net/resolver.3

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  32 .\"     @(#)resolver.3  8.1 (Berkeley) 6/4/93
  33 .\" $FreeBSD: src/lib/libc/net/resolver.3,v 1.21 2001/10/01 16:08:56 ru Exp $
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  35 .Dd June 4, 1993
  36 .Dt RESOLVER 3
  37 .Os
  38 .Sh NAME
  39 .Nm res_query ,
  40 .Nm res_search ,
  41 .Nm res_mkquery ,
  42 .Nm res_send ,
  43 .Nm res_init ,
  44 .Nm dn_comp ,
  45 .Nm dn_expand ,
  46 .Nm dn_skipname ,
  47 .Nm ns_get16 ,
  48 .Nm ns_get32 ,
  49 .Nm ns_put16 ,
  50 .Nm ns_put32
  51 .Nd resolver routines
  52 .Sh LIBRARY
  53 .Lb libc
  54 .Sh SYNOPSIS
  55 .In sys/types.h
  56 .In netinet/in.h
  57 .In arpa/nameser.h
  58 .In resolv.h
  59 .Ft int
  60 .Fo res_query
  61 .Fa "const char *dname"
  62 .Fa "int class"
  63 .Fa "int type"
  64 .Fa "u_char *answer"
  65 .Fa "int anslen"
  66 .Fc
  67 .Ft int
  68 .Fo res_search
  69 .Fa "const char *dname"
  70 .Fa "int class"
  71 .Fa "int type"
  72 .Fa "u_char *answer"
  73 .Fa "int anslen"
  74 .Fc
  75 .Ft int
  76 .Fo res_mkquery
  77 .Fa "int op"
  78 .Fa "const char *dname"
  79 .Fa "int class"
  80 .Fa "int type"
  81 .Fa "const u_char *data"
  82 .Fa "int datalen"
  83 .Fa "const u_char *newrr_in"
  84 .Fa "u_char *buf"
  85 .Fa "int buflen"
  86 .Fc
  87 .Ft int
  88 .Fo res_send
  89 .Fa "const u_char *msg"
  90 .Fa "int msglen"
  91 .Fa "u_char *answer"
  92 .Fa "int anslen"
  93 .Fc
  94 .Ft int
  95 .Fn res_init
  96 .Ft int
  97 .Fo dn_comp
  98 .Fa "const char *exp_dn"
  99 .Fa "u_char *comp_dn"
 100 .Fa "int length"
 101 .Fa "u_char **dnptrs"
 102 .Fa "u_char **lastdnptr"
 103 .Fc
 104 .Ft int
 105 .Fo dn_expand
 106 .Fa "const u_char *msg"
 107 .Fa "const u_char *eomorig"
 108 .Fa "const u_char *comp_dn"
 109 .Fa "char *exp_dn"
 110 .Fa "int length"
 111 .Fc
 112 .Ft int
 113 .Fn dn_skipname "const u_char *comp_dn" "const u_char *eom"
 114 .Ft u_int
 115 .Fn ns_get16 "const u_char *src"
 116 .Ft u_long
 117 .Fn ns_get32 "const u_char *src"
 118 .Ft void
 119 .Fn ns_put16 "u_int src" "u_char *dst"
 120 .Ft void
 121 .Fn ns_put32 "u_long src" "u_char *dst"
 122 .Sh DESCRIPTION
 123 These routines are used for making, sending and interpreting
 124 query and reply messages with Internet domain name servers.
 125 .Pp
 126 Global configuration and state information that is used by the
 127 resolver routines is kept in the structure
 128 .Em _res .
 129 Most of the values have reasonable defaults and can be ignored.
 130 Options
 131 stored in
 132 .Em _res.options
 133 are defined in
 134 .Pa resolv.h
 135 and are as follows.
 136 Options are stored as a simple bit mask containing the bitwise ``or''
 137 of the options enabled.
 138 .Bl -tag -width RES_USE_INET6
 139 .It Dv RES_INIT
 140 True if the initial name server address and default domain name are
 141 initialized (i.e.,
 142 .Fn res_init
 143 has been called).
 144 .It Dv RES_DEBUG
 145 Print debugging messages.
 146 .It Dv RES_AAONLY
 147 Accept authoritative answers only.
 148 With this option,
 149 .Fn res_send
 150 should continue until it finds an authoritative answer or finds an error.
 151 Currently this is not implemented.
 152 .It Dv RES_USEVC
 153 Use
 154 .Tn TCP
 155 connections for queries instead of
 156 .Tn UDP
 157 datagrams.
 158 .It Dv RES_STAYOPEN
 159 Used with
 160 .Dv RES_USEVC
 161 to keep the
 162 .Tn TCP
 163 connection open between
 164 queries.
 165 This is useful only in programs that regularly do many queries.
 166 .Tn UDP
 167 should be the normal mode used.
 168 .It Dv RES_IGNTC
 169 Unused currently (ignore truncation errors, i.e., don't retry with
 170 .Tn TCP ) .
 171 .It Dv RES_RECURSE
 172 Set the recursion-desired bit in queries.
 173 This is the default.
 174 .Pf ( Fn res_send
 175 does not do iterative queries and expects the name server
 176 to handle recursion.)
 177 .It Dv RES_DEFNAMES
 178 If set,
 179 .Fn res_search
 180 will append the default domain name to single-component names
 181 (those that do not contain a dot).
 182 This option is enabled by default.
 183 .It Dv RES_DNSRCH
 184 If this option is set,
 185 .Fn res_search
 186 will search for host names in the current domain and in parent domains; see
 187 .Xr hostname 7 .
 188 This is used by the standard host lookup routine
 189 .Xr gethostbyname 3 .
 190 This option is enabled by default.
 191 .It Dv RES_NOALIASES
 192 This option turns off the user level aliasing feature controlled by the
 193 .Dq Ev HOSTALIASES
 194 environment variable.  Network daemons should set this option.
 195 .It Dv RES_USE_INET6
 196 Enables support for IPv6-only applications.
 197 This causes IPv4 addresses to be returned as an IPv4 mapped address.
 198 For example,
 199 .Li 10.1.1.1
 200 will be returned as
 201 .Li ::ffff:10.1.1.1 .
 202 The option is meaningful with certain kernel configuration only.
 203 .It Dv RES_USE_EDNS0
 204 Enables support for OPT pseudo-RR for EDNS0 extension.
 205 With the option, resolver code will attach OPT pseudo-RR into DNS queries,
 206 to inform of our receive buffer size.
 207 The option will allow DNS servers to take advantage of non-default receive
 208 buffer size, and to send larger replies.
 209 DNS query packets with EDNS0 extension is not compatible with
 210 non-EDNS0 DNS servers.
 211 .El
 212 .Pp
 213 The
 214 .Fn res_init
 215 routine
 216 reads the configuration file (if any; see
 217 .Xr resolver 5 )
 218 to get the default domain name,
 219 search list and
 220 the Internet address of the local name server(s).
 221 If no server is configured, the host running
 222 the resolver is tried.
 223 The current domain name is defined by the hostname
 224 if not specified in the configuration file;
 225 it can be overridden by the environment variable
 226 .Ev LOCALDOMAIN .
 227 This environment variable may contain several blank-separated
 228 tokens if you wish to override the
 229 .Em "search list"
 230 on a per-process basis.  This is similar to the
 231 .Em search
 232 command in the configuration file.
 233 Another environment variable
 234 .Dq Ev RES_OPTIONS
 235 can be set to
 236 override certain internal resolver options which are otherwise
 237 set by changing fields in the
 238 .Em _res
 239 structure or are inherited from the configuration file's
 240 .Em options
 241 command.  The syntax of the
 242 .Dq Ev RES_OPTIONS
 243 environment variable is explained in
 244 .Xr resolver 5 .
 245 Initialization normally occurs on the first call
 246 to one of the following routines.
 247 .Pp
 248 The
 249 .Fn res_query
 250 function provides an interface to the server query mechanism.
 251 It constructs a query, sends it to the local server,
 252 awaits a response, and makes preliminary checks on the reply.
 253 The query requests information of the specified
 254 .Fa type
 255 and
 256 .Fa class
 257 for the specified fully-qualified domain name
 258 .Fa dname .
 259 The reply message is left in the
 260 .Fa answer
 261 buffer with length
 262 .Fa anslen
 263 supplied by the caller.
 264 .Pp
 265 The
 266 .Fn res_search
 267 routine makes a query and awaits a response like
 268 .Fn res_query ,
 269 but in addition, it implements the default and search rules
 270 controlled by the
 271 .Dv RES_DEFNAMES
 272 and
 273 .Dv RES_DNSRCH
 274 options.
 275 It returns the first successful reply.
 276 .Pp
 277 The remaining routines are lower-level routines used by
 278 .Fn res_query .
 279 The
 280 .Fn res_mkquery
 281 function
 282 constructs a standard query message and places it in
 283 .Fa buf .
 284 It returns the size of the query, or \-1 if the query is
 285 larger than
 286 .Fa buflen .
 287 The query type
 288 .Fa op
 289 is usually
 290 .Dv QUERY ,
 291 but can be any of the query types defined in
 292 .Aq Pa arpa/nameser.h .
 293 The domain name for the query is given by
 294 .Fa dname .
 295 .Fa Newrr
 296 is currently unused but is intended for making update messages.
 297 .Pp
 298 The
 299 .Fn res_send
 300 routine
 301 sends a pre-formatted query and returns an answer.
 302 It will call
 303 .Fn res_init
 304 if
 305 .Dv RES_INIT
 306 is not set, send the query to the local name server, and
 307 handle timeouts and retries.
 308 The length of the reply message is returned, or
 309 \-1 if there were errors.
 310 .Pp
 311 The
 312 .Fn dn_comp
 313 function
 314 compresses the domain name
 315 .Fa exp_dn
 316 and stores it in
 317 .Fa comp_dn .
 318 The size of the compressed name is returned or \-1 if there were errors.
 319 The size of the array pointed to by
 320 .Fa comp_dn
 321 is given by
 322 .Fa length .
 323 The compression uses
 324 an array of pointers
 325 .Fa dnptrs
 326 to previously-compressed names in the current message.
 327 The first pointer points to
 328 the beginning of the message and the list ends with
 329 .Dv NULL .
 330 The limit to the array is specified by
 331 .Fa lastdnptr .
 332 A side effect of
 333 .Fn dn_comp
 334 is to update the list of pointers for
 335 labels inserted into the message
 336 as the name is compressed.
 337 If
 338 .Em dnptr
 339 is
 340 .Dv NULL ,
 341 names are not compressed.
 342 If
 343 .Fa lastdnptr
 344 is
 345 .Dv NULL ,
 346 the list of labels is not updated.
 347 .Pp
 348 The
 349 .Fn dn_expand
 350 entry
 351 expands the compressed domain name
 352 .Fa comp_dn
 353 to a full domain name
 354 The compressed name is contained in a query or reply message;
 355 .Fa msg
 356 is a pointer to the beginning of the message.
 357 The uncompressed name is placed in the buffer indicated by
 358 .Fa exp_dn
 359 which is of size
 360 .Fa length .
 361 The size of compressed name is returned or \-1 if there was an error.
 362 .Pp
 363 The
 364 .Fn dn_skipname
 365 function skips over a compressed domain name, which starts at a location
 366 pointed to by
 367 .Fa comp_dn .
 368 The compressed name is contained in a query or reply message;
 369 .Fa eom
 370 is a pointer to the end of the message.
 371 The size of compressed name is returned or \-1 if there was
 372 an error.
 373 .Pp
 374 The
 375 .Fn ns_get16
 376 function gets a 16-bit quantity from a buffer pointed to by
 377 .Fa src .
 378 .Pp
 379 The
 380 .Fn ns_get32
 381 function gets a 32-bit quantity from a buffer pointed to by
 382 .Fa src .
 383 .Pp
 384 The
 385 .Fn ns_put16
 386 function puts a 16-bit quantity
 387 .Fa src
 388 to a buffer pointed to by
 389 .Fa dst .
 390 .Pp
 391 The
 392 .Fn ns_put32
 393 function puts a 32-bit quantity
 394 .Fa src
 395 to a buffer pointed to by
 396 .Fa dst .
 397 .Sh FILES
 398 .Bl -tag -width /etc/resolv.conf
 399 .It Pa /etc/resolv.conf
 400 The configuration file,
 401 see
 402 .Xr resolver 5 .
 403 .El
 404 .Sh SEE ALSO
 405 .Xr gethostbyname 3 ,
 406 .Xr resolver 5 ,
 407 .Xr hostname 7 ,
 408 .Xr named 8
 409 .Pp
 410 .%T RFC1032 ,
 411 .%T RFC1033 ,
 412 .%T RFC1034 ,
 413 .%T RFC1035 ,
 414 .%T RFC974
 415 .Rs
 416 .%T "Name Server Operations Guide for BIND"
 417 .Re
 418 .Sh HISTORY
 419 The
 420 .Nm
 421 function appeared in
 422 .Bx 4.3 .