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 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.
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
(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).
Copyright (C) 2004, 2005, 2007 Internet Systems Consortium, Inc.
Copyright (C) 2000, 2001 Internet Software Consortium.