man-pages/man3/getaddrinfo.3

.\" Copyright 2000 Sam Varshavchik <mrsam@courier-mta.com>
.\" and Copyright (c) 2007 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Permission is granted to copy and distribute modified versions of this
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one.
.\"
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date.  The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein.  The author(s) may not
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\"
.\" References: RFC 2553
.\"
.\" 2005-08-09, mtk, added AI_ALL, AI_ADDRCONFIG, AI_V4MAPPED,
.\"			and AI_NUMERICSERV.
.\" 2007-06-08, mtk: added example programs
.\"
.TH GETADDRINFO 3 2007-11-15 "GNU" "Linux Programmer's Manual"
.SH NAME
getaddrinfo, freeaddrinfo, gai_strerror \- network address and service translation
.SH SYNOPSIS
.nf
.B #include <sys/types.h>
.B #include <sys/socket.h>
.B #include <netdb.h>
.sp
.BI "int getaddrinfo(const char *" "node" ", const char *" "service" ,
.BI "                const struct addrinfo *" "hints" ,
.BI "                struct addrinfo **" "res" );
.sp
.BI "void freeaddrinfo(struct addrinfo *" "res" );
.sp
.BI "const char *gai_strerror(int " "errcode" );
.fi
.SH DESCRIPTION
The
.BR getaddrinfo ()
function combines the functionality provided by the
.\" .BR getipnodebyname (3),
.\" .BR getipnodebyaddr (3),
.BR getservbyname (3)
and
.BR getservbyport (3)
functions into a single interface.
The thread-safe
.BR getaddrinfo ()
function creates one or more socket address structures
that can be used by the
.BR bind (2)
and
.BR connect (2)
system calls to create a client or a server socket.
.PP
The
.BR getaddrinfo ()
function is not limited to creating IPv4 socket address structures;
IPv6 socket address structures can be created if IPv6 support is available.
These socket address structures can be used directly by
.BR bind (2)
or
.BR connect (2),
to prepare a client or a server socket.
.PP
The
.I addrinfo
structure used by this function contains the following members:
.sp
.in +4n
.nf
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;
};
.fi
.in
.PP
.BR getaddrinfo ()
sets
.I res
to point to a dynamically allocated linked list of
.I addrinfo
structures, linked by the
.I ai_next
member.
There are several reasons why
the linked list may have more than one
.I addrinfo
structure, including: if the network host is
multi-homed; or if the same service
is available from multiple socket protocols (one
.B SOCK_STREAM
address and another
.B SOCK_DGRAM
address, for example).
.PP
The members
.IR ai_family ,
.IR ai_socktype ,
and
.I ai_protocol
have the same meaning as the corresponding parameters in the
.BR socket (2)
system call.
The
.BR getaddrinfo ()
function returns socket addresses in either IPv4 or IPv6
address family
.RI "(" "ai_family"
will be set to either
.B AF_INET
or
.BR AF_INET6 ).
.PP
The
.I hints
parameter specifies
the preferred socket type, or protocol.
A NULL
.I hints
specifies that any network address or protocol is acceptable.
If this parameter is not NULL it points to an
.I addrinfo
structure
whose
.IR ai_family ,
.IR ai_socktype ,
and
.I ai_protocol
members specify the preferred socket type.
.B AF_UNSPEC
in
.I ai_family
specifies any protocol family (either IPv4 or IPv6, for example).
0 in
.I ai_socktype
or
.I ai_protocol
specifies that any socket type or protocol is acceptable as well.
The
.I ai_flags
member
specifies additional options, defined below.
Multiple flags are specified by logically OR-ing them together.
All the other members in the
.I hints
parameter must contain either 0, or a null pointer.
.PP
The
.I node
or
.I service
parameter, but not both, may be NULL.
.I 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
.I hints.ai_flags
contains the
.B AI_NUMERICHOST
flag then the
.I node
parameter must be a numerical network address.
The
.B AI_NUMERICHOST
flag suppresses any potentially lengthy network host address lookups.
.PP
The
.BR getaddrinfo ()
function creates a linked list of
.I addrinfo
structures, one for each network address subject to any restrictions
imposed by the
.I hints
parameter.
The
.I ai_canonname
field of the first of these
.I addrinfo
structures is set to point to the official name of the host, if
.I hints.ai_flags
includes the
.B AI_CANONNAME
flag.
.\" In glibc prior to 2.3.4, the ai_canonname of each addrinfo
.\" structure was set pointing to the canonical name; that was
.\" more than POSIX.1-2001 specified, or other implementations provided.
.\" MTK, Aug 05
.IR ai_family ,
.IR ai_socktype ,
and
.I ai_protocol
specify the socket creation parameters.
A pointer to the socket address is placed in the
.I ai_addr
member, and the length of the socket address, in bytes,
is placed in the
.I ai_addrlen
member.
.PP
If
.I node
is NULL,
the
network address in each socket structure is initialized according to the
.B AI_PASSIVE
flag, which is set in
.IR hints.ai_flags .
The network address in each socket structure will be left unspecified
if
.B 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
.B 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.
.PP
If
.I hints.ai_flags
includes the
.B AI_ADDRCONFIG
flag, then IPv4 addresses are returned in the list pointed to by
.I 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.
.PP
If
.I hint.ai_flags
specifies the
.B AI_V4MAPPED
flag, and
.I hints.ai_family
was specified as
.BR AF_INET6 ,
and no matching IPv6 addresses could be found,
then return IPv4-mapped IPv6 addresses in the list pointed to by
.IR result .
If both
.B AI_V4MAPPED
and
.B AI_ALL
are specified in
.IR hints.ai_family ,
then return both IPv6 and IPv4-mapped IPv6 addresses
in the list pointed to by
.IR result .
.B AI_ALL
is ignored if
.B AI_V4MAPPED
is not also specified.
.PP
.I service
sets the port number in the network address of each socket structure.
If
.I service
is NULL the port number will be left uninitialized.
If
.B AI_NUMERICSERV
is specified in
.I hints.ai_flags
and
.I service
is not NULL, then
.I 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.
.PP
The
.BR freeaddrinfo ()
function frees the memory that was allocated
for the dynamically allocated linked list
.IR res .
.SS "Extensions to getaddrinfo() for Internationalized Domain Names"
.PP
Starting with glibc 2.3.4,
.BR 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,
.IR "Internationalizing Domain Names in Applications (IDNA)" ).
Four new flags are defined:
.TP
.B AI_IDN
If this flag is specified, then the node name given in
.I 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.
.\" Implementation Detail:
.\" To minimize effects on system performance the implementation might
.\" want to check whether the input string contains any non-ASCII
.\" characters.  If there are none the IDN step can be skipped completely.
.\" On systems which allow not-ASCII safe encodings for a locale this
.\" might be a problem.
.TP
.B AI_CANONIDN
After a successful name lookup, and if the
.B AI_CANONNAME
flag was specified,
.BR getaddrinfo ()
will return the canonical name of the
node corresponding to the
.I 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
.I xn\-\-
prefix for one or more components of the name.
To convert these components into a readable form the
.B AI_CANONIDN
flag can be passed in addition to
.BR AI_CANONNAME .
The resulting string is encoded using the current locale's encoding.
.\"
.\"Implementation Detail:
.\"If no component of the returned name starts with xn\-\- the IDN
.\"step can be skipped, therefore avoiding unnecessary slowdowns.
.TP
.BR 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.
.SH "RETURN VALUE"
.\" FIXME glibc defines the following additional errors, some which
.\" can probably be returned by getaddrinfo(); they need to
.\" be documented.
.\" #ifdef __USE_GNU
.\" #define EAI_INPROGRESS  -100  /* Processing request in progress.  */
.\" #define EAI_CANCELED    -101  /* Request canceled.  */
.\" #define EAI_NOTCANCELED -102  /* Request not canceled.  */
.\" #define EAI_ALLDONE     -103  /* All requests done.  */
.\" #define EAI_INTR        -104  /* Interrupted by a signal.  */
.\" #define EAI_IDN_ENCODE  -105  /* IDN encoding failed.  */
.\" #endif
.BR getaddrinfo ()
returns 0 if it succeeds, or one of the following nonzero error codes:
.TP
.B EAI_ADDRFAMILY
The specified network host does not have any network addresses in the
requested address family.
.TP
.B EAI_AGAIN
The name server returned a temporary failure indication.
Try again later.
.TP
.B EAI_BADFLAGS
.I ai_flags
contains invalid flags.
.TP
.B EAI_FAIL
The name server returned a permanent failure indication.
.TP
.B EAI_FAMILY
The requested address family is not supported at all.
.TP
.B EAI_MEMORY
Out of memory.
.TP
.B EAI_NODATA
The specified network host exists, but does not have any
network addresses defined.
.TP
.B EAI_NONAME
The
.I node
or
.I service
is not known; or both
.I node
and
.I service
are NULL; or
.B AI_NUMERICSERV
was specified in
.I hints.ai_flags
and
.I service
was not a numeric port-number string.
.TP
.B EAI_SERVICE
The requested service is not available for the requested socket type.
It may be available through another socket type.
.TP
.B EAI_SOCKTYPE
The requested socket type is not supported at all.
.TP
.B EAI_SYSTEM
Other system error, check
.I errno
for details.
.PP
The
.BR gai_strerror ()
function translates these error codes to a human readable string,
suitable for error reporting.
.SH "CONFORMING TO"
POSIX.1-2001.
The
.BR getaddrinfo ()
function is documented in RFC\ 2553.
.SH "NOTES"
.BR AI_ADDRCONFIG ,
.BR AI_ALL ,
and
.B AI_V4MAPPED
are available since glibc 2.3.3.
.B AI_NUMERICSERV
is available since glibc 2.3.4.
.SH EXAMPLE
.\" getnameinfo.3 refers to this example
.\" socket.2 refers to this example
.\" bind.2 refers to this example
.\" connect.2 refers to this example
.\" recvfrom.2 refers to this example
.\" sendto.2 refers to this example
The following programs demonstrate the use of
.BR getaddrinfo (),
.BR gai_strerror (),
.BR freeaddrinfo (),
and
.BR getnameinfo (3).
The programs are an echo server and client for UDP datagrams.

This is the server:
.in +0.25i
.nf

#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");
    }
}
.fi
.in

This is the client:
.in +0.25i
.nf

#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);
}
.fi
.in
.SH "SEE ALSO"
.\" .BR getipnodebyaddr (3),
.\" .BR getipnodebyname (3),
.BR getnameinfo (3)