summaryrefslogtreecommitdiffstats
path: root/man3/gethostbyname.3
blob: 01cc0f74209b0b1715490c7f84e6aea6fb1aeb64 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
'\" t
.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" References consulted:
.\"     Linux libc source code
.\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
.\"     386BSD man pages
.\" Modified 1993-05-22, David Metcalfe
.\" Modified 1993-07-25, Rik Faith (faith@cs.unc.edu)
.\" Modified 1997-02-16, Andries Brouwer (aeb@cwi.nl)
.\" Modified 1998-12-21, Andries Brouwer (aeb@cwi.nl)
.\" Modified 2000-08-12, Andries Brouwer (aeb@cwi.nl)
.\" Modified 2001-05-19, Andries Brouwer (aeb@cwi.nl)
.\" Modified 2002-08-05, Michael Kerrisk
.\" Modified 2004-10-31, Andries Brouwer
.\"
.TH gethostbyname 3 2023-10-31 "Linux man-pages 6.7"
.SH NAME
gethostbyname, gethostbyaddr, sethostent, gethostent, endhostent,
h_errno,
herror, hstrerror,
gethostbyaddr_r,
gethostbyname2, gethostbyname2_r, gethostbyname_r,
gethostent_r \- get network host entry
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <netdb.h>
.P
.BI "void sethostent(int " stayopen );
.B void endhostent(void);
.P
.B [[deprecated]] extern int h_errno;
.P
.BI "[[deprecated]] struct hostent *gethostbyname(const char *" name );
.BI "[[deprecated]] struct hostent *gethostbyaddr(const void " addr [. len ],
.BI "                                             socklen_t " len ", int " type );
.P
.BI "[[deprecated]] void herror(const char *" s );
.BI "[[deprecated]] const char *hstrerror(int " err );
.P
/* System V/POSIX extension */
.B struct hostent *gethostent(void);
.P
/* GNU extensions */
.B [[deprecated]]
.BI "struct hostent *gethostbyname2(const char *" name ", int " af );
.P
.BI "int gethostent_r(struct hostent *restrict " ret ,
.BI "                 char " buf "[restrict ." buflen "], size_t " buflen ,
.BI "                 struct hostent **restrict " result ,
.BI "                 int *restrict " h_errnop );
.P
.B [[deprecated]]
.BI "int gethostbyaddr_r(const void " addr "[restrict ." len "], socklen_t " len ,
.BI "                 int " type ,
.BI "                 struct hostent *restrict " ret ,
.BI "                 char " buf "[restrict ." buflen "], size_t " buflen ,
.BI "                 struct hostent **restrict " result ,
.BI "                 int *restrict " h_errnop );
.B [[deprecated]]
.BI "int gethostbyname_r(const char *restrict " name ,
.BI "                 struct hostent *restrict " ret ,
.BI "                 char " buf "[restrict ." buflen "], size_t " buflen ,
.BI "                 struct hostent **restrict " result ,
.BI "                 int *restrict " h_errnop );
.B [[deprecated]]
.BI "int gethostbyname2_r(const char *restrict " name ", int " af,
.BI "                 struct hostent *restrict " ret ,
.BI "                 char " buf "[restrict ." buflen "], size_t " buflen ,
.BI "                 struct hostent **restrict " result ,
.BI "                 int *restrict " h_errnop );
.fi
.P
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.P
.BR gethostbyname2 (),
.BR gethostent_r (),
.BR gethostbyaddr_r (),
.BR gethostbyname_r (),
.BR gethostbyname2_r ():
.nf
    Since glibc 2.19:
        _DEFAULT_SOURCE
    glibc up to and including 2.19:
        _BSD_SOURCE || _SVID_SOURCE
.fi
.P
.BR herror (),
.BR hstrerror ():
.nf
    Since glibc 2.19:
        _DEFAULT_SOURCE
    glibc 2.8 to glibc 2.19:
        _BSD_SOURCE || _SVID_SOURCE
    Before glibc 2.8:
        none
.fi
.P
.BR h_errno :
.nf
    Since glibc 2.19
        _DEFAULT_SOURCE || _POSIX_C_SOURCE < 200809L
    glibc 2.12 to glibc 2.19:
        _BSD_SOURCE || _SVID_SOURCE || _POSIX_C_SOURCE < 200809L
    Before glibc 2.12:
        none
.fi
.SH DESCRIPTION
The
.BR gethostbyname* (),
.BR gethostbyaddr* (),
.BR herror (),
and
.BR hstrerror ()
functions are obsolete.
Applications should use
.BR getaddrinfo (3),
.BR getnameinfo (3),
and
.BR gai_strerror (3)
instead.
.P
The
.BR sethostent ()
function specifies, if \fIstayopen\fP is true (1),
that a connected TCP socket should be used for the name server queries and
that the connection should remain open during successive queries.
Otherwise, name server queries will use UDP datagrams.
.P
The
.BR endhostent ()
function ends the use of a TCP connection for name
server queries.
.P
The
.BR gethostbyname ()
function returns a structure of type
.I hostent
for the given host
.IR name .
Here
.I name
is either a hostname or an IPv4 address in standard dot notation (as for
.BR inet_addr (3)).
If
.I name
is an IPv4 address, no lookup is performed and
.BR gethostbyname ()
simply copies
.I name
into the
.I h_name
field and its
.I struct in_addr
equivalent into the
.I h_addr_list[0]
field of the returned
.I hostent
structure.
If
.I name
doesn't end in a dot and the environment variable
.B HOSTALIASES
is set, the alias file pointed to by
.B HOSTALIASES
will first be searched for
.I name
(see
.BR hostname (7)
for the file format).
The current domain and its parents are searched unless \fIname\fP
ends in a dot.
.P
The
.BR gethostbyaddr ()
function returns a structure of type \fIhostent\fP
for the given host address \fIaddr\fP of length \fIlen\fP and address type
\fItype\fP.
Valid address types are
.B AF_INET
and
.B AF_INET6
(defined in
.IR <sys/socket.h> ).
The host address argument is a pointer to a struct of a type depending
on the address type, for example a \fIstruct in_addr *\fP (probably
obtained via a call to
.BR inet_addr (3))
for address type
.BR AF_INET .
.P
The (obsolete)
.BR herror ()
function prints the error message associated
with the current value of \fIh_errno\fP on \fIstderr\fP.
.P
The (obsolete)
.BR hstrerror ()
function takes an error number
(typically \fIh_errno\fP) and returns the corresponding message string.
.P
The domain name queries carried out by
.BR gethostbyname ()
and
.BR gethostbyaddr ()
rely on the Name Service Switch
.RB ( nsswitch.conf (5))
configured sources or a local name server
.RB ( named (8)).
The default action is to query the Name Service Switch
.RB ( nsswitch.conf (5))
configured sources, failing that, a local name server
.RB ( named (8)).
.\"
.SS Historical
The
.BR nsswitch.conf (5)
file is the modern way of controlling the order of host lookups.
.P
In glibc 2.4 and earlier, the
.I order
keyword was used to control the order of host lookups as defined in
.I /etc/host.conf
.RB ( host.conf (5)).
.P
The \fIhostent\fP structure is defined in \fI<netdb.h>\fP as follows:
.P
.in +4n
.EX
struct hostent {
    char  *h_name;            /* official name of host */
    char **h_aliases;         /* alias list */
    int    h_addrtype;        /* host address type */
    int    h_length;          /* length of address */
    char **h_addr_list;       /* list of addresses */
}
#define h_addr h_addr_list[0] /* for backward compatibility */
.EE
.in
.P
The members of the \fIhostent\fP structure are:
.TP
.I h_name
The official name of the host.
.TP
.I h_aliases
An array of alternative names for the host, terminated by a null pointer.
.TP
.I h_addrtype
The type of address; always
.B AF_INET
or
.B AF_INET6
at present.
.TP
.I h_length
The length of the address in bytes.
.TP
.I h_addr_list
An array of pointers to network addresses for the host (in network byte
order), terminated by a null pointer.
.TP
.I h_addr
The first address in \fIh_addr_list\fP for backward compatibility.
.SH RETURN VALUE
The
.BR gethostbyname ()
and
.BR gethostbyaddr ()
functions return the
.I hostent
structure or a null pointer if an error occurs.
On error, the
.I h_errno
variable holds an error number.
When non-NULL, the return value may point at static data, see the notes below.
.SH ERRORS
The variable \fIh_errno\fP can have the following values:
.TP
.B HOST_NOT_FOUND
The specified host is unknown.
.TP
.B NO_DATA
The requested name is valid but does not have an IP address.
Another type of request to the name server for this domain
may return an answer.
The constant
.B NO_ADDRESS
is a synonym for
.BR NO_DATA .
.TP
.B NO_RECOVERY
A nonrecoverable name server error occurred.
.TP
.B TRY_AGAIN
A temporary error occurred on an authoritative name server.
Try again later.
.SH FILES
.TP
.I /etc/host.conf
resolver configuration file
.TP
.I /etc/hosts
host database file
.TP
.I /etc/nsswitch.conf
name service switch configuration
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lb lb lbx
l l l.
Interface	Attribute	Value
T{
.na
.nh
.BR gethostbyname ()
T}	Thread safety	T{
.na
.nh
MT-Unsafe race:hostbyname env
locale
T}
T{
.na
.nh
.BR gethostbyaddr ()
T}	Thread safety	T{
.na
.nh
MT-Unsafe race:hostbyaddr env
locale
T}
T{
.na
.nh
.BR sethostent (),
.BR endhostent (),
.BR gethostent_r ()
T}	Thread safety	T{
.na
.nh
MT-Unsafe race:hostent env
locale
T}
T{
.na
.nh
.BR herror (),
.BR hstrerror ()
T}	Thread safety	MT-Safe
T{
.na
.nh
.BR gethostent ()
T}	Thread safety	T{
.na
.nh
MT-Unsafe race:hostent
race:hostentbuf env locale
T}
T{
.na
.nh
.BR gethostbyname2 ()
T}	Thread safety	T{
.na
.nh
MT-Unsafe race:hostbyname2
env locale
T}
T{
.na
.nh
.BR gethostbyaddr_r (),
.BR gethostbyname_r (),
.BR gethostbyname2_r ()
T}	Thread safety	MT-Safe env locale
.TE
.P
In the above table,
.I hostent
in
.I race:hostent
signifies that if any of the functions
.BR sethostent (),
.BR gethostent (),
.BR gethostent_r (),
or
.BR \%endhostent ()
are used in parallel in different threads of a program,
then data races could occur.
.SH STANDARDS
.TP
.BR sethostent ()
.TQ
.BR endhostent ()
.TQ
.BR gethostent ()
POSIX.1-2008.
.TP
.BR gethostent_r ()
GNU.
.TP
Others:
None.
.SH HISTORY
.TP
.BR sethostent ()
.TQ
.BR endhostent ()
.TQ
.BR gethostent ()
POSIX.1-2001.
.TP
.BR gethostbyname ()
.TQ
.BR gethostbyaddr ()
.TQ
.I h_errno
Marked obsolescent in POSIX.1-2001.
Removed in POSIX.1-2008,
recommending the use of
.BR getaddrinfo (3)
and
.BR getnameinfo (3)
instead.
.SH NOTES
The functions
.BR gethostbyname ()
and
.BR gethostbyaddr ()
may return pointers to static data, which may be overwritten by
later calls.
Copying the
.I struct hostent
does not suffice, since it contains pointers; a deep copy is required.
.P
In the original BSD implementation the
.I len
argument
of
.BR gethostbyname ()
was an
.IR int .
The SUSv2 standard is buggy and declares the
.I len
argument of
.BR gethostbyaddr ()
to be of type
.IR size_t .
(That is wrong, because it has to be
.IR int ,
and
.I size_t
is not.
POSIX.1-2001 makes it
.IR socklen_t ,
which is OK.)
See also
.BR accept (2).
.P
The BSD prototype for
.BR gethostbyaddr ()
uses
.I "const char\ *"
for the first argument.
.SS System V/POSIX extension
POSIX requires the
.BR gethostent ()
call, which should return the next entry in the host data base.
When using DNS/BIND this does not make much sense, but it may
be reasonable if the host data base is a file that can be read
line by line.
On many systems, a routine of this name reads
from the file
.IR /etc/hosts .
.\" e.g., Linux, FreeBSD, UnixWare, HP-UX
It may be available only when the library was built without DNS support.
.\" e.g., FreeBSD, AIX
The glibc version will ignore ipv6 entries.
This function is not reentrant,
and glibc adds a reentrant version
.BR gethostent_r ().
.SS GNU extensions
glibc2 also has a
.BR gethostbyname2 ()
that works like
.BR gethostbyname (),
but permits to specify the address family to which the address must belong.
.P
glibc2 also has reentrant versions
.BR gethostent_r (),
.BR gethostbyaddr_r (),
.BR gethostbyname_r (),
and
.BR gethostbyname2_r ().
The caller supplies a
.I hostent
structure
.I ret
which will be filled in on success, and a temporary work buffer
.I buf
of size
.IR buflen .
After the call,
.I result
will point to the result on success.
In case of an error
or if no entry is found
.I result
will be NULL.
The functions return 0 on success and a nonzero error number on failure.
In addition to the errors returned by the nonreentrant
versions of these functions, if
.I buf
is too small, the functions will return
.BR ERANGE ,
and the call should be retried with a larger buffer.
The global variable
.I h_errno
is not modified, but the address of a variable in which to store error numbers
is passed in
.IR h_errnop .
.SH BUGS
.BR gethostbyname ()
does not recognize components of a dotted IPv4 address string
that are expressed in hexadecimal.
.\" http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=482973
.SH SEE ALSO
.BR getaddrinfo (3),
.\" .BR getipnodebyaddr (3),
.\" .BR getipnodebyname (3),
.BR getnameinfo (3),
.BR inet (3),
.BR inet_ntop (3),
.BR inet_pton (3),
.BR resolver (3),
.BR hosts (5),
.BR nsswitch.conf (5),
.BR hostname (7),
.BR named (8)
.\" .BR resolv+ (8)