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 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.
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.