path: root/man3/getaddrinfo.3

.\" 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>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" 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 2021-08-27 "Linux man-pages (unreleased)"
.SH NAME
getaddrinfo, freeaddrinfo, gai_strerror \- network address and
service translation
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <sys/types.h>
.B #include <sys/socket.h>
.B #include <netdb.h>
.PP
.BI "int getaddrinfo(const char *restrict " node ,
.BI "                const char *restrict " service ,
.BI "                const struct addrinfo *restrict " hints ,
.BI "                struct addrinfo **restrict " res );
.PP
.BI "void freeaddrinfo(struct addrinfo *" res );
.PP
.BI "const char *gai_strerror(int " errcode );
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR getaddrinfo (),
.BR freeaddrinfo (),
.BR gai_strerror ():
.nf
    Since glibc 2.22:
        _POSIX_C_SOURCE >= 200112L
    Glibc 2.21 and earlier:
        _POSIX_C_SOURCE
.fi
.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
.I ai_family
This field specifies the desired address family for the returned addresses.
Valid values for this field include
.B 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
.B 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,
.B 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,
.B 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
.B 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
.I /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
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
.B 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.,
.B SOCK_DGRAM
and
.BR IPPROTO_TCP ,
respectively).
.TP
.B EAI_SYSTEM
Other system error;
.I errno
is set to indicate the error.
.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).
.ad l
.nh
.TS
allbox;
lbx 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
.hy
.ad
.sp 1
.SH STANDARDS
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
.B (AI_V4MAPPED\~|\~AI_ADDRCONFIG)
for this case,
since this value is considered an improvement on the specification.
.SH EXAMPLES
.\" 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 <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <unistd.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\en", argv[0]);
        exit(EXIT_FAILURE);
    }

    memset(&hints, 0, sizeof(hints));
    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\en", 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);
    }

    freeaddrinfo(result);           /* No longer needed */

    if (rp == NULL) {               /* No address succeeded */
        fprintf(stderr, "Could not bind\en");
        exit(EXIT_FAILURE);
    }

    /* Read datagrams and echo them back to sender. */

    for (;;) {
        peer_addr_len = sizeof(peer_addr);
        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\en",
                    nread, host, service);
        else
            fprintf(stderr, "getnameinfo: %s\en", gai_strerror(s));

        if (sendto(sfd, buf, nread, 0,
                    (struct sockaddr *) &peer_addr,
                    peer_addr_len) != nread)
            fprintf(stderr, "Error sending response\en");
    }
}
.EE
.SS Client program
\&
.EX
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <unistd.h>

#define BUF_SIZE 500

int
main(int argc, char *argv[])
{
    struct addrinfo hints;
    struct addrinfo *result, *rp;
    int sfd, s;
    size_t len;
    ssize_t nread;
    char buf[BUF_SIZE];

    if (argc < 3) {
        fprintf(stderr, "Usage: %s host port msg...\en", argv[0]);
        exit(EXIT_FAILURE);
    }

    /* Obtain address(es) matching host/port. */

    memset(&hints, 0, sizeof(hints));
    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\en", 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);
    }

    freeaddrinfo(result);           /* No longer needed */

    if (rp == NULL) {               /* No address succeeded */
        fprintf(stderr, "Could not connect\en");
        exit(EXIT_FAILURE);
    }

    /* Send remaining command\-line arguments as separate
       datagrams, and read responses from server. */

    for (int 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\en", j);
            continue;
        }

        if (write(sfd, argv[j], len) != len) {
            fprintf(stderr, "partial/failed write\en");
            exit(EXIT_FAILURE);
        }

        nread = read(sfd, buf, BUF_SIZE);
        if (nread == \-1) {
            perror("read");
            exit(EXIT_FAILURE);
        }

        printf("Received %zd bytes: %s\en", 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)