.\" $NetBSD: getaddrinfo.3,v 1.60 2020/06/04 11:28:00 nia Exp $
.\" $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 $
.\"
.\" Copyright (C) 2004 Internet Systems Consortium, Inc. ("ISC")
.\" Copyright (C) 2000, 2001 Internet Software Consortium.
.\"
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.\"
.Dd June 4, 2020
.Dt GETADDRINFO 3
.Os
.Sh NAME
.Nm getaddrinfo ,
.Nm freeaddrinfo ,
.Nm allocaddrinfo
.Nd host and service name to socket address structure
.Sh SYNOPSIS
.In netdb.h
.Ft int
.Fn getaddrinfo "const char * restrict hostname" \
"const char * restrict servname" \
"const struct addrinfo * restrict hints" "struct addrinfo ** restrict res"
.Ft void
.Fn freeaddrinfo "struct addrinfo *ai"
.Ft struct addrinfo *
.Fn allocaddrinfo "socklen_t len"
.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
.Li 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 */
char *ai_canonname; /* canonical name for service location */
struct sockaddr *ai_addr; /* socket-address for socket */
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
.Pq and protocol
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.
Note that while address families
.Pq Dv AF_*
and protocol families
.Pq Dv PF_*
are theoretically distinct, in practice the distinction has been lost.
.\" (.Dv !? Consistent with usage below though...)
.Dv "RFC 3493"
defines
.Fn getaddrinfo
in terms of the address family constants
.Dv AF_*
even though
.Fa ai_family
is to be passed as a protocol family to
.Xr socket 2 .
.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_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, it indicates that the
.Fa servname
string contains a numeric port number.
This is used to prevent service name resolution.
.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_SRV
Perform SRV (RFC 2782) record lookups first.
This is a non-portable extension.
.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.
The three members
.Fa ai_family ,
.Fa ai_socktype ,
and
.Fa ai_protocol
in each returned
.Li addrinfo
structure are suitable for a call to
.Xr socket 2 .
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
By default IPv6 address entries are ordered before IPv4 ones, but
the order of the entries in the list can be controlled using
.Xr ip6addrctl 8 .
.Pp
This implementation of
.Fn getaddrinfo
allows numeric IPv6 address notation with scope identifier,
as documented in chapter 11 of draft-ietf-ipv6-scoping-arch-02.txt.
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 an
.Li addrinfo
structure created by a call to
.Fn getaddrinfo
or
.Fn allocaddrinfo .
The
.Fn allocaddrinfo
function is intended primarily for authors of
.Xr nsdispatch 3
plugins implementing
.Fn getaddrinfo
backends.
.Fn allocaddrinfo
allocates a
.Li struct addrinfo
in a way that is compatible with being returned from
.Fn getaddrinfo
and being ultimately freed by
.Fn freeaddrinfo .
The returned structure is zeroed, except for the
.Fa ai_addr
field, which
will point to
.Fa len
bytes of memory for storage of a socket address.
It is safe to allocate memory separately for
.Fa ai_canonname
with
.Xr malloc 3 ,
or in any other way that is compatible with deallocation by
.Xr free 3 .
.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 hosts 5 ,
.Xr resolv.conf 5 ,
.Xr services 5 ,
.Xr hostname 7 ,
.Xr ip6addrctl 8 ,
.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 internet draft
.%N draft-ietf-ipv6-scoping-arch-02.txt
.%O work in progress material
.Re
.Rs
.%A Craig Metz
.%T "Protocol Independence Using the Sockets API"
.%I USENIX Association
.%B Proceedings of the FREENIX Track: 2000 USENIX Annual Technical Conference
.%D June 18-23, 2000
.%P 99-108
.%U http://www.usenix.org/events/usenix2000/freenix/metzprotocol/metzprotocol.pdf
.Re
.Sh STANDARDS
The
.Fn getaddrinfo
function is defined by the
.St -p1003.1g-2000
draft specification and documented in
.Dv "RFC 3493" ,
.Dq Basic Socket Interface Extensions for IPv6 .