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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>