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< a name = "Bv9ARM.ch01" > < / a > Chapter<EFBFBD> 1.<2E> Introduction < / h2 > < / div > < / div > < / div >
< div class = "toc" >
< p > < b > Table of Contents< / b > < / p >
< dl >
< dt > < span class = "sect1" > < a href = "Bv9ARM.ch01.html#id2545879" > Scope of Document< / a > < / span > < / dt >
< dt > < span class = "sect1" > < a href = "Bv9ARM.ch01.html#id2545905" > Organization of This Document< / a > < / span > < / dt >
< dt > < span class = "sect1" > < a href = "Bv9ARM.ch01.html#id2545976" > Conventions Used in This Document< / a > < / span > < / dt >
< dt > < span class = "sect1" > < a href = "Bv9ARM.ch01.html#id2546234" > The Domain Name System (< span class = "acronym" > DNS< / span > )< / a > < / span > < / dt >
< dd > < dl >
< dt > < span class = "sect2" > < a href = "Bv9ARM.ch01.html#id2546254" > DNS Fundamentals< / a > < / span > < / dt >
< dt > < span class = "sect2" > < a href = "Bv9ARM.ch01.html#id2544105" > Domains and Domain Names< / a > < / span > < / dt >
< dt > < span class = "sect2" > < a href = "Bv9ARM.ch01.html#id2546579" > Zones< / a > < / span > < / dt >
< dt > < span class = "sect2" > < a href = "Bv9ARM.ch01.html#id2546653" > Authoritative Name Servers< / a > < / span > < / dt >
< dt > < span class = "sect2" > < a href = "Bv9ARM.ch01.html#id2546950" > Caching Name Servers< / a > < / span > < / dt >
< dt > < span class = "sect2" > < a href = "Bv9ARM.ch01.html#id2547076" > Name Servers in Multiple Roles< / a > < / span > < / dt >
< / dl > < / dd >
< / dl >
< / div >
< p > The Internet Domain Name System (< span class = "acronym" > DNS< / span > ) consists of the syntax
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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
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addresses. < span class = "acronym" > DNS< / span > data is maintained in a group of distributed
hierarchical databases.< / p >
< div class = "sect1" lang = "en" >
< div class = "titlepage" > < div > < div > < h2 class = "title" style = "clear: both" >
< a name = "id2545879" > < / a > Scope of Document< / h2 > < / div > < / div > < / div >
< p > The Berkeley Internet Name Domain (< span class = "acronym" > BIND< / span > ) implements an
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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 (< span class = "acronym" > ISC< / span > )
< span class = "acronym" > BIND< / span > 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" lang = "en" >
< div class = "titlepage" > < div > < div > < h2 class = "title" style = "clear: both" >
< a name = "id2545905" > < / a > Organization of This Document< / h2 > < / div > < / div > < / div >
< p > In this document, < span class = "emphasis" > < em > Section 1< / em > < / span > introduces
the basic < span class = "acronym" > DNS< / span > and < span class = "acronym" > BIND< / span > concepts. < span class = "emphasis" > < em > Section 2< / em > < / span >
describes resource requirements for running < span class = "acronym" > BIND< / span > in various
environments. Information in < span class = "emphasis" > < em > Section 3< / em > < / span > is
< span class = "emphasis" > < em > task-oriented< / em > < / span > in its presentation and is
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organized functionally, to aid in the process of installing the
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< span class = "acronym" > BIND< / span > 9 software. The task-oriented section is followed by
< span class = "emphasis" > < em > Section 4< / em > < / span > , which contains more advanced
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concepts that the system administrator may need for implementing
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certain options. < span class = "emphasis" > < em > Section 5< / em > < / span >
describes the < span class = "acronym" > BIND< / span > 9 lightweight
resolver. The contents of < span class = "emphasis" > < em > Section 6< / em > < / span > are
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organized as in a reference manual to aid in the ongoing
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maintenance of the software. < span class = "emphasis" > < em > Section 7
< / em > < / span > addresses security considerations, and
< span class = "emphasis" > < em > Section 8< / em > < / span > contains troubleshooting help. The
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main body of the document is followed by several
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< span class = "emphasis" > < em > Appendices< / em > < / span > which contain useful reference
information, such as a < span class = "emphasis" > < em > Bibliography< / em > < / span > and
historic information related to < span class = "acronym" > BIND< / span > and the Domain Name
System.< / p >
< / div >
< div class = "sect1" lang = "en" >
< div class = "titlepage" > < div > < div > < h2 class = "title" style = "clear: both" >
< a name = "id2545976" > < / a > Conventions Used in This Document< / h2 > < / div > < / div > < / div >
< p > In this document, we use the following general typographic
conventions:< / p >
< div class = "informaltable" > < table border = "1" >
< colgroup >
< col >
< col >
< / colgroup >
< tbody >
< tr >
< td >
< p > < span class = "emphasis" > < em > To
describe:< / em > < / span > < / p >
< / td >
< td >
< p > < span class = "emphasis" > < em > We use the style:< / em > < / span > < / p >
< / td >
< / tr >
< tr >
< td >
< p > a pathname, filename, URL, hostname,
mailing list name, or new term or concept< / p >
< / td >
< td > < p > < code class = "filename" > Fixed width< / code > < / p > < / td >
< / tr >
< tr >
< td > < p > literal user
input< / p > < / td >
< td > < p > < strong class = "userinput" > < code > Fixed Width Bold< / code > < / strong > < / p > < / td >
< / tr >
< tr >
< td > < p > program output< / p > < / td >
< td > < p > < code class = "computeroutput" > Fixed Width< / code > < / p > < / td >
< / tr >
< / tbody >
< / table > < / div >
< p > The following conventions are used in descriptions of the
< span class = "acronym" > BIND< / span > configuration file:< / p >
< div class = "informaltable" > < table border = "1" >
< colgroup >
< col >
< col >
< / colgroup >
< tbody >
< tr >
< td > < p > < span class = "emphasis" > < em > To
describe:< / em > < / span > < / p > < / td >
< td > < p > < span class = "emphasis" > < em > We use the style:< / em > < / span > < / p > < / td >
< / tr >
< tr >
< td > < p > keywords< / p > < / td >
< td > < p > < code class = "literal" > Fixed Width< / code > < / p > < / td >
< / tr >
< tr >
< td > < p > variables< / p > < / td >
< td > < p > < code class = "varname" > Fixed Width< / code > < / p > < / td >
< / tr >
< tr >
< td > < p > Optional input< / p > < / td >
< td > < p > [< span class = "optional" > Text is enclosed in square brackets< / span > ]< / p > < / td >
< / tr >
< / tbody >
< / table > < / div >
< / div >
< div class = "sect1" lang = "en" >
< div class = "titlepage" > < div > < div > < h2 class = "title" style = "clear: both" >
< a name = "id2546234" > < / a > The Domain Name System (< span class = "acronym" > DNS< / span > )< / h2 > < / div > < / div > < / div >
< p > The purpose of this document is to explain the installation
and upkeep of the < span class = "acronym" > BIND< / span > software package, and we
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begin by reviewing the fundamentals of the Domain Name System
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(< span class = "acronym" > DNS< / span > ) as they relate to < span class = "acronym" > BIND< / span > .
< / p >
< div class = "sect2" lang = "en" >
< div class = "titlepage" > < div > < div > < h3 class = "title" >
< a name = "id2546254" > < / a > DNS Fundamentals< / h3 > < / div > < / div > < / div >
< p > The Domain Name System (DNS) is the hierarchical, distributed
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database. It stores information for mapping Internet host names to IP
addresses and vice versa, mail routing information, and other data
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used by Internet applications.< / p >
< p > Clients look up information in the DNS by calling a
< span class = "emphasis" > < em > resolver< / em > < / span > library, which sends queries to one or
more < span class = "emphasis" > < em > name servers< / em > < / span > and interprets the responses.
The < span class = "acronym" > BIND< / span > 9 software distribution contains a
name server, < span > < strong class = "command" > named< / strong > < / span > , and two resolver
libraries, < span > < strong class = "command" > liblwres< / strong > < / span > and < span > < strong class = "command" > libbind< / strong > < / span > .
< / p >
< / div >
< div class = "sect2" lang = "en" >
< div class = "titlepage" > < div > < div > < h3 class = "title" >
< a name = "id2544105" > < / a > Domains and Domain Names< / h3 > < / div > < / div > < / div >
< p > The data stored in the DNS is identified by < span class = "emphasis" > < em > domain
names< / em > < / span > that are organized as a tree according to
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organizational or administrative boundaries. Each node of the tree,
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called a < span class = "emphasis" > < em > domain< / em > < / span > , is given a label. The domain name of the
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node is the concatenation of all the labels on the path from the
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node to the < span class = "emphasis" > < em > root< / em > < / span > node. This is represented
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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
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domain.< / p >
< p > For example, a domain name for a host at the
company < span class = "emphasis" > < em > Example, Inc.< / em > < / span > could be
< code class = "literal" > mail.example.com< / code > ,
where < code class = "literal" > com< / code > is the
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top level domain to which
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< code class = "literal" > ourhost.example.com< / code > belongs,
< code class = "literal" > example< / code > is
a subdomain of < code class = "literal" > com< / code > , and
< code class = "literal" > ourhost< / code > is the
name of the host.< / p >
< p > For administrative purposes, the name space is partitioned into
areas called < span class = "emphasis" > < em > zones< / em > < / span > , each starting at a node and
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extending down to the leaf nodes or to nodes where other zones start.
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The data for each zone is stored in a < span class = "emphasis" > < em > name
server< / em > < / span > , which answers queries about the zone using the
< span class = "emphasis" > < em > DNS protocol< / em > < / span > .
< / p >
< p > The data associated with each domain name is stored in the
form of < span class = "emphasis" > < em > resource records< / em > < / span > (< span class = "acronym" > RR< / span > s).
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Some of the supported resource record types are described in
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< a href = "Bv9ARM.ch06.html#types_of_resource_records_and_when_to_use_them" title = "Types of Resource Records and When to Use Them" > the section called “ Types of Resource Records and When to Use Them” < / a > .< / p >
< p > For more detailed information about the design of the DNS and
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the DNS protocol, please refer to the standards documents listed in
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< a href = "Bv9ARM.ch09.html#rfcs" title = "Request for Comments (RFCs)" > the section called “ Request for Comments (RFCs)” < / a > .< / p >
< / div >
< div class = "sect2" lang = "en" >
< div class = "titlepage" > < div > < div > < h3 class = "title" >
< a name = "id2546579" > < / a > Zones< / h3 > < / div > < / div > < / div >
< p > To properly operate a name server, it is important to understand
the difference between a < span class = "emphasis" > < em > zone< / em > < / span >
and a < span class = "emphasis" > < em > domain< / em > < / span > .< / p >
< p > As we stated previously, a zone is a point of delegation in
the < span class = "acronym" > DNS< / span > tree. A zone consists of
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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
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< span class = "emphasis" > < em > NS records< / em > < / span > in the
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parent zone, which should be matched by equivalent NS records at
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the root of the delegated zone.< / p >
< p > For instance, consider the < code class = "literal" > example.com< / code >
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domain which includes names
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such as < code class = "literal" > host.aaa.example.com< / code > and
< code class = "literal" > host.bbb.example.com< / code > even though
the < code class = "literal" > example.com< / code > zone includes
only delegations for the < code class = "literal" > aaa.example.com< / code > and
< code class = "literal" > bbb.example.com< / code > zones. A zone can map
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exactly to a single domain, but could also include only part of a
domain, the rest of which could be delegated to other
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name servers. Every name in the < span class = "acronym" > DNS< / span > tree is a
< span class = "emphasis" > < em > domain< / em > < / span > , even if it is
< span class = "emphasis" > < em > terminal< / em > < / span > , that is, has no
< span class = "emphasis" > < em > subdomains< / em > < / span > . Every subdomain is a domain and
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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
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topic.< / p >
< p > Though < span class = "acronym" > BIND< / span > is called a "domain name server",
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it deals primarily in terms of zones. The master and slave
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declarations in the < code class = "filename" > named.conf< / code > file specify
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zones, not domains. When you ask some other site if it is willing to
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be a slave server for your < span class = "emphasis" > < em > domain< / em > < / span > , you are
actually asking for slave service for some collection of zones.< / p >
< / div >
< div class = "sect2" lang = "en" >
< div class = "titlepage" > < div > < div > < h3 class = "title" >
< a name = "id2546653" > < / a > Authoritative Name Servers< / h3 > < / div > < / div > < / div >
< p > Each zone is served by at least
one < span class = "emphasis" > < em > authoritative name server< / em > < / span > ,
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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.
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< / p >
< p > 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
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< span > < strong class = "command" > dig< / strong > < / span > (< a href = "Bv9ARM.ch03.html#diagnostic_tools" title = "Diagnostic Tools" > the section called “ Diagnostic Tools” < / a > ).< / p >
< div class = "sect3" lang = "en" >
< div class = "titlepage" > < div > < div > < h4 class = "title" >
< a name = "id2546676" > < / a > The Primary Master< / h4 > < / div > < / div > < / div >
< p >
The authoritative server where the master copy of the zone data is maintained is
called the < span class = "emphasis" > < em > primary master< / em > < / span > server, or simply the
< span class = "emphasis" > < em > primary< / em > < / span > . It loads the zone contents from some
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local file edited by humans or perhaps generated mechanically from
some other local file which is edited by humans. This file is called
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the < span class = "emphasis" > < em > zone file< / em > < / span > or < span class = "emphasis" > < em > master file< / em > < / span > .< / p >
< / div >
< div class = "sect3" lang = "en" >
< div class = "titlepage" > < div > < div > < h4 class = "title" >
< a name = "id2546902" > < / a > Slave Servers< / h4 > < / div > < / div > < / div >
< p > The other authoritative servers, the < span class = "emphasis" > < em > slave< / em > < / span >
servers (also known as < span class = "emphasis" > < em > secondary< / em > < / span > servers) load
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the zone contents from another server using a replication process
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known as a < span class = "emphasis" > < em > zone transfer< / em > < / span > . Typically the data are
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transferred directly from the primary master, but it is also possible
to transfer it from another slave. In other words, a slave server
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may itself act as a master to a subordinate slave server.< / p >
< / div >
< div class = "sect3" lang = "en" >
< div class = "titlepage" > < div > < div > < h4 class = "title" >
< a name = "id2546921" > < / a > Stealth Servers< / h4 > < / div > < / div > < / div >
< p > Usually all of the zone's authoritative servers are listed in
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NS records in the parent zone. These NS records constitute
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a < span class = "emphasis" > < em > delegation< / em > < / span > of the zone from the parent.
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The authoritative servers are also listed in the zone file itself,
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at the < span class = "emphasis" > < em > top level< / em > < / span > or < span class = "emphasis" > < em > apex< / em > < / span >
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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
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the zone's top level.< / p >
< p > A < span class = "emphasis" > < em > stealth server< / em > < / span > is a server that is
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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
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inaccessible.< / p >
< p > A configuration where the primary master server itself is a
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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
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with the outside world.< / p >
< / div >
< / div >
< div class = "sect2" lang = "en" >
< div class = "titlepage" > < div > < div > < h3 class = "title" >
< a name = "id2546950" > < / a > Caching Name Servers< / h3 > < / div > < / div > < / div >
< p > The resolver libraries provided by most operating systems are
< span class = "emphasis" > < em > stub resolvers< / em > < / span > , meaning that they are not capable of
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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
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is called a < span class = "emphasis" > < em > recursive< / em > < / span > name server; it performs
< span class = "emphasis" > < em > recursive lookups< / em > < / span > for local clients.< / p >
< p > To improve performance, recursive servers cache the results of
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the lookups they perform. Since the processes of recursion and
caching are intimately connected, the terms
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< span class = "emphasis" > < em > recursive server< / em > < / span > and
< span class = "emphasis" > < em > caching server< / em > < / span > are often used synonymously.< / p >
< p > The length of time for which a record may be retained in
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in the cache of a caching name server is controlled by the
Time To Live (TTL) field associated with each resource record.
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< / p >
< div class = "sect3" lang = "en" >
< div class = "titlepage" > < div > < div > < h4 class = "title" >
< a name = "id2547050" > < / a > Forwarding< / h4 > < / div > < / div > < / div >
< p > Even a caching name server does not necessarily perform
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the complete recursive lookup itself. Instead, it can
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< span class = "emphasis" > < em > forward< / em > < / span > some or all of the queries
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that it cannot satisfy from its cache to another caching name server,
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commonly referred to as a < span class = "emphasis" > < em > forwarder< / em > < / span > .
< / p >
< p > There may be one or more forwarders,
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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
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of internal < span class = "acronym" > DNS< / span > servers and an Internet firewall. Servers unable
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to pass packets through the firewall would forward to the server
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that can do it, and that server would query the Internet < span class = "acronym" > DNS< / span > servers
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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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of.< / p >
< / div >
< / div >
< div class = "sect2" lang = "en" >
< div class = "titlepage" > < div > < div > < h3 class = "title" >
< a name = "id2547076" > < / a > Name Servers in Multiple Roles< / h3 > < / div > < / div > < / div >
< p > The < span class = "acronym" > BIND< / span > name server can simultaneously act as
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a master for some zones, a slave for other zones, and as a caching
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(recursive) server for a set of local clients.< / p >
< p > However, since the functions of authoritative name service
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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
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(an < span class = "emphasis" > < em > authoritative-only< / em > < / span > server) can run with
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recursion disabled, improving reliability and security.
A server that is not authoritative for any zones and only provides
recursive service to local
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clients (a < span class = "emphasis" > < em > caching-only< / em > < / span > server)
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does not need to be reachable from the Internet at large and can
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be placed inside a firewall.< / p >
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