LWRES(3)                      BIND9                      LWRES(3)



NAME
       lwres - introduction to the lightweight resolver library

SYNOPSIS
       #include 

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  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).



BIND9                      Jun 30, 2000                  LWRES(3)