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>BIND 9 Administrator Reference Manual</TH
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><A
NAME="ch05"
>Chapter 5. The <SPAN
CLASS="acronym"
>BIND</SPAN
> 9 Lightweight Resolver</A
></H1
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>Table of Contents</B
></DT
><DT
>5.1. <A
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>The Lightweight Resolver Library</A
></DT
><DT
>5.2. <A
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>Running a Resolver Daemon</A
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>5.1. The Lightweight Resolver Library</A
></H1
><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 introduces new complexity into the resolution process,
such as following A6 chains and DNAME records, and simultaneous
lookup of IPv4 and IPv6 addresses.  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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><H1
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><A
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NAME="lwresd"
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>5.2. Running a Resolver Daemon</A
></H1
><P
>To use the lightweight resolver interface, the system must
run the resolver daemon <B
CLASS="command"
>lwresd</B
>.</P
><P
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>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 overriden by <B
CLASS="command"
>lwserver</B
> lines in
<TT
CLASS="filename"
>/etc/resolv.conf</TT
>.
The daemon will try to find the answer to the questions "what are the
addresses for host
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<TT
CLASS="literal"
>foo.example.com</TT
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>?" and "what are
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the names for IPv4 address 10.1.2.3?"</P
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><P
>The daemon currently only looks in the DNS, but in the future
it may use other sources such as <TT
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CLASS="filename"
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>/etc/hosts</TT
>,
NIS, etc.</P
><P
>The <B
CLASS="command"
>lwresd</B
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> daemon is essentially a
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caching-only name server that answers 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.
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Unless configured otherwise, it uses the name servers listed on
<B
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CLASS="command"
>nameserver</B
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> lines in <TT
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CLASS="filename"
>/etc/resolv.conf</TT
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>
as forwarders, but is also capable of doing the resolution autonomously if
none are specified.</P
><P
>The <B
CLASS="command"
>lwresd</B
> daemon may also be configured with a
<TT
CLASS="filename"
>named.conf</TT
> style configuration file, in
<TT
CLASS="filename"
>/etc/lwresd.conf</TT
> by default.  A name server may also
be configured to act as a lightweight resolver daemon using the
<B
CLASS="command"
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>lwres</B
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> statement in <TT
CLASS="filename"
>named.conf</TT
>.</P
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