lwres(3)

LWRES(3)                      BIND9                      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
     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.

ISC                  Last change: 2007-06-18                    1

LWRES(3)                      BIND9                      LWRES(3)

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.

ISC                  Last change: 2007-06-18                    2

LWRES(3)                      BIND9                      LWRES(3)

     (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).

AUTHOR

     Internet Systems Consortium, Inc.

COPYRIGHT

     Copyright 8c9 2004, 2005, 2007, 2014-2016 Internet Systems
     Consortium, Inc. ("ISC")
     Copyright 8c9 2000, 2001 Internet Software Consortium.

ISC                  Last change: 2007-06-18                    3