.\" Copyright (c) 2007, 2008 Michael Kerrisk <mtk.manpages@gmail.com>

.\" and Copyright (c) 2006 Ulrich Drepper <drepper@redhat.com>

.\" A few pieces of an earlier version remain:

.\" Copyright 2000, Sam Varshavchik <mrsam@courier-mta.com>

.\"

.\" %%%LICENSE_START(VERBATIM)

.\" 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.\" manual page may be incorrect or out-of-date.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.

.\" %%%LICENSE_END

.\"

.\" References: RFC 2553

.\"

.\" 2005-08-09, mtk, added AI_ALL, AI_ADDRCONFIG, AI_V4MAPPED,

.\"			and AI_NUMERICSERV.

.\" 2006-11-25, Ulrich Drepper <drepper@redhat.com>

.\"     Add text describing Internationalized Domain Name extensions.

.\" 2007-06-08, mtk: added example programs

.\" 2008-02-26, mtk; clarify discussion of NULL 'hints' argument; other

.\"     minor rewrites.

.\" 2008-06-18, mtk: many parts rewritten

.\" 2008-12-04, Petr Baudis <pasky@suse.cz>

.\"	Describe results ordering and reference /etc/gai.conf.

.\"

.\" FIXME . glibc's 2.9 NEWS file documents DCCP and UDP-lite support

.\"           and is SCTP support now also there?

.\"

.TH GETADDRINFO 3 2017-09-15 "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>

.PP

.BI "int getaddrinfo(const char *" "node" ", const char *" "service" ,

.BI "                const struct addrinfo *" "hints" ,

.BI "                struct addrinfo **" "res" );

.PP

.BI "void freeaddrinfo(struct addrinfo *" "res" );

.PP

.BI "const char *gai_strerror(int " "errcode" );

.fi

.PP

.in -4n

Feature Test Macro Requirements for glibc (see

.BR feature_test_macros (7)):

.ad l

.in

.PP

.BR getaddrinfo (),

.BR freeaddrinfo (),

.BR gai_strerror ():

    Since glibc 2.22: _POSIX_C_SOURCE >= 200112L

    Glibc 2.21 and earlier: _POSIX_C_SOURCE

.ad b

.SH DESCRIPTION

Given

.I node

and

.IR service ,

which identify an Internet host and a service,

.BR getaddrinfo ()

returns one or more

.I addrinfo

structures, each of which contains an Internet address

that can be specified in a call to

.BR bind (2)

or

.BR connect (2).

The

.BR getaddrinfo ()

function combines the functionality provided by the

.\" .BR getipnodebyname (3),

.\" .BR getipnodebyaddr (3),

.BR gethostbyname (3)

and

.BR getservbyname (3)

functions into a single interface, but unlike the latter functions,

.BR getaddrinfo ()

is reentrant and allows programs to eliminate IPv4-versus-IPv6 dependencies.

.PP

The

.I addrinfo

structure used by

.BR getaddrinfo ()

contains the following fields:

.PP

.in +4n

.EX

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;

};

.EE

.in

.PP

The

.I hints

argument points to an

.I addrinfo

structure that specifies criteria for selecting the socket address

structures returned in the list pointed to by

.IR res .

If

.I hints

is not NULL it points to an

.I addrinfo

structure whose

.IR ai_family ,

.IR ai_socktype ,

and

.I ai_protocol

specify criteria that limit the set of socket addresses returned by

.BR getaddrinfo (),

as follows:

.TP 12

.I ai_family

This field specifies the desired address family for the returned addresses.

Valid values for this field include

.BR AF_INET

and

.BR AF_INET6 .

The value

.B AF_UNSPEC

indicates that

.BR getaddrinfo ()

should return socket addresses for any address family

(either IPv4 or IPv6, for example) that can be used with

.I node

and

.IR service .

.TP

.I ai_socktype

This field specifies the preferred socket type, for example

.BR SOCK_STREAM

or

.BR SOCK_DGRAM .

Specifying 0 in this field indicates that socket addresses of any type

can be returned by

.BR getaddrinfo ().

.TP

.I 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

.BR getaddrinfo ().

.TP

.I ai_flags

This field specifies additional options, described below.

Multiple flags are specified by bitwise OR-ing them together.

.PP

All the other fields in the structure pointed to by

.I hints

must contain either 0 or a null pointer, as appropriate.

.PP

Specifying

.I hints

as NULL is equivalent to setting

.I ai_socktype

and

.I ai_protocol

to 0;

.I ai_family

to

.BR AF_UNSPEC ;

and

.I ai_flags

to

.BR "(AI_V4MAPPED\ |\ AI_ADDRCONFIG)" .

(POSIX specifies different defaults for

.IR ai_flags ;

see NOTES.)

.I node

specifies either a numerical network address

(for IPv4, numbers-and-dots notation as supported by

.BR inet_aton (3);

for IPv6, hexadecimal string format as supported by

.BR inet_pton (3)),

or a network hostname, whose network addresses are looked up and resolved.

If

.I hints.ai_flags

contains the

.B AI_NUMERICHOST

flag, then

.I node

must be a numerical network address.

The

.B AI_NUMERICHOST

flag suppresses any potentially lengthy network host address lookups.

.PP

If the

.B AI_PASSIVE

flag is specified in

.IR hints.ai_flags ,

and

.I node

is NULL,

then the returned socket addresses will be suitable for

.BR bind (2)ing

a socket that will

.BR accept (2)

connections.

The returned socket address will contain the "wildcard address"

.RB ( INADDR_ANY

for IPv4 addresses,

.BR 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

.I node

is not NULL, then the

.B AI_PASSIVE

flag is ignored.

.PP

If the

.B AI_PASSIVE

flag is not set in

.IR hints.ai_flags ,

then the returned socket addresses will be suitable for use with

.BR connect (2),

.BR sendto (2),

or

.BR sendmsg (2).

If

.I node

is NULL,

then the network address will be set to the loopback interface address

.RB ( INADDR_LOOPBACK

for IPv4 addresses,

.BR IN6ADDR_LOOPBACK_INIT

for IPv6 address);

this is used by applications that intend to communicate

with peers running on the same host.

.PP

.I service

sets the port in each returned address structure.

If this argument is a service name (see

.BR 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

.I service

is NULL, then the port number of the returned socket addresses

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

Either

.I node

or

.IR service ,

but not both, may be NULL.

.PP

The

.BR getaddrinfo ()

function allocates and initializes a linked list of

.I addrinfo

structures, one for each network address that matches

.I node

and

.IR service ,

subject to any restrictions imposed by

.IR hints ,

and returns a pointer to the start of the list in

.IR res .

The items in the linked list are linked by the

.I ai_next

field.

.PP

There are several reasons why

the linked list may have more than one

.I addrinfo

structure, including: the network host is multihomed, accessible

over multiple protocols (e.g., both

.BR AF_INET

and

.BR AF_INET6 );

or the same service is available from multiple socket types (one

.B SOCK_STREAM

address and another

.B 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

.BR getaddrinfo ()

is defined in RFC\ 3484; the order can be tweaked for a particular

system by editing

.IR /etc/gai.conf

(available since glibc 2.5).

.PP

If

.I hints.ai_flags

includes the

.B AI_CANONNAME

flag, then the

.I ai_canonname

field of the first of the

.I addrinfo

structures in the returned list is set to point to the

official name of the host.

.\" 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

.PP

The remaining fields of each returned

.I addrinfo

structure are initialized as follows:

.IP * 2

The

.IR ai_family ,

.IR ai_socktype ,

and

.I ai_protocol

fields return the socket creation parameters (i.e., these fields have

the same meaning as the corresponding arguments of

.BR socket (2)).

For example,

.I ai_family

might return

.B AF_INET

or

.BR AF_INET6 ;

.I ai_socktype

might return

.B SOCK_DGRAM

or

.BR SOCK_STREAM ;

and

.I ai_protocol

returns the protocol for the socket.

.IP *

A pointer to the socket address is placed in the

.I ai_addr

field, and the length of the socket address, in bytes,

is placed in the

.I ai_addrlen

field.

.PP

If

.I hints.ai_flags

includes the

.B AI_ADDRCONFIG

flag, then IPv4 addresses are returned in the list pointed to by

.I 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

.BR getaddrinfo ()

does not return IPv6 socket addresses that would always fail in

.BR connect (2)

or

.BR bind (2).

.PP

If

.I hints.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 res .

If both

.B AI_V4MAPPED

and

.B AI_ALL

are specified in

.IR hints.ai_flags ,

then return both IPv6 and IPv4-mapped IPv6 addresses

in the list pointed to by

.IR res .

.B AI_ALL

is ignored if

.B AI_V4MAPPED

is not also specified.

.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

hostnames 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.

.IP

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.

.IP

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 hostname)

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

.\" Not in SUSv3

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 hints.ai_flags

contains invalid flags; or,

.I hints.ai_flags

included

.B AI_CANONNAME

and

.I name

was NULL.

.TP

.B EAI_FAIL

The name server returned a permanent failure indication.

.TP

.B EAI_FAMILY

The requested address family is not supported.

.TP

.B EAI_MEMORY

Out of memory.

.TP

.B EAI_NODATA

.\" Not in SUSv3

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.

For example, this error could occur if

.I service

was "shell" (a service available only on stream sockets), and either

.I hints.ai_protocol

was

.BR IPPROTO_UDP ,

or

.I hints.ai_socktype

was

.BR SOCK_DGRAM ;

or the error could occur if

.I service

was not NULL, and

.I hints.ai_socktype

was

.BR SOCK_RAW

(a socket type that does not support the concept of services).

.TP

.B EAI_SOCKTYPE

The requested socket type is not supported.

This could occur, for example, if

.I hints.ai_socktype

and

.I hints.ai_protocol

are inconsistent (e.g.,

.BR SOCK_DGRAM

and

.BR IPPROTO_TCP ,

respectively).

.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 FILES

.I /etc/gai.conf

.SH ATTRIBUTES

For an explanation of the terms used in this section, see

.BR attributes (7).

.TS

allbox;

lbw15 lb lb

l l l.

Interface	Attribute	Value

T{

.BR getaddrinfo ()

T}	Thread safety	MT-Safe env locale

T{

.BR freeaddrinfo (),

.BR gai_strerror ()

T}	Thread safety	MT-Safe

.TE

.sp 1

.SH CONFORMING TO

POSIX.1-2001, POSIX.1-2008.

The

.BR getaddrinfo ()

function is documented in RFC\ 2553.

.SH NOTES

.BR getaddrinfo ()

supports the

.IB address % scope-id

notation for specifying the IPv6 scope-ID.

.PP

.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.

.PP

According to POSIX.1, specifying

.\" POSIX.1-2001, POSIX.1-2008

.I hints

as NULL should cause

.I ai_flags

to be assumed as 0.

The GNU C library instead assumes a value of

.BR "(AI_V4MAPPED\ |\ AI_ADDRCONFIG)"

for this case,

since this value is considered an improvement on the specification.

.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.

.SS Server program

\&

.EX

#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");

    }

}

.EE

.SS Client program

\&

.EX

#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);

}

.EE

.SH SEE ALSO

.\" .BR getipnodebyaddr (3),

.\" .BR getipnodebyname (3),

.BR getaddrinfo_a (3),

.BR gethostbyname (3),

.BR getnameinfo (3),

.BR inet (3),

.BR gai.conf (5),

.BR hostname (7),

.BR ip (7)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I 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/.