lwres − introduction to the lightweight resolver library

#include <lwres/lwres.h>

     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.

     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.


     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

     (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

     (9) Verify that the opcode and serial match a request,
and process the packet specific information contained in the

     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

     (5) Send the resulting packet to the client.

     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).