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c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden><TABLE
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c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden>BIND 9 Administrator Reference Manual</TH
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c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden><A
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c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden>Prev</A
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c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden>Next</A
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c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden><HR
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c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil MaddenCLASS="chapter"
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden><H1
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden><A
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil MaddenNAME="ch01"
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden>Chapter 1. Introduction </A
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden></H1
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden><DIV
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil MaddenCLASS="TOC"
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden><DL
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden><DT
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden><B
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden>Table of Contents</B
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden></DT
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden><DT
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden>1.1. <A
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil MaddenHREF="Bv9ARM.ch01.html#AEN13"
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden>Scope of Document</A
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden></DT
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden><DT
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden>1.2. <A
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil MaddenHREF="Bv9ARM.ch01.html#AEN20"
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden>Organization of This Document</A
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden></DT
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden><DT
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden>1.3. <A
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil MaddenHREF="Bv9ARM.ch01.html#AEN40"
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden>Conventions Used in This Document</A
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden></DT
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden><DT
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden>1.4. <A
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil MaddenHREF="Bv9ARM.ch01.html#AEN105"
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden>The Domain Name System (<SPAN
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil MaddenCLASS="acronym"
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden>DNS</SPAN
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden>)</A
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden></DT
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden></DL
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden></DIV
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden><P
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden>The Internet Domain Name System (<SPAN
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil MaddenCLASS="acronym"
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden>DNS</SPAN
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden>) consists of the syntax
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden to specify the names of entities in the Internet in a hierarchical
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden manner, the rules used for delegating authority over names, and the
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden system implementation that actually maps names to Internet
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden addresses. <SPAN
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil MaddenCLASS="acronym"
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden>DNS</SPAN
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden> data is maintained in a group of distributed
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden hierarchical databases.</P
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden><DIV
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil MaddenCLASS="sect1"
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden><H1
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil MaddenCLASS="sect1"
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden><A
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil MaddenNAME="AEN13"
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden>1.1. Scope of Document</A
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden></H1
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden><P
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden>The Berkeley Internet Name Domain (<SPAN
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil MaddenCLASS="acronym"
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden>BIND</SPAN
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden>) implements an
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden domain name server for a number of operating systems. This
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden document provides basic information about the installation and
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden care of the Internet Software Consortium (<SPAN
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil MaddenCLASS="acronym"
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden>ISC</SPAN
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden>)
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden <SPAN
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil MaddenCLASS="acronym"
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden>BIND</SPAN
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden> version 9 software package for system
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden administrators.</P
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden><P
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden>This version of the manual corresponds to BIND version 9.2.</P
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden></DIV
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden><DIV
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil MaddenCLASS="sect1"
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden><H1
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil MaddenCLASS="sect1"
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden><A
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil MaddenNAME="AEN20"
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden>1.2. Organization of This Document</A
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden></H1
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden><P
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden>In this document, <I
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil MaddenCLASS="emphasis"
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden>Section 1</I
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden> introduces
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden the basic <SPAN
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil MaddenCLASS="acronym"
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden>DNS</SPAN
c44bbf5d2025d672efe11a37ea6e9c867e78b691Neil Madden> and <SPAN
CLASS="acronym"
>BIND</SPAN
> concepts. <I
CLASS="emphasis"
>Section 2</I
>
describes resource requirements for running <SPAN
CLASS="acronym"
>BIND</SPAN
> in various
environments. Information in <I
CLASS="emphasis"
>Section 3</I
> is
<I
CLASS="emphasis"
>task-oriented</I
> in its presentation and is
organized functionally, to aid in the process of installing the
<SPAN
CLASS="acronym"
>BIND</SPAN
> 9 software. The task-oriented section is followed by
<I
CLASS="emphasis"
>Section 4</I
>, which contains more advanced
concepts that the system administrator may need for implementing
certain options. <I
CLASS="emphasis"
>Section 5</I
>
describes the <SPAN
CLASS="acronym"
>BIND</SPAN
> 9 lightweight
resolver. The contents of <I
CLASS="emphasis"
>Section 6</I
> are
organized as in a reference manual to aid in the ongoing
maintenance of the software. <I
CLASS="emphasis"
>Section 7
</I
>addresses security considerations, and
<I
CLASS="emphasis"
>Section 8</I
> contains troubleshooting help. The
main body of the document is followed by several
<I
CLASS="emphasis"
>Appendices</I
> which contain useful reference
information, such as a <I
CLASS="emphasis"
>Bibliography</I
> and
historic information related to <SPAN
CLASS="acronym"
>BIND</SPAN
> and the Domain Name
System.</P
></DIV
><DIV
CLASS="sect1"
><H1
CLASS="sect1"
><A
NAME="AEN40"
>1.3. Conventions Used in This Document</A
></H1
><P
>In this document, we use the following general typographic
conventions:</P
><DIV
CLASS="informaltable"
><A
NAME="AEN43"
></A
><P
></P
><TABLE
CELLPADDING="3"
BORDER="1"
CLASS="CALSTABLE"
><TBODY
><TR
><TD
WIDTH="288"
ALIGN="LEFT"
VALIGN="MIDDLE"
>&#13;<P
><I
CLASS="emphasis"
>To
describe:</I
></P
></TD
><TD
WIDTH="252"
ALIGN="LEFT"
VALIGN="MIDDLE"
>&#13;<P
><I
CLASS="emphasis"
>We use the style:</I
></P
></TD
></TR
><TR
><TD
WIDTH="288"
ALIGN="LEFT"
VALIGN="MIDDLE"
>&#13;<P
>a pathname, filename, URL, hostname,
mailing list name, or new term or concept</P
></TD
><TD
WIDTH="252"
ALIGN="LEFT"
VALIGN="MIDDLE"
><P
><TT
CLASS="filename"
>Fixed width</TT
></P
></TD
></TR
><TR
><TD
WIDTH="288"
ALIGN="LEFT"
VALIGN="MIDDLE"
><P
>literal user
input</P
></TD
><TD
WIDTH="252"
ALIGN="LEFT"
VALIGN="MIDDLE"
><P
><TT
CLASS="userinput"
><B
>Fixed Width Bold</B
></TT
></P
></TD
></TR
><TR
><TD
WIDTH="288"
ALIGN="LEFT"
VALIGN="MIDDLE"
><P
>program output</P
></TD
><TD
WIDTH="252"
ALIGN="LEFT"
VALIGN="MIDDLE"
><P
><TT
CLASS="computeroutput"
>Fixed Width</TT
></P
></TD
></TR
></TBODY
></TABLE
><P
></P
></DIV
><P
>The following conventions are used in descriptions of the
<SPAN
CLASS="acronym"
>BIND</SPAN
> configuration file:<DIV
CLASS="informaltable"
><A
NAME="AEN75"
></A
><P
></P
><TABLE
CELLPADDING="3"
BORDER="1"
CLASS="CALSTABLE"
><TBODY
><TR
><TD
WIDTH="288"
ALIGN="LEFT"
VALIGN="MIDDLE"
><P
><I
CLASS="emphasis"
>To
describe:</I
></P
></TD
><TD
WIDTH="252"
ALIGN="LEFT"
VALIGN="MIDDLE"
><P
><I
CLASS="emphasis"
>We use the style:</I
></P
></TD
></TR
><TR
><TD
WIDTH="288"
ALIGN="LEFT"
VALIGN="MIDDLE"
><P
>keywords</P
></TD
><TD
WIDTH="252"
ALIGN="LEFT"
VALIGN="MIDDLE"
><P
><TT
CLASS="literal"
>Fixed Width</TT
></P
></TD
></TR
><TR
><TD
WIDTH="288"
ALIGN="LEFT"
VALIGN="MIDDLE"
><P
>variables</P
></TD
><TD
WIDTH="252"
ALIGN="LEFT"
VALIGN="MIDDLE"
><P
><TT
CLASS="varname"
>Fixed Width</TT
></P
></TD
></TR
><TR
><TD
WIDTH="288"
ALIGN="LEFT"
VALIGN="MIDDLE"
><P
>Optional input</P
></TD
><TD
WIDTH="252"
ALIGN="LEFT"
VALIGN="MIDDLE"
><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="AEN105"
>1.4. The Domain Name System (<SPAN
CLASS="acronym"
>DNS</SPAN
>)</A
></H1
><P
>The purpose of this document is to explain the installation
and upkeep of the <SPAN
CLASS="acronym"
>BIND</SPAN
> software package, and we
begin by reviewing the fundamentals of the Domain Name System
(<SPAN
CLASS="acronym"
>DNS</SPAN
>) as they relate to <SPAN
CLASS="acronym"
>BIND</SPAN
>.
</P
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="AEN112"
>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
<I
CLASS="emphasis"
>resolver</I
> library, which sends queries to one or
more <I
CLASS="emphasis"
>name servers</I
> and interprets the responses.
The <SPAN
CLASS="acronym"
>BIND 9</SPAN
> software distribution contains both a
name server and a resolver library.</P
></DIV
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="AEN119"
>1.4.2. Domains and Domain Names</A
></H2
><P
>The data stored in the DNS is identified by <I
CLASS="emphasis"
>domain
names</I
> that are organized as a tree according to
organizational or administrative boundaries. Each node of the tree,
called a <I
CLASS="emphasis"
>domain</I
>, 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 <I
CLASS="emphasis"
>root</I
> 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 <I
CLASS="emphasis"
>Example, Inc.</I
> could be
<TT
CLASS="literal"
>mail.example.net</TT
>,
were <TT
CLASS="literal"
>com</TT
> is the
top level domain to which
<TT
CLASS="literal"
>ourhost.example.com</TT
> belongs,
<TT
CLASS="literal"
>example</TT
> is
a subdomain of <TT
CLASS="literal"
>com</TT
>, and
<TT
CLASS="literal"
>ourhost</TT
> is the
name of the host.</P
><P
>For administrative purposes, the name space is partitioned into
areas called <I
CLASS="emphasis"
>zones</I
>, 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 <I
CLASS="emphasis"
>name
server</I
>, which answers queries about the zone using the
<I
CLASS="emphasis"
>DNS protocol</I
>.
</P
><P
>The data associated with each domain name is stored in the
form of <I
CLASS="emphasis"
>resource records</I
> (<SPAN
CLASS="acronym"
>RR</SPAN
>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.4.1</A
>.</P
></DIV
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="AEN143"
>1.4.3. Zones</A
></H2
><P
>To properly operate a name server, it is important to understand
the difference between a <I
CLASS="emphasis"
>zone</I
>
and a <I
CLASS="emphasis"
>domain</I
>.</P
><P
>As we stated previously, a zone is a point of delegation in
the <SPAN
CLASS="acronym"
>DNS</SPAN
> tree. A zone consists of
those contiguous parts of the domain
tree for which a 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
<I
CLASS="emphasis"
>NS records</I
> 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 <TT
CLASS="literal"
>example.com</TT
>
domain which includes names
such as <TT
CLASS="literal"
>host.aaa.example.com</TT
> and
<TT
CLASS="literal"
>host.bbb.example.com</TT
> even though
the <TT
CLASS="literal"
>example.com</TT
> zone includes
only delegations for the <TT
CLASS="literal"
>aaa.example.com</TT
> and
<TT
CLASS="literal"
>bbb.example.com</TT
> 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 <SPAN
CLASS="acronym"
>DNS</SPAN
> tree is a
<I
CLASS="emphasis"
>domain</I
>, even if it is
<I
CLASS="emphasis"
>terminal</I
>, that is, has no
<I
CLASS="emphasis"
>subdomains</I
>. 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 <SPAN
CLASS="acronym"
>BIND</SPAN
> 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 <I
CLASS="emphasis"
>domain</I
>, you are
actually asking for slave service for some collection of zones.</P
></DIV
><DIV
CLASS="sect2"
><H2
CLASS="sect2"
><A
NAME="AEN166"
>1.4.4. Authoritative Name Servers</A
></H2
><P
>Each zone is served by at least
one <I
CLASS="emphasis"
>authoritative name server</I
>,
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 3.4.1.1</A
>).</P
><DIV
CLASS="sect3"
><H3
CLASS="sect3"
><A
NAME="AEN173"
>1.4.4.1. The Primary Master</A
></H3
><P
>&#13;The authoritative server where the master copy of the zone data is maintained is
called the <I
CLASS="emphasis"
>primary master</I
> server, or simply the
<I
CLASS="emphasis"
>primary</I
>. 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 <I
CLASS="emphasis"
>zone file</I
> or <I
CLASS="emphasis"
>master file</I
>.</P
></DIV
><DIV
CLASS="sect3"
><H3
CLASS="sect3"
><A
NAME="AEN180"
>1.4.4.2. Slave Servers</A
></H3
><P
>The other authoritative servers, the <I
CLASS="emphasis"
>slave</I
>
servers (also known as <I
CLASS="emphasis"
>secondary</I
> servers) load
the zone contents from another server using a replication process
known as a <I
CLASS="emphasis"
>zone transfer</I
>. 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="AEN186"
>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 <I
CLASS="emphasis"
>delegation</I
> of the zone from the parent.
The authoritative servers are also listed in the zone file itself,
at the <I
CLASS="emphasis"
>top level</I
> or <I
CLASS="emphasis"
>apex</I
>
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 <I
CLASS="emphasis"
>stealth server</I
> 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="AEN195"
>1.4.5. Caching Name Servers</A
></H2
><P
>The resolver libraries provided by most operating systems are
<I
CLASS="emphasis"
>stub resolvers</I
>, 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 <I
CLASS="emphasis"
>recursive</I
> name server; it performs
<I
CLASS="emphasis"
>recursive lookups</I
> 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
<I
CLASS="emphasis"
>recursive server</I
> and
<I
CLASS="emphasis"
>caching server</I
> 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="AEN205"
>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
<I
CLASS="emphasis"
>forward</I
> some or all of the queries
that it cannot satisfy from its cache to another caching name server,
commonly referred to as a <I
CLASS="emphasis"
>forwarder</I
>.
</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 <SPAN
CLASS="acronym"
>DNS</SPAN
> 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 <SPAN
CLASS="acronym"
>DNS</SPAN
> 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="AEN213"
>1.4.6. Name Servers in Multiple Roles</A
></H2
><P
>The <SPAN
CLASS="acronym"
>BIND</SPAN
> 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 <I
CLASS="emphasis"
>authoritative-only</I
> 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 <I
CLASS="emphasis"
>caching-only</I
> server)
does not need to be reachable from the Internet at large and can
be placed inside a firewall.</P
></DIV
></DIV
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