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GETADDRINFO(3) Linux Programmer's Manual GETADDRINFO(3)
getaddrinfo, freeaddrinfo, gai_strerror - network address and service
translation
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
int getaddrinfo(const char *node, const char *service,
const struct addrinfo *hints,
struct addrinfo **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
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 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 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 service.
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 getaddrinfo().
ai_protocol 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().
ai_flags 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 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 network hostname, whose network addresses are
looked up and resolved. If hints.ai_flags 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.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 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.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
argument is a service name (see services(5)), 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.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 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 ai_next 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.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_addrlen 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 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) 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
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.
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
included AI_CANONNAME and name 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
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.ai_protocol was IPPROTO_UDP, or hints.ai_socktype was
SOCK_DGRAM; or the error 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
inconsistent (e.g., SOCK_DGRAM and IPPROTO_TCP, respectively).
EAI_SYSTEM
Other system error, check errno for details.
The gai_strerror() function translates these error codes to a human
readable string, suitable for error reporting.
/etc/gai.conf
For an explanation of the terms used in this section, see
attributes(7).
┌────────────────┬───────────────┬────────────────────┐
│Interface │ Attribute │ Value │
├────────────────┼───────────────┼────────────────────┤
│getaddrinfo() │ Thread safety │ MT-Safe env locale │
├────────────────┼───────────────┼────────────────────┤
│freeaddrinfo(), │ Thread safety │ MT-Safe │
│gai_strerror() │ │ │
└────────────────┴───────────────┴────────────────────┘
POSIX.1-2001, POSIX.1-2008. The getaddrinfo() function is documented
in RFC 2553.
getaddrinfo() supports the address%scope-id notation for specifying
the IPv6 scope-ID.
AI_ADDRCONFIG, AI_ALL, and AI_V4MAPPED are available since glibc
2.3.3. AI_NUMERICSERV is available since glibc 2.3.4.
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
considered an improvement on the specification.
The following programs demonstrate the use of getaddrinfo(),
gai_strerror(), freeaddrinfo(), and getnameinfo(3). The programs are
an echo server and client for UDP datagrams.
Server program
#include <sys/types.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/socket.h>
#include <netdb.h>
#define BUF_SIZE 500
int
main(int argc, char *argv[])
{
struct addrinfo hints;
struct addrinfo *result, *rp;
int sfd, s;
struct sockaddr_storage peer_addr;
socklen_t peer_addr_len;
ssize_t nread;
char buf[BUF_SIZE];
if (argc != 2) {
fprintf(stderr, "Usage: %s port\n", argv[0]);
exit(EXIT_FAILURE);
}
memset(&hints, 0, sizeof(struct addrinfo));
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);
}
if (rp == NULL) { /* No address succeeded */
fprintf(stderr, "Could not bind\n");
exit(EXIT_FAILURE);
}
freeaddrinfo(result); /* No longer needed */
/* Read datagrams and echo them back to sender */
for (;;) {
peer_addr_len = sizeof(struct sockaddr_storage);
nread = recvfrom(sfd, buf, BUF_SIZE, 0,
(struct sockaddr *) &peer_addr, &peer_addr_len);
if (nread == -1)
continue; /* Ignore failed request */
char host[NI_MAXHOST], service[NI_MAXSERV];
s = getnameinfo((struct sockaddr *) &peer_addr,
peer_addr_len, 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_addr_len) != nread)
fprintf(stderr, "Error sending response\n");
}
}
Client program
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#define BUF_SIZE 500
int
main(int argc, char *argv[])
{
struct addrinfo hints;
struct addrinfo *result, *rp;
int sfd, s, j;
size_t len;
ssize_t nread;
char buf[BUF_SIZE];
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(struct addrinfo));
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);
}
if (rp == NULL) { /* No address succeeded */
fprintf(stderr, "Could not connect\n");
exit(EXIT_FAILURE);
}
freeaddrinfo(result); /* No longer needed */
/* Send remaining command-line arguments as separate
datagrams, and read responses from server */
for (j = 3; j < argc; j++) {
len = strlen(argv[j]) + 1;
/* +1 for terminating null byte */
if (len + 1 > BUF_SIZE) {
fprintf(stderr,
"Ignoring long message in argument %d\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);
}
getaddrinfo_a(3), gethostbyname(3), getnameinfo(3), inet(3),
gai.conf(5), hostname(7), ip(7)
This page is part of release 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
GNU 2017-07-13 GETADDRINFO(3)
Pages that refer to this page: getent(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), resolver(3), gai.conf(5), resolv.conf(5), hostname(7), agetty(8), systemd-resolved.service(8)