external/bsd/bind/dist/lib/lwres/man/lwres.html

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  21 <title>lwres</title>
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  24 <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="refentry" lang="en">
  25 <a name="id2476275"></a><div class="titlepage"></div>
  26 <div class="refnamediv">
  27 <h2>Name</h2>
  28 <p>lwres &#8212; introduction to the lightweight resolver library</p>
  29 </div>
  30 <div class="refsynopsisdiv">
  31 <h2>Synopsis</h2>
  32 <div class="funcsynopsis"><pre class="funcsynopsisinfo">#include &lt;lwres/lwres.h&gt;</pre></div>
  33 </div>
  34 <div class="refsect1" lang="en">
  35 <a name="id2543357"></a><h2>DESCRIPTION</h2>
  36 <p>
  37       The BIND 9 lightweight resolver library is a simple, name service
  38       independent stub resolver library.  It provides hostname-to-address
  39       and address-to-hostname lookup services to applications by
  40       transmitting lookup requests to a resolver daemon
  41       <span><strong class="command">lwresd</strong></span>
  42       running on the local host. The resolver daemon performs the
  43       lookup using the DNS or possibly other name service protocols,
  44       and returns the results to the application through the library.
  45       The library and resolver daemon communicate using a simple
  46       UDP-based protocol.
  47     </p>
  48 </div>
  49 <div class="refsect1" lang="en">
  50 <a name="id2543370"></a><h2>OVERVIEW</h2>
  51 <p>
  52       The lwresd library implements multiple name service APIs.
  53       The standard
  54       <code class="function">gethostbyname()</code>,
  55       <code class="function">gethostbyaddr()</code>,
  56       <code class="function">gethostbyname_r()</code>,
  57       <code class="function">gethostbyaddr_r()</code>,
  58       <code class="function">getaddrinfo()</code>,
  59       <code class="function">getipnodebyname()</code>,
  60       and
  61       <code class="function">getipnodebyaddr()</code>
  62       functions are all supported.  To allow the lwres library to coexist
  63       with system libraries that define functions of the same name,
  64       the library defines these functions with names prefixed by
  65       <code class="literal">lwres_</code>.
  66       To define the standard names, applications must include the
  67       header file
  68       <code class="filename">&lt;lwres/netdb.h&gt;</code>
  69       which contains macro definitions mapping the standard function names
  70       into
  71       <code class="literal">lwres_</code>
  72       prefixed ones.  Operating system vendors who integrate the lwres
  73       library into their base distributions should rename the functions
  74       in the library proper so that the renaming macros are not needed.
  75     </p>
  76 <p>
  77       The library also provides a native API consisting of the functions
  78       <code class="function">lwres_getaddrsbyname()</code>
  79       and
  80       <code class="function">lwres_getnamebyaddr()</code>.
  81       These may be called by applications that require more detailed
  82       control over the lookup process than the standard functions
  83       provide.
  84     </p>
  85 <p>
  86       In addition to these name service independent address lookup
  87       functions, the library implements a new, experimental API
  88       for looking up arbitrary DNS resource records, using the
  89       <code class="function">lwres_getaddrsbyname()</code>
  90       function.
  91     </p>
  92 <p>
  93       Finally, there is a low-level API for converting lookup
  94       requests and responses to and from raw lwres protocol packets.
  95       This API can be used by clients requiring nonblocking operation,
  96       and is also used when implementing the server side of the lwres
  97       protocol, for example in the
  98       <span><strong class="command">lwresd</strong></span>
  99       resolver daemon.  The use of this low-level API in clients
 100       and servers is outlined in the following sections.
 101     </p>
 102 </div>
 103 <div class="refsect1" lang="en">
 104 <a name="id2543434"></a><h2>CLIENT-SIDE LOW-LEVEL API CALL FLOW</h2>
 105 <p>
 106       When a client program wishes to make an lwres request using the
 107       native low-level API, it typically performs the following
 108       sequence of actions.
 109     </p>
 110 <p>
 111       (1) Allocate or use an existing <span class="type">lwres_packet_t</span>,
 112       called <code class="varname">pkt</code> below.
 113     </p>
 114 <p>
 115       (2) Set <em class="structfield"><code>pkt.recvlength</code></em> to the maximum length
 116       we will accept.
 117       This is done so the receiver of our packets knows how large our receive
 118       buffer is.  The "default" is a constant in
 119       <code class="filename">lwres.h</code>: <code class="constant">LWRES_RECVLENGTH = 4096</code>.
 120     </p>
 121 <p>
 122       (3) Set <em class="structfield"><code>pkt.serial</code></em>
 123       to a unique serial number.  This value is echoed
 124       back to the application by the remote server.
 125     </p>
 126 <p>
 127       (4) Set <em class="structfield"><code>pkt.pktflags</code></em>.  Usually this is set to
 128       0.
 129     </p>
 130 <p>
 131       (5) Set <em class="structfield"><code>pkt.result</code></em> to 0.
 132     </p>
 133 <p>
 134       (6) Call <code class="function">lwres_*request_render()</code>,
 135       or marshall in the data using the primitives
 136       such as <code class="function">lwres_packet_render()</code>
 137       and storing the packet data.
 138     </p>
 139 <p>
 140       (7) Transmit the resulting buffer.
 141     </p>
 142 <p>
 143       (8) Call <code class="function">lwres_*response_parse()</code>
 144       to parse any packets received.
 145     </p>
 146 <p>
 147       (9) Verify that the opcode and serial match a request, and process the
 148       packet specific information contained in the body.
 149     </p>
 150 </div>
 151 <div class="refsect1" lang="en">
 152 <a name="id2543582"></a><h2>SERVER-SIDE LOW-LEVEL API CALL FLOW</h2>
 153 <p>
 154       When implementing the server side of the lightweight resolver
 155       protocol using the lwres library, a sequence of actions like the
 156       following is typically involved in processing each request packet.
 157     </p>
 158 <p>
 159       Note that the same <span class="type">lwres_packet_t</span> is used
 160       in both the <code class="function">_parse()</code> and <code class="function">_render()</code> calls,
 161       with only a few modifications made
 162       to the packet header's contents between uses.  This method is
 163       recommended
 164       as it keeps the serial, opcode, and other fields correct.
 165     </p>
 166 <p>
 167       (1) When a packet is received, call <code class="function">lwres_*request_parse()</code> to
 168       unmarshall it.  This returns a <span class="type">lwres_packet_t</span> (also called <code class="varname">pkt</code>, below)
 169       as well as a data specific type, such as <span class="type">lwres_gabnrequest_t</span>.
 170     </p>
 171 <p>
 172       (2) Process the request in the data specific type.
 173     </p>
 174 <p>
 175       (3) Set the <em class="structfield"><code>pkt.result</code></em>,
 176       <em class="structfield"><code>pkt.recvlength</code></em> as above.  All other fields
 177       can
 178       be left untouched since they were filled in by the <code class="function">*_parse()</code> call
 179       above.  If using <code class="function">lwres_*response_render()</code>,
 180       <em class="structfield"><code>pkt.pktflags</code></em> will be set up
 181       properly.  Otherwise, the <code class="constant">LWRES_LWPACKETFLAG_RESPONSE</code> bit should be
 182       set.
 183     </p>
 184 <p>
 185       (4) Call the data specific rendering function, such as
 186       <code class="function">lwres_gabnresponse_render()</code>.
 187     </p>
 188 <p>
 189       (5) Send the resulting packet to the client.
 190     </p>
 191 <p></p>
 192 </div>
 193 <div class="refsect1" lang="en">
 194 <a name="id2543666"></a><h2>SEE ALSO</h2>
 195 <p><span class="citerefentry"><span class="refentrytitle">lwres_gethostent</span>(3)</span>,
 196
 197       <span class="citerefentry"><span class="refentrytitle">lwres_getipnode</span>(3)</span>,
 198
 199       <span class="citerefentry"><span class="refentrytitle">lwres_getnameinfo</span>(3)</span>,
 200
 201       <span class="citerefentry"><span class="refentrytitle">lwres_noop</span>(3)</span>,
 202
 203       <span class="citerefentry"><span class="refentrytitle">lwres_gabn</span>(3)</span>,
 204
 205       <span class="citerefentry"><span class="refentrytitle">lwres_gnba</span>(3)</span>,
 206
 207       <span class="citerefentry"><span class="refentrytitle">lwres_context</span>(3)</span>,
 208
 209       <span class="citerefentry"><span class="refentrytitle">lwres_config</span>(3)</span>,
 210
 211       <span class="citerefentry"><span class="refentrytitle">resolver</span>(5)</span>,
 212
 213       <span class="citerefentry"><span class="refentrytitle">lwresd</span>(8)</span>.
 214
 215     </p>
 216 </div>
 217 </div></body>
 218 </html>