man3/gethostbyname.3

   1 .\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
   2 .\"
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  12 .\" Since the Linux kernel and libraries are constantly changing, this
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  15 .\" the use of the information contained herein.  The author(s) may not
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  17 .\" which is licensed free of charge, as they might when working
  18 .\" professionally.
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  20 .\" Formatted or processed versions of this manual, if unaccompanied by
  21 .\" the source, must acknowledge the copyright and authors of this work.
  22 .\"
  23 .\" References consulted:
  24 .\"     Linux libc source code
  25 .\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
  26 .\"     386BSD man pages
  27 .\" Modified 1993-05-22, David Metcalfe
  28 .\" Modified 1993-07-25, Rik Faith (faith@cs.unc.edu)
  29 .\" Modified 1997-02-16, Andries Brouwer (aeb@cwi.nl)
  30 .\" Modified 1998-12-21, Andries Brouwer (aeb@cwi.nl)
  31 .\" Modified 2000-08-12, Andries Brouwer (aeb@cwi.nl)
  32 .\" Modified 2001-05-19, Andries Brouwer (aeb@cwi.nl)
  33 .\" Modified 2002-08-05, Michael Kerrisk
  34 .\" Modified 2004-10-31, Andries Brouwer
  35 .\"
  36 .TH GETHOSTBYNAME 3 2010-10-04 "" "Linux Programmer's Manual"
  37 .SH NAME
  38 gethostbyname, gethostbyaddr, sethostent, gethostent, endhostent,
  39 h_errno,
  40 herror, hstrerror,
  41 gethostbyaddr_r,
  42 gethostbyname2, gethostbyname2_r, gethostbyname_r,
  43 gethostent_r \- get network host entry
  44 .SH SYNOPSIS
  45 .nf
  46 .B #include <netdb.h>
  47 .B extern int h_errno;
  48 .sp
  49 .BI "struct hostent *gethostbyname(const char *" name );
  50 .sp
  51 .BR "#include <sys/socket.h>" "       /* for AF_INET */"
  52 .BI "struct hostent *gethostbyaddr(const void *" addr ,
  53 .BI "                              socklen_t " len ", int " type );
  54 .sp
  55 .BI "void sethostent(int " stayopen );
  56 .sp
  57 .B void endhostent(void);
  58 .sp
  59 .BI "void herror(const char *" s );
  60 .sp
  61 .BI "const char *hstrerror(int " err );
  62 .sp
  63 /* System V/POSIX extension */
  64 .br
  65 .B struct hostent *gethostent(void);
  66 .sp
  67 /* GNU extensions */
  68 .br
  69 .BI "struct hostent *gethostbyname2(const char *" name ", int " af );
  70 .sp
  71 .B "int gethostent_r("
  72 .BI "        struct hostent *" ret ", char *" buf ", size_t " buflen ,
  73 .BI "        struct hostent **" result ", int *" h_errnop );
  74 .sp
  75 .BI "int gethostbyaddr_r(const void *" addr ", socklen_t " len ", int " type ,
  76 .BI "        struct hostent *" ret ", char *" buf ", size_t " buflen ,
  77 .BI "        struct hostent **" result ", int *" h_errnop );
  78 .sp
  79 .BI "int gethostbyname_r(const char *" name ,
  80 .BI "        struct hostent *" ret ", char *" buf ", size_t " buflen ,
  81 .BI "        struct hostent **" result ", int *" h_errnop );
  82 .sp
  83 .BI "int gethostbyname2_r(const char *" name ", int " af,
  84 .BI "        struct hostent *" ret ", char *" buf ", size_t " buflen ,
  85 .BI "        struct hostent **" result ", int *" h_errnop );
  86 .fi
  87 .sp
  88 .in -4n
  89 Feature Test Macro Requirements for glibc (see
  90 .BR feature_test_macros (7)):
  91 .in
  92 .sp
  93 .PD 0
  94 .ad l
  95 .BR gethostbyname2 (),
  96 .BR gethostent_r (),
  97 .BR gethostbyaddr_r (),
  98 .BR gethostbyname_r (),
  99 .BR gethostbyname2_r ():
 100 .RS 4
 101 _BSD_SOURCE || _SVID_SOURCE
 102 .RE
 103
 104 .BR herror (),
 105 .BR hstrerror ():
 106 .RS 4
 107 .TP 4
 108 Since glibc 2.8:
 109 _BSD_SOURCE || _SVID_SOURCE || _GNU_SOURCE
 110 .TP
 111 Before glibc 2.8:
 112 none
 113 .RE
 114 .ad b
 115 .PD
 116 .SH DESCRIPTION
 117 The
 118 .BR gethostbyname* ()
 119 and
 120 .BR gethostbyaddr* ()
 121 functions are obsolete.
 122 Applications should use
 123 .BR getaddrinfo (3)
 124 and
 125 .BR getnameinfo (3)
 126 instead.
 127
 128 The
 129 .BR gethostbyname ()
 130 function returns a structure of type
 131 .I hostent
 132 for the given host
 133 .IR name .
 134 Here
 135 .I name
 136 is either a hostname, or an IPv4 address in standard dot notation (as for
 137 .BR inet_addr (3)),
 138 or an IPv6 address in colon (and possibly dot) notation.
 139 (See RFC\ 1884 for the description of IPv6 addresses.)
 140 If
 141 .I name
 142 is an IPv4 or IPv6 address, no lookup is performed and
 143 .BR gethostbyname ()
 144 simply copies
 145 .I name
 146 into the
 147 .I h_name
 148 field and its
 149 .I struct in_addr
 150 equivalent into the
 151 .I h_addr_list[0]
 152 field of the returned
 153 .I hostent
 154 structure.
 155 If
 156 .I name
 157 doesn't end in a dot and the environment variable
 158 .B HOSTALIASES
 159 is set, the alias file pointed to by
 160 .B HOSTALIASES
 161 will first be searched for
 162 .I name
 163 (see
 164 .BR hostname (7)
 165 for the file format).
 166 The current domain and its parents are searched unless \fIname\fP
 167 ends in a dot.
 168 .PP
 169 The
 170 .BR gethostbyaddr ()
 171 function returns a structure of type \fIhostent\fP
 172 for the given host address \fIaddr\fP of length \fIlen\fP and address type
 173 \fItype\fP.
 174 Valid address types are
 175 .B AF_INET
 176 and
 177 .BR AF_INET6 .
 178 The host address argument is a pointer to a struct of a type depending
 179 on the address type, for example a \fIstruct in_addr *\fP (probably
 180 obtained via a call to
 181 .BR inet_addr (3))
 182 for address type
 183 .BR AF_INET .
 184 .PP
 185 The
 186 .BR sethostent ()
 187 function specifies, if \fIstayopen\fP is true (1),
 188 that a connected TCP socket should be used for the name server queries and
 189 that the connection should remain open during successive queries.
 190 Otherwise, name server queries will use UDP datagrams.
 191 .PP
 192 The
 193 .BR endhostent ()
 194 function ends the use of a TCP connection for name
 195 server queries.
 196 .PP
 197 The (obsolete)
 198 .BR herror ()
 199 function prints the error message associated
 200 with the current value of \fIh_errno\fP on \fIstderr\fP.
 201 .PP
 202 The (obsolete)
 203 .BR hstrerror ()
 204 function takes an error number
 205 (typically \fIh_errno\fP) and returns the corresponding message string.
 206 .PP
 207 The domain name queries carried out by
 208 .BR gethostbyname ()
 209 and
 210 .BR gethostbyaddr ()
 211 use a combination of any or all of the name server
 212 .BR named (8),
 213 a broken out line from \fI/etc/hosts\fP, and the Network
 214 Information Service (NIS or YP), depending upon the contents of the
 215 \fIorder\fP line in
 216 .IR /etc/host.conf .
 217 .\" (See
 218 .\" .BR resolv+ (8)).
 219 The default action is to query
 220 .BR named (8),
 221 followed by
 222 .IR /etc/hosts .
 223 .PP
 224 The \fIhostent\fP structure is defined in \fI<netdb.h>\fP as follows:
 225 .sp
 226 .in +4n
 227 .nf
 228 .ne 7
 229 struct hostent {
 230     char  *h_name;            /* official name of host */
 231     char **h_aliases;         /* alias list */
 232     int    h_addrtype;        /* host address type */
 233     int    h_length;          /* length of address */
 234     char **h_addr_list;       /* list of addresses */
 235 }
 236 #define h_addr h_addr_list[0] /* for backward compatibility */
 237 .fi
 238 .in
 239 .PP
 240 The members of the \fIhostent\fP structure are:
 241 .TP
 242 .I h_name
 243 The official name of the host.
 244 .TP
 245 .I h_aliases
 246 An array of alternative names for the host, terminated by a NULL pointer.
 247 .TP
 248 .I h_addrtype
 249 The type of address; always
 250 .B AF_INET
 251 or
 252 .B AF_INET6
 253 at present.
 254 .TP
 255 .I h_length
 256 The length of the address in bytes.
 257 .TP
 258 .I h_addr_list
 259 An array of pointers to network addresses for the host (in network byte
 260 order), terminated by a NULL pointer.
 261 .TP
 262 .I h_addr
 263 The first address in \fIh_addr_list\fP for backward compatibility.
 264 .SH RETURN VALUE
 265 The
 266 .BR gethostbyname ()
 267 and
 268 .BR gethostbyaddr ()
 269 functions return the
 270 .I hostent
 271 structure or a NULL pointer if an error occurs.
 272 On error, the
 273 .I h_errno
 274 variable holds an error number.
 275 When non-NULL, the return value may point at static data, see the notes below.
 276 .SH ERRORS
 277 The variable \fIh_errno\fP can have the following values:
 278 .TP
 279 .B HOST_NOT_FOUND
 280 The specified host is unknown.
 281 .TP
 282 .BR NO_ADDRESS " or " NO_DATA
 283 The requested name is valid but does not have an IP address.
 284 .TP
 285 .B NO_RECOVERY
 286 A nonrecoverable name server error occurred.
 287 .TP
 288 .B TRY_AGAIN
 289 A temporary error occurred on an authoritative name server.
 290 Try again later.
 291 .SH FILES
 292 .TP
 293 .I /etc/host.conf
 294 resolver configuration file
 295 .TP
 296 .I /etc/hosts
 297 host database file
 298 .TP
 299 .I /etc/nsswitch.conf
 300 name service switch configuration
 301 .SH CONFORMING TO
 302 POSIX.1-2001 specifies
 303 .BR gethostbyname (),
 304 .BR gethostbyaddr (),
 305 .BR sethostent (),
 306 .BR endhostent (),
 307 .BR gethostent (),
 308 and
 309 .IR h_errno ;
 310 .BR gethostbyname (),
 311 .BR gethostbyaddr (),
 312 and
 313 .IR h_errno
 314 are marked obsolescent in that standard.
 315 POSIX.1-2008 removes the specifications of
 316 .BR gethostbyname (),
 317 .BR gethostbyaddr (),
 318 and
 319 .IR h_errno ,
 320 recommending the use of
 321 .BR getaddrinfo (3)
 322 and
 323 .BR getnameinfo (3)
 324 instead.
 325 .SH NOTES
 326 The functions
 327 .BR gethostbyname ()
 328 and
 329 .BR gethostbyaddr ()
 330 may return pointers to static data, which may be overwritten by
 331 later calls.
 332 Copying the
 333 .I struct hostent
 334 does not suffice, since it contains pointers; a deep copy is required.
 335 .LP
 336 In the original BSD implementation the
 337 .I len
 338 argument
 339 of
 340 .BR gethostbyname ()
 341 was an
 342 .IR int .
 343 The SUSv2 standard is buggy and declares the
 344 .I len
 345 argument of
 346 .BR gethostbyaddr ()
 347 to be of type
 348 .IR size_t .
 349 (That is wrong, because it has to be
 350 .IR int ,
 351 and
 352 .I size_t
 353 is not.
 354 POSIX.1-2001 makes it
 355 .IR socklen_t ,
 356 which is OK.)
 357 See also
 358 .BR accept (2).
 359 .LP
 360 The BSD prototype for
 361 .BR gethostbyaddr ()
 362 uses
 363 .I const char *
 364 for the first argument.
 365 .SS "System V/POSIX extension"
 366 POSIX requires the
 367 .BR gethostent ()
 368 call, that should return the next entry in the host data base.
 369 When using DNS/BIND this does not make much sense, but it may
 370 be reasonable if the host data base is a file that can be read
 371 line by line.
 372 On many systems a routine of this name reads
 373 from the file
 374 .IR /etc/hosts .
 375 .\" e.g., Linux, FreeBSD, UnixWare, HP-UX
 376 It may be available only when the library was built without DNS support.
 377 .\" e.g., FreeBSD, AIX
 378 The glibc version will ignore ipv6 entries.
 379 This function is not reentrant,
 380 and glibc adds a reentrant version
 381 .BR gethostent_r ().
 382 .SS "GNU extensions"
 383 Glibc2 also has a
 384 .BR gethostbyname2 ()
 385 that works like
 386 .BR gethostbyname (),
 387 but permits to specify the address family to which the address must belong.
 388 .LP
 389 Glibc2 also has reentrant versions
 390 .BR gethostent_r (),
 391 .BR gethostbyaddr_r (),
 392 .BR gethostbyname_r ()
 393 and
 394 .BR gethostbyname2_r ().
 395 The caller supplies a
 396 .I hostent
 397 structure
 398 .I ret
 399 which will be filled in on success, and a temporary work buffer
 400 .I buf
 401 of size
 402 .IR buflen .
 403 After the call,
 404 .I result
 405 will point to the result on success.
 406 In case of an error
 407 or if no entry is found
 408 .I result
 409 will be NULL.
 410 The functions return 0 on success and a nonzero error number on failure.
 411 In addition to the errors returned by the nonreentrant
 412 versions of these functions, if
 413 .I buf
 414 is too small, the functions will return
 415 .BR ERANGE ,
 416 and the call should be retried with a larger buffer.
 417 The global variable
 418 .I h_errno
 419 is not modified, but the address of a variable in which to store error numbers
 420 is passed in
 421 .IR h_errnop .
 422 .SH BUGS
 423 .BR gethostbyname ()
 424 does not recognize components of a dotted IPv4 address string
 425 that are expressed in hexadecimal.
 426 .\" http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=482973
 427 .SH SEE ALSO
 428 .BR getaddrinfo (3),
 429 .\" .BR getipnodebyaddr (3),
 430 .\" .BR getipnodebyname (3),
 431 .BR getnameinfo (3),
 432 .BR inet (3),
 433 .BR inet_ntop (3),
 434 .BR inet_pton (3),
 435 .BR resolver (3),
 436 .BR hosts (5),
 437 .BR nsswitch.conf (5),
 438 .BR hostname (7),
 439 .BR named (8)
 440 .\" .BR resolv+ (8)