lwres

LWRES(3)							     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	stan-
       dard  gethostbyname(),  gethostbyaddr(),	 gethostbyname_r(),  gethost-
       byaddr_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	func-
       tions,  the  library implements a new, experimental API for looking up
       arbitrary DNS resource records, using the lwres_getaddrsbyname() func-
       tion.

       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 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-
       sponse_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)