Manpage of GETADDRINFO

GETADDRINFO

Section: Linux Programmer's Manual (3)
Updated: 2016-03-15
Index
 

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);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

getaddrinfo(), freeaddrinfo(), gai_strerror():
    Since glibc 2.22: _POSIX_C_SOURCE >= 201112L
    Glibc 2.21 and earlier: _POSIX_C_SOURCE  

DESCRIPTION

Given nodeand service, which identify an Internet host and a service, getaddrinfo() returns one or more addrinfostructures, 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 addrinfostructure 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 hintsargument points to an addrinfostructure that specifies criteria for selecting the socket address structures returned in the list pointed to by res. If hintsis not NULL it points to an addrinfostructure whose ai_family, ai_socktype, and ai_protocolspecify 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_INETand AF_INET6. The value AF_UNSPECindicates that getaddrinfo() should return socket addresses for any address family (either IPv4 or IPv6, for example) that can be used with nodeand service.
ai_socktype
This field specifies the preferred socket type, for example SOCK_STREAMor 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 hintsmust contain either 0 or a null pointer, as appropriate.

Specifying hintsas NULL is equivalent to setting ai_socktypeand ai_protocolto 0; ai_familyto AF_UNSPEC; and ai_flagsto (AI_V4MAPPED | AI_ADDRCONFIG). (POSIX specifies different defaults for ai_flags; see NOTES.) nodespecifies 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_flagscontains the AI_NUMERICHOSTflag, then nodemust be a numerical network address. The AI_NUMERICHOSTflag suppresses any potentially lengthy network host address lookups.

If the AI_PASSIVEflag is specified in hints.ai_flags, and nodeis 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_ANYfor IPv4 addresses, IN6ADDR_ANY_INITfor 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 nodeis not NULL, then the AI_PASSIVEflag is ignored.

If the AI_PASSIVEflag 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 nodeis NULL, then the network address will be set to the loopback interface address (INADDR_LOOPBACKfor IPv4 addresses, IN6ADDR_LOOPBACK_INITfor IPv6 address); this is used by applications that intend to communicate with peers running on the same host.

servicesets 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 serviceis NULL, then the port number of the returned socket addresses will be left uninitialized. If AI_NUMERICSERVis specified in hints.ai_flagsand serviceis not NULL, then servicemust 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 nodeor service, but not both, may be NULL.

The getaddrinfo() function allocates and initializes a linked list of addrinfostructures, one for each network address that matches nodeand 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_nextfield.

There are several reasons why the linked list may have more than one addrinfostructure, including: the network host is multihomed, accessible over multiple protocols (e.g., both AF_INETand AF_INET6); or the same service is available from multiple socket types (one SOCK_STREAMaddress and another SOCK_DGRAMaddress, 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_flagsincludes the AI_CANONNAMEflag, then the ai_canonnamefield of the first of the addrinfostructures in the returned list is set to point to the official name of the host.

The remaining fields of each returned addrinfostructure are initialized as follows:

*
The ai_family, ai_socktype, and ai_protocolfields return the socket creation parameters (i.e., these fields have the same meaning as the corresponding arguments of socket(2)). For example, ai_familymight return AF_INETor AF_INET6; ai_socktypemight return SOCK_DGRAMor SOCK_STREAM; and ai_protocolreturns the protocol for the socket.
*
A pointer to the socket address is placed in the ai_addrfield, and the length of the socket address, in bytes, is placed in the ai_addrlenfield.

If hints.ai_flagsincludes the AI_ADDRCONFIGflag, then IPv4 addresses are returned in the list pointed to by resonly 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_flagsspecifies the AI_V4MAPPEDflag, and hints.ai_familywas 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_V4MAPPEDand AI_ALLare specified in hints.ai_flags, then return both IPv6 and IPv4-mapped IPv6 addresses in the list pointed to by res. AI_ALLis ignored if AI_V4MAPPEDis 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 nodeis 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_CANONNAMEflag was specified, getaddrinfo() will return the canonical name of the node corresponding to the addrinfostructure 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_CANONIDNflag 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

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_flagscontains invalid flags; or, hints.ai_flagsincluded AI_CANONNAMEand namewas 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 nodeor serviceis not known; or both nodeand serviceare NULL; or AI_NUMERICSERVwas specified in hints.ai_flagsand servicewas 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 servicewas "shell" (a service available only on stream sockets), and either hints.ai_protocolwas IPPROTO_UDP, or hints.ai_socktypewas SOCK_DGRAM; or the error could occur if servicewas not NULL, and hints.ai_socktypewas 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_socktypeand hints.ai_protocolare inconsistent (e.g., SOCK_DGRAMand IPPROTO_TCP, respectively).
EAI_SYSTEM
Other system error, check errnofor details.

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).
InterfaceAttributeValue
getaddrinfo() Thread safetyMT-Safe env locale
freeaddrinfo(), gai_strerror() Thread safetyMT-Safe

 

CONFORMING TO

POSIX.1-2001, POSIX.1-2008. The getaddrinfo() function is documented in RFC 2553.  

NOTES

getaddrinfo() supports the address%scope-idnotation for specifying the IPv6 scope-ID.

AI_ADDRCONFIG, AI_ALL, and AI_V4MAPPEDare available since glibc 2.3.3. AI_NUMERICSERVis available since glibc 2.3.4.

According to POSIX.1, specifying hintsas NULL should cause ai_flagsto 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.  

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.  

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);
}
 

SEE ALSO

getaddrinfo_a(3), gethostbyname(3), getnameinfo(3), inet(3), gai.conf(5), hostname(7), ip(7)


 

Index

NAME
SYNOPSIS
DESCRIPTION
Extensions to getaddrinfo() for Internationalized Domain Names
RETURN VALUE
FILES
ATTRIBUTES
CONFORMING TO
NOTES
EXAMPLE
Server program
Client program
SEE ALSO

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Time: 22:27:59 GMT, June 20, 2016