path: root/lib/libc/net/getaddrinfo.3

.\"	$OpenBSD: getaddrinfo.3,v 1.61 2022/09/11 06:38:10 jmc Exp $
.\"	$KAME: getaddrinfo.3,v 1.36 2005/01/05 03:23:05 itojun Exp $
.\"
.\" Copyright (C) 2004  Internet Systems Consortium, Inc. ("ISC")
.\" Copyright (C) 2000, 2001  Internet Software Consortium.
.\"
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.Dd $Mdocdate: September 11 2022 $
.Dt GETADDRINFO 3
.Os
.Sh NAME
.Nm getaddrinfo ,
.Nm freeaddrinfo
.Nd host and service name to socket address structure
.Sh SYNOPSIS
.In sys/types.h
.In sys/socket.h
.In netdb.h
.Ft int
.Fn getaddrinfo "const char *hostname" "const char *servname" \
    "const struct addrinfo *hints" "struct addrinfo **res"
.Ft void
.Fn freeaddrinfo "struct addrinfo *ai"
.Sh DESCRIPTION
The
.Fn getaddrinfo
function is used to get a list of
.Tn IP
addresses and port numbers for host
.Fa hostname
and service
.Fa servname .
It is a replacement for and provides more flexibility than the
.Xr gethostbyname 3
and
.Xr getservbyname 3
functions.
.Pp
The
.Fa hostname
and
.Fa servname
arguments are either pointers to NUL-terminated strings or the null pointer.
An acceptable value for
.Fa hostname
is either a valid host name or a numeric host address string consisting
of a dotted decimal IPv4 address or an IPv6 address.
The
.Fa servname
is either a decimal port number or a service name listed in
.Xr services 5 .
At least one of
.Fa hostname
and
.Fa servname
must be non-null.
.Pp
.Fa hints
is an optional pointer to a
.Vt struct addrinfo ,
as defined by
.In netdb.h :
.Bd -literal
struct addrinfo {
	int ai_flags;		/* input flags */
	int ai_family;		/* address family for socket */
	int ai_socktype;	/* socket type */
	int ai_protocol;	/* protocol for socket */
	socklen_t ai_addrlen;	/* length of socket-address */
	struct sockaddr *ai_addr; /* socket-address for socket */
	char *ai_canonname;	/* canonical name for service location */
	struct addrinfo *ai_next; /* pointer to next in list */
};
.Ed
.Pp
This structure can be used to provide hints concerning the type of socket
that the caller supports or wishes to use.
The caller can supply the following structure elements in
.Fa hints :
.Bl -tag -width "ai_socktypeXX"
.It Fa ai_family
The address family that should be used.
When
.Fa ai_family
is set to
.Dv AF_UNSPEC ,
it means the caller will accept any address family supported by the
operating system.
.It Fa ai_socktype
Denotes the type of socket that is wanted:
.Dv SOCK_STREAM ,
.Dv SOCK_DGRAM ,
or
.Dv SOCK_RAW .
When
.Fa ai_socktype
is zero, the caller will accept any socket type.
.It Fa ai_protocol
Indicates which transport protocol is desired,
.Dv IPPROTO_UDP
or
.Dv IPPROTO_TCP .
If
.Fa ai_protocol
is zero, the caller will accept any protocol.
.It Fa ai_flags
.Fa ai_flags
is formed by
.Tn OR Ns 'ing
the following values:
.Bl -tag -width "AI_CANONNAMEXX"
.It Dv AI_ADDRCONFIG
If the
.Dv AI_ADDRCONFIG
bit is set, IPv4 addresses will be returned only if an IPv4 address is
configured on an interface, and IPv6 addresses will be returned only if an IPv6
address is configured on an interface.
Addresses on a loopback interface and link-local IPv6 addresses are not
considered valid as configured addresses.
This bit is only considered when determining whether a DNS query should
be performed or not.
.It Dv AI_CANONNAME
If the
.Dv AI_CANONNAME
bit is set, a successful call to
.Fn getaddrinfo
will return a NUL-terminated string containing the canonical name
of the specified host name in the
.Fa ai_canonname
element of the first
.Vt addrinfo
structure returned.
.It Dv AI_FQDN
If the
.Dv AI_FQDN
bit is set, a successful call to
.Fn getaddrinfo
will return a NUL-terminated string containing the fully qualified domain name
of the specified host name in the
.Fa ai_canonname
element of the first
.Vt addrinfo
structure returned.
.Pp
This is different from the
.Dv AI_CANONNAME
bit flag that returns the canonical name registered in DNS,
which may be different from the fully qualified domain name
that the host name resolved to.
Only one of the
.Dv AI_FQDN
and
.Dv AI_CANONNAME
bits can be set.
.It Dv AI_NUMERICHOST
If the
.Dv AI_NUMERICHOST
bit is set, it indicates that
.Fa hostname
should be treated as a numeric string defining an IPv4 or IPv6 address
and no name resolution should be attempted.
.It Dv AI_NUMERICSERV
If the
.Dv AI_NUMERICSERV
bit is set, it indicates that
.Fa servname
should be treated as a numeric port string
and no service name resolution should be attempted.
.It Dv AI_PASSIVE
If the
.Dv AI_PASSIVE
bit is set, it indicates that the returned socket address structure
is intended for use in a call to
.Xr bind 2 .
In this case, if the
.Fa hostname
argument is the null pointer, then the IP address portion of the
socket address structure will be set to
.Dv INADDR_ANY
for an IPv4 address or
.Dv IN6ADDR_ANY_INIT
for an IPv6 address.
.Pp
If the
.Dv AI_PASSIVE
bit is not set, the returned socket address structure will be ready
for use in a call to
.Xr connect 2
for a connection-oriented protocol or
.Xr connect 2 ,
.Xr sendto 2 ,
or
.Xr sendmsg 2
if a connectionless protocol was chosen.
The
.Tn IP
address portion of the socket address structure will be set to the
loopback address if
.Fa hostname
is the null pointer and
.Dv AI_PASSIVE
is not set.
.El
.El
.Pp
All other elements of the
.Vt addrinfo
structure passed via
.Fa hints
must be zero or the null pointer.
.Pp
If
.Fa hints
is the null pointer,
.Fn getaddrinfo
behaves as if the caller provided a
.Vt struct addrinfo
with
.Fa ai_family
set to
.Dv AF_UNSPEC ,
.Fa ai_flags
set to
.Dv AI_ADDRCONFIG ,
and all other elements set to zero or
.Dv NULL .
.Pp
After a successful call to
.Fn getaddrinfo ,
.Fa *res
is a pointer to a linked list of one or more
.Vt addrinfo
structures.
The list can be traversed by following the
.Fa ai_next
pointer in each
.Vt addrinfo
structure until a null pointer is encountered.
The three members
.Fa ai_family ,
.Fa ai_socktype ,
and
.Fa ai_protocol
in each returned
.Vt addrinfo
structure are suitable for a call to
.Xr socket 2 .
For each
.Vt addrinfo
structure in the list, the
.Fa ai_addr
member points to a filled-in socket address structure of length
.Fa ai_addrlen .
.Pp
This implementation of
.Fn getaddrinfo
allows numeric IPv6 address notation with scope identifier,
as documented in RFC 4007.
By appending the percent character and scope identifier to addresses,
one can fill the
.Li sin6_scope_id
field for addresses.
This would make management of scoped addresses easier
and allows cut-and-paste input of scoped addresses.
.Pp
At this moment the code supports only link-local addresses with the format.
The scope identifier is hardcoded to the name of the hardware interface
associated
with the link
.Po
such as
.Li ne0
.Pc .
An example is
.Dq Li fe80::1%ne0 ,
which means
.Do
.Li fe80::1
on the link associated with the
.Li ne0
interface
.Dc .
.Pp
The current implementation assumes a one-to-one relationship between
the interface and link, which is not necessarily true from the specification.
.Pp
All of the information returned by
.Fn getaddrinfo
is dynamically allocated: the
.Vt addrinfo
structures themselves as well as the socket address structures and
the canonical host name strings included in the
.Vt addrinfo
structures.
.Pp
Memory allocated for the dynamically allocated structures created by
a successful call to
.Fn getaddrinfo
is released by the
.Fn freeaddrinfo
function.
The
.Fa ai
pointer should be an
.Vt addrinfo
structure created by a call to
.Fn getaddrinfo .
.Sh RETURN VALUES
.Fn getaddrinfo
returns zero on success or one of the error codes listed in
.Xr gai_strerror 3
if an error occurs.
If an error occurs, no memory is allocated by
.Fn getaddrinfo ,
therefore it is not necessary to release the
.Vt addrinfo
structure(s).
.Sh EXAMPLES
The following code tries to connect to
.Dq Li www.kame.net
service
.Dq Li www
via a stream socket.
It loops through all the addresses available, regardless of address family.
If the destination resolves to an IPv4 address, it will use an
.Dv AF_INET
socket.
Similarly, if it resolves to IPv6, an
.Dv AF_INET6
socket is used.
Observe that there is no hardcoded reference to a particular address family.
The code works even if
.Fn getaddrinfo
returns addresses that are not IPv4/v6.
.Bd -literal -offset indent
struct addrinfo hints, *res, *res0;
int error;
int save_errno;
int s;
const char *cause = NULL;

memset(&hints, 0, sizeof(hints));
hints.ai_family = AF_UNSPEC;
hints.ai_socktype = SOCK_STREAM;
error = getaddrinfo("www.kame.net", "www", &hints, &res0);
if (error)
	errx(1, "%s", gai_strerror(error));
s = -1;
for (res = res0; res; res = res->ai_next) {
	s = socket(res->ai_family, res->ai_socktype,
	    res->ai_protocol);
	if (s == -1) {
		cause = "socket";
		continue;
	}

	if (connect(s, res->ai_addr, res->ai_addrlen) == -1) {
		cause = "connect";
		save_errno = errno;
		close(s);
		errno = save_errno;
		s = -1;
		continue;
	}

	break;	/* okay we got one */
}
if (s == -1)
	err(1, "%s", cause);
freeaddrinfo(res0);
.Ed
.Pp
The following example tries to open a wildcard listening socket onto service
.Dq Li www ,
for all the address families available.
.Bd -literal -offset indent
struct addrinfo hints, *res, *res0;
int error;
int save_errno;
int s[MAXSOCK];
int nsock;
const char *cause = NULL;

memset(&hints, 0, sizeof(hints));
hints.ai_family = AF_UNSPEC;
hints.ai_socktype = SOCK_STREAM;
hints.ai_flags = AI_PASSIVE;
error = getaddrinfo(NULL, "www", &hints, &res0);
if (error)
	errx(1, "%s", gai_strerror(error));
nsock = 0;
for (res = res0; res && nsock < MAXSOCK; res = res->ai_next) {
	s[nsock] = socket(res->ai_family, res->ai_socktype,
	    res->ai_protocol);
	if (s[nsock] == -1) {
		cause = "socket";
		continue;
	}

	if (bind(s[nsock], res->ai_addr, res->ai_addrlen) == -1) {
		cause = "bind";
		save_errno = errno;
		close(s[nsock]);
		errno = save_errno;
		continue;
	}
	(void) listen(s[nsock], 5);

	nsock++;
}
if (nsock == 0)
	err(1, "%s", cause);
freeaddrinfo(res0);
.Ed
.Sh SEE ALSO
.Xr bind 2 ,
.Xr connect 2 ,
.Xr send 2 ,
.Xr socket 2 ,
.Xr gai_strerror 3 ,
.Xr gethostbyname 3 ,
.Xr getnameinfo 3 ,
.Xr getservbyname 3 ,
.Xr res_init 3 ,
.Xr hosts 5 ,
.Xr resolv.conf 5 ,
.Xr services 5 ,
.Xr hostname 7
.Rs
.%A Craig Metz
.%B Proceedings of the Freenix Track: 2000 USENIX Annual Technical Conference
.%D June 2000
.%T Protocol Independence Using the Sockets API
.Re
.Sh STANDARDS
The
.Fn getaddrinfo
function is defined by the
.St -p1003.1g-2000
draft specification and documented in RFC 3493.
.Pp
The
.Dv AI_FQDN
flag bit first appeared in Windows 7.
.Pp
.Rs
.%A R. Gilligan
.%A S. Thomson
.%A J. Bound
.%A J. McCann
.%A W. Stevens
.%D February 2003
.%R RFC 3493
.%T Basic Socket Interface Extensions for IPv6
.Re
.Pp
.Rs
.%A S. Deering
.%A B. Haberman
.%A T. Jinmei
.%A E. Nordmark
.%A B. Zill
.%D March 2005
.%R RFC 4007
.%T IPv6 Scoped Address Architecture
.Re