1GETADDRINFO(3) Linux Programmer's Manual GETADDRINFO(3)
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6 getaddrinfo, freeaddrinfo, gai_strerror - network address and service
7 translation
8
10 #include <sys/types.h>
11 #include <sys/socket.h>
12 #include <netdb.h>
13 int getaddrinfo(const char *node, const char *service,
15 const struct addrinfo *hints,
16 struct addrinfo **res);
17 void freeaddrinfo(struct addrinfo *res);
19 const char *gai_strerror(int errcode);
21 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
23 getaddrinfo(), freeaddrinfo(), gai_strerror():
25 Since glibc 2.22: _POSIX_C_SOURCE >= 200112L
26 Glibc 2.21 and earlier: _POSIX_C_SOURCE
27
29 Given node and service, which identify an Internet host and a service,
30 getaddrinfo() returns one or more addrinfo structures, each of which
31 contains an Internet address that can be specified in a call to bind(2)
32 or connect(2). The getaddrinfo() function combines the functionality
33 provided by the gethostbyname(3) and getservbyname(3) functions into a
34 single interface, but unlike the latter functions, getaddrinfo() is
35 reentrant and allows programs to eliminate IPv4-versus-IPv6 dependen‐
36 cies.
37 The addrinfo structure used by getaddrinfo() contains the following
39 fields:
40 struct addrinfo {
42 int ai_flags;
43 int ai_family;
44 int ai_socktype;
45 int ai_protocol;
46 socklen_t ai_addrlen;
47 struct sockaddr *ai_addr;
48 char *ai_canonname;
49 struct addrinfo *ai_next;
50 };
51 The hints argument points to an addrinfo structure that specifies cri‐
53 teria for selecting the socket address structures returned in the list
54 pointed to by res. If hints is not NULL it points to an addrinfo
55 structure whose ai_family, ai_socktype, and ai_protocol specify crite‐
56 ria that limit the set of socket addresses returned by getaddrinfo(),
57 as follows:
58 ai_family This field specifies the desired address family for the
60 returned addresses. Valid values for this field include
61 AF_INET and AF_INET6. The value AF_UNSPEC indicates that
62 getaddrinfo() should return socket addresses for any
63 address family (either IPv4 or IPv6, for example) that can
64 be used with node and service.
65 ai_socktype This field specifies the preferred socket type, for example
67 SOCK_STREAM or SOCK_DGRAM. Specifying 0 in this field
68 indicates that socket addresses of any type can be returned
69 by getaddrinfo().
70 ai_protocol This field specifies the protocol for the returned socket
72 addresses. Specifying 0 in this field indicates that
73 socket addresses with any protocol can be returned by
74 getaddrinfo().
75 ai_flags This field specifies additional options, described below.
77 Multiple flags are specified by bitwise OR-ing them
78 together.
79 All the other fields in the structure pointed to by hints must contain
81 either 0 or a null pointer, as appropriate.
82 Specifying hints as NULL is equivalent to setting ai_socktype and
84 ai_protocol to 0; ai_family to AF_UNSPEC; and ai_flags to
85 (AI_V4MAPPED | AI_ADDRCONFIG). (POSIX specifies different defaults for
86 ai_flags; see NOTES.) node specifies either a numerical network
87 address (for IPv4, numbers-and-dots notation as supported by
88 inet_aton(3); for IPv6, hexadecimal string format as supported by
89 inet_pton(3)), or a network hostname, whose network addresses are
90 looked up and resolved. If hints.ai_flags contains the AI_NUMERICHOST
91 flag, then node must be a numerical network address. The AI_NUMERI‐
92 CHOST flag suppresses any potentially lengthy network host address
93 lookups.
94 If the AI_PASSIVE flag is specified in hints.ai_flags, and node is
96 NULL, then the returned socket addresses will be suitable for
97 bind(2)ing a socket that will accept(2) connections. The returned
98 socket address will contain the "wildcard address" (INADDR_ANY for IPv4
99 addresses, IN6ADDR_ANY_INIT for IPv6 address). The wildcard address is
100 used by applications (typically servers) that intend to accept connec‐
101 tions on any of the host's network addresses. If node is not NULL,
102 then the AI_PASSIVE flag is ignored.
103 If the AI_PASSIVE flag is not set in hints.ai_flags, then the returned
105 socket addresses will be suitable for use with connect(2), sendto(2),
106 or sendmsg(2). If node is NULL, then the network address will be set
107 to the loopback interface address (INADDR_LOOPBACK for IPv4 addresses,
108 IN6ADDR_LOOPBACK_INIT for IPv6 address); this is used by applications
109 that intend to communicate with peers running on the same host.
110 service sets the port in each returned address structure. If this
112 argument is a service name (see services(5)), it is translated to the
113 corresponding port number. This argument can also be specified as a
114 decimal number, which is simply converted to binary. If service is
115 NULL, then the port number of the returned socket addresses will be
116 left uninitialized. If AI_NUMERICSERV is specified in hints.ai_flags
117 and service is not NULL, then service must point to a string containing
118 a numeric port number. This flag is used to inhibit the invocation of
119 a name resolution service in cases where it is known not to be
120 required.
121 Either node or service, but not both, may be NULL.
123 The getaddrinfo() function allocates and initializes a linked list of
125 addrinfo structures, one for each network address that matches node and
126 service, subject to any restrictions imposed by hints, and returns a
127 pointer to the start of the list in res. The items in the linked list
128 are linked by the ai_next field.
129 There are several reasons why the linked list may have more than one
131 addrinfo structure, including: the network host is multihomed, accessi‐
132 ble over multiple protocols (e.g., both AF_INET and AF_INET6); or the
133 same service is available from multiple socket types (one SOCK_STREAM
134 address and another SOCK_DGRAM address, for example). Normally, the
135 application should try using the addresses in the order in which they
136 are returned. The sorting function used within getaddrinfo() is
137 defined in RFC 3484; the order can be tweaked for a particular system
138 by editing /etc/gai.conf (available since glibc 2.5).
139 If hints.ai_flags includes the AI_CANONNAME flag, then the ai_canonname
141 field of the first of the addrinfo structures in the returned list is
142 set to point to the official name of the host.
143 The remaining fields of each returned addrinfo structure are initial‐
145 ized as follows:
146 * The ai_family, ai_socktype, and ai_protocol fields return the socket
148 creation parameters (i.e., these fields have the same meaning as the
149 corresponding arguments of socket(2)). For example, ai_family might
150 return AF_INET or AF_INET6; ai_socktype might return SOCK_DGRAM or
151 SOCK_STREAM; and ai_protocol returns the protocol for the socket.
152 * A pointer to the socket address is placed in the ai_addr field, and
154 the length of the socket address, in bytes, is placed in the
155 ai_addrlen field.
156 If hints.ai_flags includes the AI_ADDRCONFIG flag, then IPv4 addresses
158 are returned in the list pointed to by res only if the local system has
159 at least one IPv4 address configured, and IPv6 addresses are returned
160 only if the local system has at least one IPv6 address configured. The
161 loopback address is not considered for this case as valid as a config‐
162 ured address. This flag is useful on, for example, IPv4-only systems,
163 to ensure that getaddrinfo() does not return IPv6 socket addresses that
164 would always fail in connect(2) or bind(2).
165 If hints.ai_flags specifies the AI_V4MAPPED flag, and hints.ai_family
167 was specified as AF_INET6, and no matching IPv6 addresses could be
168 found, then return IPv4-mapped IPv6 addresses in the list pointed to by
169 res. If both AI_V4MAPPED and AI_ALL are specified in hints.ai_flags,
170 then return both IPv6 and IPv4-mapped IPv6 addresses in the list
171 pointed to by res. AI_ALL is ignored if AI_V4MAPPED is not also speci‐
172 fied.
173 The freeaddrinfo() function frees the memory that was allocated for the
175 dynamically allocated linked list res.
176 Extensions to getaddrinfo() for Internationalized Domain Names
178 Starting with glibc 2.3.4, getaddrinfo() has been extended to selec‐
179 tively allow the incoming and outgoing hostnames to be transparently
180 converted to and from the Internationalized Domain Name (IDN) format
181 (see RFC 3490, Internationalizing Domain Names in Applications (IDNA)).
182 Four new flags are defined:
183 AI_IDN If this flag is specified, then the node name given in node is
185 converted to IDN format if necessary. The source encoding is
186 that of the current locale.
187 If the input name contains non-ASCII characters, then the IDN
189 encoding is used. Those parts of the node name (delimited by
190 dots) that contain non-ASCII characters are encoded using ASCII
191 Compatible Encoding (ACE) before being passed to the name reso‐
192 lution functions.
193 AI_CANONIDN
195 After a successful name lookup, and if the AI_CANONNAME flag was
196 specified, getaddrinfo() will return the canonical name of the
197 node corresponding to the addrinfo structure value passed back.
198 The return value is an exact copy of the value returned by the
199 name resolution function.
200 If the name is encoded using ACE, then it will contain the xn--
202 prefix for one or more components of the name. To convert these
203 components into a readable form the AI_CANONIDN flag can be
204 passed in addition to AI_CANONNAME. The resulting string is
205 encoded using the current locale's encoding.
206 AI_IDN_ALLOW_UNASSIGNED, AI_IDN_USE_STD3_ASCII_RULES
208 Setting these flags will enable the IDNA_ALLOW_UNASSIGNED (allow
209 unassigned Unicode code points) and IDNA_USE_STD3_ASCII_RULES
210 (check output to make sure it is a STD3 conforming hostname)
211 flags respectively to be used in the IDNA handling.
212
214 getaddrinfo() returns 0 if it succeeds, or one of the following nonzero
215 error codes:
216 EAI_ADDRFAMILY
218 The specified network host does not have any network addresses
219 in the requested address family.
220 EAI_AGAIN
222 The name server returned a temporary failure indication. Try
223 again later.
224 EAI_BADFLAGS
226 hints.ai_flags contains invalid flags; or, hints.ai_flags
227 included AI_CANONNAME and name was NULL.
228 EAI_FAIL
230 The name server returned a permanent failure indication.
231 EAI_FAMILY
233 The requested address family is not supported.
234 EAI_MEMORY
236 Out of memory.
237 EAI_NODATA
239 The specified network host exists, but does not have any network
240 addresses defined.
241 EAI_NONAME
243 The node or service is not known; or both node and service are
244 NULL; or AI_NUMERICSERV was specified in hints.ai_flags and ser‐
245 vice was not a numeric port-number string.
246 EAI_SERVICE
248 The requested service is not available for the requested socket
249 type. It may be available through another socket type. For
250 example, this error could occur if service was "shell" (a ser‐
251 vice available only on stream sockets), and either hints.ai_pro‐
252 tocol was IPPROTO_UDP, or hints.ai_socktype was SOCK_DGRAM; or
253 the error could occur if service was not NULL, and
254 hints.ai_socktype was SOCK_RAW (a socket type that does not sup‐
255 port the concept of services).
256 EAI_SOCKTYPE
258 The requested socket type is not supported. This could occur,
259 for example, if hints.ai_socktype and hints.ai_protocol are
260 inconsistent (e.g., SOCK_DGRAM and IPPROTO_TCP, respectively).
261 EAI_SYSTEM
263 Other system error, check errno for details.
264 The gai_strerror() function translates these error codes to a human
266 readable string, suitable for error reporting.
267
269 /etc/gai.conf
270
272 For an explanation of the terms used in this section, see
273 attributes(7).
274 ┌────────────────┬───────────────┬────────────────────┐
276 │Interface │ Attribute │ Value │
277 ├────────────────┼───────────────┼────────────────────┤
278 │getaddrinfo() │ Thread safety │ MT-Safe env locale │
279 ├────────────────┼───────────────┼────────────────────┤
280 │freeaddrinfo(), │ Thread safety │ MT-Safe │
281 │gai_strerror() │ │ │
282 └────────────────┴───────────────┴────────────────────┘
283
285 POSIX.1-2001, POSIX.1-2008. The getaddrinfo() function is documented
286 in RFC 2553.
287
289 getaddrinfo() supports the address%scope-id notation for specifying the
290 IPv6 scope-ID.
291 AI_ADDRCONFIG, AI_ALL, and AI_V4MAPPED are available since glibc 2.3.3.
293 AI_NUMERICSERV is available since glibc 2.3.4.
294 According to POSIX.1, specifying hints as NULL should cause ai_flags to
296 be assumed as 0. The GNU C library instead assumes a value of
297 (AI_V4MAPPED | AI_ADDRCONFIG) for this case, since this value is con‐
298 sidered an improvement on the specification.
299
301 The following programs demonstrate the use of getaddrinfo(), gai_str‐
302 error(), freeaddrinfo(), and getnameinfo(3). The programs are an echo
303 server and client for UDP datagrams.
304 Server program
306 #include <sys/types.h>
308 #include <stdio.h>
309 #include <stdlib.h>
310 #include <unistd.h>
311 #include <string.h>
312 #include <sys/socket.h>
313 #include <netdb.h>
314 #define BUF_SIZE 500
316 int
318 main(int argc, char *argv[])
319 {
320 struct addrinfo hints;
321 struct addrinfo *result, *rp;
322 int sfd, s;
323 struct sockaddr_storage peer_addr;
324 socklen_t peer_addr_len;
325 ssize_t nread;
326 char buf[BUF_SIZE];
327 if (argc != 2) {
329 fprintf(stderr, "Usage: %s port\n", argv[0]);
330 exit(EXIT_FAILURE);
331 }
332 memset(&hints, 0, sizeof(struct addrinfo));
334 hints.ai_family = AF_UNSPEC; /* Allow IPv4 or IPv6 */
335 hints.ai_socktype = SOCK_DGRAM; /* Datagram socket */
336 hints.ai_flags = AI_PASSIVE; /* For wildcard IP address */
337 hints.ai_protocol = 0; /* Any protocol */
338 hints.ai_canonname = NULL;
339 hints.ai_addr = NULL;
340 hints.ai_next = NULL;
341 s = getaddrinfo(NULL, argv[1], &hints, &result);
343 if (s != 0) {
344 fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(s));
345 exit(EXIT_FAILURE);
346 }
347 /* getaddrinfo() returns a list of address structures.
349 Try each address until we successfully bind(2).
350 If socket(2) (or bind(2)) fails, we (close the socket
351 and) try the next address. */
352 for (rp = result; rp != NULL; rp = rp->ai_next) {
354 sfd = socket(rp->ai_family, rp->ai_socktype,
355 rp->ai_protocol);
356 if (sfd == -1)
357 continue;
358 if (bind(sfd, rp->ai_addr, rp->ai_addrlen) == 0)
360 break; /* Success */
361 close(sfd);
363 }
364 if (rp == NULL) { /* No address succeeded */
366 fprintf(stderr, "Could not bind\n");
367 exit(EXIT_FAILURE);
368 }
369 freeaddrinfo(result); /* No longer needed */
371 /* Read datagrams and echo them back to sender */
373 for (;;) {
375 peer_addr_len = sizeof(struct sockaddr_storage);
376 nread = recvfrom(sfd, buf, BUF_SIZE, 0,
377 (struct sockaddr *) &peer_addr, &peer_addr_len);
378 if (nread == -1)
379 continue; /* Ignore failed request */
380 char host[NI_MAXHOST], service[NI_MAXSERV];
382 s = getnameinfo((struct sockaddr *) &peer_addr,
384 peer_addr_len, host, NI_MAXHOST,
385 service, NI_MAXSERV, NI_NUMERICSERV);
386 if (s == 0)
387 printf("Received %zd bytes from %s:%s\n",
388 nread, host, service);
389 else
390 fprintf(stderr, "getnameinfo: %s\n", gai_strerror(s));
391 if (sendto(sfd, buf, nread, 0,
393 (struct sockaddr *) &peer_addr,
394 peer_addr_len) != nread)
395 fprintf(stderr, "Error sending response\n");
396 }
397 }
398 Client program
400 #include <sys/types.h>
402 #include <sys/socket.h>
403 #include <netdb.h>
404 #include <stdio.h>
405 #include <stdlib.h>
406 #include <unistd.h>
407 #include <string.h>
408 #define BUF_SIZE 500
410 int
412 main(int argc, char *argv[])
413 {
414 struct addrinfo hints;
415 struct addrinfo *result, *rp;
416 int sfd, s, j;
417 size_t len;
418 ssize_t nread;
419 char buf[BUF_SIZE];
420 if (argc < 3) {
422 fprintf(stderr, "Usage: %s host port msg...\n", argv[0]);
423 exit(EXIT_FAILURE);
424 }
425 /* Obtain address(es) matching host/port */
427 memset(&hints, 0, sizeof(struct addrinfo));
429 hints.ai_family = AF_UNSPEC; /* Allow IPv4 or IPv6 */
430 hints.ai_socktype = SOCK_DGRAM; /* Datagram socket */
431 hints.ai_flags = 0;
432 hints.ai_protocol = 0; /* Any protocol */
433 s = getaddrinfo(argv[1], argv[2], &hints, &result);
435 if (s != 0) {
436 fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(s));
437 exit(EXIT_FAILURE);
438 }
439 /* getaddrinfo() returns a list of address structures.
441 Try each address until we successfully connect(2).
442 If socket(2) (or connect(2)) fails, we (close the socket
443 and) try the next address. */
444 for (rp = result; rp != NULL; rp = rp->ai_next) {
446 sfd = socket(rp->ai_family, rp->ai_socktype,
447 rp->ai_protocol);
448 if (sfd == -1)
449 continue;
450 if (connect(sfd, rp->ai_addr, rp->ai_addrlen) != -1)
452 break; /* Success */
453 close(sfd);
455 }
456 if (rp == NULL) { /* No address succeeded */
458 fprintf(stderr, "Could not connect\n");
459 exit(EXIT_FAILURE);
460 }
461 freeaddrinfo(result); /* No longer needed */
463 /* Send remaining command-line arguments as separate
465 datagrams, and read responses from server */
466 for (j = 3; j < argc; j++) {
468 len = strlen(argv[j]) + 1;
469 /* +1 for terminating null byte */
470 if (len > BUF_SIZE) {
472 fprintf(stderr,
473 "Ignoring long message in argument %d\n", j);
474 continue;
475 }
476 if (write(sfd, argv[j], len) != len) {
478 fprintf(stderr, "partial/failed write\n");
479 exit(EXIT_FAILURE);
480 }
481 nread = read(sfd, buf, BUF_SIZE);
483 if (nread == -1) {
484 perror("read");
485 exit(EXIT_FAILURE);
486 }
487 printf("Received %zd bytes: %s\n", nread, buf);
489 }
490 exit(EXIT_SUCCESS);
492 }
493
495 getaddrinfo_a(3), gethostbyname(3), getnameinfo(3), inet(3),
496 gai.conf(5), hostname(7), ip(7)
497
499 This page is part of release 5.04 of the Linux man-pages project. A
500 description of the project, information about reporting bugs, and the
501 latest version of this page, can be found at
502 https://www.kernel.org/doc/man-pages/.
503GNU 2019-03-06 GETADDRINFO(3)