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e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="refentry" lang="en">
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer<a name="id2463721"></a><div class="titlepage"></div>
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer<p>lwres — introduction to the lightweight resolver library</p>
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer<div class="funcsynopsis"><pre class="funcsynopsisinfo">#include <lwres/lwres.h></pre></div>
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer The BIND 9 lightweight resolver library is a simple, name service
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer independent stub resolver library. It provides hostname-to-address
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer and address-to-hostname lookup services to applications by
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer transmitting lookup requests to a resolver daemon
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer <span><strong class="command">lwresd</strong></span>
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer running on the local host. The resover daemon performs the
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer lookup using the DNS or possibly other name service protocols,
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer and returns the results to the application through the library.
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer The library and resolver daemon communicate using a simple
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer UDP-based protocol.
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer The lwresd library implements multiple name service APIs.
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer The standard
744e50acbaaf22481b25b921b2bc199c8dd4a35cStephen Brooks <code class="function">gethostbyname()</code>,
744e50acbaaf22481b25b921b2bc199c8dd4a35cStephen Brooks <code class="function">gethostbyaddr()</code>,
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer <code class="function">gethostbyname_r()</code>,
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer <code class="function">gethostbyaddr_r()</code>,
7656b92221e26376547b0dc52b67e7a414ea9775Stephen Brooks <code class="function">getipnodebyname()</code>,
7656b92221e26376547b0dc52b67e7a414ea9775Stephen Brooks <code class="function">getipnodebyaddr()</code>
7656b92221e26376547b0dc52b67e7a414ea9775Stephen Brooks functions are all supported. To allow the lwres library to coexist
7a0878494fd11b7a861d0b3e2034534c10b5fb46Stephen Brooks with system libraries that define functions of the same name,
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer the library defines these functions with names prefixed by
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer To define the standard names, applications must include the
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer <code class="filename"><lwres/netdb.h></code>
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer which contains macro definitions mapping the standard function names
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer prefixed ones. Operating system vendors who integrate the lwres
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer library into their base distributions should rename the functions
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer in the library proper so that the renaming macros are not needed.
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer The library also provides a native API consisting of the functions
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer <code class="function">lwres_getaddrsbyname()</code>
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer <code class="function">lwres_getnamebyaddr()</code>.
7656b92221e26376547b0dc52b67e7a414ea9775Stephen Brooks These may be called by applications that require more detailed
7656b92221e26376547b0dc52b67e7a414ea9775Stephen Brooks control over the lookup process than the standard functions
7656b92221e26376547b0dc52b67e7a414ea9775Stephen Brooks In addition to these name service independent address lookup
7656b92221e26376547b0dc52b67e7a414ea9775Stephen Brooks functions, the library implements a new, experimental API
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer for looking up arbitrary DNS resource records, using the
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer <code class="function">lwres_getaddrsbyname()</code>
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer Finally, there is a low-level API for converting lookup
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer requests and responses to and from raw lwres protocol packets.
7a0878494fd11b7a861d0b3e2034534c10b5fb46Stephen Brooks This API can be used by clients requiring nonblocking operation,
7a0878494fd11b7a861d0b3e2034534c10b5fb46Stephen Brooks and is also used when implementing the server side of the lwres
7a0878494fd11b7a861d0b3e2034534c10b5fb46Stephen Brooks protocol, for example in the
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer <span><strong class="command">lwresd</strong></span>
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer resolver daemon. The use of this low-level API in clients
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer and servers is outlined in the following sections.
7656b92221e26376547b0dc52b67e7a414ea9775Stephen Brooks<a name="id2525917"></a><h2>CLIENT-SIDE LOW-LEVEL API CALL FLOW</h2>
7656b92221e26376547b0dc52b67e7a414ea9775Stephen Brooks When a client program wishes to make an lwres request using the
7656b92221e26376547b0dc52b67e7a414ea9775Stephen Brooks native low-level API, it typically performs the following
7656b92221e26376547b0dc52b67e7a414ea9775Stephen Brooks sequence of actions.
7656b92221e26376547b0dc52b67e7a414ea9775Stephen Brooks (1) Allocate or use an existing <span class="type">lwres_packet_t</span>,
7656b92221e26376547b0dc52b67e7a414ea9775Stephen Brooks called <code class="varname">pkt</code> below.
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer (2) Set <em class="structfield"><code>pkt.recvlength</code></em> to the maximum length
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer we will accept.
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer This is done so the receiver of our packets knows how large our receive
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer buffer is. The "default" is a constant in
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer <code class="filename">lwres.h</code>: <code class="constant">LWRES_RECVLENGTH = 4096</code>.
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer (3) Set <em class="structfield"><code>pkt.serial</code></em>
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer to a unique serial number. This value is echoed
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer back to the application by the remote server.
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer (4) Set <em class="structfield"><code>pkt.pktflags</code></em>. Usually this is set to
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer (5) Set <em class="structfield"><code>pkt.result</code></em> to 0.
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer (6) Call <code class="function">lwres_*request_render()</code>,
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer or marshall in the data using the primitives
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer such as <code class="function">lwres_packet_render()</code>
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer and storing the packet data.
7656b92221e26376547b0dc52b67e7a414ea9775Stephen Brooks (7) Transmit the resulting buffer.
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer (8) Call <code class="function">lwres_*response_parse()</code>
7a0878494fd11b7a861d0b3e2034534c10b5fb46Stephen Brooks to parse any packets received.
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer (9) Verify that the opcode and serial match a request, and process the
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer packet specific information contained in the body.
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer<a name="id2526066"></a><h2>SERVER-SIDE LOW-LEVEL API CALL FLOW</h2>
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer When implementing the server side of the lightweight resolver
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer protocol using the lwres library, a sequence of actions like the
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer following is typically involved in processing each request packet.
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer Note that the same <span class="type">lwres_packet_t</span> is used
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer in both the <code class="function">_parse()</code> and <code class="function">_render()</code> calls,
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer with only a few modifications made
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer to the packet header's contents between uses. This method is
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer as it keeps the serial, opcode, and other fields correct.
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer (1) When a packet is received, call <code class="function">lwres_*request_parse()</code> to
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer unmarshall it. This returns a <span class="type">lwres_packet_t</span> (also called <code class="varname">pkt</code>, below)
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer as well as a data specific type, such as <span class="type">lwres_gabnrequest_t</span>.
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer (2) Process the request in the data specific type.
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer (3) Set the <em class="structfield"><code>pkt.result</code></em>,
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer <em class="structfield"><code>pkt.recvlength</code></em> as above. All other fields
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer be left untouched since they were filled in by the <code class="function">*_parse()</code> call
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer above. If using <code class="function">lwres_*response_render()</code>,
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer <em class="structfield"><code>pkt.pktflags</code></em> will be set up
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer properly. Otherwise, the <code class="constant">LWRES_LWPACKETFLAG_RESPONSE</code> bit should be
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer (4) Call the data specific rendering function, such as
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer <code class="function">lwres_gabnresponse_render()</code>.
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer (5) Send the resulting packet to the client.
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer<p><span class="citerefentry"><span class="refentrytitle">lwres_gethostent</span>(3)</span>,
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer <span class="citerefentry"><span class="refentrytitle">lwres_getipnode</span>(3)</span>,
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer <span class="citerefentry"><span class="refentrytitle">lwres_getnameinfo</span>(3)</span>,
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer <span class="citerefentry"><span class="refentrytitle">lwres_noop</span>(3)</span>,
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer <span class="citerefentry"><span class="refentrytitle">lwres_gabn</span>(3)</span>,
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer <span class="citerefentry"><span class="refentrytitle">lwres_gnba</span>(3)</span>,
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer <span class="citerefentry"><span class="refentrytitle">lwres_context</span>(3)</span>,
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer <span class="citerefentry"><span class="refentrytitle">lwres_config</span>(3)</span>,
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer <span class="citerefentry"><span class="refentrytitle">resolver</span>(5)</span>,
e4f5a11d4a234623168c1558fcdf4341e11769e1James Kremer <span class="citerefentry"><span class="refentrytitle">lwresd</span>(8)</span>.