)">the section called &#8220;Request for Comments (RFCs)&#8221;</a>.
</p>
</div>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="id2567271"></a>Zones</h3></div></div></div>
<p>
To properly operate a name server, it is important to understand
the difference between a <span class="emphasis"><em>zone</em></span>
and a <span class="emphasis"><em>domain</em></span>.
</p>
<p>
As stated previously, a zone is a point of delegation in
the <acronym class="acronym">DNS</acronym> 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
<span class="emphasis"><em>NS records</em></span> in the
parent zone, which should be matched by equivalent NS records at
the root of the delegated zone.
</p>
<p>
For instance, consider the <code class="literal">example.com</code>
domain which includes names
such as <code class="literal">host.aaa.example.com</code> and
<code class="literal">host.bbb.example.com</code> even though
the <code class="literal">example.com</code> zone includes
only delegations for the <code class="literal">aaa.example.com</code> and
<code class="literal">bbb.example.com</code> 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 <acronym class="acronym">DNS</acronym>
tree is a
<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
to
gain a complete understanding of this difficult and subtle
topic.
</p>
<p>
Though <acronym class="acronym">BIND</acronym> is called a "domain name
server",
it deals primarily in terms of zones. The master and slave
declarations in the <code class="filename">named.conf</code> file
specify
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.
</p>
</div>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="id2567348"></a>Authoritative Name Servers</h3></div></div></div>
<p>
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
different networks.
</p>
<p>
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 &#8220;Diagnostic Tools&#8221;</a>).
</p>
<div class="sect3" lang="en">
<div class="titlepage"><div><div><h4 class="title">
<a name="id2567371"></a>The Primary Master</h4></div></div></div>
<p>
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>.
</p>
<p>
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.
</p>
</div>
<div class="sect3" lang="en">
<div class="titlepage"><div><div><h4 class="title">
<a name="id2567401"></a>Slave Servers</h4></div></div></div>
<p>
The other authoritative servers, the <span class="emphasis"><em>slave</em></span>
servers (also known as <span class="emphasis"><em>secondary</em></span> servers)
load
the zone contents from another server using a replication process
known as a <span class="emphasis"><em>zone transfer</em></span>. 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.
</p>
</div>
<div class="sect3" lang="en">
<div class="titlepage"><div><div><h4 class="title">
<a name="id2567422"></a>Stealth Servers</h4></div></div></div>
<p>
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
the zone's top level.
</p>
<p>
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
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.
</p>
<p>
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.
</p>
</div>
</div>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="id2567589"></a>Caching Name Servers</h3></div></div></div>
<p>
The resolver libraries provided by most operating systems are
<span class="emphasis"><em>stub resolvers</em></span>, 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 <span class="emphasis"><em>recursive</em></span> name server; it performs
<span class="emphasis"><em>recursive lookups</em></span> for local clients.
</p>
<p>
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.
</p>
<p>
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.
</p>
<div class="sect3" lang="en">
<div class="titlepage"><div><div><h4 class="title">
<a name="id2567624"></a>Forwarding</h4></div></div></div>
<p>
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
server,
commonly referred to as a <span class="emphasis"><em>forwarder</em></span>.
</p>
<p>
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 <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.
</p>
</div>
</div>
<div class="sect2" lang="en">
<div class="titlepage"><div><div><h3 class="title">
<a name="id2567651"></a>Name Servers in Multiple Roles</h3></div></div></div>
<p>
The <acronym class="acronym">BIND</acronym> 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.
</p>
<p>
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 <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.
</p>
</div>
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