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<div class="titlepage"><div><div><h2 class="title">
<a name="Bv9ARM.ch05"></a>Chapter 5. The <span class="acronym">BIND</span> 9 Lightweight Resolver</h2></div></div></div>
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<dl>
<dt><span class="sect1"><a href="Bv9ARM.ch05.html#id2539034">The Lightweight Resolver Library</a></span></dt>
<dt><span class="sect1"><a href="Bv9ARM.ch05.html#lwresd">Running a Resolver Daemon</a></span></dt>
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="id2539034"></a>The Lightweight Resolver Library</h2></div></div></div>
<p>Traditionally applications have been linked with a stub resolver
library that sends recursive DNS queries to a local caching name
server.</p>
<p>IPv6 once introduced new complexity into the resolution process,
such as following A6 chains and DNAME records, and simultaneous
lookup of IPv4 and IPv6 addresses. Though most of the complexity was
then removed, these are hard or impossible
to implement in a traditional stub resolver.</p>
<p>Instead, <span class="acronym">BIND</span> 9 provides resolution services to local clients
using a combination of a lightweight resolver library and a resolver
daemon process running on the local host. These communicate using
a simple UDP-based protocol, the "lightweight resolver protocol"
that is distinct from and simpler than the full DNS protocol.</p>
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<div class="sect1" lang="en">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="lwresd"></a>Running a Resolver Daemon</h2></div></div></div>
<p>To use the lightweight resolver interface, the system must
run the resolver daemon <span><strong class="command">lwresd</strong></span> or a local
name server configured with a <span><strong class="command">lwres</strong></span> statement.</p>
<p>By default, applications using the lightweight resolver library will make
UDP requests to the IPv4 loopback address (127.0.0.1) on port 921. The
address can be overridden by <span><strong class="command">lwserver</strong></span> lines in
<code class="filename">/etc/resolv.conf</code>.</p>
<p>The daemon currently only looks in the DNS, but in the future
it may use other sources such as <code class="filename">/etc/hosts</code>,
NIS, etc.</p>
<p>The <span><strong class="command">lwresd</strong></span> daemon is essentially a
caching-only name server that responds to requests using the lightweight
resolver protocol rather than the DNS protocol. Because it needs
to run on each host, it is designed to require no or minimal configuration.
Unless configured otherwise, it uses the name servers listed on
<span><strong class="command">nameserver</strong></span> lines in <code class="filename">/etc/resolv.conf</code>
as forwarders, but is also capable of doing the resolution autonomously if
none are specified.</p>
<p>The <span><strong class="command">lwresd</strong></span> daemon may also be configured with a
<code class="filename">named.conf</code> style configuration file, in
<code class="filename">/etc/lwresd.conf</code> by default. A name server may also
be configured to act as a lightweight resolver daemon using the
<span><strong class="command">lwres</strong></span> statement in <code class="filename">named.conf</code>.</p>
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