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.
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 (), gai_strerror () |
Thread safety |
MT-Safe |
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.
#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");
}
}
#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 > 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)