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<title>BIND 9 Administrator Reference Manual</title>

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<holder>Internet Systems Consortium, Inc. ("ISC")</holder>
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<holder>Internet Software Consortium</holder>

  <chapter id="ch01">
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  <title>Introduction </title>
  <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>

    <title>Scope of Document</title>

    <para>The Berkeley Internet Name Domain (<acronym>BIND</acronym>) implements an
    domain name server for a number of operating systems. This
    document provides basic information about the installation and
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    care of the Internet Software Consortium (<acronym>ISC</acronym>)
    <acronym>BIND</acronym> version 9 software package for system

    <para>This version of the manual corresponds to BIND version 9.3.</para>
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  <sect1><title>Organization of This Document</title>
    <para>In this document, <emphasis>Section 1</emphasis> introduces
    the basic <acronym>DNS</acronym> and <acronym>BIND</acronym> concepts. <emphasis>Section 2</emphasis>
    describes resource requirements for running <acronym>BIND</acronym> in various
    environments. Information in <emphasis>Section 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>Section 4</emphasis>, which contains more advanced
    concepts that the system administrator may need for implementing
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    certain options. <emphasis>Section 5</emphasis>
    describes the <acronym>BIND</acronym> 9 lightweight
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    resolver.  The contents of <emphasis>Section 6</emphasis> are
    organized as in a reference manual to aid in the ongoing
    maintenance of the software. <emphasis>Section 7
    </emphasis>addresses security considerations, and
    <emphasis>Section 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
  <sect1><title>Conventions Used in This Document</title>

    <para>In this document, we use the following general typographic

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        <tgroup cols = "2">
          <colspec colname = "1" colnum = "1" colwidth = "3.000in"/>
          <colspec colname = "2" colnum = "2" colwidth = "2.625in"/>
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              <entry colname = "1">
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              <entry colname = "2">
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<para><emphasis>We use the style:</emphasis></para></entry>
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              <entry colname = "1">
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<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>
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              <entry colname = "1"><para>literal user
              <entry colname = "2"><para><userinput>Fixed Width Bold</userinput></para></entry>
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              <entry colname = "1"><para>program output</para></entry>
              <entry colname = "2"><para><computeroutput>Fixed Width</computeroutput></para></entry>
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    <para>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"/>
            <row rowsep = "0">
              <entry colname = "1" colsep = "1" rowsep = "1"><para><emphasis>To
              <entry colname = "2" rowsep = "1"><para><emphasis>We use the style:</emphasis></para></entry>
            <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>
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            <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>
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<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>
<sect1><title>The Domain Name System (<acronym>DNS</acronym>)</title>
<para>The purpose of this document is to explain the installation
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and upkeep of the <acronym>BIND</acronym> software package, and we
begin by reviewing the fundamentals of the Domain Name System
(<acronym>DNS</acronym>) as they relate to <acronym>BIND</acronym>.

<title>DNS Fundamentals</title>

<para>The Domain Name System (DNS) is the 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.
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The <acronym>BIND</acronym> 9 software distribution contains a
name server, <command>named</command>, and two resolver
libraries, <command>liblwres</command> and <command>libbind</command>.
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<title>Domains and Domain Names</title>

<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

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

<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 we stated previously, a zone is a point of delegation in
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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
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it has authority. It contains all domain names from a certain point
downward in the domain tree except those which are delegated to
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other zones. A delegation point is marked by one or more
<emphasis>NS records</emphasis> in the
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parent zone, which should be matched by equivalent NS records at
the root of the delegated zone.</para>
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<para>For instance, consider the <literal></literal>
domain which includes names
such as <literal></literal> and
<literal></literal> even though
the <literal></literal> zone includes
only delegations for the <literal></literal> and
<literal></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

<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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<sect2><title>Authoritative Name Servers</title>

<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.

<para>Responses from authoritative servers have the "authoritative
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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>

<sect3><title>The Primary Master</title>

The authoritative server where the master copy of the zone data is maintained is
called the <emphasis>primary master</emphasis> server, or simply the
<emphasis>primary</emphasis>.  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 <emphasis>zone file</emphasis> or <emphasis>master file</emphasis>.</para>

<sect3><title>Slave Servers</title>
<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>

<sect3><title>Stealth Servers</title>

<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
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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

<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>



<title>Caching Name Servers</title>

<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
in the cache of a caching name server is controlled by the 
Time To Live (TTL) field associated with each resource record.


<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>There may be one or more forwarders,
and they are queried in turn until the list is exhausted or an answer
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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
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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. An added benefit of using the forwarding
feature is that the central machine develops a much more complete
cache of information that all the clients can take advantage
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<sect2><title>Name Servers in Multiple Roles</title>

<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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<chapter id="ch02"><title><acronym>BIND</acronym> Resource Requirements</title>

<title>Hardware requirements</title>

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<para><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.</para>
<para>The DNSSEC and IPv6 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
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multiprocessor systems for installations that need it.</para></sect1>
<sect1><title>CPU Requirements</title>
<para>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.</para></sect1>
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<sect1><title>Memory Requirements</title>
<para>The memory of the server has to be large enough to fit the
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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>
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Additionally, if additional section caching
(<xref linkend="acache"/>) is enabled, 
the <command>max-acache-size</command> can be used to limit the amount
of memory used by the mechanism.
It is still good practice to have enough memory to load
all zone and cache data into memory &mdash; unfortunately, the best way
to determine this for a given installation is to watch the name server
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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.</para></sect1>

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<sect1><title>Name Server Intensive Environment Issues</title>
<para>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
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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.
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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,
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as none of the name servers share their cached data.</para></sect1>

<sect1><title>Supported Operating Systems</title>
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<para>ISC <acronym>BIND</acronym> 9 compiles and runs on a large number
of Unix-like operating system and on Windows NT / 2000.  For an up-to-date
list of supported systems, see the README file in the top level directory
of the BIND 9 source distribution.</para>

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<chapter id="ch03">
<title>Name Server Configuration</title>
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<para>In this section we provide some suggested configurations along
with guidelines for their use. We also address the topic of reasonable
option setting.</para>
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<sect1 id="sample_configuration">
<title>Sample Configurations</title>
<title>A Caching-only Name Server</title>
<para>The following sample configuration is appropriate for a caching-only
name server for use by clients internal to a corporation.  All queries
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from outside clients are refused using the <command>allow-query</command>
option.  Alternatively, the same effect could be achieved using suitable
firewall rules.</para>

// Two corporate subnets we wish to allow queries from.
acl corpnets {;; };
options {
     directory "/etc/namedb";           // Working directory
     allow-query { corpnets; };
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// Provide a reverse mapping for the loopback address
zone "" {
     type master;
     file "localhost.rev";
     notify no;
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<title>An Authoritative-only Name Server</title>
<para>This sample configuration is for an authoritative-only server
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that is the master server for "<filename></filename>"
and a slave for the subdomain "<filename></filename>".</para>
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options {
     directory "/etc/namedb";           // Working directory
     allow-query-cache { none; };       // Do not allow access to cache
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     allow-query { any; };              // This is the default
     recursion no;                      // Do not provide recursive service
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// Provide a reverse mapping for the loopback address
zone "" {
     type master;
     file "localhost.rev";
     notify no;
// We are the master server for
zone "" {
     type master;
     file "";
     // IP addresses of slave servers allowed to transfer
     allow-transfer {
// We are a slave server for
zone "" {
     type slave;
     file "";
     // IP address of master server
     masters {; };
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<title>Load Balancing</title>

<para>A primitive form of load balancing can be achieved in
the <acronym>DNS</acronym> by using multiple A records for one name.</para>

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<para>For example, if you have three WWW servers with network addresses
of, and, a set of records such as the
following means that clients will connect to each machine one third
of the time:</para>
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<informaltable colsep = "0" rowsep = "0">
<tgroup cols = "5" colsep = "0" rowsep = "0" tgroupstyle = "2Level-table">
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<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"/>
<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 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></literal></para></entry>
<row rowsep = "0">
<entry colname = "1"><para></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></literal></para></entry>
<row rowsep = "0">
<entry colname = "1"><para></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></literal></para></entry>
    <para>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.</para>
    <para>For more detail on ordering responses, check the
    <command>rrset-order</command> substatement in the
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    <command>options</command> statement, see
    <xref endterm="rrset_ordering_title" linkend="rrset_ordering"/>.
    This substatement is not supported in
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    <acronym>BIND</acronym> 9, and only the ordering scheme described above is


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<title>Name Server Operations</title>

<title>Tools for Use With the Name Server Daemon</title>
<para>There are several indispensable diagnostic, administrative
and monitoring tools available to the system administrator for controlling
and debugging the name server daemon. We describe several in this
section </para>
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<sect3 id="diagnostic_tools">
<title>Diagnostic Tools</title>
<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.

<term id="dig"><command>dig</command></term>
<para>The domain information groper (<command>dig</command>)
is the most versatile and complete of these lookup tools.
It has two modes: simple interactive
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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">
        <arg choice="plain"><replaceable>domain</replaceable></arg>
<para>The usual simple use of dig 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>
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<para>The <command>host</command> utility emphasizes simplicity
and ease of use.  By default, it converts
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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">
        <arg>-c <replaceable>class</replaceable></arg>
        <arg>-N <replaceable>ndots</replaceable></arg>
        <arg>-t <replaceable>type</replaceable></arg>
        <arg>-W <replaceable>timeout</replaceable></arg>
        <arg>-R <replaceable>retries</replaceable></arg>
        <arg choice="plain"><replaceable>hostname</replaceable></arg>
<para>For more information and a list of available commands and
options, see the <command>host</command> man page.</para>
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<para><command>nslookup</command> has two modes: interactive
and non-interactive. Interactive mode allows the user to query name servers
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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">
        <arg rep="repeat">-option</arg>
                <arg>- <arg>server</arg></arg>
<para>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>
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<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>
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<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.</para>
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<sect3 id="admin_tools">
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        <title>Administrative Tools</title>
        <para>Administrative tools play an integral part in the management
of a server.</para>
          <varlistentry id="named-checkconf" xreflabel="Named Configuration Checking application">
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              <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">
                <arg>-t <replaceable>directory</replaceable></arg>
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          <varlistentry id="named-checkzone" xreflabel="Zone Checking application">
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              <para>The <command>named-checkzone</command> program checks a master file for
              syntax and consistency.</para>
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              <cmdsynopsis label="Usage">
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                <arg>-c <replaceable>class</replaceable></arg>
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                <arg>-o <replaceable>output</replaceable></arg>
                <arg>-t <replaceable>directory</replaceable></arg>
                <arg>-w <replaceable>directory</replaceable></arg>
                <arg>-k <replaceable>(ignore|warn|fail)</replaceable></arg>
                <arg>-n <replaceable>(ignore|warn|fail)</replaceable></arg>
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                <arg choice="plain"><replaceable>zone</replaceable></arg>
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          <varlistentry id="rndc" xreflabel="Remote Name Daemon Control application">
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              <para>The remote name daemon control
              (<command>rndc</command>) program allows the system
              administrator to control the operation of a name server.
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              If you run <command>rndc</command> without any options
              it will display a usage message as follows:</para>
              <cmdsynopsis label="Usage">
                <arg>-c <replaceable>config</replaceable></arg>
                <arg>-s <replaceable>server</replaceable></arg>
                <arg>-p <replaceable>port</replaceable></arg>
                <arg>-y <replaceable>key</replaceable></arg>
                <arg choice="plain"><replaceable>command</replaceable></arg>
                <arg rep="repeat"><replaceable>command</replaceable></arg>
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              <para><command>command</command> is one of the following:</para>


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   <listitem><para>Reload configuration file and zones.</para></listitem>

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   <varlistentry><term><userinput>reload <replaceable>zone</replaceable>
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   <listitem><para>Reload the given zone.</para></listitem>

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   <varlistentry><term><userinput>refresh <replaceable>zone</replaceable>
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   <listitem><para>Schedule zone maintenance for the given zone.</para></listitem>

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   <varlistentry><term><userinput>retransfer <replaceable>zone</replaceable>
   <listitem><para>Retransfer the given zone from the master.</para></listitem>

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   <varlistentry><term><userinput>freeze <replaceable>zone</replaceable>
   <listitem><para>Suspend updates to a dynamic zone.  This allows manual
    edits to be made to a zone normally updated by dynamic update.  It
    also causes changes in the journal file to be synced into the master
    and the journal file to be removed.  All dynamic update attempts will
    be refused while the zone is frozen.</para></listitem>

   <varlistentry><term><userinput>unfreeze <replaceable>zone</replaceable>
   <listitem><para>Enable updates to a frozen dynamic zone.  This causes
    the server to reload the zone from disk, and re-enables dynamic updates
    after the load has completed.  After a zone is unfrozen, dynamic updates
    will no longer be refused.</para></listitem>

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   <listitem><para>Reload the configuration file and load new zones,
   but do not reload existing zone files even if they have changed.
   This is faster than a full <command>reload</command> when there
   is a large number of zones because it avoids the need to examine the
   modification times of the zones files.

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   <listitem><para>Write server statistics to the statistics file.</para></listitem>

   <listitem><para>Toggle query logging. Query logging can also be enabled
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   by explicitly directing the <command>queries</command>
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   <command>category</command> to a <command>channel</command> in the
   <command>logging</command> section of

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   <listitem><para>Dump the server's caches to the dump file. </para></listitem></varlistentry>

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   <listitem><para>Stop the server,
   making sure any recent changes
   made through dynamic update or IXFR are first saved to the master files
   of the updated zones.</para></listitem></varlistentry>

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   <listitem><para>Stop the server immediately.  Recent changes
   made through dynamic update or IXFR are not saved to the master files,
   but will be rolled forward from the journal files when the server
   is restarted.</para></listitem></varlistentry>

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   <listitem><para>Increment the servers debugging level by one. </para></listitem></varlistentry>

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   <varlistentry><term><userinput>trace <replaceable>level</replaceable></userinput></term>
   <listitem><para>Sets the server's debugging level to an explicit

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   <listitem><para>Sets the server's debugging level to 0.</para></listitem></varlistentry>

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   <listitem><para>Flushes the server's cache.</para></listitem></varlistentry>

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   <listitem><para>Display status of the server.
Note the number of zones includes the internal <command>bind/CH</command> zone
and the default <command>./IN</command> hint zone if there is not a
explicit root zone configured.</para></listitem></varlistentry>


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<para>In <acronym>BIND</acronym> 9.2, <command>rndc</command>
supports all the commands of the BIND 8 <command>ndc</command>
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utility except <command>ndc start</command> and
<command>ndc restart</command>, which were also
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not supported in <command>ndc</command>'s channel mode.</para>

<para>A configuration file is required, 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
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<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
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<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
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

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<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
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host name or address argument  and represents the server that will
be contacted if no <option>-s</option>
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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.
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<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>

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<para>The <command>key</command> statement defines an 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 named.conf.
The keyword <userinput>key</userinput> is
followed by a key name, which must be a valid
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domain name, though it need not actually be hierarchical; thus,
a string like "<userinput>rndc_key</userinput>" is a valid name.
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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 string "<userinput>hmac-md5</userinput>"
has any meaning.  The secret is a base-64 encoded string.</para>

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<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>
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key rndc_key {
     algorithm "hmac-md5";
     secret "c3Ryb25nIGVub3VnaCBmb3IgYSBtYW4gYnV0IG1hZGUgZm9yIGEgd29tYW4K";
options {
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     default-key    rndc_key;

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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 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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controls {
        inet allow { localhost; } keys { rndc_key; };
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<para>and it had an identical key statement for
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<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.

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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">
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<colspec colname = "1" colnum = "1" colsep = "0" colwidth = "1.125in"/>
<colspec colname = "2" colnum = "2" colsep = "0" colwidth = "4.000in"/>
<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 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 rowsep = "0">
              <entry colname = "1">
              <entry colname = "2"><para>Causes the server to clean up and exit.</para></entry>

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<chapter id="ch04">
<title>Advanced DNS Features</title>

<sect1 id="notify">

<para><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.</para>

For more information about
<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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<sect1 id="dynamic_update">
<title>Dynamic Update</title>

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    <para>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
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    in RFC 2136.</para>

    <para>Dynamic update is enabled by
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    including an <command>allow-update</command> or
    <command>update-policy</command> clause in the
    <command>zone</command> statement.</para>

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

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    <sect2 id="journal">
    <title>The journal file</title>

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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
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    by the server when the first dynamic update takes place.
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    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
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    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
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    zone is updated frequently.  Instead, the dump is delayed by
    up to 15 minutes, allowing additional updates to take place.</para>
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    <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
    dynamic changes - those are only in the journal file.
    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>
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    <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
    <command>rndc freeze <replaceable>zone</replaceable></command>.
    This will also remove the zone's <filename>.jnl</filename> file
    and update the master file.  Edit the zone file.  Run
    <command>rndc unfreeze <replaceable>zone</replaceable></command>
    to reload the changed zone and re-enable dynamic updates.</para>

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<sect1 id="incremental_zone_transfers">
<title>Incremental Zone Transfers (IXFR)</title>

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<para>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"/>.</para>

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<para>When acting as a master, <acronym>BIND</acronym> 9
supports IXFR for those zones
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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
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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>When acting as a slave, <acronym>BIND</acronym> 9 will 
attempt to use IXFR unless
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it is explicitly disabled. For more information about disabling
IXFR, see the description of the <command>request-ixfr</command> clause
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of the <command>server</command> statement.</para>

<sect1><title>Split DNS</title>
<para>Setting up different views, or visibility, of the DNS space to
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internal and external resolvers is usually referred to as a <emphasis>Split
DNS</emphasis> setup. There are several reasons an organization
would want to set up its DNS this way.</para>
<para>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.</para>
<para>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.</para>
<para>Here is an example of a split DNS setup:</para>
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<para>Let's say a company named <emphasis>Example, Inc.</emphasis>
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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
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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></filename>,
and <filename></filename>, to the servers in the
DMZ. These internal servers will have complete sets of information
for <filename></filename>, <filename></filename>,<emphasis> </emphasis><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
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to these domains from any external hosts, including the bastion
<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></filename> zones.
This could include things such as the host records for public servers
(<filename></filename> and <filename></filename>),
and mail exchange (MX)  records (<filename></filename> and <filename></filename>).</para>
<para>In addition, the public <filename>site1</filename> and <filename></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><literal>*   IN MX 10</literal></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>
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<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
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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>
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        <simpara>Look up any hostnames in the <literal>site1</literal> and 
<literal></literal> zones.</simpara></listitem>
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        <simpara>Look up any hostnames in the <literal>site1.internal</literal> and 
<literal>site2.internal</literal> domains.</simpara></listitem>
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        <simpara>Look up any hostnames on the Internet.</simpara></listitem>
        <simpara>Exchange mail with internal AND external people.</simpara></listitem></itemizedlist>
<para>Hosts on the Internet will be able to:</para>
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        <simpara>Look up any hostnames in the <literal>site1</literal> and 
<literal></literal> zones.</simpara></listitem>
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        <simpara>Exchange mail with anyone in the <literal>site1</literal> and 
<literal></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
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<para>Internal DNS server config:</para>
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