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LWRES(3)                             BIND9                            LWRES(3)



NAME
       lwres - introduction to the lightweight resolver library

SYNOPSIS
       #include <lwres/lwres.h>

DESCRIPTION
       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 lwresd running on the local host. The resover 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.

OVERVIEW
       The lwresd library implements multiple name service APIs. The standard gethostbyname(), gethostbyaddr(),
       gethostbyname_r(), gethostbyaddr_r(), getaddrinfo(), getipnodebyname(), and getipnodebyaddr() 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 lwres_. To define the standard names, applications
       must include the header file <lwres/netdb.h> which contains macro definitions mapping the standard function
       names into lwres_ 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.

       The library also provides a native API consisting of the functions lwres_getaddrsbyname() and
       lwres_getnamebyaddr(). These may be called by applications that require more detailed control over the lookup
       process than the standard functions provide.

       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 lwres_getaddrsbyname() function.

       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 lwresd resolver daemon. The use of this low-level API
       in clients and servers is outlined in the following sections.

CLIENT-SIDE LOW-LEVEL API CALL FLOW
       When a client program wishes to make an lwres request using the native low-level API, it typically performs the
       following sequence of actions.

       (1) Allocate or use an existing lwres_packet_t, called pkt below.

       (2) Set pkt.recvlength 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 lwres.h: LWRES_RECVLENGTH = 4096.

       (3) Set pkt.serial to a unique serial number. This value is echoed back to the application by the remote
       server.

       (4) Set pkt.pktflags. Usually this is set to 0.

       (5) Set pkt.result to 0.

       (6) Call lwres_*request_render(), or marshall in the data using the primitives such as lwres_packet_render()
       and storing the packet data.

       (7) Transmit the resulting buffer.

       (8) Call lwres_*response_parse() to parse any packets received.

       (9) Verify that the opcode and serial match a request, and process the packet specific information contained in
       the body.

SERVER-SIDE LOW-LEVEL API CALL FLOW
       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.

       Note that the same lwres_packet_t is used in both the _parse() and _render() 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.

       (1) When a packet is received, call lwres_*request_parse() to unmarshall it. This returns a lwres_packet_t
       (also called pkt, below) as well as a data specific type, such as lwres_gabnrequest_t.

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

       (3) Set the pkt.result, pkt.recvlength as above. All other fields can be left untouched since they were filled
       in by the *_parse() call above. If using lwres_*response_render(), pkt.pktflags will be set up properly.
       Otherwise, the LWRES_LWPACKETFLAG_RESPONSE bit should be set.

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

       (5) Send the resulting packet to the client.


SEE ALSO
       lwres_gethostent(3), lwres_getipnode(3), lwres_getnameinfo(3), lwres_noop(3), lwres_gabn(3), lwres_gnba(3),
       lwres_context(3), lwres_config(3), resolver(5), lwresd(8).

COPYRIGHT
       Copyright (C) 2004, 2005, 2007 Internet Systems Consortium, Inc. ("ISC")
       Copyright (C) 2000, 2001 Internet Software Consortium.



BIND9                            Jun 30, 2000                         LWRES(3)