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 inde-
       pendent 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 pro-
       cess 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  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)