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IN NO EVENT SHALL ISC BE LIABLE FOR ANY SPECIAL, DIRECT, 12 - INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM 13 - LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE 14 - OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR 15 - PERFORMANCE OF THIS SOFTWARE. 16--> 17<!-- Id --> 18<html> 19<head> 20<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"> 21<title>lwres</title> 22<meta name="generator" content="DocBook XSL Stylesheets V1.71.1"> 23</head> 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 — 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 <lwres/lwres.h></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"><lwres/netdb.h></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> 219