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LWRES(3)							      LWRES(3)

       lwres - introduction to the lightweight resolver	library

       #include	<lwres/lwres.h>

       The BIND	9 lightweight resolver library is a simple, name service inde-
       pendent stub resolver library. It provides hostname-to-address and  ad-
       dress-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.

       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 li-
       braries 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 im-
       plementing 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.

       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.

       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	unmar-
       shall 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_gabnre-

       (5) Send	the resulting packet to	the client.

       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)


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