lwres - introduction to the lightweight resolver library
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
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.
The lwresd library implements multiple name service APIs. The standard
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
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
The library also provides a native API consisting of the functions
. 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()
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
The use of this low-level API in clients and servers is outlined in the
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
(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 =
(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()
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()
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
, 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.
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