1GETADDRINFO(3) Linux Programmer's Manual GETADDRINFO(3)
2
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 *restrict node,
15 const char *restrict service,
16 const struct addrinfo *restrict hints,
17 struct addrinfo **restrict res);
18 void freeaddrinfo(struct addrinfo *res);
20 const char *gai_strerror(int errcode);
22 Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
24 getaddrinfo(), freeaddrinfo(), gai_strerror():
26 Since glibc 2.22:
27 _POSIX_C_SOURCE >= 200112L
28 Glibc 2.21 and earlier:
29 _POSIX_C_SOURCE
30
32 Given node and service, which identify an Internet host and a service,
33 getaddrinfo() returns one or more addrinfo structures, each of which
34 contains an Internet address that can be specified in a call to bind(2)
35 or connect(2). The getaddrinfo() function combines the functionality
36 provided by the gethostbyname(3) and getservbyname(3) functions into a
37 single interface, but unlike the latter functions, getaddrinfo() is
38 reentrant and allows programs to eliminate IPv4-versus-IPv6 dependen‐
39 cies.
40 The addrinfo structure used by getaddrinfo() contains the following
42 fields:
43 struct addrinfo {
45 int ai_flags;
46 int ai_family;
47 int ai_socktype;
48 int ai_protocol;
49 socklen_t ai_addrlen;
50 struct sockaddr *ai_addr;
51 char *ai_canonname;
52 struct addrinfo *ai_next;
53 };
54 The hints argument points to an addrinfo structure that specifies cri‐
56 teria for selecting the socket address structures returned in the list
57 pointed to by res. If hints is not NULL it points to an addrinfo
58 structure whose ai_family, ai_socktype, and ai_protocol specify crite‐
59 ria that limit the set of socket addresses returned by getaddrinfo(),
60 as follows:
61 ai_family
63 This field specifies the desired address family for the returned
64 addresses. Valid values for this field include AF_INET and
65 AF_INET6. The value AF_UNSPEC indicates that getaddrinfo()
66 should return socket addresses for any address family (either
67 IPv4 or IPv6, for example) that can be used with node and ser‐
68 vice.
69 ai_socktype
71 This field specifies the preferred socket type, for example
72 SOCK_STREAM or SOCK_DGRAM. Specifying 0 in this field indicates
73 that socket addresses of any type can be returned by getad‐
74 drinfo().
75 ai_protocol
77 This field specifies the protocol for the returned socket ad‐
78 dresses. Specifying 0 in this field indicates that socket ad‐
79 dresses with any protocol can be returned by getaddrinfo().
80 ai_flags
82 This field specifies additional options, described below. Mul‐
83 tiple flags are specified by bitwise OR-ing them together.
84 All the other fields in the structure pointed to by hints must contain
86 either 0 or a null pointer, as appropriate.
87 Specifying hints as NULL is equivalent to setting ai_socktype and
89 ai_protocol to 0; ai_family to AF_UNSPEC; and ai_flags to
90 (AI_V4MAPPED | AI_ADDRCONFIG). (POSIX specifies different defaults for
91 ai_flags; see NOTES.) node specifies either a numerical network ad‐
92 dress (for IPv4, numbers-and-dots notation as supported by
93 inet_aton(3); for IPv6, hexadecimal string format as supported by
94 inet_pton(3)), or a network hostname, whose network addresses are
95 looked up and resolved. If hints.ai_flags contains the AI_NUMERICHOST
96 flag, then node must be a numerical network address. The AI_NUMERI‐
97 CHOST flag suppresses any potentially lengthy network host address
98 lookups.
99 If the AI_PASSIVE flag is specified in hints.ai_flags, and node is
101 NULL, then the returned socket addresses will be suitable for
102 bind(2)ing a socket that will accept(2) connections. The returned
103 socket address will contain the "wildcard address" (INADDR_ANY for IPv4
104 addresses, IN6ADDR_ANY_INIT for IPv6 address). The wildcard address is
105 used by applications (typically servers) that intend to accept connec‐
106 tions on any of the host's network addresses. If node is not NULL,
107 then the AI_PASSIVE flag is ignored.
108 If the AI_PASSIVE flag is not set in hints.ai_flags, then the returned
110 socket addresses will be suitable for use with connect(2), sendto(2),
111 or sendmsg(2). If node is NULL, then the network address will be set
112 to the loopback interface address (INADDR_LOOPBACK for IPv4 addresses,
113 IN6ADDR_LOOPBACK_INIT for IPv6 address); this is used by applications
114 that intend to communicate with peers running on the same host.
115 service sets the port in each returned address structure. If this ar‐
117 gument is a service name (see services(5)), it is translated to the
118 corresponding port number. This argument can also be specified as a
119 decimal number, which is simply converted to binary. If service is
120 NULL, then the port number of the returned socket addresses will be
121 left uninitialized. If AI_NUMERICSERV is specified in hints.ai_flags
122 and service is not NULL, then service must point to a string containing
123 a numeric port number. This flag is used to inhibit the invocation of
124 a name resolution service in cases where it is known not to be re‐
125 quired.
126 Either node or service, but not both, may be NULL.
128 The getaddrinfo() function allocates and initializes a linked list of
130 addrinfo structures, one for each network address that matches node and
131 service, subject to any restrictions imposed by hints, and returns a
132 pointer to the start of the list in res. The items in the linked list
133 are linked by the ai_next field.
134 There are several reasons why the linked list may have more than one
136 addrinfo structure, including: the network host is multihomed, accessi‐
137 ble over multiple protocols (e.g., both AF_INET and AF_INET6); or the
138 same service is available from multiple socket types (one SOCK_STREAM
139 address and another SOCK_DGRAM address, for example). Normally, the
140 application should try using the addresses in the order in which they
141 are returned. The sorting function used within getaddrinfo() is de‐
142 fined in RFC 3484; the order can be tweaked for a particular system by
143 editing /etc/gai.conf (available since glibc 2.5).
144 If hints.ai_flags includes the AI_CANONNAME flag, then the ai_canonname
146 field of the first of the addrinfo structures in the returned list is
147 set to point to the official name of the host.
148 The remaining fields of each returned addrinfo structure are initial‐
150 ized as follows:
151 * The ai_family, ai_socktype, and ai_protocol fields return the socket
153 creation parameters (i.e., these fields have the same meaning as the
154 corresponding arguments of socket(2)). For example, ai_family might
155 return AF_INET or AF_INET6; ai_socktype might return SOCK_DGRAM or
156 SOCK_STREAM; and ai_protocol returns the protocol for the socket.
157 * A pointer to the socket address is placed in the ai_addr field, and
159 the length of the socket address, in bytes, is placed in the ai_ad‐
160 drlen field.
161 If hints.ai_flags includes the AI_ADDRCONFIG flag, then IPv4 addresses
163 are returned in the list pointed to by res only if the local system has
164 at least one IPv4 address configured, and IPv6 addresses are returned
165 only if the local system has at least one IPv6 address configured. The
166 loopback address is not considered for this case as valid as a config‐
167 ured address. This flag is useful on, for example, IPv4-only systems,
168 to ensure that getaddrinfo() does not return IPv6 socket addresses that
169 would always fail in connect(2) or bind(2).
170 If hints.ai_flags specifies the AI_V4MAPPED flag, and hints.ai_family
172 was specified as AF_INET6, and no matching IPv6 addresses could be
173 found, then return IPv4-mapped IPv6 addresses in the list pointed to by
174 res. If both AI_V4MAPPED and AI_ALL are specified in hints.ai_flags,
175 then return both IPv6 and IPv4-mapped IPv6 addresses in the list
176 pointed to by res. AI_ALL is ignored if AI_V4MAPPED is not also speci‐
177 fied.
178 The freeaddrinfo() function frees the memory that was allocated for the
180 dynamically allocated linked list res.
181 Extensions to getaddrinfo() for Internationalized Domain Names
183 Starting with glibc 2.3.4, getaddrinfo() has been extended to selec‐
184 tively allow the incoming and outgoing hostnames to be transparently
185 converted to and from the Internationalized Domain Name (IDN) format
186 (see RFC 3490, Internationalizing Domain Names in Applications (IDNA)).
187 Four new flags are defined:
188 AI_IDN If this flag is specified, then the node name given in node is
190 converted to IDN format if necessary. The source encoding is
191 that of the current locale.
192 If the input name contains non-ASCII characters, then the IDN
194 encoding is used. Those parts of the node name (delimited by
195 dots) that contain non-ASCII characters are encoded using ASCII
196 Compatible Encoding (ACE) before being passed to the name reso‐
197 lution functions.
198 AI_CANONIDN
200 After a successful name lookup, and if the AI_CANONNAME flag was
201 specified, getaddrinfo() will return the canonical name of the
202 node corresponding to the addrinfo structure value passed back.
203 The return value is an exact copy of the value returned by the
204 name resolution function.
205 If the name is encoded using ACE, then it will contain the xn--
207 prefix for one or more components of the name. To convert these
208 components into a readable form the AI_CANONIDN flag can be
209 passed in addition to AI_CANONNAME. The resulting string is en‐
210 coded using the current locale's encoding.
211 AI_IDN_ALLOW_UNASSIGNED, AI_IDN_USE_STD3_ASCII_RULES
213 Setting these flags will enable the IDNA_ALLOW_UNASSIGNED (allow
214 unassigned Unicode code points) and IDNA_USE_STD3_ASCII_RULES
215 (check output to make sure it is a STD3 conforming hostname)
216 flags respectively to be used in the IDNA handling.
217
219 getaddrinfo() returns 0 if it succeeds, or one of the following nonzero
220 error codes:
221 EAI_ADDRFAMILY
223 The specified network host does not have any network addresses
224 in the requested address family.
225 EAI_AGAIN
227 The name server returned a temporary failure indication. Try
228 again later.
229 EAI_BADFLAGS
231 hints.ai_flags contains invalid flags; or, hints.ai_flags in‐
232 cluded AI_CANONNAME and name was NULL.
233 EAI_FAIL
235 The name server returned a permanent failure indication.
236 EAI_FAMILY
238 The requested address family is not supported.
239 EAI_MEMORY
241 Out of memory.
242 EAI_NODATA
244 The specified network host exists, but does not have any network
245 addresses defined.
246 EAI_NONAME
248 The node or service is not known; or both node and service are
249 NULL; or AI_NUMERICSERV was specified in hints.ai_flags and ser‐
250 vice was not a numeric port-number string.
251 EAI_SERVICE
253 The requested service is not available for the requested socket
254 type. It may be available through another socket type. For ex‐
255 ample, this error could occur if service was "shell" (a service
256 available only on stream sockets), and either hints.ai_protocol
257 was IPPROTO_UDP, or hints.ai_socktype was SOCK_DGRAM; or the er‐
258 ror could occur if service was not NULL, and hints.ai_socktype
259 was SOCK_RAW (a socket type that does not support the concept of
260 services).
261 EAI_SOCKTYPE
263 The requested socket type is not supported. This could occur,
264 for example, if hints.ai_socktype and hints.ai_protocol are in‐
265 consistent (e.g., SOCK_DGRAM and IPPROTO_TCP, respectively).
266 EAI_SYSTEM
268 Other system error; errno is set to indicate the error.
269 The gai_strerror() function translates these error codes to a human
271 readable string, suitable for error reporting.
272
274 /etc/gai.conf
275
277 For an explanation of the terms used in this section, see at‐
278 tributes(7).
279 ┌─────────────────────────────────┬───────────────┬────────────────────┐
281 │Interface │ Attribute │ Value │
282 ├─────────────────────────────────┼───────────────┼────────────────────┤
283 │getaddrinfo() │ Thread safety │ MT-Safe env locale │
284 ├─────────────────────────────────┼───────────────┼────────────────────┤
285 │freeaddrinfo(), gai_strerror() │ Thread safety │ MT-Safe │
286 └─────────────────────────────────┴───────────────┴────────────────────┘
287
289 POSIX.1-2001, POSIX.1-2008. The getaddrinfo() function is documented
290 in RFC 2553.
291
293 getaddrinfo() supports the address%scope-id notation for specifying the
294 IPv6 scope-ID.
295 AI_ADDRCONFIG, AI_ALL, and AI_V4MAPPED are available since glibc 2.3.3.
297 AI_NUMERICSERV is available since glibc 2.3.4.
298 According to POSIX.1, specifying hints as NULL should cause ai_flags to
300 be assumed as 0. The GNU C library instead assumes a value of
301 (AI_V4MAPPED | AI_ADDRCONFIG) for this case, since this value is con‐
302 sidered an improvement on the specification.
303
305 The following programs demonstrate the use of getaddrinfo(), gai_str‐
306 error(), freeaddrinfo(), and getnameinfo(3). The programs are an echo
307 server and client for UDP datagrams.
308 Server program
310 #include <sys/types.h>
312 #include <stdio.h>
313 #include <stdlib.h>
314 #include <unistd.h>
315 #include <string.h>
316 #include <sys/socket.h>
317 #include <netdb.h>
318 #define BUF_SIZE 500
320 int
322 main(int argc, char *argv[])
323 {
324 struct addrinfo hints;
325 struct addrinfo *result, *rp;
326 int sfd, s;
327 struct sockaddr_storage peer_addr;
328 socklen_t peer_addr_len;
329 ssize_t nread;
330 char buf[BUF_SIZE];
331 if (argc != 2) {
333 fprintf(stderr, "Usage: %s port\n", argv[0]);
334 exit(EXIT_FAILURE);
335 }
336 memset(&hints, 0, sizeof(hints));
338 hints.ai_family = AF_UNSPEC; /* Allow IPv4 or IPv6 */
339 hints.ai_socktype = SOCK_DGRAM; /* Datagram socket */
340 hints.ai_flags = AI_PASSIVE; /* For wildcard IP address */
341 hints.ai_protocol = 0; /* Any protocol */
342 hints.ai_canonname = NULL;
343 hints.ai_addr = NULL;
344 hints.ai_next = NULL;
345 s = getaddrinfo(NULL, argv[1], &hints, &result);
347 if (s != 0) {
348 fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(s));
349 exit(EXIT_FAILURE);
350 }
351 /* getaddrinfo() returns a list of address structures.
353 Try each address until we successfully bind(2).
354 If socket(2) (or bind(2)) fails, we (close the socket
355 and) try the next address. */
356 for (rp = result; rp != NULL; rp = rp->ai_next) {
358 sfd = socket(rp->ai_family, rp->ai_socktype,
359 rp->ai_protocol);
360 if (sfd == -1)
361 continue;
362 if (bind(sfd, rp->ai_addr, rp->ai_addrlen) == 0)
364 break; /* Success */
365 close(sfd);
367 }
368 freeaddrinfo(result); /* No longer needed */
370 if (rp == NULL) { /* No address succeeded */
372 fprintf(stderr, "Could not bind\n");
373 exit(EXIT_FAILURE);
374 }
375 /* Read datagrams and echo them back to sender. */
377 for (;;) {
379 peer_addr_len = sizeof(peer_addr);
380 nread = recvfrom(sfd, buf, BUF_SIZE, 0,
381 (struct sockaddr *) &peer_addr, &peer_addr_len);
382 if (nread == -1)
383 continue; /* Ignore failed request */
384 char host[NI_MAXHOST], service[NI_MAXSERV];
386 s = getnameinfo((struct sockaddr *) &peer_addr,
388 peer_addr_len, host, NI_MAXHOST,
389 service, NI_MAXSERV, NI_NUMERICSERV);
390 if (s == 0)
391 printf("Received %zd bytes from %s:%s\n",
392 nread, host, service);
393 else
394 fprintf(stderr, "getnameinfo: %s\n", gai_strerror(s));
395 if (sendto(sfd, buf, nread, 0,
397 (struct sockaddr *) &peer_addr,
398 peer_addr_len) != nread)
399 fprintf(stderr, "Error sending response\n");
400 }
401 }
402 Client program
404 #include <sys/types.h>
406 #include <sys/socket.h>
407 #include <netdb.h>
408 #include <stdio.h>
409 #include <stdlib.h>
410 #include <unistd.h>
411 #include <string.h>
412 #define BUF_SIZE 500
414 int
416 main(int argc, char *argv[])
417 {
418 struct addrinfo hints;
419 struct addrinfo *result, *rp;
420 int sfd, s;
421 size_t len;
422 ssize_t nread;
423 char buf[BUF_SIZE];
424 if (argc < 3) {
426 fprintf(stderr, "Usage: %s host port msg...\n", argv[0]);
427 exit(EXIT_FAILURE);
428 }
429 /* Obtain address(es) matching host/port. */
431 memset(&hints, 0, sizeof(hints));
433 hints.ai_family = AF_UNSPEC; /* Allow IPv4 or IPv6 */
434 hints.ai_socktype = SOCK_DGRAM; /* Datagram socket */
435 hints.ai_flags = 0;
436 hints.ai_protocol = 0; /* Any protocol */
437 s = getaddrinfo(argv[1], argv[2], &hints, &result);
439 if (s != 0) {
440 fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(s));
441 exit(EXIT_FAILURE);
442 }
443 /* getaddrinfo() returns a list of address structures.
445 Try each address until we successfully connect(2).
446 If socket(2) (or connect(2)) fails, we (close the socket
447 and) try the next address. */
448 for (rp = result; rp != NULL; rp = rp->ai_next) {
450 sfd = socket(rp->ai_family, rp->ai_socktype,
451 rp->ai_protocol);
452 if (sfd == -1)
453 continue;
454 if (connect(sfd, rp->ai_addr, rp->ai_addrlen) != -1)
456 break; /* Success */
457 close(sfd);
459 }
460 freeaddrinfo(result); /* No longer needed */
462 if (rp == NULL) { /* No address succeeded */
464 fprintf(stderr, "Could not connect\n");
465 exit(EXIT_FAILURE);
466 }
467 /* Send remaining command-line arguments as separate
469 datagrams, and read responses from server. */
470 for (int j = 3; j < argc; j++) {
472 len = strlen(argv[j]) + 1;
473 /* +1 for terminating null byte */
474 if (len > BUF_SIZE) {
476 fprintf(stderr,
477 "Ignoring long message in argument %d\n", j);
478 continue;
479 }
480 if (write(sfd, argv[j], len) != len) {
482 fprintf(stderr, "partial/failed write\n");
483 exit(EXIT_FAILURE);
484 }
485 nread = read(sfd, buf, BUF_SIZE);
487 if (nread == -1) {
488 perror("read");
489 exit(EXIT_FAILURE);
490 }
491 printf("Received %zd bytes: %s\n", nread, buf);
493 }
494 exit(EXIT_SUCCESS);
496 }
497
499 getaddrinfo_a(3), gethostbyname(3), getnameinfo(3), inet(3),
500 gai.conf(5), hostname(7), ip(7)
501
503 This page is part of release 5.13 of the Linux man-pages project. A
504 description of the project, information about reporting bugs, and the
505 latest version of this page, can be found at
506 https://www.kernel.org/doc/man-pages/.
507GNU 2021-08-27 GETADDRINFO(3)