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<!-- Converted by db4-upgrade version 1.0 -->
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<book xmlns:db="http://docbook.org/ns/docbook" version="5.0">
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  <info>
    <title>BIND 9 Administrator Reference Manual</title>
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    <!-- insert copyright start -->
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    <copyright>
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      <year>2000</year>
      <year>2001</year>
      <year>2002</year>
      <year>2003</year>
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      <year>2004</year>
      <year>2005</year>
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      <year>2006</year>
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      <year>2007</year>
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      <year>2008</year>
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      <year>2009</year>
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      <year>2010</year>
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      <year>2011</year>
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      <year>2012</year>
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      <year>2013</year>
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      <year>2014</year>
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      <year>2015</year>
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      <year>2016</year>
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      <holder>Internet Systems Consortium, Inc. ("ISC")</holder>
    </copyright>
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    <!-- insert copyright end -->
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    <xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="releaseinfo.xml"/>
  </info>
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  <chapter xml:id="Bv9ARM.ch01"><info><title>Introduction</title></info>
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    <para>
      The Internet Domain Name System (<acronym>DNS</acronym>)
      consists of the syntax
      to specify the names of entities in the Internet in a hierarchical
      manner, the rules used for delegating authority over names, and the
      system implementation that actually maps names to Internet
      addresses.  <acronym>DNS</acronym> data is maintained in a
      group of distributed
      hierarchical databases.
    </para>

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    <section xml:id="doc_scope"><info><title>Scope of Document</title></info>
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      <para>
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	The Berkeley Internet Name Domain
	(<acronym>BIND</acronym>) implements a
	domain name server for a number of operating systems. This
	document provides basic information about the installation and
	care of the Internet Systems Consortium (<acronym>ISC</acronym>)
	<acronym>BIND</acronym> version 9 software package for
	system administrators.
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      </para>
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      <xi:include xmlns:xi="http://www.w3.org/2001/XInclude" href="pkgversion.xml"/>
    </section>
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    <section xml:id="organization"><info><title>Organization of This Document</title></info>

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      <para>
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	In this document, <emphasis>Chapter 1</emphasis> introduces
	the basic <acronym>DNS</acronym> and <acronym>BIND</acronym> concepts. <emphasis>Chapter 2</emphasis>
	describes resource requirements for running <acronym>BIND</acronym> in various
	environments. Information in <emphasis>Chapter 3</emphasis> is
	<emphasis>task-oriented</emphasis> in its presentation and is
	organized functionally, to aid in the process of installing the
	<acronym>BIND</acronym> 9 software. The task-oriented
	section is followed by
	<emphasis>Chapter 4</emphasis>, which contains more advanced
	concepts that the system administrator may need for implementing
	certain options. <emphasis>Chapter 5</emphasis>
	describes the <acronym>BIND</acronym> 9 lightweight
	resolver.  The contents of <emphasis>Chapter 6</emphasis> are
	organized as in a reference manual to aid in the ongoing
	maintenance of the software. <emphasis>Chapter 7</emphasis> addresses
	security considerations, and
	<emphasis>Chapter 8</emphasis> contains troubleshooting help. The
	main body of the document is followed by several
	<emphasis>appendices</emphasis> which contain useful reference
	information, such as a <emphasis>bibliography</emphasis> and
	historic information related to <acronym>BIND</acronym>
	and the Domain Name
	System.
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      </para>
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    </section>
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    <section xml:id="conventions"><info><title>Conventions Used in This Document</title></info>
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      <para>
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	In this document, we use the following general typographic
	conventions:
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      </para>

      <informaltable>
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	<tgroup cols="2">
	  <colspec colname="1" colnum="1" colwidth="3.000in"/>
	  <colspec colname="2" colnum="2" colwidth="2.625in"/>
	  <tbody>
	    <row>
	      <entry colname="1">
		<para>
		  <emphasis>To describe:</emphasis>
		</para>
	      </entry>
	      <entry colname="2">
		<para>
		  <emphasis>We use the style:</emphasis>
		</para>
	      </entry>
	    </row>
	    <row>
	      <entry colname="1">
		<para>
		  a pathname, filename, URL, hostname,
		  mailing list name, or new term or concept
		</para>
	      </entry>
	      <entry colname="2">
		<para>
		  <filename>Fixed width</filename>
		</para>
	      </entry>
	    </row>
	    <row>
	      <entry colname="1">
		<para>
		  literal user
		  input
		</para>
	      </entry>
	      <entry colname="2">
		<para>
		  <userinput>Fixed Width Bold</userinput>
		</para>
	      </entry>
	    </row>
	    <row>
	      <entry colname="1">
		<para>
		  program output
		</para>
	      </entry>
	      <entry colname="2">
		<para>
		  <computeroutput>Fixed Width</computeroutput>
		</para>
	      </entry>
	    </row>
	  </tbody>
	</tgroup>
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      </informaltable>

      <para>
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	The following conventions are used in descriptions of the
	<acronym>BIND</acronym> configuration file:<informaltable colsep="0" frame="all" rowsep="0">
		  <tgroup cols="2" colsep="0" rowsep="0" tgroupstyle="2Level-table">
		      <colspec colname="1" colnum="1" colsep="0" colwidth="3.000in"/>
	    <colspec colname="2" colnum="2" colsep="0" colwidth="2.625in"/>
	    <tbody>
	      <row rowsep="0">
		<entry colname="1" colsep="1" rowsep="1">
		  <para>
		    <emphasis>To describe:</emphasis>
		  </para>
		</entry>
		<entry colname="2" rowsep="1">
		  <para>
		    <emphasis>We use the style:</emphasis>
		  </para>
		</entry>
	      </row>
	      <row rowsep="0">
		<entry colname="1" colsep="1" rowsep="1">
		  <para>
		    keywords
		  </para>
		</entry>
		<entry colname="2" rowsep="1">
		  <para>
		    <literal>Fixed Width</literal>
		  </para>
		</entry>
	      </row>
	      <row rowsep="0">
		<entry colname="1" colsep="1" rowsep="1">
		  <para>
		    variables
		  </para>
		</entry>
		<entry colname="2" rowsep="1">
		  <para>
		    <varname>Fixed Width</varname>
		  </para>
		</entry>
	      </row>
	      <row rowsep="0">
		<entry colname="1" colsep="1">
		  <para>
		    Optional input
		  </para>
		</entry>
		<entry colname="2">
		  <para>
		    <optional>Text is enclosed in square brackets</optional>
		  </para>
		</entry>
	      </row>
	    </tbody>
	  </tgroup>
	</informaltable>
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      </para>
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    </section>
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    <section xml:id="dns_overview"><info><title>The Domain Name System (<acronym>DNS</acronym>)</title></info>

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

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      <section xml:id="dns_fundamentals"><info><title>DNS Fundamentals</title></info>
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	<para>
	  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.
	</para>

	<para>
	  Clients look up information in the DNS by calling a
	  <emphasis>resolver</emphasis> library, which sends queries to one or
	  more <emphasis>name servers</emphasis> and interprets the responses.
	  The <acronym>BIND</acronym> 9 software distribution
	  contains a
	  name server, <command>named</command>, and a resolver
	  library, <command>liblwres</command>.  The older
	  <command>libbind</command> resolver library is also available
	  from ISC as a separate download.
	</para>

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	</section>
	<section xml:id="domain_names"><info><title>Domains and Domain Names</title></info>
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	<para>
	  The data stored in the DNS is identified by <emphasis>domain names</emphasis> that are organized as a tree according to
	  organizational or administrative boundaries. Each node of the tree,
	  called a <emphasis>domain</emphasis>, 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 <emphasis>root</emphasis> 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.
	</para>

	<para>
	  For example, a domain name for a host at the
	  company <emphasis>Example, Inc.</emphasis> could be
	  <literal>ourhost.example.com</literal>,
	  where <literal>com</literal> is the
	  top level domain to which
	  <literal>ourhost.example.com</literal> belongs,
	  <literal>example</literal> is
	  a subdomain of <literal>com</literal>, and
	  <literal>ourhost</literal> is the
	  name of the host.
	</para>

	<para>
	  For administrative purposes, the name space is partitioned into
	  areas called <emphasis>zones</emphasis>, 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 <emphasis>name server</emphasis>, which answers queries about the zone using the
	  <emphasis>DNS protocol</emphasis>.
	</para>

	<para>
	  The data associated with each domain name is stored in the
	  form of <emphasis>resource records</emphasis> (<acronym>RR</acronym>s).
	  Some of the supported resource record types are described in
	  <xref linkend="types_of_resource_records_and_when_to_use_them"/>.
	</para>

	<para>
	  For more detailed information about the design of the DNS and
	  the DNS protocol, please refer to the standards documents listed in
	  <xref linkend="rfcs"/>.
	</para>
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      </section>
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      <section xml:id="zones"><info><title>Zones</title></info>

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	<para>
	  To properly operate a name server, it is important to understand
	  the difference between a <emphasis>zone</emphasis>
	  and a <emphasis>domain</emphasis>.
	</para>

	<para>
	  As stated previously, a zone is a point of delegation in
	  the <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
	  <emphasis>NS records</emphasis> in the
	  parent zone, which should be matched by equivalent NS records at
	  the root of the delegated zone.
	</para>

	<para>
	  For instance, consider the <literal>example.com</literal>
	  domain which includes names
	  such as <literal>host.aaa.example.com</literal> and
	  <literal>host.bbb.example.com</literal> even though
	  the <literal>example.com</literal> zone includes
	  only delegations for the <literal>aaa.example.com</literal> and
	  <literal>bbb.example.com</literal> 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>DNS</acronym>
	  tree is a
	  <emphasis>domain</emphasis>, even if it is
	  <emphasis>terminal</emphasis>, that is, has no
	  <emphasis>subdomains</emphasis>.  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.
	</para>

	<para>
	  Though <acronym>BIND</acronym> is called a "domain name
	  server",
	  it deals primarily in terms of zones. The master and slave
	  declarations in the <filename>named.conf</filename> file
	  specify
	  zones, not domains. When you ask some other site if it is willing to
	  be a slave server for your <emphasis>domain</emphasis>, you are
	  actually asking for slave service for some collection of zones.
	</para>
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      </section>
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      <section xml:id="auth_servers"><info><title>Authoritative Name Servers</title></info>
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	<para>
	  Each zone is served by at least
	  one <emphasis>authoritative name server</emphasis>,
	  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.
	</para>

	<para>
	  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
	  <command>dig</command> (<xref linkend="diagnostic_tools"/>).
	</para>

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	<section xml:id="primary_master"><info><title>The Primary Master</title></info>
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	  <para>
	    The authoritative server where the master copy of the zone
	    data is maintained is called the
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	    <emphasis>primary master</emphasis> server, or simply the
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	    <emphasis>primary</emphasis>.  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
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	    <emphasis>zone file</emphasis> or
	    <emphasis>master file</emphasis>.
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	  </para>
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	  <para>
	    In some cases, however, the master file may not be edited
	    by humans at all, but may instead be the result of
	    <emphasis>dynamic update</emphasis> operations.
	  </para>
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	</section>
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	<section xml:id="slave_server"><info><title>Slave Servers</title></info>

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	  <para>
	    The other authoritative servers, the <emphasis>slave</emphasis>
	    servers (also known as <emphasis>secondary</emphasis> servers)
	    load
	    the zone contents from another server using a replication process
	    known as a <emphasis>zone transfer</emphasis>.  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.
	  </para>
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	</section>
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	<section xml:id="stealth_server"><info><title>Stealth Servers</title></info>
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	  <para>
	    Usually all of the zone's authoritative servers are listed in
	    NS records in the parent zone.  These NS records constitute
	    a <emphasis>delegation</emphasis> of the zone from the parent.
	    The authoritative servers are also listed in the zone file itself,
	    at the <emphasis>top level</emphasis> or <emphasis>apex</emphasis>
	    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.
	  </para>

	  <para>
	    A <emphasis>stealth server</emphasis> 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.
	  </para>

	  <para>
	    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.
	  </para>

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	</section>
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      </section>
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      <section xml:id="cache_servers"><info><title>Caching Name Servers</title></info>
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	<!--
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	  - Terminology here is inconsistent.  Probably ought to
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	  - convert to using "recursive name server" everywhere
	  - with just a note about "caching" terminology.
	  -->

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	<para>
	  The resolver libraries provided by most operating systems are
	  <emphasis>stub resolvers</emphasis>, 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 <emphasis>recursive</emphasis> name server; it performs
	  <emphasis>recursive lookups</emphasis> for local clients.
	</para>

	<para>
	  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
	  <emphasis>recursive server</emphasis> and
	  <emphasis>caching server</emphasis> are often used synonymously.
	</para>

	<para>
	  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.
	</para>

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	<section xml:id="forwarder"><info><title>Forwarding</title></info>
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	  <para>
	    Even a caching name server does not necessarily perform
	    the complete recursive lookup itself.  Instead, it can
	    <emphasis>forward</emphasis> some or all of the queries
	    that it cannot satisfy from its cache to another caching name
	    server,
	    commonly referred to as a <emphasis>forwarder</emphasis>.
	  </para>

	  <para>
	    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>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>DNS</acronym> servers
	    on the internal server's behalf.
	  </para>
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	</section>
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      </section>
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      <section xml:id="multi_role"><info><title>Name Servers in Multiple Roles</title></info>
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	<para>
	  The <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.
	</para>

	<para>
	  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 <emphasis>authoritative-only</emphasis> 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 <emphasis>caching-only</emphasis> server)
	  does not need to be reachable from the Internet at large and can
	  be placed inside a firewall.
	</para>
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      </section>
    </section>
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  </chapter>

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  <chapter xml:id="Bv9ARM.ch02"><info><title><acronym>BIND</acronym> Resource Requirements</title></info>
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    <section xml:id="hw_req"><info><title>Hardware requirements</title></info>
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      <para>
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	<acronym>DNS</acronym> hardware requirements have
	traditionally been quite modest.
	For many installations, servers that have been pensioned off from
	active duty have performed admirably as <acronym>DNS</acronym> servers.
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      </para>
      <para>
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	The DNSSEC features of <acronym>BIND</acronym> 9
	may prove to be quite
	CPU intensive however, so organizations that make heavy use of these
	features may wish to consider larger systems for these applications.
	<acronym>BIND</acronym> 9 is fully multithreaded, allowing
	full utilization of
	multiprocessor systems for installations that need it.
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      </para>
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    </section>
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    <section xml:id="cpu_req"><info><title>CPU Requirements</title></info>
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      <para>
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	CPU requirements for <acronym>BIND</acronym> 9 range from
	i486-class machines
	for serving of static zones without caching, to enterprise-class
	machines if you intend to process many dynamic updates and DNSSEC
	signed zones, serving many thousands of queries per second.
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      </para>
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    </section>
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    <section xml:id="mem_req"><info><title>Memory Requirements</title></info>
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      <para>
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	The memory of the server has to be large enough to fit the
	cache and zones loaded off disk.  The <command>max-cache-size</command>
	option can be used to limit the amount of memory used by the cache,
	at the expense of reducing cache hit rates and causing more <acronym>DNS</acronym>
	traffic.
	Additionally, if additional section caching
	(<xref linkend="acache"/>) is enabled,
	the <command>max-acache-size</command> option can be used to
	limit the amount
	of memory used by the mechanism.
	It is still good practice to have enough memory to load
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	all zone and cache data into memory — unfortunately, the best
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	way
	to determine this for a given installation is to watch the name server
	in operation. After a few weeks the server process should reach
	a relatively stable size where entries are expiring from the cache as
	fast as they are being inserted.
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      </para>
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      <!--
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	- Add something here about leaving overhead for attacks?
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	- How much overhead?  Percentage?
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	-->
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    </section>
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    <section xml:id="intensive_env"><info><title>Name Server Intensive Environment Issues</title></info>

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      <para>
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	For name server intensive environments, there are two alternative
	configurations that may be used. The first is where clients and
	any second-level internal name servers query a main name server, which
	has enough memory to build a large cache. This approach minimizes
	the bandwidth used by external name lookups. The second alternative
	is to set up second-level internal name servers to make queries
	independently.
	In this configuration, none of the individual machines needs to
	have as much memory or CPU power as in the first alternative, but
	this has the disadvantage of making many more external queries,
	as none of the name servers share their cached data.
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      </para>
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    </section>
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    <section xml:id="supported_os"><info><title>Supported Operating Systems</title></info>

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      <para>
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	ISC <acronym>BIND</acronym> 9 compiles and runs on a large
	number
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	of Unix-like operating systems and on
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	Microsoft Windows Server 2003 and 2008, and Windows XP and Vista.
	For an up-to-date
	list of supported systems, see the README file in the top level
	directory
	of the BIND 9 source distribution.
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      </para>
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    </section>
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  </chapter>

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  <chapter xml:id="Bv9ARM.ch03"><info><title>Name Server Configuration</title></info>
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    <para>
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      In this chapter we provide some suggested configurations along
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      with guidelines for their use.  We suggest reasonable values for
      certain option settings.
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    </para>

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    <section xml:id="sample_configuration"><info><title>Sample Configurations</title></info>
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      <section xml:id="cache_only_sample"><info><title>A Caching-only Name Server</title></info>

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	<para>
	  The following sample configuration is appropriate for a caching-only
	  name server for use by clients internal to a corporation.  All
	  queries
	  from outside clients are refused using the <command>allow-query</command>
	  option.  Alternatively, the same effect could be achieved using
	  suitable
	  firewall rules.
	</para>
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<programlisting>
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// Two corporate subnets we wish to allow queries from.
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acl corpnets { 192.168.4.0/24; 192.168.7.0/24; };
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options {
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     // Working directory
     directory "/etc/namedb";

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     allow-query { corpnets; };
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};
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// Provide a reverse mapping for the loopback
// address 127.0.0.1
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zone "0.0.127.in-addr.arpa" {
     type master;
     file "localhost.rev";
     notify no;
};
</programlisting>
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      </section>
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      <section xml:id="auth_only_sample"><info><title>An Authoritative-only Name Server</title></info>

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	<para>
	  This sample configuration is for an authoritative-only server
	  that is the master server for "<filename>example.com</filename>"
	  and a slave for the subdomain "<filename>eng.example.com</filename>".
	</para>
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<programlisting>
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options {
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     // Working directory
     directory "/etc/namedb";
     // Do not allow access to cache
     allow-query-cache { none; };
     // This is the default
     allow-query { any; };
     // Do not provide recursive service
     recursion no;
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};

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// Provide a reverse mapping for the loopback
// address 127.0.0.1
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zone "0.0.127.in-addr.arpa" {
     type master;
     file "localhost.rev";
     notify no;
};
// We are the master server for example.com
zone "example.com" {
     type master;
     file "example.com.db";
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     // IP addresses of slave servers allowed to
     // transfer example.com
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     allow-transfer {
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	  192.168.4.14;
	  192.168.5.53;
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     };
};
// We are a slave server for eng.example.com
zone "eng.example.com" {
     type slave;
     file "eng.example.com.bk";
     // IP address of eng.example.com master server
     masters { 192.168.4.12; };
};
</programlisting>
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      </section>
    </section>
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    <section xml:id="load_balancing"><info><title>Load Balancing</title></info>

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	- and completely pointless in the general case.
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	-->
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      <para>
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	A primitive form of load balancing can be achieved in
	the <acronym>DNS</acronym> by using multiple records
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	(such as multiple A records) for one name.
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      </para>

      <para>
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	For example, if you have three WWW servers with network addresses
	of 10.0.0.1, 10.0.0.2 and 10.0.0.3, a set of records such as the
	following means that clients will connect to each machine one third
	of the time:
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      </para>

      <informaltable colsep="0" rowsep="0">
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	<tgroup cols="5" colsep="0" rowsep="0" tgroupstyle="2Level-table">
	  <colspec colname="1" colnum="1" colsep="0" colwidth="0.875in"/>
	  <colspec colname="2" colnum="2" colsep="0" colwidth="0.500in"/>
	  <colspec colname="3" colnum="3" colsep="0" colwidth="0.750in"/>
	  <colspec colname="4" colnum="4" colsep="0" colwidth="0.750in"/>
	  <colspec colname="5" colnum="5" colsep="0" colwidth="2.028in"/>
	  <tbody>
	    <row rowsep="0">
	      <entry colname="1">
		<para>
		  Name
		</para>
	      </entry>
	      <entry colname="2">
		<para>
		  TTL
		</para>
	      </entry>
	      <entry colname="3">
		<para>
		  CLASS
		</para>
	      </entry>
	      <entry colname="4">
		<para>
		  TYPE
		</para>
	      </entry>
	      <entry colname="5">
		<para>
		  Resource Record (RR) Data
		</para>
	      </entry>
	    </row>
	    <row rowsep="0">
	      <entry colname="1">
		<para>
		  <literal>www</literal>
		</para>
	      </entry>
	      <entry colname="2">
		<para>
		  <literal>600</literal>
		</para>
	      </entry>
	      <entry colname="3">
		<para>
		  <literal>IN</literal>
		</para>
	      </entry>
	      <entry colname="4">
		<para>
		  <literal>A</literal>
		</para>
	      </entry>
	      <entry colname="5">
		<para>
		  <literal>10.0.0.1</literal>
		</para>
	      </entry>
	    </row>
	    <row rowsep="0">
	      <entry colname="1">
		<para/>
	      </entry>
	      <entry colname="2">
		<para>
		  <literal>600</literal>
		</para>
	      </entry>
	      <entry colname="3">
		<para>
		  <literal>IN</literal>
		</para>
	      </entry>
	      <entry colname="4">
		<para>
		  <literal>A</literal>
		</para>
	      </entry>
	      <entry colname="5">
		<para>
		  <literal>10.0.0.2</literal>
		</para>
	      </entry>
	    </row>
	    <row rowsep="0">
	      <entry colname="1">
		<para/>
	      </entry>
	      <entry colname="2">
		<para>
		  <literal>600</literal>
		</para>
	      </entry>
	      <entry colname="3">
		<para>
		  <literal>IN</literal>
		</para>
	      </entry>
	      <entry colname="4">
		<para>
		  <literal>A</literal>
		</para>
	      </entry>
	      <entry colname="5">
		<para>
		  <literal>10.0.0.3</literal>
		</para>
	      </entry>
	    </row>
	  </tbody>
	</tgroup>
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      </informaltable>
      <para>
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	When a resolver queries for these records, <acronym>BIND</acronym> will rotate
	them and respond to the query with the records in a different
	order.  In the example above, clients will randomly receive
	records in the order 1, 2, 3; 2, 3, 1; and 3, 1, 2. Most clients
	will use the first record returned and discard the rest.
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      </para>
      <para>
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	For more detail on ordering responses, check the
	<command>rrset-order</command> sub-statement in the
	<command>options</command> statement, see
	<xref endterm="rrset_ordering_title" linkend="rrset_ordering"/>.
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      </para>

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    </section>
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    <section xml:id="ns_operations"><info><title>Name Server Operations</title></info>
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      <section xml:id="tools"><info><title>Tools for Use With the Name Server Daemon</title></info>
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	<para>
	  This section describes several indispensable diagnostic,
	  administrative and monitoring tools available to the system
	  administrator for controlling and debugging the name server
	  daemon.
	</para>
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	<section xml:id="diagnostic_tools"><info><title>Diagnostic Tools</title></info>
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	  <para>
	    The <command>dig</command>, <command>host</command>, and
	    <command>nslookup</command> programs are all command
	    line tools
	    for manually querying name servers.  They differ in style and
	    output format.
	  </para>

	  <variablelist>
	    <varlistentry>
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	      <term xml:id="dig"><command>dig</command></term>
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	      <listitem>
		<para>
		  The domain information groper (<command>dig</command>)
		  is the most versatile and complete of these lookup tools.
		  It has two modes: simple interactive
		  mode for a single query, and batch mode which executes a
		  query for
		  each in a list of several query lines. All query options are
		  accessible
		  from the command line.
		</para>
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		<cmdsynopsis label="Usage" sepchar=" ">
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		  <command>dig</command>
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		  <arg choice="opt" rep="norepeat">@<replaceable>server</replaceable></arg>
		  <arg choice="plain" rep="norepeat"><replaceable>domain</replaceable></arg>
		  <arg choice="opt" rep="norepeat"><replaceable>query-type</replaceable></arg>
		  <arg choice="opt" rep="norepeat"><replaceable>query-class</replaceable></arg>
		  <arg choice="opt" rep="norepeat">+<replaceable>query-option</replaceable></arg>
		  <arg choice="opt" rep="norepeat">-<replaceable>dig-option</replaceable></arg>
		  <arg choice="opt" rep="norepeat">%<replaceable>comment</replaceable></arg>
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		</cmdsynopsis>
		<para>
		  The usual simple use of <command>dig</command> will take the form
		</para>
		<simpara>
		  <command>dig @server domain query-type query-class</command>
		</simpara>
		<para>
		  For more information and a list of available commands and
		  options, see the <command>dig</command> man
		  page.
		</para>
	      </listitem>
	    </varlistentry>

	    <varlistentry>
	      <term><command>host</command></term>
	      <listitem>
		<para>
		  The <command>host</command> utility emphasizes
		  simplicity
		  and ease of use.  By default, it converts
		  between host names and Internet addresses, but its
		  functionality
		  can be extended with the use of options.
		</para>
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		<cmdsynopsis label="Usage" sepchar=" ">
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		  <command>host</command>
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		  <arg choice="opt" rep="norepeat">-aCdlnrsTwv</arg>
		  <arg choice="opt" rep="norepeat">-c <replaceable>class</replaceable></arg>
		  <arg choice="opt" rep="norepeat">-N <replaceable>ndots</replaceable></arg>
		  <arg choice="opt" rep="norepeat">-t <replaceable>type</replaceable></arg>
		  <arg choice="opt" rep="norepeat">-W <replaceable>timeout</replaceable></arg>
		  <arg choice="opt" rep="norepeat">-R <replaceable>retries</replaceable></arg>
		  <arg choice="opt" rep="norepeat">-m <replaceable>flag</replaceable></arg>
		  <arg choice="opt" rep="norepeat">-4</arg>
		  <arg choice="opt" rep="norepeat">-6</arg>
		  <arg choice="plain" rep="norepeat"><replaceable>hostname</replaceable></arg>
		  <arg choice="opt" rep="norepeat"><replaceable>server</replaceable></arg>
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		</cmdsynopsis>
		<para>
		  For more information and a list of available commands and
		  options, see the <command>host</command> man
		  page.
		</para>
	      </listitem>
	    </varlistentry>

	    <varlistentry>
	      <term><command>nslookup</command></term>
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	      <listitem>
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		<para><command>nslookup</command>
		  has two modes: interactive and
		  non-interactive. Interactive mode allows the user to
		  query name servers for information about various
		  hosts and domains or to print a list of hosts in a
		  domain. Non-interactive mode is used to print just
		  the name and requested information for a host or
		  domain.
		</para>
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		<cmdsynopsis label="Usage" sepchar=" ">
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		  <command>nslookup</command>
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		  <arg rep="repeat" choice="opt">-option</arg>
		  <group choice="opt" rep="norepeat">
		    <arg choice="opt" rep="norepeat"><replaceable>host-to-find</replaceable></arg>
		    <arg choice="opt" rep="norepeat">- <arg choice="opt" rep="norepeat">server</arg></arg>
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		  </group>
		</cmdsynopsis>
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		<para>
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		  Interactive mode is entered when no arguments are given (the
		  default name server will be used) or when the first argument
		  is a
		  hyphen (`-') and the second argument is the host name or
		  Internet address
		  of a name server.
		</para>
		<para>
		  Non-interactive mode is used when the name or Internet
		  address
		  of the host to be looked up is given as the first argument.
		  The
		  optional second argument specifies the host name or address
		  of a name server.
		</para>
		<para>
		  Due to its arcane user interface and frequently inconsistent
		  behavior, we do not recommend the use of <command>nslookup</command>.
		  Use <command>dig</command> instead.
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		</para>
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	      </listitem>
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	    </varlistentry>
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	  </variablelist>
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	</section>
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	<section xml:id="admin_tools"><info><title>Administrative Tools</title></info>
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	  <para>
	    Administrative tools play an integral part in the management
	    of a server.
	  </para>
	  <variablelist>
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	    <varlistentry xml:id="named-checkconf" xreflabel="Named Configuration Checking application">
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	      <term><command>named-checkconf</command></term>
	      <listitem>
		<para>
		  The <command>named-checkconf</command> program
		  checks the syntax of a <filename>named.conf</filename> file.
		</para>
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		<cmdsynopsis label="Usage" sepchar=" ">
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		  <command>named-checkconf</command>
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		  <arg choice="opt" rep="norepeat">-jvz</arg>
		  <arg choice="opt" rep="norepeat">-t <replaceable>directory</replaceable></arg>
		  <arg choice="opt" rep="norepeat"><replaceable>filename</replaceable></arg>
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		</cmdsynopsis>
	      </listitem>
	    </varlistentry>
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	    <varlistentry xml:id="named-checkzone" xreflabel="Zone Checking application">
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	      <term><command>named-checkzone</command></term>
	      <listitem>
		<para>
		  The <command>named-checkzone</command> program
		  checks a master file for
		  syntax and consistency.
		</para>
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		<cmdsynopsis label="Usage" sepchar=" ">
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		  <command>named-checkzone</command>
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		  <arg choice="opt" rep="norepeat">-djqvD</arg>
		  <arg choice="opt" rep="norepeat">-c <replaceable>class</replaceable></arg>
		  <arg choice="opt" rep="norepeat">-o <replaceable>output</replaceable></arg>
		  <arg choice="opt" rep="norepeat">-t <replaceable>directory</replaceable></arg>
		  <arg choice="opt" rep="norepeat">-w <replaceable>directory</replaceable></arg>
		  <arg choice="opt" rep="norepeat">-k <replaceable>(ignore|warn|fail)</replaceable></arg>
		  <arg choice="opt" rep="norepeat">-n <replaceable>(ignore|warn|fail)</replaceable></arg>
		  <arg choice="opt" rep="norepeat">-W <replaceable>(ignore|warn)</replaceable></arg>
		  <arg choice="plain" rep="norepeat"><replaceable>zone</replaceable></arg>
		  <arg choice="opt" rep="norepeat"><replaceable>filename</replaceable></arg>
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		</cmdsynopsis>
	      </listitem>
	    </varlistentry>
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	    <varlistentry xml:id="named-compilezone" xreflabel="Zone Compilation application">
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	      <term><command>named-compilezone</command></term>
	      <listitem>
		<para>
		  Similar to <command>named-checkzone,</command> but
		  it always dumps the zone content to a specified file
		  (typically in a different format).
		</para>
	      </listitem>
	    </varlistentry>
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	    <varlistentry xml:id="rndc" xreflabel="Remote Name Daemon Control application">
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	      <term><command>rndc</command></term>
	      <listitem>
		<para>
		  The remote name daemon control
		  (<command>rndc</command>) program allows the
		  system
		  administrator to control the operation of a name server.
		  Since <acronym>BIND</acronym> 9.2, <command>rndc</command>
		  supports all the commands of the BIND 8 <command>ndc</command>
		  utility except <command>ndc start</command> and
		  <command>ndc restart</command>, which were also
		  not supported in <command>ndc</command>'s
		  channel mode.
		  If you run <command>rndc</command> without any
		  options
		  it will display a usage message as follows:
		</para>
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		<cmdsynopsis label="Usage" sepchar=" ">
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		  <command>rndc</command>
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		  <arg choice="opt" rep="norepeat">-c <replaceable>config</replaceable></arg>
		  <arg choice="opt" rep="norepeat">-s <replaceable>server</replaceable></arg>
		  <arg choice="opt" rep="norepeat">-p <replaceable>port</replaceable></arg>
		  <arg choice="opt" rep="norepeat">-y <replaceable>key</replaceable></arg>
		  <arg choice="plain" rep="norepeat"><replaceable>command</replaceable></arg>
		  <arg rep="repeat" choice="opt"><replaceable>command</replaceable></arg>
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		</cmdsynopsis>

		<para>See <xref linkend="man.rndc"/> for details of
		  the available <command>rndc</command> commands.
		</para>

		<para>
		  <command>rndc</command> requires a configuration file,
		  since all
		  communication with the server is authenticated with
		  digital signatures that rely on a shared secret, and
		  there is no way to provide that secret other than with a
		  configuration file.  The default location for the
		  <command>rndc</command> configuration file is
		  <filename>/etc/rndc.conf</filename>, but an
		  alternate
		  location can be specified with the <option>-c</option>
		  option.  If the configuration file is not found,
		  <command>rndc</command> will also look in
		  <filename>/etc/rndc.key</filename> (or whatever
		  <varname>sysconfdir</varname> was defined when
		  the <acronym>BIND</acronym> build was
		  configured).
		  The <filename>rndc.key</filename> file is
		  generated by
		  running <command>rndc-confgen -a</command> as
		  described in
		  <xref linkend="controls_statement_definition_and_usage"/>.
		</para>

		<para>
		  The format of the configuration file is similar to
		  that of <filename>named.conf</filename>, but
		  limited to
		  only four statements, the <command>options</command>,
		  <command>key</command>, <command>server</command> and
		  <command>include</command>
		  statements.  These statements are what associate the
		  secret keys to the servers with which they are meant to
		  be shared.  The order of statements is not
		  significant.
		</para>

		<para>
		  The <command>options</command> statement has
		  three clauses:
		  <command>default-server</command>, <command>default-key</command>,
		  and <command>default-port</command>.
		  <command>default-server</command> takes a
		  host name or address argument  and represents the server
		  that will
		  be contacted if no <option>-s</option>
		  option is provided on the command line.
		  <command>default-key</command> takes
		  the name of a key as its argument, as defined by a <command>key</command> statement.
		  <command>default-port</command> specifies the
		  port to which
		  <command>rndc</command> should connect if no
		  port is given on the command line or in a
		  <command>server</command> statement.
		</para>

		<para>
		  The <command>key</command> statement defines a
		  key to be used
		  by <command>rndc</command> when authenticating
		  with
		  <command>named</command>.  Its syntax is
		  identical to the
		  <command>key</command> statement in <filename>named.conf</filename>.
		  The keyword <userinput>key</userinput> is
		  followed by a key name, which must be a valid
		  domain name, though it need not actually be hierarchical;
		  thus,
		  a string like "<userinput>rndc_key</userinput>" is a valid
		  name.
		  The <command>key</command> statement has two
		  clauses:
		  <command>algorithm</command> and <command>secret</command>.
		  While the configuration parser will accept any string as the
		  argument
		  to algorithm, currently only the strings
		  "<userinput>hmac-md5</userinput>",
		  "<userinput>hmac-sha1</userinput>",
		  "<userinput>hmac-sha224</userinput>",
		  "<userinput>hmac-sha256</userinput>",
		  "<userinput>hmac-sha384</userinput>"
		  and "<userinput>hmac-sha512</userinput>"
		  have any meaning.  The secret is a base-64 encoded string
		  as specified in RFC 3548.
		</para>

		<para>
		  The <command>server</command> statement
		  associates a key
		  defined using the <command>key</command>
		  statement with a server.
		  The keyword <userinput>server</userinput> is followed by a
		  host name or address.  The <command>server</command> statement
		  has two clauses: <command>key</command> and <command>port</command>.
		  The <command>key</command> clause specifies the
		  name of the key
		  to be used when communicating with this server, and the
		  <command>port</command> clause can be used to
		  specify the port <command>rndc</command> should
		  connect
		  to on the server.
		</para>

		<para>
		  A sample minimal configuration file is as follows:
		</para>

<programlisting>
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key rndc_key {
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     algorithm "hmac-sha256";
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     secret
       "c3Ryb25nIGVub3VnaCBmb3IgYSBtYW4gYnV0IG1hZGUgZm9yIGEgd29tYW4K";
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};
options {
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     default-server 127.0.0.1;
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     default-key    rndc_key;
};
</programlisting>
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		<para>
		  This file, if installed as <filename>/etc/rndc.conf</filename>,
		  would allow the command:
		</para>
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		<para>
		  <prompt>$ </prompt><userinput>rndc reload</userinput>
		</para>
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		<para>
		  to connect to 127.0.0.1 port 953 and cause the name server
		  to reload, if a name server on the local machine were
		  running with
		  following controls statements:
		</para>
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<programlisting>
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controls {
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	inet 127.0.0.1
	    allow { localhost; } keys { rndc_key; };
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};
</programlisting>

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		<para>
		  and it had an identical key statement for
		  <literal>rndc_key</literal>.
		</para>

		<para>
		  Running the <command>rndc-confgen</command>
		  program will
		  conveniently create a <filename>rndc.conf</filename>
		  file for you, and also display the
		  corresponding <command>controls</command>
		  statement that you need to
		  add to <filename>named.conf</filename>.
		  Alternatively,
		  you can run <command>rndc-confgen -a</command>
		  to set up
		  a <filename>rndc.key</filename> file and not
		  modify
		  <filename>named.conf</filename> at all.
		</para>

	      </listitem>
	    </varlistentry>
	  </variablelist>

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	</section>
      </section>
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      <section xml:id="signals"><info><title>Signals</title></info>
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	<para>
	  Certain UNIX signals cause the name server to take specific
	  actions, as described in the following table.  These signals can
	  be sent using the <command>kill</command> command.
	</para>
	<informaltable frame="all">
	  <tgroup cols="2">
	    <colspec colname="1" colnum="1" colsep="0" colwidth="1.125in"/>
	    <colspec colname="2" colnum="2" colsep="0" colwidth="4.000in"/>
	    <tbody>
	      <row rowsep="0">
		<entry colname="1">
		  <para><command>SIGHUP</command></para>
		</entry>
		<entry colname="2">
		  <para>
		    Causes the server to read <filename>named.conf</filename> and
		    reload the database.
		  </para>
		</entry>
	      </row>
	      <row rowsep="0">
		<entry colname="1">
		  <para><command>SIGTERM</command></para>
		</entry>
		<entry colname="2">
		  <para>
		    Causes the server to clean up and exit.
		  </para>
		</entry>
	      </row>
	      <row rowsep="0">
		<entry colname="1">
		  <para><command>SIGINT</command></para>
		</entry>
		<entry colname="2">
		  <para>
		    Causes the server to clean up and exit.
		  </para>
		</entry>
	      </row>
	    </tbody>
	  </tgroup>
	</informaltable>
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      </section>
    </section>
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  </chapter>

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  <chapter xml:id="Bv9ARM.ch04"><info><title>Advanced DNS Features</title></info>
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    <section xml:id="notify"><info><title>Notify</title></info>
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      <para>
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	<acronym>DNS</acronym> NOTIFY is a mechanism that allows master
	servers to notify their slave servers of changes to a zone's data. In
	response to a <command>NOTIFY</command> from a master server, the
	slave will check to see that its version of the zone is the
	current version and, if not, initiate a zone transfer.
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      </para>

      <para>
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	For more information about <acronym>DNS</acronym>
	<command>NOTIFY</command>, see the description of the
	<command>notify</command> option in <xref linkend="boolean_options"/> and
	the description of the zone option <command>also-notify</command> in
	<xref linkend="zone_transfers"/>.  The <command>NOTIFY</command>
	protocol is specified in RFC 1996.
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      </para>

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      <note><simpara>
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	As a slave zone can also be a master to other slaves, <command>named</command>,
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	by default, sends <command>NOTIFY</command> messages for every zone
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	it loads.  Specifying <command>notify master-only;</command> will
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	cause <command>named</command> to only send <command>NOTIFY</command> for master
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	zones that it loads.
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      </simpara></note>
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    </section>
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    <section xml:id="dynamic_update"><info><title>Dynamic Update</title></info>
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      <para>
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	Dynamic Update is a method for adding, replacing or deleting
	records in a master server by sending it a special form of DNS
	messages.  The format and meaning of these messages is specified
	in RFC 2136.
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      </para>

      <para>
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	Dynamic update is enabled by including an
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	<command>allow-update</command> or an <command>update-policy</command>
	clause in the <command>zone</command> statement.
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      </para>
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      <para>
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	If the zone's <command>update-policy</command> is set to
	<userinput>local</userinput>, updates to the zone
	will be permitted for the key <varname>local-ddns</varname>,
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	which will be generated by <command>named</command> at startup.
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	See <xref linkend="dynamic_update_policies"/> for more details.
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      </para>

      <para>
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	Dynamic updates using Kerberos signed requests can be made
	using the TKEY/GSS protocol by setting either the
	<command>tkey-gssapi-keytab</command> option, or alternatively
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	by setting both the <command>tkey-gssapi-credential</command>
	and <command>tkey-domain</command> options. Once enabled,
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	Kerberos signed requests will be matched against the update
	policies for the zone, using the Kerberos principal as the
	signer for the request.
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      </para>

      <para>
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	Updating of secure zones (zones using DNSSEC) follows RFC
	3007: RRSIG, NSEC and NSEC3 records affected by updates are
	automatically regenerated by the server using an online
	zone key.  Update authorization is based on transaction
	signatures and an explicit server policy.
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      </para>

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      <section xml:id="journal"><info><title>The journal file</title></info>
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	<para>
	  All changes made to a zone using dynamic update are stored
	  in the zone's journal file.  This file is automatically created
	  by the server when the first dynamic update takes place.
	  The name of the journal file is formed by appending the extension
	  <filename>.jnl</filename> to the name of the
	  corresponding zone
	  file unless specifically overridden.  The journal file is in a
	  binary format and should not be edited manually.
	</para>

	<para>
	  The server will also occasionally write ("dump")
	  the complete contents of the updated zone to its zone file.
	  This is not done immediately after
	  each dynamic update, because that would be too slow when a large
	  zone is updated frequently.  Instead, the dump is delayed by
	  up to 15 minutes, allowing additional updates to take place.
	  During the dump process, transient files will be created
	  with the extensions <filename>.jnw</filename> and
	  <filename>.jbk</filename>; under ordinary circumstances, these
	  will be removed when the dump is complete, and can be safely
	  ignored.
	</para>

	<para>
	  When a server is restarted after a shutdown or crash, it will replay
	      the journal file to incorporate into the zone any updates that
	  took
	  place after the last zone dump.
	</para>

	<para>
	  Changes that result from incoming incremental zone transfers are
	  also
	  journalled in a similar way.
	</para>

	<para>
	  The zone files of dynamic zones cannot normally be edited by
	  hand because they are not guaranteed to contain the most recent
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	  dynamic changes — those are only in the journal file.
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	  The only way to ensure that the zone file of a dynamic zone
	  is up to date is to run <command>rndc stop</command>.
	</para>

	<para>
	  If you have to make changes to a dynamic zone
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	  manually, the following procedure will work:
	  Disable dynamic updates to the zone using
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	  <command>rndc freeze <replaceable>zone</replaceable></command>.
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	  This will update the zone's master file with the changes
	  stored in its <filename>.jnl</filename> file.
	  Edit the zone file.  Run
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	  <command>rndc thaw <replaceable>zone</replaceable></command>
	  to reload the changed zone and re-enable dynamic updates.
	</para>
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	<para>
	  <command>rndc sync <replaceable>zone</replaceable></command>
	  will update the zone file with changes from the journal file
	  without stopping dynamic updates; this may be useful for viewing
	  the current zone state.  To remove the <filename>.jnl</filename>
	  file after updating the zone file, use
	  <command>rndc sync -clean</command>.
	</para>

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      </section>
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    </section>
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    <section xml:id="incremental_zone_transfers"><info><title>Incremental Zone Transfers (IXFR)</title></info>
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      <para>
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	The incremental zone transfer (IXFR) protocol is a way for
	slave servers to transfer only changed data, instead of having to
	transfer the entire zone. The IXFR protocol is specified in RFC
	1995. See <xref linkend="proposed_standards"/>.
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      </para>

      <para>
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	When acting as a master, <acronym>BIND</acronym> 9
	supports IXFR for those zones
	where the necessary change history information is available. These
	include master zones maintained by dynamic update and slave zones
	whose data was obtained by IXFR.  For manually maintained master
	zones, and for slave zones obtained by performing a full zone
	transfer (AXFR), IXFR is supported only if the option
	<command>ixfr-from-differences</command> is set
	to <userinput>yes</userinput>.
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      </para>

      <para>
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	When acting as a slave, <acronym>BIND</acronym> 9 will
	attempt to use IXFR unless
	it is explicitly disabled. For more information about disabling
	IXFR, see the description of the <command>request-ixfr</command> clause
	of the <command>server</command> statement.
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      </para>
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    </section>
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    <section xml:id="split_dns"><info><title>Split DNS</title></info>

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      <para>
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	Setting up different views, or visibility, of the DNS space to
	internal and external resolvers is usually referred to as a
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	<emphasis>Split DNS</emphasis> setup. There are several
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	reasons an organization would want to set up its DNS this way.
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      </para>
      <para>
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	One common reason for setting up a DNS system this way is
	to hide "internal" DNS information from "external" clients on the
	Internet. There is some debate as to whether or not this is actually
	useful.
	Internal DNS information leaks out in many ways (via email headers,
	for example) and most savvy "attackers" can find the information
	they need using other means.
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	However, since listing addresses of internal servers that
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	external clients cannot possibly reach can result in
	connection delays and other annoyances, an organization may
	choose to use a Split DNS to present a consistent view of itself
	to the outside world.
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      </para>
      <para>
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	Another common reason for setting up a Split DNS system is
	to allow internal networks that are behind filters or in RFC 1918
	space (reserved IP space, as documented in RFC 1918) to resolve DNS
	on the Internet. Split DNS can also be used to allow mail from outside
	back in to the internal network.
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      </para>
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      <section xml:id="split_dns_sample"><info><title>Example split DNS setup</title></info>
	<para>
	  Let's say a company named <emphasis>Example, Inc.</emphasis>
	  (<literal>example.com</literal>)
	  has several corporate sites that have an internal network with
	  reserved
	  Internet Protocol (IP) space and an external demilitarized zone (DMZ),
	  or "outside" section of a network, that is available to the public.
	</para>
	<para>
	  <emphasis>Example, Inc.</emphasis> wants its internal clients
	  to be able to resolve external hostnames and to exchange mail with
	  people on the outside. The company also wants its internal resolvers
	  to have access to certain internal-only zones that are not available
	  at all outside of the internal network.
	</para>
	<para>
	  In order to accomplish this, the company will set up two sets
	  of name servers. One set will be on the inside network (in the
	  reserved
	  IP space) and the other set will be on bastion hosts, which are
	  "proxy"
	  hosts that can talk to both sides of its network, in the DMZ.
	</para>
	<para>
	  The internal servers will be configured to forward all queries,
	  except queries for <filename>site1.internal</filename>, <filename>site2.internal</filename>, <filename>site1.example.com</filename>,
	  and <filename>site2.example.com</filename>, to the servers
	  in the
	  DMZ. These internal servers will have complete sets of information
	  for <filename>site1.example.com</filename>, <filename>site2.example.com</filename>, <filename>site1.internal</filename>,
	  and <filename>site2.internal</filename>.
	</para>
	<para>
	  To protect the <filename>site1.internal</filename> and <filename>site2.internal</filename> domains,
	  the internal name servers must be configured to disallow all queries
	  to these domains from any external hosts, including the bastion
	  hosts.
	</para>
	<para>
	  The external servers, which are on the bastion hosts, will
	  be configured to serve the "public" version of the <filename>site1</filename> and <filename>site2.example.com</filename> zones.
	  This could include things such as the host records for public servers
	  (<filename>www.example.com</filename> and <filename>ftp.example.com</filename>),
	  and mail exchange (MX)  records (<filename>a.mx.example.com</filename> and <filename>b.mx.example.com</filename>).
	</para>
	<para>
	  In addition, the public <filename>site1</filename> and <filename>site2.example.com</filename> zones
	  should have special MX records that contain wildcard (`*') records
	  pointing to the bastion hosts. This is needed because external mail
	  servers do not have any other way of looking up how to deliver mail
	  to those internal hosts. With the wildcard records, the mail will
	  be delivered to the bastion host, which can then forward it on to
	  internal hosts.
	</para>
	<para>
	  Here's an example of a wildcard MX record:
	</para>
	<programlisting>*   IN MX 10 external1.example.com.</programlisting>
	<para>
	  Now that they accept mail on behalf of anything in the internal
	  network, the bastion hosts will need to know how to deliver mail
	  to internal hosts. In order for this to work properly, the resolvers
	  on
	  the bastion hosts will need to be configured to point to the internal
	  name servers for DNS resolution.
	</para>
	<para>
	  Queries for internal hostnames will be answered by the internal
	  servers, and queries for external hostnames will be forwarded back
	  out to the DNS servers on the bastion hosts.
	</para>
	<para>
	  In order for all this to work properly, internal clients will
	  need to be configured to query <emphasis>only</emphasis> the internal
	  name servers for DNS queries. This could also be enforced via
	  selective
	  filtering on the network.
	</para>
	<para>
	  If everything has been set properly, <emphasis>Example, Inc.</emphasis>'s
	  internal clients will now be able to:
	</para>
	<itemizedlist>
	  <listitem>
	    <simpara>
	      Look up any hostnames in the <literal>site1</literal>
	      and
	      <literal>site2.example.com</literal> zones.
	    </simpara>
	  </listitem>
	  <listitem>
	    <simpara>
	      Look up any hostnames in the <literal>site1.internal</literal> and
	      <literal>site2.internal</literal> domains.
	    </simpara>
	  </listitem>
	  <listitem>
	    <simpara>Look up any hostnames on the Internet.</simpara>
	  </listitem>
	  <listitem>
	    <simpara>Exchange mail with both internal and external people.</simpara>
	  </listitem>
	</itemizedlist>
	<para>
	  Hosts on the Internet will be able to:
	</para>
	<itemizedlist>
	  <listitem>
	    <simpara>
	      Look up any hostnames in the <literal>site1</literal>
	      and
	      <literal>site2.example.com</literal> zones.
	    </simpara>
	  </listitem>
	  <listitem>
	    <simpara>
	      Exchange mail with anyone in the <literal>site1</literal> and
	      <literal>site2.example.com</literal> zones.
	    </simpara>
	  </listitem>
	</itemizedlist>
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	<para>
	  Here is an example configuration for the setup we just
	  described above. Note that this is only configuration information;
	  for information on how to configure your zone files, see <xref linkend="sample_configuration"/>.
	</para>
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	<para>
	  Internal DNS server config:
	</para>
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<programlisting>
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acl internals { 172.16.72.0/24; 192.168.1.0/24; };

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acl externals { <varname>bastion-ips-go-here</varname>; };
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options {
    ...
    ...
    forward only;
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    // forward to external servers
    forwarders {
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	<varname>bastion-ips-go-here</varname>;
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    };
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    // sample allow-transfer (no one)
    allow-transfer { none; };
    // restrict query access
    allow-query { internals; externals; };
    // restrict recursion
    allow-recursion { internals; };
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    ...
    ...
};
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// sample master zone
zone "site1.example.com" {
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  type master;
  file "m/site1.example.com";
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  // do normal iterative resolution (do not forward)
  forwarders { };
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  allow-query { internals; externals; };
  allow-transfer { internals; };
};
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// sample slave zone
zone "site2.example.com" {
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  type slave;
  file "s/site2.example.com";
  masters { 172.16.72.3; };
  forwarders { };
  allow-query { internals; externals; };
  allow-transfer { internals; };
};
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zone "site1.internal" {
  type master;
  file "m/site1.internal";
  forwarders { };
  allow-query { internals; };
  allow-transfer { internals; }
};
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zone "site2.internal" {
  type slave;
  file "s/site2.internal";
  masters { 172.16.72.3; };
  forwarders { };
  allow-query { internals };
  allow-transfer { internals; }
};
</programlisting>
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	<para>
	  External (bastion host) DNS server config:
	</para>
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<programlisting>
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