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<refentry>

<refentryinfo>

<date>June 18, 2007</date>

</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>2004</year>

<year>2005</year>

<year>2007</year>

<year>2014</year>

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

</copyright>

<copyright>

<year>2000</year>

<year>2001</year>

<holder>Internet Software Consortium.</holder>

</copyright>

</docinfo>

<refsynopsisdiv>

<funcsynopsis>

<funcsynopsisinfo>#include &lt;lwres/lwres.h&gt;</funcsynopsisinfo>

</funcsynopsis>

</refsynopsisdiv>

<refsect1>

<title>DESCRIPTION</title>

<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>

</refsect1>

<refsect1>

<title>OVERVIEW</title>

<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>&lt;lwres/netdb.h&gt;</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>

</refsect1>

<refsect1>

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

<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 <structfield>pkt.recvlength</structfield> 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 <structfield>pkt.serial</structfield>

to a unique serial number. This value is echoed

back to the application by the remote server.

</para>

<para>

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

0.

</para>

<para>

(5) Set <structfield>pkt.result</structfield> 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>

</refsect1>

<refsect1>

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

<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 <structfield>pkt.result</structfield>,

<structfield>pkt.recvlength</structfield> 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>,

<structfield>pkt.pktflags</structfield> 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></para>

</refsect1>

<refsect1>

<title>SEE ALSO</title>

<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>

</refsect1>

</refentry><!--