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
  16 .\" have taken the same level of care in the production of this manual,
  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 2009-12-03 "" "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 .ad l
  94 .BR gethostbyname2 (),
  95 .BR gethostent_r (),
  96 .BR gethostbyaddr_r (),
  97 .BR gethostbyname_r (),
  98 .BR gethostbyname2_r ():
  99 .RS 4
 100 _BSD_SOURCE || _SVID_SOURCE
 101 .RE
 102
 103 .BR herror (),
 104 .BR hstrerror ()
 105 .RS 4
 106 (since glibc 2.8): _BSD_SOURCE || _SVID_SOURCE || _GNU_SOURCE
 107 .RE
 108 .ad b
 109 .SH DESCRIPTION
 110 The
 111 .BR gethostbyname* ()
 112 and
 113 .BR gethostbyaddr* ()
 114 functions are obsolete.
 115 Applications should use
 116 .BR getaddrinfo (3)
 117 and
 118 .BR getnameinfo (3)
 119 instead.
 120
 121 The
 122 .BR gethostbyname ()
 123 function returns a structure of type
 124 .I hostent
 125 for the given host
 126 .IR name .
 127 Here
 128 .I name
 129 is either a hostname, or an IPv4 address in standard dot notation (as for
 130 .BR inet_addr (3)),
 131 or an IPv6 address in colon (and possibly dot) notation.
 132 (See RFC\ 1884 for the description of IPv6 addresses.)
 133 If
 134 .I name
 135 is an IPv4 or IPv6 address, no lookup is performed and
 136 .BR gethostbyname ()
 137 simply copies
 138 .I name
 139 into the
 140 .I h_name
 141 field and its
 142 .I struct in_addr
 143 equivalent into the
 144 .I h_addr_list[0]
 145 field of the returned
 146 .I hostent
 147 structure.
 148 If
 149 .I name
 150 doesn't end in a dot and the environment variable
 151 .B HOSTALIASES
 152 is set, the alias file pointed to by
 153 .B HOSTALIASES
 154 will first be searched for
 155 .I name
 156 (see
 157 .BR hostname (7)
 158 for the file format).
 159 The current domain and its parents are searched unless \fIname\fP
 160 ends in a dot.
 161 .PP
 162 The
 163 .BR gethostbyaddr ()
 164 function returns a structure of type \fIhostent\fP
 165 for the given host address \fIaddr\fP of length \fIlen\fP and address type
 166 \fItype\fP.
 167 Valid address types are
 168 .B AF_INET
 169 and
 170 .BR AF_INET6 .
 171 The host address argument is a pointer to a struct of a type depending
 172 on the address type, for example a \fIstruct in_addr *\fP (probably
 173 obtained via a call to
 174 .BR inet_addr (3))
 175 for address type
 176 .BR AF_INET .
 177 .PP
 178 The
 179 .BR sethostent ()
 180 function specifies, if \fIstayopen\fP is true (1),
 181 that a connected TCP socket should be used for the name server queries and
 182 that the connection should remain open during successive queries.
 183 Otherwise, name server queries will use UDP datagrams.
 184 .PP
 185 The
 186 .BR endhostent ()
 187 function ends the use of a TCP connection for name
 188 server queries.
 189 .PP
 190 The (obsolete)
 191 .BR herror ()
 192 function prints the error message associated
 193 with the current value of \fIh_errno\fP on \fIstderr\fP.
 194 .PP
 195 The (obsolete)
 196 .BR hstrerror ()
 197 function takes an error number
 198 (typically \fIh_errno\fP) and returns the corresponding message string.
 199 .PP
 200 The domain name queries carried out by
 201 .BR gethostbyname ()
 202 and
 203 .BR gethostbyaddr ()
 204 use a combination of any or all of the name server
 205 .BR named (8),
 206 a broken out line from \fI/etc/hosts\fP, and the Network
 207 Information Service (NIS or YP), depending upon the contents of the
 208 \fIorder\fP line in
 209 .IR /etc/host.conf .
 210 .\" (See
 211 .\" .BR resolv+ (8)).
 212 The default action is to query
 213 .BR named (8),
 214 followed by
 215 .IR /etc/hosts .
 216 .PP
 217 The \fIhostent\fP structure is defined in \fI<netdb.h>\fP as follows:
 218 .sp
 219 .in +4n
 220 .nf
 221 .ne 7
 222 struct hostent {
 223     char  *h_name;            /* official name of host */
 224     char **h_aliases;         /* alias list */
 225     int    h_addrtype;        /* host address type */
 226     int    h_length;          /* length of address */
 227     char **h_addr_list;       /* list of addresses */
 228 }
 229 #define h_addr h_addr_list[0] /* for backward compatibility */
 230 .fi
 231 .in
 232 .PP
 233 The members of the \fIhostent\fP structure are:
 234 .TP
 235 .I h_name
 236 The official name of the host.
 237 .TP
 238 .I h_aliases
 239 An array of alternative names for the host, terminated by a NULL pointer.
 240 .TP
 241 .I h_addrtype
 242 The type of address; always
 243 .B AF_INET
 244 or
 245 .B AF_INET6
 246 at present.
 247 .TP
 248 .I h_length
 249 The length of the address in bytes.
 250 .TP
 251 .I h_addr_list
 252 An array of pointers to network addresses for the host (in network byte
 253 order), terminated by a NULL pointer.
 254 .TP
 255 .I h_addr
 256 The first address in \fIh_addr_list\fP for backward compatibility.
 257 .SH "RETURN VALUE"
 258 The
 259 .BR gethostbyname ()
 260 and
 261 .BR gethostbyaddr ()
 262 functions return the
 263 .I hostent
 264 structure or a NULL pointer if an error occurs.
 265 On error, the
 266 .I h_errno
 267 variable holds an error number.
 268 When non-NULL, the return value may point at static data, see the notes below.
 269 .SH ERRORS
 270 The variable \fIh_errno\fP can have the following values:
 271 .TP
 272 .B HOST_NOT_FOUND
 273 The specified host is unknown.
 274 .TP
 275 .BR NO_ADDRESS " or " NO_DATA
 276 The requested name is valid but does not have an IP address.
 277 .TP
 278 .B NO_RECOVERY
 279 A nonrecoverable name server error occurred.
 280 .TP
 281 .B TRY_AGAIN
 282 A temporary error occurred on an authoritative name server.
 283 Try again later.
 284 .SH FILES
 285 .TP
 286 .I /etc/host.conf
 287 resolver configuration file
 288 .TP
 289 .I /etc/hosts
 290 host database file
 291 .TP
 292 .I /etc/nsswitch.conf
 293 name service switch configuration
 294 .SH "CONFORMING TO"
 295 POSIX.1-2001 specifies
 296 .BR gethostbyname (),
 297 .BR gethostbyaddr (),
 298 .BR sethostent (),
 299 .BR endhostent (),
 300 .BR gethostent (),
 301 and
 302 .IR h_errno ;
 303 .BR gethostbyname (),
 304 .BR gethostbyaddr (),
 305 and
 306 .IR h_errno
 307 are marked obsolescent in that standard.
 308 POSIX.1-2008 removes the specifications of
 309 .BR gethostbyname (),
 310 .BR gethostbyaddr (),
 311 and
 312 .IR h_errno ,
 313 recommending the use of
 314 .BR getaddrinfo (3)
 315 and
 316 .BR getnameinfo (3)
 317 instead.
 318 .SH NOTES
 319 The functions
 320 .BR gethostbyname ()
 321 and
 322 .BR gethostbyaddr ()
 323 may return pointers to static data, which may be overwritten by
 324 later calls.
 325 Copying the
 326 .I struct hostent
 327 does not suffice, since it contains pointers; a deep copy is required.
 328 .LP
 329 In the original BSD implementation the
 330 .I len
 331 argument
 332 of
 333 .BR gethostbyname ()
 334 was an
 335 .IR int .
 336 The SUSv2 standard is buggy and declares the
 337 .I len
 338 argument of
 339 .BR gethostbyaddr ()
 340 to be of type
 341 .IR size_t .
 342 (That is wrong, because it has to be
 343 .IR int ,
 344 and
 345 .I size_t
 346 is not.
 347 POSIX.1-2001 makes it
 348 .IR socklen_t ,
 349 which is OK.)
 350 See also
 351 .BR accept (2).
 352 .LP
 353 The BSD prototype for
 354 .BR gethostbyaddr ()
 355 uses
 356 .I const char *
 357 for the first argument.
 358 .SS "System V/POSIX Extension"
 359 POSIX requires the
 360 .BR gethostent ()
 361 call, that should return the next entry in the host data base.
 362 When using DNS/BIND this does not make much sense, but it may
 363 be reasonable if the host data base is a file that can be read
 364 line by line.
 365 On many systems a routine of this name reads
 366 from the file
 367 .IR /etc/hosts .
 368 .\" e.g., Linux, FreeBSD, Unixware, HP-UX
 369 It may be available only when the library was built without DNS support.
 370 .\" e.g., FreeBSD, AIX
 371 The glibc version will ignore ipv6 entries.
 372 This function is not reentrant,
 373 and glibc adds a reentrant version
 374 .BR gethostent_r ().
 375 .SS "GNU Extensions"
 376 Glibc2 also has a
 377 .BR gethostbyname2 ()
 378 that works like
 379 .BR gethostbyname (),
 380 but permits to specify the address family to which the address must belong.
 381 .LP
 382 Glibc2 also has reentrant versions
 383 .BR gethostent_r (),
 384 .BR gethostbyaddr_r (),
 385 .BR gethostbyname_r ()
 386 and
 387 .BR gethostbyname2_r ().
 388 The caller supplies a
 389 .I hostent
 390 structure
 391 .I ret
 392 which will be filled in on success, and a temporary work buffer
 393 .I buf
 394 of size
 395 .IR buflen .
 396 After the call,
 397 .I result
 398 will point to the result on success.
 399 In case of an error
 400 or if no entry is found
 401 .I result
 402 will be NULL.
 403 The functions return 0 on success and a nonzero error number on failure.
 404 In addition to the errors returned by the nonreentrant
 405 versions of these functions, if
 406 .I buf
 407 is too small, the functions will return
 408 .BR ERANGE ,
 409 and the call should be retried with a larger buffer.
 410 The global variable
 411 .I h_errno
 412 is not modified, but the address of a variable in which to store error numbers
 413 is passed in
 414 .IR h_errnop .
 415 .SH BUGS
 416 .BR gethostbyname ()
 417 does not recognize components of a dotted IPv4 address string
 418 that are expressed in hexadecimal.
 419 .\" http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=482973
 420 .SH "SEE ALSO"
 421 .BR getaddrinfo (3),
 422 .\" .BR getipnodebyaddr (3),
 423 .\" .BR getipnodebyname (3),
 424 .BR getnameinfo (3),
 425 .BR inet (3),
 426 .BR inet_ntop (3),
 427 .BR inet_pton (3),
 428 .BR resolver (3),
 429 .BR hosts (5),
 430 .BR nsswitch.conf (5),
 431 .BR hostname (7),
 432 .BR named (8)
 433 .\" .BR resolv+ (8)