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 libresolv
  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.  Constants for
 264 .Fa type
 265 and
 266 .Fa class
 267 are defined in
 268 .Aq Pa arpa/nameser.h .
 269 .Pp
 270 The
 271 .Fn res_search
 272 routine makes a query and awaits a response like
 273 .Fn res_query ,
 274 but in addition, it implements the default and search rules
 275 controlled by the
 276 .Dv RES_DEFNAMES
 277 and
 278 .Dv RES_DNSRCH
 279 options.
 280 It returns the first successful reply.
 281 .Pp
 282 The remaining routines are lower-level routines used by
 283 .Fn res_query .
 284 The
 285 .Fn res_mkquery
 286 function
 287 constructs a standard query message and places it in
 288 .Fa buf .
 289 It returns the size of the query, or \-1 if the query is
 290 larger than
 291 .Fa buflen .
 292 The query type
 293 .Fa op
 294 is usually
 295 .Dv QUERY ,
 296 but can be any of the query types defined in
 297 .Aq Pa arpa/nameser.h .
 298 The domain name for the query is given by
 299 .Fa dname .
 300 .Fa Newrr
 301 is currently unused but is intended for making update messages.
 302 .Pp
 303 The
 304 .Fn res_send
 305 routine
 306 sends a pre-formatted query and returns an answer.
 307 It will call
 308 .Fn res_init
 309 if
 310 .Dv RES_INIT
 311 is not set, send the query to the local name server, and
 312 handle timeouts and retries.
 313 The length of the reply message is returned, or
 314 \-1 if there were errors.
 315 .Pp
 316 The
 317 .Fn dn_comp
 318 function
 319 compresses the domain name
 320 .Fa exp_dn
 321 and stores it in
 322 .Fa comp_dn .
 323 The size of the compressed name is returned or \-1 if there were errors.
 324 The size of the array pointed to by
 325 .Fa comp_dn
 326 is given by
 327 .Fa length .
 328 The compression uses
 329 an array of pointers
 330 .Fa dnptrs
 331 to previously-compressed names in the current message.
 332 The first pointer points to
 333 the beginning of the message and the list ends with
 334 .Dv NULL .
 335 The limit to the array is specified by
 336 .Fa lastdnptr .
 337 A side effect of
 338 .Fn dn_comp
 339 is to update the list of pointers for
 340 labels inserted into the message
 341 as the name is compressed.
 342 If
 343 .Em dnptr
 344 is
 345 .Dv NULL ,
 346 names are not compressed.
 347 If
 348 .Fa lastdnptr
 349 is
 350 .Dv NULL ,
 351 the list of labels is not updated.
 352 .Pp
 353 The
 354 .Fn dn_expand
 355 entry
 356 expands the compressed domain name
 357 .Fa comp_dn
 358 to a full domain name.
 359 The compressed name is contained in a query or reply message;
 360 .Fa msg
 361 is a pointer to the beginning of the message.
 362 The uncompressed name is placed in the buffer indicated by
 363 .Fa exp_dn
 364 which is of size
 365 .Fa length .
 366 The size of compressed name is returned or \-1 if there was an error.
 367 .Pp
 368 The
 369 .Fn dn_skipname
 370 function skips over a compressed domain name, which starts at a location
 371 pointed to by
 372 .Fa comp_dn .
 373 The compressed name is contained in a query or reply message;
 374 .Fa eom
 375 is a pointer to the end of the message.
 376 The size of compressed name is returned or \-1 if there was
 377 an error.
 378 .Pp
 379 The
 380 .Fn ns_get16
 381 function gets a 16-bit quantity from a buffer pointed to by
 382 .Fa src .
 383 .Pp
 384 The
 385 .Fn ns_get32
 386 function gets a 32-bit quantity from a buffer pointed to by
 387 .Fa src .
 388 .Pp
 389 The
 390 .Fn ns_put16
 391 function puts a 16-bit quantity
 392 .Fa src
 393 to a buffer pointed to by
 394 .Fa dst .
 395 .Pp
 396 The
 397 .Fn ns_put32
 398 function puts a 32-bit quantity
 399 .Fa src
 400 to a buffer pointed to by
 401 .Fa dst .
 402 .Sh FILES
 403 .Bl -tag -width /etc/resolv.conf
 404 .It Pa /etc/resolv.conf
 405 The configuration file,
 406 see
 407 .Xr resolver 5 .
 408 .El
 409 .Sh SEE ALSO
 410 .Xr gethostbyname 3 ,
 411 .Xr resolver 5 ,
 412 .Xr hostname 7 ,
 413 .Xr named 8
 414 .Pp
 415 .%T RFC1032 ,
 416 .%T RFC1033 ,
 417 .%T RFC1034 ,
 418 .%T RFC1035 ,
 419 .%T RFC974
 420 .Rs
 421 .%T "Name Server Operations Guide for BIND"
 422 .Re
 423 .Sh HISTORY
 424 The
 425 .Nm
 426 function appeared in
 427 .Bx 4.3 .