> version 9 software package for system
>This version of the manual corresponds to BIND version 9.2.</
P>1.2. Organization of This Document</
A describes resource requirements for running <
SPAN environments. Information in <
I> in its presentation and is
organized functionally, to aid in the process of installing the
> 9 software. The task-oriented section is followed by
>, which contains more advanced
concepts that the system administrator may need for implementing
resolver. The contents of <
I organized as in a reference manual to aid in the ongoing
maintenance of the software. <
I>addresses security considerations, and
> contains troubleshooting help. The
main body of the document is followed by several
> which contain useful reference
information, such as a <
I historic information related to <
SPAN>1.3. Conventions Used in This Document</
A>In this document, we use the following general typographic
>a pathname, filename, URL, hostname,
mailing list name, or new term or concept</
P>The following conventions are used in descriptions of the
> configuration file:<
DIV>Text is enclosed in square brackets</
SPAN>1.4. The Domain Name System (<
SPAN>The purpose of this document is to explain the installation
> software package, and we
begin by reviewing the fundamentals of the Domain Name System
>) as they relate to <
SPAN>1.4.1. DNS Fundamentals</
A>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>Clients look up information in the DNS by calling a
> library, which sends queries to one or
> and interprets the responses.
> software distribution contains both a
name server and a resolver library.</
P>1.4.2. Domains and Domain Names</
A>The data stored in the DNS is identified by <
I> that are organized as a tree according to
organizational or administrative boundaries. Each node of the tree,
>, is given a label. The domain name of the
node is the concatenation of all the labels on the path from the
> 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
>For example, a domain name for a host at the
top level domain to which
>For administrative purposes, the name space is partitioned into
>, 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>, which answers queries about the zone using the
>The data associated with each domain name is stored in the
Some of the supported resource record types are described in
>For more detailed information about the design of the DNS and
the DNS protocol, please refer to the standards documents listed in
>To properly operate a name server, it is important to understand
the difference between a <
I>As we stated previously, a zone is a point of delegation in
> 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
parent zone, which should be matched by equivalent NS records at
the root of the delegated zone.</
P>For instance, consider the <
TTdomain which includes names
only delegations for the <
TTexactly 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>. 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
> is called a "domain name server",
it deals primarily in terms of zones. The master and slave
zones, not domains. When you ask some other site if it is willing to
be a slave server for your <
Iactually asking for slave service for some collection of zones.</
P>1.4.4. Authoritative Name Servers</
A>Each zone is served by at least
>authoritative name server</
Iwhich 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.
>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
>1.4.4.1. The Primary Master</
A> The authoritative server where the master copy of the zone data is maintained is
>. 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
>1.4.4.2. Slave Servers</
A>The other authoritative servers, the <
Iservers (also known as <
Ithe zone contents from another server using a replication process
>. 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>1.4.4.3. Stealth Servers</
A>Usually all of the zone's authoritative servers are listed in
NS records in the parent zone. These NS records constitute
> of the zone from the parent.
The authoritative servers are also listed in the zone file itself,
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
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
>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>1.4.5. Caching Name Servers</
A>The resolver libraries provided by most operating systems are
>, 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
> name server; it performs
>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
> are often used synonymously.</
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.
>Even a caching name server does not necessarily perform
the complete recursive lookup itself. Instead, it can
> some or all of the queries
that it cannot satisfy from its cache to another caching name server,
commonly referred to as a <
I>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
> 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 <
SPANon 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
>1.4.6. Name Servers in Multiple Roles</
A> 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>However, since the functions of authoritative name service
often advantageous to run them on separate server machines.
A server that only provides authoritative name service
recursion disabled, improving reliability and security.
A server that is not authoritative for any zones and only provides
recursive service to local
does not need to be reachable from the Internet at large and can
be placed inside a firewall.</
P>BIND 9 Administrator Reference Manual</
TD> Resource Requirements</
TD