Section (3) getaddrinfo
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) ; |
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Note | ||||
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DESCRIPTION
Given node
and
service
, which
identify an Internet host and a service, getaddrinfo
() returns one or more
addrinfo structures, 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 addrinfo structure
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 hints
argument
points to an addrinfo
structure that specifies criteria for selecting the socket
address structures returned in the list pointed to by
res
. If hints
is not NULL it points to
an addrinfo structure whose
ai_family
, ai_socktype
, and ai_protocol
specify 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_INET
andAF_INET6
. The valueAF_UNSPEC
indicates thatgetaddrinfo
() should return socket addresses for any address family (either IPv4 or IPv6, for example) that can be used withnode
andservice
. ai_socktype
-
This field specifies the preferred socket type, for example
SOCK_STREAM
orSOCK_DGRAM
. Specifying 0 in this field indicates that socket addresses of any type can be returned bygetaddrinfo
(). 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
hints
must contain
either 0 or a null pointer, as appropriate.
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).
(POSIX specifies different defaults for ai_flags
; see NOTES.)
node
specifies 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_flags
contains the AI_NUMERICHOST
flag, then node
must
be a numerical network address. The AI_NUMERICHOST
flag suppresses any
potentially lengthy network host address lookups.
If the AI_PASSIVE
flag is
specified in hints.ai_flags
, and
node
is 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_ANY
for IPv4 addresses,
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_zsingle_quotesz_s network addresses. If node
is not NULL, then the
AI_PASSIVE
flag is ignored.
If the AI_PASSIVE
flag 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 node
is NULL, then the network
address will be set to the loopback interface address
(INADDR_LOOPBACK
for IPv4
addresses, IN6ADDR_LOOPBACK_INIT
for IPv6 address);
this is used by applications that intend to communicate with
peers running on the same host.
service
sets 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 service
is NULL, then the port
number of the returned socket addresses 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.
Either node
or
service
, but not
both, may be NULL.
The getaddrinfo
() function
allocates and initializes a linked list of addrinfo structures, one for each network
address that matches node
and 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_next
field.
There are several reasons why the linked list may have
more than one addrinfo
structure, including: the network host is multihomed,
accessible over multiple protocols (e.g., both AF_INET
and AF_INET6
); or the same service is available
from multiple socket types (one SOCK_STREAM
address and another
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 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_flags
includes the
AI_CANONNAME
flag, then the
ai_canonname
field of
the first of the addrinfo
structures in the returned list is set to point to the
official name of the host.
The remaining fields of each returned addrinfo structure are initialized as follows:
-
The
ai_family
,ai_socktype
, andai_protocol
fields return the socket creation parameters (i.e., these fields have the same meaning as the corresponding arguments of socket(2)). For example,ai_family
might returnAF_INET
orAF_INET6
;ai_socktype
might returnSOCK_DGRAM
orSOCK_STREAM
; andai_protocol
returns the protocol for the socket. -
A pointer to the socket address is placed in the
ai_addr
field, and the length of the socket address, in bytes, is placed in theai_addrlen
field.
If hints.ai_flags
includes the
AI_ADDRCONFIG
flag, then IPv4
addresses are returned in the list pointed to by 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
getaddrinfo
() does not return
IPv6 socket addresses that would always fail in connect(2) or bind(2).
If hints.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
res
. If both
AI_V4MAPPED
and AI_ALL
are specified in hints.ai_flags
, then return
both IPv6 and IPv4-mapped IPv6 addresses in the list pointed
to by res
.
AI_ALL
is ignored if
AI_V4MAPPED
is 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
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 theAI_CANONIDN
flag can be passed in addition toAI_CANONNAME
. The resulting string is encoded using the current locale_zsingle_quotesz_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_flags
contains invalid flags; or,hints.ai_flags
includedAI_CANONNAME
andname
was 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
node
orservice
is not known; or bothnode
andservice
are NULL; orAI_NUMERICSERV
was specified inhints.ai_flags
andservice
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. For example, this error could occur if
service
was shell (a service available only on stream sockets), and eitherhints.ai_protocol
wasIPPROTO_UDP
, orhints.ai_socktype
wasSOCK_DGRAM
; or the error could occur ifservice
was not NULL, andhints.ai_socktype
wasSOCK_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_socktype
andhints.ai_protocol
are inconsistent (e.g.,SOCK_DGRAM
andIPPROTO_TCP
, respectively). 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.
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
Interface | Attribute | Value |
getaddrinfo () |
Thread safety | MT-Safe env locale |
freeaddrinfo (), gai_strerror () |
Thread safety | MT-Safe |
NOTES
getaddrinfo
() supports the
address
%scope-id
notation for
specifying the IPv6 scope-ID.
AI_ADDRCONFIG
, AI_ALL
, and AI_V4MAPPED
are available since glibc
2.3.3. AI_NUMERICSERV
is
available since glibc 2.3.4.
According to POSIX.1, specifying hints
as NULL should cause
ai_flags
to 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 , 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 , 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 ); 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 , nread, host, service); else fprintf(stderr, getnameinfo: %s , gai_strerror(s)); if (sendto(sfd, buf, nread, 0, (struct sockaddr *) &peer_addr, peer_addr_len) != nread) fprintf(stderr, Error sending response ); } }
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... , 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 , 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 ); 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 > BUF_SIZE) { fprintf(stderr, Ignoring long message in argument %d , j); continue; } if (write(sfd, argv[j], len) != len) { fprintf(stderr, partial/failed write ); exit(EXIT_FAILURE); } nread = read(sfd, buf, BUF_SIZE); if (nread == −1) { perror(read); exit(EXIT_FAILURE); } printf(Received %zd bytes: %s , nread, buf); } exit(EXIT_SUCCESS); }
SEE ALSO
getaddrinfo_a(3), gethostbyname(3), getnameinfo(3), inet(3), gai.conf(5), hostname(7), ip(7)
COLOPHON
This page is part of release 5.04 of the Linux 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/.
Copyright (c) 2007, 2008 Michael Kerrisk <mtk.manpagesgmail.com> and Copyright (c) 2006 Ulrich Drepper <drepperredhat.com> A few pieces of an earlier version remain: Copyright 2000, Sam Varshavchik <mrsamcourier-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 <drepperredhat.com> Add text describing Internationalized Domain Name extensions. 2007-06-08, mtk: added example programs 2008-02-26, mtk; clarify discussion of NULL _zsingle_quotesz_hints_zsingle_quotesz_ argument; other minor rewrites. 2008-06-18, mtk: many parts rewritten 2008-12-04, Petr Baudis <paskysuse.cz> Describe results ordering and reference /etc/gai.conf. FIXME . glibc_zsingle_quotesz_s 2.9 NEWS file documents DCCP and UDP-lite support and is SCTP support now also there? |