libc/net/getaddrinfo.3

.\"	$KAME: getaddrinfo.3,v 1.36 2005/01/05 03:23:05 itojun Exp $
.\"	$OpenBSD: getaddrinfo.3,v 1.35 2004/12/21 03:40:31 jaredy Exp $
.\"
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.\" Copyright (C) 2000, 2001  Internet Software Consortium.
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.\" $FreeBSD$
.\"
.Dd February 10, 2019
.Dt GETADDRINFO 3
.Os
.Sh NAME
.Nm getaddrinfo ,
.Nm freeaddrinfo
.Nd socket address structure to host and service name
.Sh SYNOPSIS
.In sys/types.h
.In sys/socket.h
.In netdb.h
.Ft int
.Fo getaddrinfo
.Fa "const char *hostname" "const char *servname"
.Fa "const struct addrinfo *hints" "struct addrinfo **res"
.Fc
.Ft void
.Fn freeaddrinfo "struct addrinfo *ai"
.Sh DESCRIPTION
The
.Fn getaddrinfo
function is used to get a list of
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,
an IPv6 address,
or a UNIX-domain 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
.Li struct addrinfo ,
as defined by
.Aq Pa netdb.h :
.Bd -literal
struct addrinfo {
        int     ai_flags;       /* AI_PASSIVE, AI_CANONNAME, .. */
        int     ai_family;      /* AF_xxx */
        int     ai_socktype;    /* SOCK_xxx */
        int     ai_protocol;    /* 0 or IPPROTO_xxx for IPv4 and IPv6 */
        socklen_t ai_addrlen;   /* length of ai_addr */
        char    *ai_canonname;  /* canonical name for hostname */
        struct  sockaddr *ai_addr;      /* binary address */
        struct  addrinfo *ai_next;      /* next structure in linked 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 ,
.Dv SOCK_SEQPACKET ,
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 ,
.Dv IPPROTO_TCP ,
.Dv IPPROTO_SCTP ,
or
.Dv IPPROTO_UDPLITE .
If
.Fa ai_protocol
is zero the caller will accept any protocol.
.It Fa ai_flags
The
.Fa ai_flags
field to which the
.Fa hints
parameter points shall be set to zero
or be the bitwise-inclusive OR of one or more of the values
.Dv AI_ADDRCONFIG ,
.Dv AI_ALL ,
.Dv AI_CANONNAME ,
.Dv AI_NUMERICHOST ,
.Dv AI_NUMERICSERV ,
.Dv AI_PASSIVE
and
.Dv AI_V4MAPPED .
For a UNIX-domain address,
.Fa ai_flags
is ignored.
.Bl -tag -width "AI_CANONNAMEXX"
.It Dv AI_ADDRCONFIG
If the
.Dv AI_ADDRCONFIG
bit is set, IPv4 addresses shall be returned only if
an IPv4 address is configured on the local system,
and IPv6 addresses shall be returned only if
an IPv6 address is configured on the local system.
.It Dv AI_ALL
If the
.Dv AI_ALL
flag is used with the
.Dv AI_V4MAPPED
flag, then
.Fn getaddrinfo
shall return all matching IPv6 and IPv4 addresses.
.Pp
For example, when using the DNS, queries are made for both AAAA records and A records, and
.Fn getaddrinfo
returns the combined results of both queries.
Any IPv4 addresses found are returned as IPv4-mapped IPv6 addresses.
.Pp
The
.Dv AI_ALL
flag without the
.Dv AI_V4MAPPED
flag is ignored.
.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 hostname in the
.Fa ai_canonname
element of the first
.Li addrinfo
structure returned.
.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,
then a non-null
.Fa servname
string supplied shall be a numeric port string.
Otherwise, an
.Dv EAI_NONAME
error shall be returned.
This bit shall prevent any type of name resolution service
(for example, NIS+) from being invoked.
.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.
.It Dv AI_V4MAPPED
If the
.Dv AI_V4MAPPED
flag is specified along with an ai_family of
.Dv AF_INET6 ,
then
.Fn getaddrinfo
shall return IPv4-mapped IPv6 addresses on finding no matching IPv6 addresses (
.Fa ai_addrlen
shall be 16).
.Pp
For example, when using the DNS, if no AAAA records are found then a query is made for A records and any found are returned as IPv4-mapped IPv6 addresses.
.Pp
The
.Dv AI_V4MAPPED
flag shall be ignored unless
.Fa ai_family
equals
.Dv AF_INET6 .
.El
.El
.Pp
All other elements of the
.Li 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
.Li struct addrinfo
with
.Fa ai_family
set to
.Dv AF_UNSPEC
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
.Li addrinfo
structures.
The list can be traversed by following the
.Fa ai_next
pointer in each
.Li addrinfo
structure until a null pointer is encountered.
Each returned
.Li addrinfo
structure contains three members that are suitable for a call to
.Xr socket 2 :
.Fa ai_family ,
.Fa ai_socktype ,
and
.Fa ai_protocol .
For each
.Li 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 chapter 11 of 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
.Li addrinfo
structures themselves as well as the socket address structures and
the canonical host name strings included in the
.Li 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 a
.Li addrinfo
structure created by a call to
.Fn getaddrinfo .
.Sh IMPLEMENTATION NOTES
The behavior of
.Li freeadrinfo(NULL)
is left unspecified by both
.St -susv4
and
.Dv "RFC 3493" .
The current implementation ignores a
.Dv NULL
argument for compatibility with programs that rely on the implementation
details of other operating systems.
.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.
.Sh EXAMPLES
The following code tries to connect to
.Dq Li www.kame.net
service
.Dq Li http
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 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", "http", &hints, &res0);
if (error) {
	errx(1, "%s", gai_strerror(error));
	/* NOTREACHED */
}
s = -1;
for (res = res0; res; res = res->ai_next) {
	s = socket(res->ai_family, res->ai_socktype,
	    res->ai_protocol);
	if (s < 0) {
		cause = "socket";
		continue;
	}

	if (connect(s, res->ai_addr, res->ai_addrlen) < 0) {
		cause = "connect";
		close(s);
		s = -1;
		continue;
	}

	break;	/* okay we got one */
}
if (s < 0) {
	err(1, "%s", cause);
	/* NOTREACHED */
}
freeaddrinfo(res0);
.Ed
.Pp
The following example tries to open a wildcard listening socket onto service
.Dq Li http ,
for all the address families available.
.Bd -literal -offset indent
struct addrinfo hints, *res, *res0;
int error;
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, "http", &hints, &res0);
if (error) {
	errx(1, "%s", gai_strerror(error));
	/* NOTREACHED */
}
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] < 0) {
		cause = "socket";
		continue;
	}

	if (bind(s[nsock], res->ai_addr, res->ai_addrlen) < 0) {
		cause = "bind";
		close(s[nsock]);
		continue;
	}
	(void) listen(s[nsock], 5);

	nsock++;
}
if (nsock == 0) {
	err(1, "%s", cause);
	/* NOTREACHED */
}
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 resolver 3 ,
.Xr inet 4 ,
.Xr inet6 4 ,
.Xr unix 4 ,
.Xr hosts 5 ,
.Xr resolv.conf 5 ,
.Xr services 5 ,
.Xr hostname 7 ,
.Xr named 8
.Rs
.%A R. Gilligan
.%A S. Thomson
.%A J. Bound
.%A J. McCann
.%A W. Stevens
.%T Basic Socket Interface Extensions for IPv6
.%R RFC 3493
.%D February 2003
.Re
.Rs
.%A S. Deering
.%A B. Haberman
.%A T. Jinmei
.%A E. Nordmark
.%A B. Zill
.%T "IPv6 Scoped Address Architecture"
.%R RFC 4007
.%D March 2005
.Re
.Rs
.%A Craig Metz
.%T Protocol Independence Using the Sockets API
.%B "Proceedings of the freenix track: 2000 USENIX annual technical conference"
.%D June 2000
.Re
.Sh STANDARDS
The
.Fn getaddrinfo
function is defined by the
.St -p1003.1-2004
specification and documented in
.Dv "RFC 3493" ,
.Dq Basic Socket Interface Extensions for IPv6 .