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<H2><A NAME="Heading1"></A><FONT COLOR="#000077">Chapter 25<BR>Configuring Domain Name Service

</FONT></H2>

<P><I>by Steve Burnett</I></P>

<DL>

<DT><B><BIG>I</BIG>n this chapter

</B><DT>Introducing DNS

<DT>Configuring the Resolver

<DT>Using the <TT>named</TT> Daemon to Set Up the Server 

<DT>Troubleshooting

</DL>

<P>Originally, when the Internet was first formed, the number of hosts on the Net was very small. It was fairly easy to maintain the name/address mapping. Each host simply had a complete list of all host names and addresses in a local file. As the growth of the Internet accelerated, this system quickly became unwieldy. When a new host was added, it was necessary to update every host file on every computer. Also, because each new computer resulted in a new line in every host file, the size of the host files began to grow to quite a large size. Clearly, a new solution was needed.

</P>

<P>Mapping Internet system names to IP addresses is a task that requires a good degree of consideration. With the explosive growth of the Internet over the past few years, the original system of maintaining host name to IP address mappings in a local flat ASCII file quickly proved impractical. With thousands of computers on the Net and more being added daily, a new system was needed. That new system was a network-wide distributed database known as BIND, the Berkeley Internet Domain server. Also referred to variously as the Domain Name Service, the Domain Name System, or DNS, this system provides an effective, relatively transparent host name to the IP address mapping mechanism.</P>

<P>DNS is notoriously hard to configure, but when you&#146;re successful, it&#146;s fairly easy to maintain. This chapter provides a basic overview of how to set up and configure a DNS system. It is, by no means, a complete reference; whole books are available on the subject.</P>

<H3><A NAME="Heading2"></A><FONT COLOR="#000077">Introducing DNS</FONT></H3>

<P>DNS provides a mechanism for converting IP addresses into mnemonic names that represent hosts, networks, and mail aliases. It does this by dividing the entire Internet IP and name space into different logical groups. Each group has authority for its own computers and other information.

</P>

<P>Because DNS is a complicated topic, it has its own specialized set of terms. Table 25.1 lists the definitions of some commonly used DNS terms.</P>

<TABLE WIDTH="100%"><CAPTION ALIGN=LEFT><B>Table 25.1</B> Commonly Used DNS Terms

<TR>

<TH COLSPAN="2"><HR>

<TR>

<TH WIDTH="25%" ALIGN="LEFT">Term

<TH WIDTH="75%" ALIGN="LEFT">Definition

<TR>

<TD COLSPAN="2"><HR>

<TR>

<TD VALIGN="TOP">domain

<TD>The logical entity or organization that represents a part of a network. For example, unc.edu is the name of the primary domain for the University of North Carolina at Chapel Hill.

<TR>

<TD VALIGN="TOP">domain name

<TD>The name portion of a host name that represents the domain that contains the host. For example, in the address <B>sunsite.unc.edu</B>, the domain name is unc.edu. Also used interchangeably with <I>domain</I>.

<TR>

<TD VALIGN="TOP">host

<TD>A computer on a network.

<TR>

<TD VALIGN="TOP">node

<TD>A computer on a network.

<TR>

<TD VALIGN="TOP">name server

<TD>A computer that provides DNS services to map DNS names to IP addresses.

<TR>

<TD VALIGN="TOP">resolve

<TD>The act of translating a DNS name into its corresponding IP address.

<TR>

<TD VALIGN="TOP">resolver

<TD>A program or library routine that extracts DNS information from a name server.

<TR>

<TD VALIGN="TOP">reverse resolution

<TD>Matching a given IP address to its DNS name. This is also called reverse DNS.

<TR>

<TD VALIGN="TOP">spoof

<TD>The act of appearing to the network as having a different IP address or domain name.

<TR>

<TD COLSPAN="2"><HR>

</TABLE>

<P>DNS can be conceptually divided into the following three parts:

</P>

<DL>

<DD><B>&#149;</B>&nbsp;&nbsp;<I>Domain name space.</I> This is a specification for a tree structure that identifies a set of hosts and provides information about them. Conceptually, each node in the tree has a database of information about the hosts under its authority. Queries attempt to extract the appropriate information from this database. In simple terms, this is just the listing of all different types of information, names, IP addresses, mail aliases, and such that are available for lookup in the DNS system.

<DD><B>&#149;</B>&nbsp;&nbsp;<I>Name servers.</I> These are programs that hold and maintain the data located in the domain name space. Each name server has complete information about a subset of the domain name space and cached information about other portions.

<BR>A name server has complete information for its area of authority. This authoritative information is divided into areas known as <I>zones</I>, which can be divided among different name servers to provide redundant service for a zone. Each name server knows about other name servers that are responsible for different zones. If a request comes in for information from the zone that a given name server is responsible for, the name server simply returns the information. However, if a request comes in for information from a different zone, the name server contacts the appropriate server with authority for that zone.

<DD><B>&#149;</B>&nbsp;&nbsp;<I>Resolvers.</I> These are simply programs or library routines that extract information from the name servers in response to a query about a host in the domain name space.

</DL>

<H3><A NAME="Heading3"></A><FONT COLOR="#000077">Configuring the Resolver</FONT></H3>

<P>The first step in using DNS is to configure the resolver library on your computer. You must configure your local resolver if you intend to use DNS name resolution, even if you&#146;re not going to run a local domain name server.

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