<HTML ><HEAD ><TITLE >Introduction </TITLE ><META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.73 "><LINK REL="HOME" TITLE="BIND 9 Administrator Reference Manual" HREF="Bv9ARM.html"><LINK REL="PREVIOUS" TITLE="BIND 9 Administrator Reference Manual" HREF="Bv9ARM.html"><LINK REL="NEXT" TITLE="BIND Resource Requirements" HREF="Bv9ARM.ch02.html"></HEAD ><BODY CLASS="chapter" BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#0000FF" VLINK="#840084" ALINK="#0000FF" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><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" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="80%" ALIGN="center" VALIGN="bottom" ></TD ><TD WIDTH="10%" ALIGN="right" VALIGN="bottom" ><A HREF="Bv9ARM.ch02.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="chapter" ><H1 ><A NAME="ch01" >Chapter 1. Introduction </A ></H1 ><DIV CLASS="TOC" ><DL ><DT ><B >Table of Contents</B ></DT ><DT >1.1. <A HREF="Bv9ARM.ch01.html#AEN13" >Scope of Document</A ></DT ><DT >1.2. <A HREF="Bv9ARM.ch01.html#AEN20" >Organization of This Document</A ></DT ><DT >1.3. <A HREF="Bv9ARM.ch01.html#AEN40" >Conventions Used in This Document</A ></DT ><DT >1.4. <A HREF="Bv9ARM.ch01.html#AEN105" >The Domain Name System (<SPAN CLASS="acronym" >DNS</SPAN >)</A ></DT ></DL ></DIV ><P >The Internet Domain Name System (<SPAN CLASS="acronym" >DNS</SPAN >) 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. <SPAN CLASS="acronym" >DNS</SPAN > data is maintained in a group of distributed hierarchical databases.</P ><DIV CLASS="sect1" ><H1 CLASS="sect1" ><A NAME="AEN13" >1.1. Scope of Document</A ></H1 ><P >The Berkeley Internet Name Domain (<SPAN CLASS="acronym" >BIND</SPAN >) 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 (<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 9.2.</P ></DIV ><DIV CLASS="sect1" ><H1 CLASS="sect1" ><A NAME="AEN20" >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 <SPAN CLASS="acronym" >DNS</SPAN > and <SPAN CLASS="acronym" >BIND</SPAN > concepts. <SPAN CLASS="emphasis" ><I CLASS="emphasis" >Section 2</I ></SPAN > describes resource requirements for running <SPAN CLASS="acronym" >BIND</SPAN > 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 <SPAN CLASS="acronym" >BIND</SPAN > 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 <SPAN CLASS="acronym" >BIND</SPAN > 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 <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" > <P ><SPAN CLASS="emphasis" ><I CLASS="emphasis" >To describe:</I ></SPAN ></P ></TD ><TD WIDTH="252" ALIGN="LEFT" VALIGN="MIDDLE" > <P ><SPAN CLASS="emphasis" ><I CLASS="emphasis" >We use the style:</I ></SPAN ></P ></TD ></TR ><TR ><TD WIDTH="288" ALIGN="LEFT" VALIGN="MIDDLE" > <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 ><SPAN CLASS="emphasis" ><I CLASS="emphasis" >To describe:</I ></SPAN ></P ></TD ><TD WIDTH="252" ALIGN="LEFT" VALIGN="MIDDLE" ><P ><SPAN CLASS="emphasis" ><I CLASS="emphasis" >We use the style:</I ></SPAN ></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 <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 <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 <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 <TT CLASS="literal" >mail.example.com</TT >, where <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 <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 > (<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 <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 <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 <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 <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 <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 <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 <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="AEN166" >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 3.4.1.1</A >).</P ><DIV CLASS="sect3" ><H3 CLASS="sect3" ><A NAME="AEN173" >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="AEN180" >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="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 <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="AEN195" >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="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 <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 <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 <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 ></DIV ></DIV ><DIV CLASS="NAVFOOTER" ><HR ALIGN="LEFT" WIDTH="100%"><TABLE SUMMARY="Footer navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><A HREF="Bv9ARM.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="Bv9ARM.html" ACCESSKEY="H" >Home</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><A HREF="Bv9ARM.ch02.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >BIND 9 Administrator Reference Manual</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" > </TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><SPAN CLASS="acronym" >BIND</SPAN > Resource Requirements</TD ></TR ></TABLE ></DIV ></BODY ></HTML >