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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 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(4), lwresd(8).

Index for Section 3

Alphabetical listing for L

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