lib/liblwres/man/lwres.3

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  17 .TH "LWRES" "3" "Jun 30, 2000" "BIND9" ""
  18 .SH NAME
  19 lwres \- introduction to the lightweight resolver library
  20 .SH SYNOPSIS
  21 \fB#include <lwres/lwres.h>\fR
  22 .SH "DESCRIPTION"
  23 .PP
  24 The BIND 9 lightweight resolver library is a simple, name service
  25 independent stub resolver library. It provides hostname-to-address
  26 and address-to-hostname lookup services to applications by
  27 transmitting lookup requests to a resolver daemon
  28 \fBlwresd\fR
  29 running on the local host. The resover daemon performs the
  30 lookup using the DNS or possibly other name service protocols,
  31 and returns the results to the application through the library.
  32 The library and resolver daemon communicate using a simple
  33 UDP-based protocol.
  34 .SH "OVERVIEW"
  35 .PP
  36 The lwresd library implements multiple name service APIs.
  37 The standard
  38 \fBgethostbyname()\fR,
  39 \fBgethostbyaddr()\fR,
  40 \fBgethostbyname_r()\fR,
  41 \fBgethostbyaddr_r()\fR,
  42 \fBgetaddrinfo()\fR,
  43 \fBgetipnodebyname()\fR,
  44 and
  45 \fBgetipnodebyaddr()\fR
  46 functions are all supported. To allow the lwres library to coexist
  47 with system libraries that define functions of the same name,
  48 the library defines these functions with names prefixed by
  49 lwres_.
  50 To define the standard names, applications must include the
  51 header file
  52 \fI<lwres/netdb.h>\fR
  53 which contains macro definitions mapping the standard function names
  54 into
  55 lwres_
  56 prefixed ones. Operating system vendors who integrate the lwres
  57 library into their base distributions should rename the functions
  58 in the library proper so that the renaming macros are not needed.
  59 .PP
  60 The library also provides a native API consisting of the functions
  61 \fBlwres_getaddrsbyname()\fR
  62 and
  63 \fBlwres_getnamebyaddr()\fR.
  64 These may be called by applications that require more detailed
  65 control over the lookup process than the standard functions
  66 provide.
  67 .PP
  68 In addition to these name service independent address lookup
  69 functions, the library implements a new, experimental API
  70 for looking up arbitrary DNS resource records, using the
  71 \fBlwres_getaddrsbyname()\fR
  72 function.
  73 .PP
  74 Finally, there is a low-level API for converting lookup
  75 requests and responses to and from raw lwres protocol packets.
  76 This API can be used by clients requiring nonblocking operation,
  77 and is also used when implementing the server side of the lwres
  78 protocol, for example in the
  79 \fBlwresd\fR
  80 resolver daemon. The use of this low-level API in clients
  81 and servers is outlined in the following sections.
  82 .SH "CLIENT-SIDE LOW-LEVEL API CALL FLOW"
  83 .PP
  84 When a client program wishes to make an lwres request using the
  85 native low-level API, it typically performs the following
  86 sequence of actions.
  87 .PP
  88 (1) Allocate or use an existing \fBlwres_packet_t\fR,
  89 called pkt below.
  90 .PP
  91 (2) Set \fBpkt.recvlength\fR to the maximum length we will accept.
  92 This is done so the receiver of our packets knows how large our receive
  93 buffer is. The "default" is a constant in
  94 \fIlwres.h\fR: LWRES_RECVLENGTH = 4096.
  95 .PP
  96 (3) Set \fBpkt.serial\fR
  97 to a unique serial number. This value is echoed
  98 back to the application by the remote server.
  99 .PP
 100 (4) Set \fBpkt.pktflags\fR. Usually this is set to 0.
 101 .PP
 102 (5) Set \fBpkt.result\fR to 0.
 103 .PP
 104 (6) Call \fBlwres_*request_render()\fR,
 105 or marshall in the data using the primitives
 106 such as \fBlwres_packet_render()\fR
 107 and storing the packet data.
 108 .PP
 109 (7) Transmit the resulting buffer.
 110 .PP
 111 (8) Call \fBlwres_*response_parse()\fR
 112 to parse any packets received.
 113 .PP
 114 (9) Verify that the opcode and serial match a request, and process the
 115 packet specific information contained in the body.
 116 .SH "SERVER-SIDE LOW-LEVEL API CALL FLOW"
 117 .PP
 118 When implementing the server side of the lightweight resolver
 119 protocol using the lwres library, a sequence of actions like the
 120 following is typically involved in processing each request packet.
 121 .PP
 122 Note that the same \fBlwres_packet_t\fR is used
 123 in both the \fB_parse()\fR and \fB_render()\fR calls,
 124 with only a few modifications made
 125 to the packet header's contents between uses. This method is recommended
 126 as it keeps the serial, opcode, and other fields correct.
 127 .PP
 128 (1) When a packet is received, call \fBlwres_*request_parse()\fR to
 129 unmarshall it. This returns a \fBlwres_packet_t\fR (also called pkt, below)
 130 as well as a data specific type, such as \fBlwres_gabnrequest_t\fR.
 131 .PP
 132 (2) Process the request in the data specific type.
 133 .PP
 134 (3) Set the \fBpkt.result\fR,
 135 \fBpkt.recvlength\fR as above. All other fields can
 136 be left untouched since they were filled in by the \fB*_parse()\fR call
 137 above. If using \fBlwres_*response_render()\fR,
 138 \fBpkt.pktflags\fR will be set up
 139 properly. Otherwise, the LWRES_LWPACKETFLAG_RESPONSE bit should be
 140 set.
 141 .PP
 142 (4) Call the data specific rendering function, such as
 143 \fBlwres_gabnresponse_render()\fR.
 144 .PP
 145 (5) Send the resulting packet to the client.
 146 .PP
 147 .SH "SEE ALSO"
 148 .PP
 149 \fBlwres_gethostent\fR(3),
 150 \fBlwres_getipnode\fR(3),
 151 \fBlwres_getnameinfo\fR(3),
 152 \fBlwres_noop\fR(3),
 153 \fBlwres_gabn\fR(3),
 154 \fBlwres_gnba\fR(3),
 155 \fBlwres_context\fR(3),
 156 \fBlwres_config\fR(3),
 157 \fBresolver\fR(5),
 158 \fBlwresd\fR(8).