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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="refentry" lang="en">
<a name="id2463721"></a><div class="titlepage"></div>
<div class="refnamediv">
<h2>Name</h2>
<p>lwres — introduction to the lightweight resolver library</p>
</div>
<div class="refsynopsisdiv">
<h2>Synopsis</h2>
<div class="funcsynopsis"><pre class="funcsynopsisinfo">#include <lwres/lwres.h></pre></div>
</div>
<div class="refsect1" lang="en">
<a name="id2525832"></a><h2>DESCRIPTION</h2>
<p>
The BIND 9 lightweight resolver library is a simple, name service
independent stub resolver library. It provides hostname-to-address
and address-to-hostname lookup services to applications by
transmitting lookup requests to a resolver daemon
<span><strong class="command">lwresd</strong></span>
running on the local host. The resover daemon performs the
lookup using the DNS or possibly other name service protocols,
and returns the results to the application through the library.
The library and resolver daemon communicate using a simple
UDP-based protocol.
</p>
</div>
<div class="refsect1" lang="en">
<a name="id2525845"></a><h2>OVERVIEW</h2>
<p>
The lwresd library implements multiple name service APIs.
The standard
<code class="function">gethostbyname()</code>,
<code class="function">gethostbyaddr()</code>,
<code class="function">gethostbyname_r()</code>,
<code class="function">gethostbyaddr_r()</code>,
<code class="function">getaddrinfo()</code>,
<code class="function">getipnodebyname()</code>,
and
<code class="function">getipnodebyaddr()</code>
functions are all supported. To allow the lwres library to coexist
with system libraries that define functions of the same name,
the library defines these functions with names prefixed by
<code class="literal">lwres_</code>.
To define the standard names, applications must include the
header file
<code class="filename"><lwres/netdb.h></code>
which contains macro definitions mapping the standard function names
into
<code class="literal">lwres_</code>
prefixed ones. Operating system vendors who integrate the lwres
library into their base distributions should rename the functions
in the library proper so that the renaming macros are not needed.
</p>
<p>
The library also provides a native API consisting of the functions
<code class="function">lwres_getaddrsbyname()</code>
and
<code class="function">lwres_getnamebyaddr()</code>.
These may be called by applications that require more detailed
control over the lookup process than the standard functions
provide.
</p>
<p>
In addition to these name service independent address lookup
functions, the library implements a new, experimental API
for looking up arbitrary DNS resource records, using the
<code class="function">lwres_getaddrsbyname()</code>
function.
</p>
<p>
Finally, there is a low-level API for converting lookup
requests and responses to and from raw lwres protocol packets.
This API can be used by clients requiring nonblocking operation,
and is also used when implementing the server side of the lwres
protocol, for example in the
<span><strong class="command">lwresd</strong></span>
resolver daemon. The use of this low-level API in clients
and servers is outlined in the following sections.
</p>
</div>
<div class="refsect1" lang="en">
<a name="id2525909"></a><h2>CLIENT-SIDE LOW-LEVEL API CALL FLOW</h2>
<p>
When a client program wishes to make an lwres request using the
native low-level API, it typically performs the following
sequence of actions.
</p>
<p>
(1) Allocate or use an existing <span class="type">lwres_packet_t</span>,
called <code class="varname">pkt</code> below.
</p>
<p>
(2) Set <em class="structfield"><code>pkt.recvlength</code></em> to the maximum length we will accept.
This is done so the receiver of our packets knows how large our receive
buffer is. The "default" is a constant in
<code class="filename">lwres.h</code>: <code class="constant">LWRES_RECVLENGTH = 4096</code>.
</p>
<p>
(3) Set <em class="structfield"><code>pkt.serial</code></em>
to a unique serial number. This value is echoed
back to the application by the remote server.
</p>
<p>
(4) Set <em class="structfield"><code>pkt.pktflags</code></em>. Usually this is set to 0.
</p>
<p>
(5) Set <em class="structfield"><code>pkt.result</code></em> to 0.
</p>
<p>
(6) Call <code class="function">lwres_*request_render()</code>,
or marshall in the data using the primitives
such as <code class="function">lwres_packet_render()</code>
and storing the packet data.
</p>
<p>
(7) Transmit the resulting buffer.
</p>
<p>
(8) Call <code class="function">lwres_*response_parse()</code>
to parse any packets received.
</p>
<p>
(9) Verify that the opcode and serial match a request, and process the
packet specific information contained in the body.
</p>
</div>
<div class="refsect1" lang="en">
<a name="id2526056"></a><h2>SERVER-SIDE LOW-LEVEL API CALL FLOW</h2>
<p>
When implementing the server side of the lightweight resolver
protocol using the lwres library, a sequence of actions like the
following is typically involved in processing each request packet.
</p>
<p>
Note that the same <span class="type">lwres_packet_t</span> is used
in both the <code class="function">_parse()</code> and <code class="function">_render()</code> calls,
with only a few modifications made
to the packet header's contents between uses. This method is recommended
as it keeps the serial, opcode, and other fields correct.
</p>
<p>
(1) When a packet is received, call <code class="function">lwres_*request_parse()</code> to
unmarshall it. This returns a <span class="type">lwres_packet_t</span> (also called <code class="varname">pkt</code>, below)
as well as a data specific type, such as <span class="type">lwres_gabnrequest_t</span>.
</p>
<p>
(2) Process the request in the data specific type.
</p>
<p>
(3) Set the <em class="structfield"><code>pkt.result</code></em>,
<em class="structfield"><code>pkt.recvlength</code></em> as above. All other fields can
be left untouched since they were filled in by the <code class="function">*_parse()</code> call
above. If using <code class="function">lwres_*response_render()</code>,
<em class="structfield"><code>pkt.pktflags</code></em> will be set up
properly. Otherwise, the <code class="constant">LWRES_LWPACKETFLAG_RESPONSE</code> bit should be
set.
</p>
<p>
(4) Call the data specific rendering function, such as
<code class="function">lwres_gabnresponse_render()</code>.
</p>
<p>
(5) Send the resulting packet to the client.
</p>
<p>
</p>
</div>
<div class="refsect1" lang="en">
<a name="id2526141"></a><h2>SEE ALSO</h2>
<p>
<span class="citerefentry"><span class="refentrytitle">lwres_gethostent</span>(3)</span>,
<span class="citerefentry"><span class="refentrytitle">lwres_getipnode</span>(3)</span>,
<span class="citerefentry"><span class="refentrytitle">lwres_getnameinfo</span>(3)</span>,
<span class="citerefentry"><span class="refentrytitle">lwres_noop</span>(3)</span>,
<span class="citerefentry"><span class="refentrytitle">lwres_gabn</span>(3)</span>,
<span class="citerefentry"><span class="refentrytitle">lwres_gnba</span>(3)</span>,
<span class="citerefentry"><span class="refentrytitle">lwres_context</span>(3)</span>,
<span class="citerefentry"><span class="refentrytitle">lwres_config</span>(3)</span>,
<span class="citerefentry"><span class="refentrytitle">resolver</span>(5)</span>,
<span class="citerefentry"><span class="refentrytitle">lwresd</span>(8)</span>.
</p>
</div>
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