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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  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 2007-10-16 "" "Linux Programmer's Manual"
  37 .SH NAME
  38 gethostbyname, gethostbyaddr, sethostent, gethostent, endhostent,
  39 herror, hstrerror,
  40 gethostbyaddr_r,
  41 gethostbyname2, gethostbyname2_r, gethostbyname_r,
  42 gethostent_r \- get network host entry
  43 .SH SYNOPSIS
  44 .nf
  45 .B #include <netdb.h>
  46 .B extern int h_errno;
  47 .sp
  48 .BI "struct hostent *gethostbyname(const char *" name );
  49 .sp
  50 .BR "#include <sys/socket.h>" "       /* for AF_INET */"
  51 .BI "struct hostent *gethostbyaddr(const void *" addr ,
  52 .BI "                              socklen_t " len ", int " type );
  53 .sp
  54 .BI "void sethostent(int " stayopen );
  55 .sp
  56 .B void endhostent(void);
  57 .sp
  58 .BI "void herror(const char *" s );
  59 .sp
  60 .BI "const char *hstrerror(int " err );
  61 .sp
  62 /* System V/POSIX extension */
  63 .br
  64 .B struct hostent *gethostent(void);
  65 .sp
  66 /* GNU extensions */
  67 .br
  68 .BI "struct hostent *gethostbyname2(const char *" name ", int " af );
  69 .sp
  70 .B "int gethostent_r("
  71 .BI "        struct hostent *" ret ", char *" buf ", size_t " buflen ,
  72 .BI "        struct hostent **" result ", int *" h_errnop );
  73 .sp
  74 .BI "int gethostbyaddr_r(const void *" addr ", socklen_t " len ", int " type ,
  75 .BI "        struct hostent *" ret ", char *" buf ", size_t " buflen ,
  76 .BI "        struct hostent **" result ", int *" h_errnop );
  77 .sp
  78 .BI "int gethostbyname_r(const char *" name ,
  79 .BI "        struct hostent *" ret ", char *" buf ", size_t " buflen ,
  80 .BI "        struct hostent **" result ", int *" h_errnop );
  81 .sp
  82 .BI "int gethostbyname2_r(const char *" name ", int " af,
  83 .BI "        struct hostent *" ret ", char *" buf ", size_t " buflen ,
  84 .BI "        struct hostent **" result ", int *" h_errnop );
  85 .fi
  86 .sp
  87 .in -4n
  88 Feature Test Macro Requirements for glibc (see
  89 .BR feature_test_macros (7)):
  90 .in
  91 .sp
  92 .ad l
  93 .BR gethostbyname2 (),
  94 .BR gethostent_r (),
  95 .BR gethostbyaddr_r (),
  96 .BR gethostbyname_r (),
  97 .BR gethostbyname2_r ():
  98 _BSD_SOURCE || _SVID_SOURCE
  99 .ad b
 100 .SH DESCRIPTION
 101 These functions are obsolete.
 102 Applications should use
 103 .BR getaddrinfo (3)
 104 and
 105 .BR getnameinfo (3)
 106 instead.
 107
 108 The
 109 .BR gethostbyname ()
 110 function returns a structure of type
 111 .I hostent
 112 for the given host
 113 .IR name .
 114 Here
 115 .I name
 116 is either a host name, or an IPv4 address in standard dot notation,
 117 or an IPv6 address in colon (and possibly dot) notation.
 118 (See RFC\ 1884 for the description of IPv6 addresses.)
 119 If
 120 .I name
 121 is an IPv4 or IPv6 address, no lookup is performed and
 122 .BR gethostbyname ()
 123 simply copies
 124 .I name
 125 into the
 126 .I h_name
 127 field and its
 128 .I struct in_addr
 129 equivalent into the
 130 .I h_addr_list[0]
 131 field of the returned
 132 .I hostent
 133 structure.
 134 If
 135 .I name
 136 doesn't end in a dot and the environment variable
 137 .B HOSTALIASES
 138 is set, the alias file pointed to by
 139 .B HOSTALIASES
 140 will first be searched for
 141 .I name
 142 (see
 143 .BR hostname (7)
 144 for the file format).
 145 The current domain and its parents are searched unless \fIname\fP
 146 ends in a dot.
 147 .PP
 148 The
 149 .BR gethostbyaddr ()
 150 function returns a structure of type \fIhostent\fP
 151 for the given host address \fIaddr\fP of length \fIlen\fP and address type
 152 \fItype\fP.
 153 Valid address types are
 154 .B AF_INET
 155 and
 156 .BR AF_INET6 .
 157 The host address argument is a pointer to a struct of a type depending
 158 on the address type, for example a \fIstruct in_addr *\fP (probably
 159 obtained via a call to
 160 .BR inet_addr (3))
 161 for address type
 162 .BR AF_INET .
 163 .PP
 164 The
 165 .BR sethostent ()
 166 function specifies, if \fIstayopen\fP is true (1),
 167 that a connected TCP socket should be used for the name server queries and
 168 that the connection should remain open during successive queries.
 169 Otherwise, name server queries will use UDP datagrams.
 170 .PP
 171 The
 172 .BR endhostent ()
 173 function ends the use of a TCP connection for name
 174 server queries.
 175 .PP
 176 The (obsolete)
 177 .BR herror ()
 178 function prints the error message associated
 179 with the current value of \fIh_errno\fP on \fIstderr\fP.
 180 .PP
 181 The (obsolete)
 182 .BR hstrerror ()
 183 function takes an error number
 184 (typically \fIh_errno\fP) and returns the corresponding message string.
 185 .PP
 186 The domain name queries carried out by
 187 .BR gethostbyname ()
 188 and
 189 .BR gethostbyaddr ()
 190 use a combination of any or all of the name server
 191 .BR named (8),
 192 a broken out line from \fI/etc/hosts\fP, and the Network
 193 Information Service (NIS or YP), depending upon the contents of the
 194 \fIorder\fP line in
 195 .IR /etc/host.conf .
 196 .\" (See
 197 .\" .BR resolv+ (8)).
 198 The default action is to query
 199 .BR named (8),
 200 followed by
 201 .IR /etc/hosts .
 202 .PP
 203 The \fIhostent\fP structure is defined in \fI<netdb.h>\fP as follows:
 204 .sp
 205 .in +4n
 206 .nf
 207 .ne 7
 208 struct hostent {
 209     char  *h_name;            /* official name of host */
 210     char **h_aliases;         /* alias list */
 211     int    h_addrtype;        /* host address type */
 212     int    h_length;          /* length of address */
 213     char **h_addr_list;       /* list of addresses */
 214 }
 215 #define h_addr h_addr_list[0] /* for backward compatibility */
 216 .fi
 217 .in
 218 .PP
 219 The members of the \fIhostent\fP structure are:
 220 .TP
 221 .I h_name
 222 The official name of the host.
 223 .TP
 224 .I h_aliases
 225 An array of alternative names for the host, terminated by a NULL pointer.
 226 .TP
 227 .I h_addrtype
 228 The type of address; always
 229 .B AF_INET
 230 or
 231 .B AF_INET6
 232 at present.
 233 .TP
 234 .I h_length
 235 The length of the address in bytes.
 236 .TP
 237 .I h_addr_list
 238 An array of pointers to network addresses for the host (in network byte
 239 order), terminated by a NULL pointer.
 240 .TP
 241 .I h_addr
 242 The first address in \fIh_addr_list\fP for backward compatibility.
 243 .SH "RETURN VALUE"
 244 The
 245 .BR gethostbyname ()
 246 and
 247 .BR gethostbyaddr ()
 248 functions return the
 249 .I hostent
 250 structure or a NULL pointer if an error occurs.
 251 On error, the
 252 .I h_errno
 253 variable holds an error number.
 254 When non-NULL, the return value may point at static data, see the notes below.
 255 .SH ERRORS
 256 The variable \fIh_errno\fP can have the following values:
 257 .TP
 258 .B HOST_NOT_FOUND
 259 The specified host is unknown.
 260 .TP
 261 .BR NO_ADDRESS " or " NO_DATA
 262 The requested name is valid but does not have an IP address.
 263 .TP
 264 .B NO_RECOVERY
 265 A non-recoverable name server error occurred.
 266 .TP
 267 .B TRY_AGAIN
 268 A temporary error occurred on an authoritative name server.
 269 Try again later.
 270 .SH FILES
 271 .TP
 272 .I /etc/host.conf
 273 resolver configuration file
 274 .TP
 275 .I /etc/hosts
 276 host database file
 277 .TP
 278 .I /etc/nsswitch.conf
 279 name service switch configuration
 280 .SH "CONFORMING TO"
 281 4.3BSD, POSIX.1-2001.
 282 .SH NOTES
 283 The functions
 284 .BR gethostbyname ()
 285 and
 286 .BR gethostbyaddr ()
 287 may return pointers to static data, which may be overwritten by
 288 later calls.
 289 Copying the
 290 .I struct hostent
 291 does not suffice, since it contains pointers; a deep copy is required.
 292 .LP
 293 In the original BSD implementation the
 294 .I len
 295 argument
 296 of
 297 .BR gethostbyname ()
 298 was an
 299 .IR int .
 300 The SUSv2 standard is buggy and declares the
 301 .I len
 302 parameter of
 303 .BR gethostbyaddr ()
 304 to be of type
 305 .IR size_t .
 306 (That is wrong, because it has to be
 307 .IR int ,
 308 and
 309 .I size_t
 310 is not.
 311 POSIX.1-2001 makes it
 312 .IR socklen_t ,
 313 which is OK.)
 314 See also
 315 .BR accept (2).
 316 .LP
 317 The BSD prototype for
 318 .BR gethostbyaddr ()
 319 uses
 320 .I const char *
 321 for the first argument.
 322 .LP
 323 POSIX.1-2001 marks
 324 .BR gethostbyaddr ()
 325 and
 326 .BR gethostbyname ()
 327 obsolescent.
 328 See
 329 .BR getaddrinfo (3),
 330 .BR getnameinfo (3),
 331 .BR gai_strerror (3).
 332 .SS "System V/POSIX Extension"
 333 POSIX requires the
 334 .BR gethostent ()
 335 call, that should return the next entry in the host data base.
 336 When using DNS/BIND this does not make much sense, but it may
 337 be reasonable if the host data base is a file that can be read
 338 line by line.
 339 On many systems a routine of this name reads
 340 from the file
 341 .IR /etc/hosts .
 342 .\" e.g., Linux, FreeBSD, Unixware, HP-UX
 343 It may be available only when the library was built without DNS support.
 344 .\" e.g., FreeBSD, AIX
 345 The glibc version will ignore ipv6 entries.
 346 This function is not reentrant,
 347 and glibc adds a reentrant version
 348 .BR gethostent_r ().
 349 .SS "GNU Extensions"
 350 Glibc2 also has a
 351 .BR gethostbyname2 ()
 352 that works like
 353 .BR gethostbyname (),
 354 but permits to specify the address family to which the address must belong.
 355 .LP
 356 Glibc2 also has reentrant versions
 357 .BR gethostent_r (),
 358 .BR gethostbyaddr_r (),
 359 .BR gethostbyname_r ()
 360 and
 361 .BR gethostbyname2_r ().
 362 The caller supplies a
 363 .I hostent
 364 structure
 365 .I ret
 366 which will be filled in on success, and a temporary work buffer
 367 .I buf
 368 of size
 369 .IR buflen .
 370 After the call,
 371 .I result
 372 will point to the result on success.  In case of an error
 373 or if no entry is found
 374 .I result
 375 will be NULL.
 376 The functions return 0 on success and a non-zero error number on failure.
 377 In addition to the errors returned by the non-reentrant
 378 versions of these functions, if
 379 .I buf
 380 is too small, the functions will return
 381 .BR ERANGE ,
 382 and the call should be retried with a larger buffer.
 383 The global variable
 384 .I h_errno
 385 is not modified, but the address of a variable in which to store error numbers
 386 is passed in
 387 .IR h_errnop .
 388 .SH "SEE ALSO"
 389 .BR getaddrinfo (3),
 390 .\" .BR getipnodebyaddr (3),
 391 .\" .BR getipnodebyname (3),
 392 .BR getnameinfo (3),
 393 .BR inet_ntop (3),
 394 .BR inet_pton (3),
 395 .BR resolver (3),
 396 .BR hosts (5),
 397 .BR nsswitch.conf (5),
 398 .BR hostname (7),
 399 .BR named (8)
 400 .\" .BR resolv+ (8)