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 -

 - This Source Code Form is subject to the terms of the Mozilla Public

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<refentry xmlns:db="http://docbook.org/ns/docbook" version="5.0">

  <info>

    <date>2007-06-18</date>

  </info>

  <refentryinfo>

    <corpname>ISC</corpname>

    <corpauthor>Internet Systems Consortium, Inc.</corpauthor>

  </refentryinfo>

  <refmeta>

    <refentrytitle>lwres</refentrytitle>

    <manvolnum>3</manvolnum>

    <refmiscinfo>BIND9</refmiscinfo>

  </refmeta>

  <refnamediv>

    <refname>lwres</refname>

    <refpurpose>introduction to the lightweight resolver library</refpurpose>

  </refnamediv>

  <docinfo>

    <copyright>

      <year>2000</year>

      <year>2001</year>

      <year>2004</year>

      <year>2005</year>

      <year>2007</year>

      <year>2014</year>

      <year>2015</year>

      <year>2016</year>

      <year>2018</year>

      <year>2019</year>

      <year>2020</year>

      <holder>Internet Systems Consortium, Inc. ("ISC")</holder>

    </copyright>

  </docinfo>

  <refsynopsisdiv>

    <funcsynopsis>

<funcsynopsisinfo>#include <lwres/lwres.h></funcsynopsisinfo>

</funcsynopsis>

  </refsynopsisdiv>

  <refsection><info><title>DESCRIPTION</title></info>

    <para>

      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

      <command>lwresd</command>

      running on the local host. The resolver 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.

    </para>

  </refsection>

  <refsection><info><title>OVERVIEW</title></info>

    <para>

      The lwresd library implements multiple name service APIs.

      The standard

      <function>gethostbyname()</function>,

      <function>gethostbyaddr()</function>,

      <function>gethostbyname_r()</function>,

      <function>gethostbyaddr_r()</function>,

      <function>getaddrinfo()</function>,

      <function>getipnodebyname()</function>,

      and

      <function>getipnodebyaddr()</function>

      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

      <literal>lwres_</literal>.

      To define the standard names, applications must include the

      header file

      <filename><lwres/netdb.h></filename>

      which contains macro definitions mapping the standard function names

      into

      <literal>lwres_</literal>

      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.

    </para>

    <para>

      The library also provides a native API consisting of the functions

      <function>lwres_getaddrsbyname()</function>

      and

      <function>lwres_getnamebyaddr()</function>.

      These may be called by applications that require more detailed

      control over the lookup process than the standard functions

      provide.

    </para>

    <para>

      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

      <function>lwres_getaddrsbyname()</function>

      function.

    </para>

    <para>

      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

      <command>lwresd</command>

      resolver daemon.  The use of this low-level API in clients

      and servers is outlined in the following sections.

    </para>

  </refsection>

  <refsection><info><title>CLIENT-SIDE LOW-LEVEL API CALL FLOW</title></info>

    <para>

      When a client program wishes to make an lwres request using the

      native low-level API, it typically performs the following

      sequence of actions.

    </para>

    <para>

      (1) Allocate or use an existing <type>lwres_packet_t</type>,

      called <varname>pkt</varname> below.

    </para>

    <para>

      (2) Set <varname remap="structfield">pkt.recvlength</varname> 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

      <filename>lwres.h</filename>: <constant>LWRES_RECVLENGTH = 4096</constant>.

    </para>

    <para>

      (3) Set <varname remap="structfield">pkt.serial</varname>

      to a unique serial number.  This value is echoed

      back to the application by the remote server.

    </para>

    <para>

      (4) Set <varname remap="structfield">pkt.pktflags</varname>.  Usually this is set to

      0.

    </para>

    <para>

      (5) Set <varname remap="structfield">pkt.result</varname> to 0.

    </para>

    <para>

      (6) Call <function>lwres_*request_render()</function>,

      or marshall in the data using the primitives

      such as <function>lwres_packet_render()</function>

      and storing the packet data.

    </para>

    <para>

      (7) Transmit the resulting buffer.

    </para>

    <para>

      (8) Call <function>lwres_*response_parse()</function>

      to parse any packets received.

    </para>

    <para>

      (9) Verify that the opcode and serial match a request, and process the

      packet specific information contained in the body.

    </para>

  </refsection>

  <refsection><info><title>SERVER-SIDE LOW-LEVEL API CALL FLOW</title></info>

    <para>

      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.

    </para>

    <para>

      Note that the same <type>lwres_packet_t</type> is used

      in both the <function>_parse()</function> and <function>_render()</function> 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.

    </para>

    <para>

      (1) When a packet is received, call <function>lwres_*request_parse()</function> to

      unmarshall it.  This returns a <type>lwres_packet_t</type> (also called <varname>pkt</varname>, below)

      as well as a data specific type, such as <type>lwres_gabnrequest_t</type>.

    </para>

    <para>

      (2) Process the request in the data specific type.

    </para>

    <para>

      (3) Set the <varname remap="structfield">pkt.result</varname>,

      <varname remap="structfield">pkt.recvlength</varname> as above.  All other fields

      can

      be left untouched since they were filled in by the <function>*_parse()</function> call

      above.  If using <function>lwres_*response_render()</function>,

      <varname remap="structfield">pkt.pktflags</varname> will be set up

      properly.  Otherwise, the <constant>LWRES_LWPACKETFLAG_RESPONSE</constant> bit should be

      set.

    </para>

    <para>

      (4) Call the data specific rendering function, such as

      <function>lwres_gabnresponse_render()</function>.

    </para>

    <para>

      (5) Send the resulting packet to the client.

    </para>

    <para/>

  </refsection>

  <refsection><info><title>SEE ALSO</title></info>

    <para><citerefentry>

        <refentrytitle>lwres_gethostent</refentrytitle><manvolnum>3</manvolnum>

      </citerefentry>,

      <citerefentry>

        <refentrytitle>lwres_getipnode</refentrytitle><manvolnum>3</manvolnum>

      </citerefentry>,

      <citerefentry>

        <refentrytitle>lwres_getnameinfo</refentrytitle><manvolnum>3</manvolnum>

      </citerefentry>,

      <citerefentry>

        <refentrytitle>lwres_noop</refentrytitle><manvolnum>3</manvolnum>

      </citerefentry>,

      <citerefentry>

        <refentrytitle>lwres_gabn</refentrytitle><manvolnum>3</manvolnum>

      </citerefentry>,

      <citerefentry>

        <refentrytitle>lwres_gnba</refentrytitle><manvolnum>3</manvolnum>

      </citerefentry>,

      <citerefentry>

        <refentrytitle>lwres_context</refentrytitle><manvolnum>3</manvolnum>

      </citerefentry>,

      <citerefentry>

        <refentrytitle>lwres_config</refentrytitle><manvolnum>3</manvolnum>

      </citerefentry>,

      <citerefentry>

        <refentrytitle>resolver</refentrytitle><manvolnum>5</manvolnum>

      </citerefentry>,

      <citerefentry>

        <refentrytitle>lwresd</refentrytitle><manvolnum>8</manvolnum>

      </citerefentry>.

    </para>

  </refsection>

</refentry>