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 resolver 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 © 2004, 2005, 2007, 2014 Internet Systems Consortium, Inc. ("ISC")
       Copyright © 2000, 2001 Internet Software Consortium.