getaddrinfo(3) - Linux manual page (original) (raw)
getaddrinfo(3) Library Functions Manual getaddrinfo(3)
NAME top
getaddrinfo, freeaddrinfo, gai_strerror - network address and
service translationLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#include <sys/types.h>**
**#include <sys/socket.h>**
**#include <netdb.h>**
**int getaddrinfo(const char *restrict** _node_**,**
**const char *restrict** _service_**,**
**const struct addrinfo *restrict** _hints_**,**
**struct addrinfo restrict** _res_**);**
**void freeaddrinfo(struct addrinfo ***_res_**);**
**const char *gai_strerror(int** _errcode_**);**Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
**getaddrinfo**(), **freeaddrinfo**(), **gai_strerror**():
Since glibc 2.22:
_POSIX_C_SOURCE >= 200112L
glibc 2.21 and earlier:
_POSIX_C_SOURCEDESCRIPTION top
Given _node_ and _service_, which identify an Internet host and a
service, **getaddrinfo**() returns one or more _addrinfo_ structures,
each of which contains an Internet address that can be specified
in a call to [bind(2)](../man2/bind.2.html) or [connect(2)](../man2/connect.2.html). The **getaddrinfo**() function
combines the functionality provided by the [gethostbyname(3)](../man3/gethostbyname.3.html) and
[getservbyname(3)](../man3/getservbyname.3.html) functions into a single interface, but unlike the
latter functions, **getaddrinfo**() is reentrant and allows programs
to eliminate IPv4-versus-IPv6 dependencies.
The _addrinfo_ structure used by **getaddrinfo**() contains the
following fields:
struct addrinfo {
int ai_flags;
int ai_family;
int ai_socktype;
int ai_protocol;
socklen_t ai_addrlen;
struct sockaddr *ai_addr;
char *ai_canonname;
struct addrinfo *ai_next;
};
The _hints_ argument points to an _addrinfo_ structure that specifies
criteria for selecting the socket address structures returned in
the list pointed to by _res_. If _hints_ is not NULL it points to an
_addrinfo_ structure whose _aifamily_, _aisocktype_, and _aiprotocol_
specify criteria that limit the set of socket addresses returned
by **getaddrinfo**(), as follows:
_aifamily_
This field specifies the desired address family for the
returned addresses. Valid values for this field include
**AF_INET** and **AF_INET6**. The value **AF_UNSPEC** indicates that
**getaddrinfo**() should return socket addresses for any
address family (either IPv4 or IPv6, for example) that can
be used with _node_ and _service_.
_aisocktype_
This field specifies the preferred socket type, for example
**SOCK_STREAM** or **SOCK_DGRAM**. Specifying 0 in this field
indicates that socket addresses of any type can be returned
by **getaddrinfo**().
_aiprotocol_
This field specifies the protocol for the returned socket
addresses. Specifying 0 in this field indicates that
socket addresses with any protocol can be returned by
**getaddrinfo**().
_aiflags_
This field specifies additional options, described below.
Multiple flags are specified by bitwise OR-ing them
together.
All the other fields in the structure pointed to by _hints_ must
contain either 0 or a null pointer, as appropriate.
Specifying _hints_ as NULL is equivalent to setting _aisocktype_ and
_aiprotocol_ to 0; _aifamily_ to **AF_UNSPEC**; and _aiflags_ to
**(AI_V4MAPPED | AI_ADDRCONFIG)**. (POSIX specifies different
defaults for _aiflags_; see NOTES.) _node_ specifies either a
numerical network address (for IPv4, numbers-and-dots notation as
supported by [inet_aton(3)](../man3/inet%5Faton.3.html); for IPv6, hexadecimal string format as
supported by [inet_pton(3)](../man3/inet%5Fpton.3.html)), or a network hostname, whose network
addresses are looked up and resolved. If _hints.aiflags_ contains
the **AI_NUMERICHOST** flag, then _node_ must be a numerical network
address. The **AI_NUMERICHOST** flag suppresses any potentially
lengthy network host address lookups.
If the **AI_PASSIVE** flag is specified in _hints.aiflags_, and _node_ is
NULL, then the returned socket addresses will be suitable for
[bind(2)](../man2/bind.2.html)ing a socket that will [accept(2)](../man2/accept.2.html) connections. The returned
socket address will contain the "wildcard address" (**INADDR_ANY** for
IPv4 addresses, **IN6ADDR_ANY_INIT** for IPv6 address). The wildcard
address is used by applications (typically servers) that intend to
accept connections on any of the host's network addresses. If
_node_ is not NULL, then the **AI_PASSIVE** flag is ignored.
If the **AI_PASSIVE** flag is not set in _hints.aiflags_, then the
returned socket addresses will be suitable for use with
[connect(2)](../man2/connect.2.html), [sendto(2)](../man2/sendto.2.html), or [sendmsg(2)](../man2/sendmsg.2.html). If _node_ is NULL, then the
network address will be set to the loopback interface address
(**INADDR_LOOPBACK** for IPv4 addresses, **IN6ADDR_LOOPBACK_INIT** for
IPv6 address); this is used by applications that intend to
communicate with peers running on the same host.
_service_ sets the port in each returned address structure. If this
argument is a service name (see [services(5)](../man5/services.5.html)), it is translated to
the corresponding port number. This argument can also be
specified as a decimal number, which is simply converted to
binary. If _service_ is NULL, then the port number of the returned
socket addresses will be left uninitialized. If **AI_NUMERICSERV** is
specified in _hints.aiflags_ and _service_ is not NULL, then _service_
must point to a string containing a numeric port number. This
flag is used to inhibit the invocation of a name resolution
service in cases where it is known not to be required.
Either _node_ or _service_, but not both, may be NULL.
The **getaddrinfo**() function allocates and initializes a linked list
of _addrinfo_ structures, one for each network address that matches
_node_ and _service_, subject to any restrictions imposed by _hints_,
and returns a pointer to the start of the list in _res_. The items
in the linked list are linked by the _ainext_ field.
There are several reasons why the linked list may have more than
one _addrinfo_ structure, including: the network host is multihomed,
accessible over multiple protocols (e.g., both **AF_INET** and
**AF_INET6**); or the same service is available from multiple socket
types (one **SOCK_STREAM** address and another **SOCK_DGRAM** address, for
example). Normally, the application should try using the
addresses in the order in which they are returned. The sorting
function used within **getaddrinfo**() is defined in RFC 3484; the
order can be tweaked for a particular system by editing
_/etc/gai.conf_ (available since glibc 2.5).
If _hints.aiflags_ includes the **AI_CANONNAME** flag, then the
_aicanonname_ field of the first of the _addrinfo_ structures in the
returned list is set to point to the official name of the host.
The remaining fields of each returned _addrinfo_ structure are
initialized as follows:
• The _aifamily_, _aisocktype_, and _aiprotocol_ fields return the
socket creation parameters (i.e., these fields have the same
meaning as the corresponding arguments of [socket(2)](../man2/socket.2.html)). For
example, _aifamily_ might return **AF_INET** or **AF_INET6**;
_aisocktype_ might return **SOCK_DGRAM** or **SOCK_STREAM**; and
_aiprotocol_ returns the protocol for the socket.
• A pointer to the socket address is placed in the _aiaddr_ field,
and the size of the socket address, in bytes, is placed in the
_aiaddrlen_ field.
If _hints.aiflags_ includes the **AI_ADDRCONFIG** flag, then IPv4
addresses are returned in the list pointed to by _res_ only if the
local system has at least one IPv4 address configured, and IPv6
addresses are returned only if the local system has at least one
IPv6 address configured. The loopback address is not considered
for this case as valid as a configured address. This flag is
useful on, for example, IPv4-only systems, to ensure that
**getaddrinfo**() does not return IPv6 socket addresses that would
always fail in [connect(2)](../man2/connect.2.html) or [bind(2)](../man2/bind.2.html).
If _hints.aiflags_ specifies the **AI_V4MAPPED** flag, and
_hints.aifamily_ was specified as **AF_INET6**, and no matching IPv6
addresses could be found, then return IPv4-mapped IPv6 addresses
in the list pointed to by _res_. If both **AI_V4MAPPED** and **AI_ALL** are
specified in _hints.aiflags_, then return both IPv6 and IPv4-mapped
IPv6 addresses in the list pointed to by _res_. **AI_ALL** is ignored
if **AI_V4MAPPED** is not also specified.
The **freeaddrinfo**() function frees the memory that was allocated
for the dynamically allocated linked list _res_.Extensions to getaddrinfo() for Internationalized Domain Names Starting with glibc 2.3.4, getaddrinfo() has been extended to selectively allow the incoming and outgoing hostnames to be transparently converted to and from the Internationalized Domain Name (IDN) format (see RFC 3490, Internationalizing Domain Names in Applications (IDNA)). Four new flags are defined:
**AI_IDN** If this flag is specified, then the node name given in _node_
is converted to IDN format if necessary. The source
encoding is that of the current locale.
If the input name contains non-ASCII characters, then the
IDN encoding is used. Those parts of the node name
(delimited by dots) that contain non-ASCII characters are
encoded using ASCII Compatible Encoding (ACE) before being
passed to the name resolution functions.
**AI_CANONIDN**
After a successful name lookup, and if the **AI_CANONNAME**
flag was specified, **getaddrinfo**() will return the canonical
name of the node corresponding to the _addrinfo_ structure
value passed back. The return value is an exact copy of
the value returned by the name resolution function.
If the name is encoded using ACE, then it will contain the
_xn--_ prefix for one or more components of the name. To
convert these components into a readable form the
**AI_CANONIDN** flag can be passed in addition to **AI_CANONNAME**.
The resulting string is encoded using the current locale's
encoding.
**AI_IDN_ALLOW_UNASSIGNED**
**AI_IDN_USE_STD3_ASCII_RULES**
Setting these flags will enable the IDNA_ALLOW_UNASSIGNED
(allow unassigned Unicode code points) and
IDNA_USE_STD3_ASCII_RULES (check output to make sure it is
a STD3 conforming hostname) flags respectively to be used
in the IDNA handling.RETURN VALUE top
**getaddrinfo**() returns 0 if it succeeds, or one of the following
nonzero error codes:
**EAI_ADDRFAMILY**
The specified network host does not have any network
addresses in the requested address family.
**EAI_AGAIN**
The name server returned a temporary failure indication.
Try again later.
**EAI_BADFLAGS**
_hints.aiflags_ contains invalid flags; or, _hints.aiflags_
included **AI_CANONNAME** and _node_ was NULL.
**EAI_FAIL**
The name server returned a permanent failure indication.
**EAI_FAMILY**
The requested address family is not supported.
**EAI_MEMORY**
Out of memory.
**EAI_NODATA**
The specified network host exists, but does not have any
network addresses defined.
**EAI_NONAME**
The _node_ or _service_ is not known; or both _node_ and _service_
are NULL; or **AI_NUMERICSERV** was specified in _hints.aiflags_
and _service_ was not a numeric port-number string.
**EAI_SERVICE**
The requested service is not available for the requested
socket type. It may be available through another socket
type. For example, this error could occur if _service_ was
"shell" (a service available only on stream sockets), and
either _hints.aiprotocol_ was **IPPROTO_UDP**, or
_hints.aisocktype_ was **SOCK_DGRAM**; or the error could occur
if _service_ was not NULL, and _hints.aisocktype_ was **SOCK_RAW**
(a socket type that does not support the concept of
services).
**EAI_SOCKTYPE**
The requested socket type is not supported. This could
occur, for example, if _hints.aisocktype_ and
_hints.aiprotocol_ are inconsistent (e.g., **SOCK_DGRAM** and
**IPPROTO_TCP**, respectively).
**EAI_SYSTEM**
Other system error; _[errno](../man3/errno.3.html)_ is set to indicate the error.
The **gai_strerror**() function translates these error codes to a
human readable string, suitable for error reporting.FILES top
_/etc/gai.conf_ATTRIBUTES top
For an explanation of the terms used in this section, see
[attributes(7)](../man7/attributes.7.html).
┌───────────────────────────┬───────────────┬────────────────────┐
│ **Interface** │ **Attribute** │ **Value** │
├───────────────────────────┼───────────────┼────────────────────┤
│ **getaddrinfo**() │ Thread safety │ MT-Safe env locale │
├───────────────────────────┼───────────────┼────────────────────┤
│ **freeaddrinfo**(), │ Thread safety │ MT-Safe │
│ **gai_strerror**() │ │ │
└───────────────────────────┴───────────────┴────────────────────┘VERSIONS top
According to POSIX.1, specifying _hints_ as NULL should cause
_aiflags_ to be assumed as 0. The GNU C library instead assumes a
value of **(AI_V4MAPPED | AI_ADDRCONFIG)** for this case, since this
value is considered an improvement on the specification.STANDARDS top
POSIX.1-2008.
**getaddrinfo**()
RFC 2553.HISTORY top
POSIX.1-2001.
**AI_ADDRCONFIG**
**AI_ALL**
**AI_V4MAPPED**
glibc 2.3.3.
**AI_NUMERICSERV**
glibc 2.3.4.NOTES top
**getaddrinfo**() supports the _address_**%**_scope-id_ notation for
specifying the IPv6 scope-ID.EXAMPLES top
The following programs demonstrate the use of **getaddrinfo**(),
**gai_strerror**(), **freeaddrinfo**(), and [getnameinfo(3)](../man3/getnameinfo.3.html). The programs
are an echo server and client for UDP datagrams.Server program
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <unistd.h>
#define BUF_SIZE 500
int
main(int argc, char *argv[])
{
int sfd, s;
char buf[BUF_SIZE];
ssize_t nread;
socklen_t peer_addrlen;
struct addrinfo hints;
struct addrinfo *result, *rp;
struct sockaddr_storage peer_addr;
if (argc != 2) {
fprintf(stderr, "Usage: %s port\n", argv[0]);
exit(EXIT_FAILURE);
}
memset(&hints, 0, sizeof(hints));
hints.ai_family = AF_UNSPEC; /* Allow IPv4 or IPv6 */
hints.ai_socktype = SOCK_DGRAM; /* Datagram socket */
hints.ai_flags = AI_PASSIVE; /* For wildcard IP address */
hints.ai_protocol = 0; /* Any protocol */
hints.ai_canonname = NULL;
hints.ai_addr = NULL;
hints.ai_next = NULL;
s = getaddrinfo(NULL, argv[1], &hints, &result);
if (s != 0) {
fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(s));
exit(EXIT_FAILURE);
}
/* getaddrinfo() returns a list of address structures.
Try each address until we successfully bind(2).
If socket(2) (or bind(2)) fails, we (close the socket
and) try the next address. */
for (rp = result; rp != NULL; rp = rp->ai_next) {
sfd = socket(rp->ai_family, rp->ai_socktype,
rp->ai_protocol);
if (sfd == -1)
continue;
if (bind(sfd, rp->ai_addr, rp->ai_addrlen) == 0)
break; /* Success */
close(sfd);
}
freeaddrinfo(result); /* No longer needed */
if (rp == NULL) { /* No address succeeded */
fprintf(stderr, "Could not bind\n");
exit(EXIT_FAILURE);
}
/* Read datagrams and echo them back to sender. */
for (;;) {
char host[NI_MAXHOST], service[NI_MAXSERV];
peer_addrlen = sizeof(peer_addr);
nread = recvfrom(sfd, buf, BUF_SIZE, 0,
(struct sockaddr *) &peer_addr, &peer_addrlen);
if (nread == -1)
continue; /* Ignore failed request */
s = getnameinfo((struct sockaddr *) &peer_addr,
peer_addrlen, host, NI_MAXHOST,
service, NI_MAXSERV, NI_NUMERICSERV);
if (s == 0)
printf("Received %zd bytes from %s:%s\n",
nread, host, service);
else
fprintf(stderr, "getnameinfo: %s\n", gai_strerror(s));
if (sendto(sfd, buf, nread, 0, (struct sockaddr *) &peer_addr,
peer_addrlen) != nread)
{
fprintf(stderr, "Error sending response\n");
}
}
}Client program
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <unistd.h>
#define BUF_SIZE 500
int
main(int argc, char *argv[])
{
int sfd, s;
char buf[BUF_SIZE];
size_t size;
ssize_t nread;
struct addrinfo hints;
struct addrinfo *result, *rp;
if (argc < 3) {
fprintf(stderr, "Usage: %s host port msg...\n", argv[0]);
exit(EXIT_FAILURE);
}
/* Obtain address(es) matching host/port. */
memset(&hints, 0, sizeof(hints));
hints.ai_family = AF_UNSPEC; /* Allow IPv4 or IPv6 */
hints.ai_socktype = SOCK_DGRAM; /* Datagram socket */
hints.ai_flags = 0;
hints.ai_protocol = 0; /* Any protocol */
s = getaddrinfo(argv[1], argv[2], &hints, &result);
if (s != 0) {
fprintf(stderr, "getaddrinfo: %s\n", gai_strerror(s));
exit(EXIT_FAILURE);
}
/* getaddrinfo() returns a list of address structures.
Try each address until we successfully connect(2).
If socket(2) (or connect(2)) fails, we (close the socket
and) try the next address. */
for (rp = result; rp != NULL; rp = rp->ai_next) {
sfd = socket(rp->ai_family, rp->ai_socktype,
rp->ai_protocol);
if (sfd == -1)
continue;
if (connect(sfd, rp->ai_addr, rp->ai_addrlen) != -1)
break; /* Success */
close(sfd);
}
freeaddrinfo(result); /* No longer needed */
if (rp == NULL) { /* No address succeeded */
fprintf(stderr, "Could not connect\n");
exit(EXIT_FAILURE);
}
/* Send remaining command-line arguments as separate
datagrams, and read responses from server. */
for (size_t j = 3; j < argc; j++) {
size = strlen(argv[j]) + 1;
/* +1 for terminating null byte */
if (size > BUF_SIZE) {
fprintf(stderr,
"Ignoring long message in argument %zu\n", j);
continue;
}
if (write(sfd, argv[j], size) != size) {
fprintf(stderr, "partial/failed write\n");
exit(EXIT_FAILURE);
}
nread = read(sfd, buf, BUF_SIZE);
if (nread == -1) {
perror("read");
exit(EXIT_FAILURE);
}
printf("Received %zd bytes: %s\n", nread, buf);
}
exit(EXIT_SUCCESS);
}SEE ALSO top
[getaddrinfo_a(3)](../man3/getaddrinfo%5Fa.3.html), [gethostbyname(3)](../man3/gethostbyname.3.html), [getnameinfo(3)](../man3/getnameinfo.3.html), [inet(3)](../man3/inet.3.html),
[gai.conf(5)](../man5/gai.conf.5.html), [hostname(7)](../man7/hostname.7.html), [ip(7)](../man7/ip.7.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-11-17 getaddrinfo(3)
Pages that refer to this page:getent(1), pmdanetcheck(1), bind(2), connect(2), recv(2), send(2), socket(2), getaddrinfo_a(3), gethostbyname(3), getipnodebyname(3), getnameinfo(3), inet(3), inet_pton(3), NULL(3const), resolver(3), sockaddr(3type), gai.conf(5), resolv.conf(5), hostname(7), agetty(8), systemd-machined.service(8), systemd-resolved.service(8)