1LWRES(3)                             BIND9                            LWRES(3)
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NAME

6       lwres - introduction to the lightweight resolver library
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SYNOPSIS

9       #include <lwres/lwres.h>
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DESCRIPTION

12       The BIND 9 lightweight resolver library is a simple, name service
13       independent stub resolver library. It provides hostname-to-address and
14       address-to-hostname lookup services to applications by transmitting
15       lookup requests to a resolver daemon lwresd running on the local host.
16       The resolver daemon performs the lookup using the DNS or possibly other
17       name service protocols, and returns the results to the application
18       through the library. The library and resolver daemon communicate using
19       a simple UDP-based protocol.
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OVERVIEW

22       The lwresd library implements multiple name service APIs. The standard
23       gethostbyname(), gethostbyaddr(), gethostbyname_r(), gethostbyaddr_r(),
24       getaddrinfo(), getipnodebyname(), and getipnodebyaddr() functions are
25       all supported. To allow the lwres library to coexist with system
26       libraries that define functions of the same name, the library defines
27       these functions with names prefixed by lwres_. To define the standard
28       names, applications must include the header file <lwres/netdb.h> which
29       contains macro definitions mapping the standard function names into
30       lwres_ prefixed ones. Operating system vendors who integrate the lwres
31       library into their base distributions should rename the functions in
32       the library proper so that the renaming macros are not needed.
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34       The library also provides a native API consisting of the functions
35       lwres_getaddrsbyname() and lwres_getnamebyaddr(). These may be called
36       by applications that require more detailed control over the lookup
37       process than the standard functions provide.
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39       In addition to these name service independent address lookup functions,
40       the library implements a new, experimental API for looking up arbitrary
41       DNS resource records, using the lwres_getaddrsbyname() function.
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43       Finally, there is a low-level API for converting lookup requests and
44       responses to and from raw lwres protocol packets. This API can be used
45       by clients requiring nonblocking operation, and is also used when
46       implementing the server side of the lwres protocol, for example in the
47       lwresd resolver daemon. The use of this low-level API in clients and
48       servers is outlined in the following sections.
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CLIENT-SIDE LOW-LEVEL API CALL FLOW

51       When a client program wishes to make an lwres request using the native
52       low-level API, it typically performs the following sequence of actions.
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54       (1) Allocate or use an existing lwres_packet_t, called pkt below.
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56       (2) Set pkt.recvlength to the maximum length we will accept. This is
57       done so the receiver of our packets knows how large our receive buffer
58       is. The "default" is a constant in lwres.h: LWRES_RECVLENGTH = 4096.
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60       (3) Set pkt.serial to a unique serial number. This value is echoed back
61       to the application by the remote server.
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63       (4) Set pkt.pktflags. Usually this is set to 0.
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65       (5) Set pkt.result to 0.
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67       (6) Call lwres_*request_render(), or marshall in the data using the
68       primitives such as lwres_packet_render() and storing the packet data.
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70       (7) Transmit the resulting buffer.
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72       (8) Call lwres_*response_parse() to parse any packets received.
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74       (9) Verify that the opcode and serial match a request, and process the
75       packet specific information contained in the body.
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SERVER-SIDE LOW-LEVEL API CALL FLOW

78       When implementing the server side of the lightweight resolver protocol
79       using the lwres library, a sequence of actions like the following is
80       typically involved in processing each request packet.
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82       Note that the same lwres_packet_t is used in both the _parse() and
83       _render() calls, with only a few modifications made to the packet
84       header's contents between uses. This method is recommended as it keeps
85       the serial, opcode, and other fields correct.
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87       (1) When a packet is received, call lwres_*request_parse() to
88       unmarshall it. This returns a lwres_packet_t (also called pkt, below)
89       as well as a data specific type, such as lwres_gabnrequest_t.
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91       (2) Process the request in the data specific type.
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93       (3) Set the pkt.result, pkt.recvlength as above. All other fields can
94       be left untouched since they were filled in by the *_parse() call
95       above. If using lwres_*response_render(), pkt.pktflags will be set up
96       properly. Otherwise, the LWRES_LWPACKETFLAG_RESPONSE bit should be set.
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98       (4) Call the data specific rendering function, such as
99       lwres_gabnresponse_render().
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101       (5) Send the resulting packet to the client.
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SEE ALSO

104       lwres_gethostent(3), lwres_getipnode(3), lwres_getnameinfo(3),
105       lwres_noop(3), lwres_gabn(3), lwres_gnba(3), lwres_context(3),
106       lwres_config(3), resolver(5), lwresd(8).
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AUTHOR

109       Internet Systems Consortium, Inc.
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112       Copyright © 2000, 2001, 2004, 2005, 2007, 2014-2016, 2018-2020 Internet
113       Systems Consortium, Inc. ("ISC")
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117ISC                               2007-06-18                          LWRES(3)
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