GETADDRINFO(3)

NAME
getaddrinfo, freeaddrinfo, gai_strerror - network address and service translation

SYNOPSIS
#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);

DESCRIPTION
Given node and service, getaddrinfo() returns one or more structures containing values that can be used by the bind(2) and connect(2) system calls to create a client or a server socket. The getaddrinfo() function combines the functionality provided by the getservbyname(3) and getservbyport(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 this function contains the following members:

struct addrinfo {
    int              ai_flags;
    int              ai_family;
    int              ai_socktype;
    int              ai_protocol;
    size_t           ai_addrlen;
    struct sockaddr *ai_addr;
    char            *ai_canonname;
    struct addrinfo *ai_next;
};

getaddrinfo() sets res to point to a dynamically allocated linked list of addrinfo structures, linked by the ai_next member. There are several reasons why the linked list may have more than one addrinfo structure, including: if the network host is multi-homed; or if the same service is available from multiple socket protocols (one SOCK_STREAM address and another SOCK_DGRAM address, for example).

The hints parameter points to an addrinfo structure that specifies criteria for selecting the socket address structures returned in the list pointed to by res. If this parameter is not NULL it points to an addrinfo structure whose ai_family, ai_socktype, and ai_protocol members specify the preferred socket type. AF_UNSPEC in ai_family specifies any protocol family (either IPv4 or IPv6, for example). 0 in ai_socktype or ai_protocol specifies that any socket type or protocol is acceptable as well. The ai_flags member specifies additional options, defined below. Multiple flags are specified by logically OR-ing them together. All the other members in the hints parameter must contain either 0, or a null pointer. 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).

The node or service parameter, but not both, may be NULL. node specifies either a numerical network address (dotted-decimal format for IPv4, hexadecimal format for IPv6) or a network hostname, whose network addresses are looked up and resolved. If hints.ai_flags contains the AI_NUMERICHOST flag then the node parameter must be a numerical network address. The AI_NUMERICHOST flag suppresses any potentially lengthy network host address lookups.

The getaddrinfo() function creates a linked list of addrinfo structures, one for each network address subject to any restrictions imposed by the hints parameter. The ai_canonname field of the first of these addrinfo structures is set to point to the official name of the host, if hints.ai_flags includes the AI_CANONNAME flag. ai_family, ai_socktype, and ai_protocol specify the socket creation parameters (i.e., these fields have the same meaning as the corresponding parameters of socket(2)). A pointer to the socket address is placed in the ai_addr member, and the length of the socket address, in bytes, is placed in the ai_addrlen member.

The getaddrinfo() function returns socket addresses in either IPv4 or IPv6 address family (ai_family will be set to either AF_INET or AF_INET6).

If node is NULL, the network address in each socket structure is initialized according to the AI_PASSIVE flag, which is set in hints.ai_flags. The network address in each socket structure will be left unspecified if AI_PASSIVE flag is set. This is used by server applications, which intend to accept client connections on any network address. The network address will be set to the loopback interface address if the AI_PASSIVE flag is not set. This is used by client applications, which intend to connect to a server running on the same network host.

If hints.ai_flags includes the AI_ADDRCONFIG flag, then IPv4 addresses are returned in the list pointed to by result only if the local system has at least one IPv4 address configured, and IPv6 addresses are only returned if the local system has at least one IPv6 address configured.

If hint.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 result. If both AI_V4MAPPED and AI_ALL are specified in hints.ai_family, then return both IPv6 and IPv4-mapped IPv6 addresses in the list pointed to by result. AI_ALL is ignored if AI_V4MAPPED is not also specified.

service sets the port number in the network address of each socket structure. If service is NULL the port number 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.

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 host names 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 host name) 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
    ai_flags contains invalid flags.

EAI_FAIL
    The name server returned a permanent failure indication.

EAI_FAMILY
    The requested address family is not supported at all.

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.

EAI_SOCKTYPE
    The requested socket type is not supported at all.

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.

CONFORMING TO
POSIX.1-2001. The getaddrinfo() function is documented in RFC 2553.

NOTES
AI_ADDRCONFIG, AI_ALL, and AI_V4MAPPED are available since glibc 2.3.3. AI_NUMERICSERV is available since glibc 2.3.4.

EXAMPLE
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. This is the server:
#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 %ld bytes from %s:%s\n",
                    (long) 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");
    }
}
This is the client:
#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 %ld bytes: %s\n", (long) nread, buf);
    }
    exit(EXIT_SUCCESS);
}

SEE ALSO
getnameinfo(3)

COLOPHON
This page is part of release 2.79 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.