getaddrinfo(3) Library Functions Manual getaddrinfo(3)
NAME
getaddrinfo, freeaddrinfo, gai_strerror - network address and service
translation
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#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_SOURCE
DESCRIPTION
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)
or connect(2). The getaddrinfo() function combines the functionality
provided by the gethostbyname(3) and getservbyname(3) functions into a
single interface, but unlike the latter functions, getaddrinfo() is
reentrant and allows programs to eliminate IPv4-versus-IPv6 dependen-
cies.
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 crite-
ria for selecting the socket address structures returned in the list
pointed to by res. If hints is not NULL it points to an addrinfo struc-
ture whose ai_family, ai_socktype, and ai_protocol specify criteria that
limit the set of socket addresses returned by getaddrinfo(), as follows:
ai_family
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 ser-
vice.
ai_socktype
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 getad-
drinfo().
ai_protocol
This field specifies the protocol for the returned socket ad-
dresses. Specifying 0 in this field indicates that socket ad-
dresses with any protocol can be returned by getaddrinfo().
ai_flags
This field specifies additional options, described below. Multi-
ple 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 ai_socktype and
ai_protocol to 0; ai_family to AF_UNSPEC; and ai_flags to
(AI_V4MAPPED | AI_ADDRCONFIG). (POSIX specifies different defaults for
ai_flags; see NOTES.) node specifies either a numerical network address
(for IPv4, numbers-and-dots notation as supported by inet_aton(3); for
IPv6, hexadecimal string format as supported by inet_pton(3)), or a net-
work hostname, whose network addresses are looked up and resolved. If
hints.ai_flags contains the AI_NUMERICHOST flag, then node must be a nu-
merical network address. The AI_NUMERICHOST flag suppresses any poten-
tially lengthy network host address lookups.
If the AI_PASSIVE flag is specified in hints.ai_flags, and node is NULL,
then the returned socket addresses will be suitable for bind(2)ing a
socket that will accept(2) 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 ap-
plications (typically servers) that intend to accept connections on any
of the host's network addresses. If node is not NULL, then the AI_PAS-
SIVE flag is ignored.
If the AI_PASSIVE flag is not set in hints.ai_flags, then the returned
socket addresses will be suitable for use with connect(2), sendto(2), or
sendmsg(2). 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 argu-
ment is a service name (see services(5)), it is translated to the corre-
sponding 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 uninitial-
ized. If AI_NUMERICSERV is specified in hints.ai_flags 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 resolu-
tion 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 ai_next field.
There are several reasons why the linked list may have more than one ad-
drinfo 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.ai_flags includes the AI_CANONNAME flag, then the ai_canonname
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 ai_family, ai_socktype, and ai_protocol fields return the socket
creation parameters (i.e., these fields have the same meaning as the
corresponding arguments of socket(2)). For example, ai_family might
return AF_INET or AF_INET6; ai_socktype might return SOCK_DGRAM or
SOCK_STREAM; and ai_protocol returns the protocol for the socket.
• A pointer to the socket address is placed in the ai_addr field, and
the length of the socket address, in bytes, is placed in the ai_ad-
drlen field.
If hints.ai_flags 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 config-
ured 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) or bind(2).
If hints.ai_flags specifies the AI_V4MAPPED flag, and hints.ai_family
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.ai_flags,
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 selec-
tively 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 en-
coding is used. Those parts of the node name (delimited by dots)
that contain non-ASCII characters are encoded using ASCII Compat-
ible 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 en-
coded 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
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.ai_flags contains invalid flags; or, hints.ai_flags in-
cluded 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.ai_flags and ser-
vice 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 ex-
ample, this error could occur if service was "shell" (a service
available only on stream sockets), and either hints.ai_protocol
was IPPROTO_UDP, or hints.ai_socktype was SOCK_DGRAM; or the er-
ror could occur if service was not NULL, and hints.ai_socktype
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.ai_socktype and hints.ai_protocol are in-
consistent (e.g., SOCK_DGRAM and IPPROTO_TCP, respectively).
EAI_SYSTEM
Other system error; errno is set to indicate the error.
The gai_strerror() function translates these error codes to a human
readable string, suitable for error reporting.
FILES
/etc/gai.conf
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
┌─────────────────────────────────┬───────────────┬────────────────────┐
│ Interface │ Attribute │ Value │
├─────────────────────────────────┼───────────────┼────────────────────┤
│ getaddrinfo() │ Thread safety │ MT-Safe env locale │
├─────────────────────────────────┼───────────────┼────────────────────┤
│ freeaddrinfo(), gai_strerror() │ Thread safety │ MT-Safe │
└─────────────────────────────────┴───────────────┴────────────────────┘
VERSIONS
According to POSIX.1, specifying hints as NULL should cause ai_flags 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 consid-
ered an improvement on the specification.
STANDARDS
POSIX.1-2008.
getaddrinfo()
RFC 2553.
HISTORY
POSIX.1-2001.
AI_ADDRCONFIG
AI_ALL
AI_V4MAPPED
glibc 2.3.3.
AI_NUMERICSERV
glibc 2.3.4.
NOTES
getaddrinfo() supports the address%scope-id notation for specifying the
IPv6 scope-ID.
EXAMPLES
The following programs demonstrate the use of getaddrinfo(), gai_str-
error(), freeaddrinfo(), and getnameinfo(3). 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 len;
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++) {
len = strlen(argv[j]) + 1;
/* +1 for terminating null byte */
if (len > BUF_SIZE) {
fprintf(stderr,
"Ignoring long message in argument %zu\n", j);
continue;
}
if (write(sfd, argv[j], len) != len) {
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
getaddrinfo_a(3), gethostbyname(3), getnameinfo(3), inet(3),
gai.conf(5), hostname(7), ip(7)
Linux man-pages 6.9.1 2024-06-15 getaddrinfo(3)
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