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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<HTML
><HEAD
><TITLE
>Introduction </TITLE
><META
NAME="GENERATOR"
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><BODY
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><DIV
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SUMMARY="Header navigation table"
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><TR
><TH
COLSPAN="3"
ALIGN="center"
>BIND 9 Administrator Reference Manual</TH
></TR
><TR
><TD
WIDTH="10%"
ALIGN="left"
VALIGN="bottom"
><A
HREF="Bv9ARM.html"
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>Prev</A
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></TD
></TR
></TABLE
><HR
ALIGN="LEFT"
WIDTH="100%"></DIV
><DIV
CLASS="chapter"
><H1
><A
NAME="ch01"
></A
>Chapter 1. Introduction </H1
><DIV
CLASS="TOC"
><DL
><DT
><B
>Table of Contents</B
></DT
><DT
>1.1. <A
HREF="Bv9ARM.ch01.html#AEN15"
>Scope of Document</A
></DT
><DT
>1.2. <A
HREF="Bv9ARM.ch01.html#AEN22"
>Organization of This Document</A
></DT
><DT
>1.3. <A
HREF="Bv9ARM.ch01.html#AEN42"
>Conventions Used in This Document</A
></DT
><DT
>1.4. <A
HREF="Bv9ARM.ch01.html#AEN107"
>The Domain Name System (<ACRONYM
CLASS="acronym"
>DNS</ACRONYM
>)</A
></DT
></DL
></DIV
><P
>The Internet Domain Name System (<ACRONYM
CLASS="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
CLASS="acronym"
>DNS</ACRONYM
> data is maintained in a group of distributed
hierarchical databases.</P
><DIV
CLASS="sect1"
><H1
CLASS="sect1"
><A
NAME="AEN15"
>1.1. Scope of Document</A
></H1
><P
>The Berkeley Internet Name Domain (<ACRONYM
CLASS="acronym"
>BIND</ACRONYM
>) implements an
domain name server for a number of operating systems. This
document provides basic information about the installation and
care of the Internet Software Consortium (<ACRONYM
CLASS="acronym"
>ISC</ACRONYM
>)
<ACRONYM
CLASS="acronym"
>BIND</ACRONYM
> version 9 software package for system
administrators.</P
><P
>This version of the manual corresponds to BIND version 9.3.</P
></DIV
><DIV
CLASS="sect1"
><H1
CLASS="sect1"
><A
NAME="AEN22"
>1.2. Organization of This Document</A
></H1
><P
>In this document, <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>Section 1</I
></SPAN
> introduces
the basic <ACRONYM
CLASS="acronym"
>DNS</ACRONYM
> and <ACRONYM
CLASS="acronym"
>BIND</ACRONYM
> concepts. <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>Section 2</I
></SPAN
>
describes resource requirements for running <ACRONYM
CLASS="acronym"
>BIND</ACRONYM
> in various
environments. Information in <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>Section 3</I
></SPAN
> is
<SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>task-oriented</I
></SPAN
> in its presentation and is
organized functionally, to aid in the process of installing the
<ACRONYM
CLASS="acronym"
>BIND</ACRONYM
> 9 software. The task-oriented section is followed by
<SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>Section 4</I
></SPAN
>, which contains more advanced
concepts that the system administrator may need for implementing
certain options. <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>Section 5</I
></SPAN
>
describes the <ACRONYM
CLASS="acronym"
>BIND</ACRONYM
> 9 lightweight
resolver. The contents of <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>Section 6</I
></SPAN
> are
organized as in a reference manual to aid in the ongoing
maintenance of the software. <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>Section 7
</I
></SPAN
>addresses security considerations, and
<SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>Section 8</I
></SPAN
> contains troubleshooting help. The
main body of the document is followed by several
<SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>Appendices</I
></SPAN
> which contain useful reference
information, such as a <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>Bibliography</I
></SPAN
> and
historic information related to <ACRONYM
CLASS="acronym"
>BIND</ACRONYM
> and the Domain Name
System.</P
></DIV
><DIV
CLASS="sect1"
><H1
CLASS="sect1"
><A
NAME="AEN42"
>1.3. Conventions Used in This Document</A
></H1
><P
>In this document, we use the following general typographic
conventions:</P
><DIV
CLASS="informaltable"
><P
></P
><A
NAME="AEN45"
></A
><TABLE
CELLPADDING="3"
BORDER="1"
CLASS="CALSTABLE"
><TBODY
><TR
><TD
> <P
><SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>To
describe:</I
></SPAN
></P
></TD
><TD
> <P
><SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>We use the style:</I
></SPAN
></P
></TD
></TR
><TR
><TD
> <P
>a pathname, filename, URL, hostname,
mailing list name, or new term or concept</P
></TD
><TD
><P
><TT
CLASS="filename"
>Fixed width</TT
></P
></TD
></TR
><TR
><TD
><P
>literal user
input</P
></TD
><TD
><P
><KBD
CLASS="userinput"
>Fixed Width Bold</KBD
></P
></TD
></TR
><TR
><TD
><P
>program output</P
></TD
><TD
><P
><SAMP
CLASS="computeroutput"
>Fixed Width</SAMP
></P
></TD
></TR
></TBODY
></TABLE
><P
></P
></DIV
><P
>The following conventions are used in descriptions of the
<ACRONYM
CLASS="acronym"
>BIND</ACRONYM
> configuration file:<DIV
CLASS="informaltable"
><P
></P
><A
NAME="AEN77"
></A
><TABLE
CELLPADDING="3"
BORDER="1"
CLASS="CALSTABLE"
><TBODY
><TR
><TD
><P
><SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>To
describe:</I
></SPAN
></P
></TD
><TD
><P
><SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>We use the style:</I
></SPAN
></P
></TD
></TR
><TR
><TD
><P
>keywords</P
></TD
><TD
><P
><VAR
CLASS="literal"
>Fixed Width</VAR
></P
></TD
></TR
><TR
><TD
><P
>variables</P
></TD
><TD
><P
><VAR
CLASS="varname"
>Fixed Width</VAR
></P
></TD
></TR
><TR
><TD
><P
>Optional input</P
></TD
><TD
><P
>[<SPAN
CLASS="optional"
>Text is enclosed in square brackets</SPAN
>]</P
></TD
></TR
></TBODY
></TABLE
><P
></P
></DIV
></P
></DIV
><DIV
CLASS="sect1"
><H1
CLASS="sect1"
><A
NAME="AEN107"
>1.4. The Domain Name System (<ACRONYM
CLASS="acronym"
>DNS</ACRONYM
>)</A
></H1
><P
>The purpose of this document is to explain the installation
and upkeep of the <ACRONYM
CLASS="acronym"
>BIND</ACRONYM
> software package, and we
begin by reviewing the fundamentals of the Domain Name System
(<ACRONYM
CLASS="acronym"
>DNS</ACRONYM
>) as they relate to <ACRONYM
CLASS="acronym"
>BIND</ACRONYM
>.
</P
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="AEN114"
>1.4.1. DNS Fundamentals</A
></H2
><P
>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.</P
><P
>Clients look up information in the DNS by calling a
<SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>resolver</I
></SPAN
> library, which sends queries to one or
more <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>name servers</I
></SPAN
> and interprets the responses.
The <ACRONYM
CLASS="acronym"
>BIND</ACRONYM
> 9 software distribution contains a
name server, <B
CLASS="command"
>named</B
>, and two resolver
libraries, <B
CLASS="command"
>liblwres</B
> and <B
CLASS="command"
>libbind</B
>.
</P
></DIV
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="AEN124"
>1.4.2. Domains and Domain Names</A
></H2
><P
>The data stored in the DNS is identified by <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>domain
names</I
></SPAN
> that are organized as a tree according to
organizational or administrative boundaries. Each node of the tree,
called a <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>domain</I
></SPAN
>, is given a label. The domain name of the
node is the concatenation of all the labels on the path from the
node to the <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>root</I
></SPAN
> node. This is represented
in written form as a string of labels listed from right to left and
separated by dots. A label need only be unique within its parent
domain.</P
><P
>For example, a domain name for a host at the
company <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>Example, Inc.</I
></SPAN
> could be
<VAR
CLASS="literal"
>mail.example.com</VAR
>,
where <VAR
CLASS="literal"
>com</VAR
> is the
top level domain to which
<VAR
CLASS="literal"
>ourhost.example.com</VAR
> belongs,
<VAR
CLASS="literal"
>example</VAR
> is
a subdomain of <VAR
CLASS="literal"
>com</VAR
>, and
<VAR
CLASS="literal"
>ourhost</VAR
> is the
name of the host.</P
><P
>For administrative purposes, the name space is partitioned into
areas called <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>zones</I
></SPAN
>, each starting at a node and
extending down to the leaf nodes or to nodes where other zones start.
The data for each zone is stored in a <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>name
server</I
></SPAN
>, which answers queries about the zone using the
<SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>DNS protocol</I
></SPAN
>.
</P
><P
>The data associated with each domain name is stored in the
form of <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>resource records</I
></SPAN
> (<ACRONYM
CLASS="acronym"
>RR</ACRONYM
>s).
Some of the supported resource record types are described in
<A
HREF="Bv9ARM.ch06.html#types_of_resource_records_and_when_to_use_them"
>Section 6.3.1</A
>.</P
><P
>For more detailed information about the design of the DNS and
the DNS protocol, please refer to the standards documents listed in
<A
HREF="Bv9ARM.ch09.html#rfcs"
>Section A.3.1</A
>.</P
></DIV
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="AEN148"
>1.4.3. Zones</A
></H2
><P
>To properly operate a name server, it is important to understand
the difference between a <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>zone</I
></SPAN
>
and a <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>domain</I
></SPAN
>.</P
><P
>As we stated previously, a zone is a point of delegation in
the <ACRONYM
CLASS="acronym"
>DNS</ACRONYM
> tree. A zone consists of
those contiguous parts of the domain
tree for which a name server has complete information and over which
it has authority. It contains all domain names from a certain point
downward in the domain tree except those which are delegated to
other zones. A delegation point is marked by one or more
<SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>NS records</I
></SPAN
> in the
parent zone, which should be matched by equivalent NS records at
the root of the delegated zone.</P
><P
>For instance, consider the <VAR
CLASS="literal"
>example.com</VAR
>
domain which includes names
such as <VAR
CLASS="literal"
>host.aaa.example.com</VAR
> and
<VAR
CLASS="literal"
>host.bbb.example.com</VAR
> even though
the <VAR
CLASS="literal"
>example.com</VAR
> zone includes
only delegations for the <VAR
CLASS="literal"
>aaa.example.com</VAR
> and
<VAR
CLASS="literal"
>bbb.example.com</VAR
> zones. A zone can map
exactly to a single domain, but could also include only part of a
domain, the rest of which could be delegated to other
name servers. Every name in the <ACRONYM
CLASS="acronym"
>DNS</ACRONYM
> tree is a
<SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>domain</I
></SPAN
>, even if it is
<SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>terminal</I
></SPAN
>, that is, has no
<SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>subdomains</I
></SPAN
>. Every subdomain is a domain and
every domain except the root is also a subdomain. The terminology is
not intuitive and we suggest that you read RFCs 1033, 1034 and 1035 to
gain a complete understanding of this difficult and subtle
topic.</P
><P
>Though <ACRONYM
CLASS="acronym"
>BIND</ACRONYM
> is called a "domain name server",
it deals primarily in terms of zones. The master and slave
declarations in the <TT
CLASS="filename"
>named.conf</TT
> file specify
zones, not domains. When you ask some other site if it is willing to
be a slave server for your <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>domain</I
></SPAN
>, you are
actually asking for slave service for some collection of zones.</P
></DIV
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="AEN171"
>1.4.4. Authoritative Name Servers</A
></H2
><P
>Each zone is served by at least
one <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>authoritative name server</I
></SPAN
>,
which contains the complete data for the zone.
To make the DNS tolerant of server and network failures,
most zones have two or more authoritative servers.
</P
><P
>Responses from authoritative servers have the "authoritative
answer" (AA) bit set in the response packets. This makes them
easy to identify when debugging DNS configurations using tools like
<B
CLASS="command"
>dig</B
> (<A
HREF="Bv9ARM.ch03.html#diagnostic_tools"
>Section 3.3.1.1</A
>).</P
><DIV
CLASS="sect3"
><H3
CLASS="sect3"
><A
NAME="AEN178"
>1.4.4.1. The Primary Master</A
></H3
><P
> The authoritative server where the master copy of the zone data is maintained is
called the <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>primary master</I
></SPAN
> server, or simply the
<SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>primary</I
></SPAN
>. It loads the zone contents from some
local file edited by humans or perhaps generated mechanically from
some other local file which is edited by humans. This file is called
the <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>zone file</I
></SPAN
> or <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>master file</I
></SPAN
>.</P
></DIV
><DIV
CLASS="sect3"
><H3
CLASS="sect3"
><A
NAME="AEN185"
>1.4.4.2. Slave Servers</A
></H3
><P
>The other authoritative servers, the <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>slave</I
></SPAN
>
servers (also known as <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>secondary</I
></SPAN
> servers) load
the zone contents from another server using a replication process
known as a <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>zone transfer</I
></SPAN
>. Typically the data are
transferred directly from the primary master, but it is also possible
to transfer it from another slave. In other words, a slave server
may itself act as a master to a subordinate slave server.</P
></DIV
><DIV
CLASS="sect3"
><H3
CLASS="sect3"
><A
NAME="AEN191"
>1.4.4.3. Stealth Servers</A
></H3
><P
>Usually all of the zone's authoritative servers are listed in
NS records in the parent zone. These NS records constitute
a <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>delegation</I
></SPAN
> of the zone from the parent.
The authoritative servers are also listed in the zone file itself,
at the <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>top level</I
></SPAN
> or <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>apex</I
></SPAN
>
of the zone. You can list servers in the zone's top-level NS
records that are not in the parent's NS delegation, but you cannot
list servers in the parent's delegation that are not present at
the zone's top level.</P
><P
>A <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>stealth server</I
></SPAN
> is a server that is
authoritative for a zone but is not listed in that zone's NS
records. Stealth servers can be used for keeping a local copy of a
zone to speed up access to the zone's records or to make sure that the
zone is available even if all the "official" servers for the zone are
inaccessible.</P
><P
>A configuration where the primary master server itself is a
stealth server is often referred to as a "hidden primary"
configuration. One use for this configuration is when the primary master
is behind a firewall and therefore unable to communicate directly
with the outside world.</P
></DIV
></DIV
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="AEN200"
>1.4.5. Caching Name Servers</A
></H2
><P
>The resolver libraries provided by most operating systems are
<SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>stub resolvers</I
></SPAN
>, meaning that they are not capable of
performing the full DNS resolution process by themselves by talking
directly to the authoritative servers. Instead, they rely on a local
name server to perform the resolution on their behalf. Such a server
is called a <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>recursive</I
></SPAN
> name server; it performs
<SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>recursive lookups</I
></SPAN
> for local clients.</P
><P
>To improve performance, recursive servers cache the results of
the lookups they perform. Since the processes of recursion and
caching are intimately connected, the terms
<SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>recursive server</I
></SPAN
> and
<SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>caching server</I
></SPAN
> are often used synonymously.</P
><P
>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.
</P
><DIV
CLASS="sect3"
><H3
CLASS="sect3"
><A
NAME="AEN210"
>1.4.5.1. Forwarding</A
></H3
><P
>Even a caching name server does not necessarily perform
the complete recursive lookup itself. Instead, it can
<SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>forward</I
></SPAN
> some or all of the queries
that it cannot satisfy from its cache to another caching name server,
commonly referred to as a <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>forwarder</I
></SPAN
>.
</P
><P
>There may be one or more forwarders,
and they are queried in turn until the list is exhausted or an answer
is found. Forwarders are typically used when you do not
wish all the servers at a given site to interact directly with the rest of
the Internet servers. A typical scenario would involve a number
of internal <ACRONYM
CLASS="acronym"
>DNS</ACRONYM
> servers and an Internet firewall. Servers unable
to pass packets through the firewall would forward to the server
that can do it, and that server would query the Internet <ACRONYM
CLASS="acronym"
>DNS</ACRONYM
> servers
on the internal server's behalf. 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
of.</P
></DIV
></DIV
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="AEN218"
>1.4.6. Name Servers in Multiple Roles</A
></H2
><P
>The <ACRONYM
CLASS="acronym"
>BIND</ACRONYM
> name server can simultaneously act as
a master for some zones, a slave for other zones, and as a caching
(recursive) server for a set of local clients.</P
><P
>However, since the functions of authoritative name service
and caching/recursive name service are logically separate, it is
often advantageous to run them on separate server machines.
A server that only provides authoritative name service
(an <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>authoritative-only</I
></SPAN
> server) can run with
recursion disabled, improving reliability and security.
A server that is not authoritative for any zones and only provides
recursive service to local
clients (a <SPAN
CLASS="emphasis"
><I
CLASS="emphasis"
>caching-only</I
></SPAN
> server)
does not need to be reachable from the Internet at large and can
be placed inside a firewall.</P
></DIV
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