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getaddrinfo.3 revision 1.34 
 $NetBSD: getaddrinfo.3,v 1.34 2004/04/14 04:42:35 itojun Exp $
 $KAME: getaddrinfo.3,v 1.32 2003/04/30 06:06:42 itojun Exp $

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 From: @(#)gethostbyname.3 8.4 (Berkeley) 5/25/95

.Dd April 14, 2004
.Dt GETADDRINFO 3
.Os

.Sh NAME
.Nm getaddrinfo ,
.Nm freeaddrinfo ,
.Nm gai_strerror
.Nd nodename-to-address translation in protocol-independent manner

.Sh LIBRARY
.Lb libc

.Sh SYNOPSIS
n netdb.h .Ft int
.Fn getaddrinfo "const char *nodename" "const char *servname" \
"const struct addrinfo *hints" "struct addrinfo **res"
.Ft void
.Fn freeaddrinfo "struct addrinfo *ai"
.Ft "char *"
.Fn gai_strerror "int ecode"

.Sh DESCRIPTION
The
.Fn getaddrinfo
function is defined for protocol-independent nodename-to-address translation.
It performs the functionality of
.Xr gethostbyname 3
and
.Xr getservbyname 3 ,
but in a more sophisticated manner.

p
The
.Li addrinfo
structure is defined as a result of including the
.Aq Pa netdb.h
header:
d -literal -offset struct addrinfo {
 int ai_flags; /* AI_xxx */
 int ai_family; /* PF_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 nodename */
 struct sockaddr *ai_addr; /* binary address */
 struct addrinfo *ai_next; /* next structure in linked list */
};
.Ed

p
The
.Fa nodename
and
.Fa servname
arguments are pointers to null-terminated strings or
.Dv NULL .
One or both of these two arguments must be a

f non Dv -NULL
pointer.
In the normal client scenario, both the
.Fa nodename
and
.Fa servname
are specified.
In the normal server scenario, only the
.Fa servname
is specified.
A

f non Dv -NULL
.Fa nodename
string can be either a node name or a numeric host address string

o
i.e., a dotted-decimal IPv4 address or an IPv6 hex address

c .
A

f non Dv -NULL
.Fa servname
string can be either a service name or a decimal port number.

p
The caller can optionally pass an
.Li addrinfo
structure, pointed to by the third argument,
to provide hints concerning the type of socket that the caller supports.
In this
.Fa hints
structure all members other than
.Fa ai_flags ,
.Fa ai_family ,
.Fa ai_socktype ,
and
.Fa ai_protocol
must be zero or a
.Dv NULL
pointer.
A value of
.Dv PF_UNSPEC
for
.Fa ai_family
means the caller will accept any protocol family.
A value of 0 for
.Fa ai_socktype
means the caller will accept any socket type.
A value of 0 for
.Fa ai_protocol
means the caller will accept any protocol.
For example, if the caller handles only TCP and not UDP, then the
.Fa ai_socktype
member of the hints structure should be set to
.Dv SOCK_STREAM
when
.Fn getaddrinfo
is called.
If the caller handles only IPv4 and not IPv6, then the
.Fa ai_family
member of the
.Fa hints
structure should be set to
.Dv PF_INET
when
.Fn getaddrinfo
is called.
If the third argument to
.Fn getaddrinfo
is a
.Dv NULL
pointer, this is the same as if the caller had filled in an
.Li addrinfo
structure initialized to zero with
.Fa ai_family
set to
.Dv PF_UNSPEC .

p
Upon successful return a pointer to a linked list of one or more
.Li addrinfo
structures is returned through the final argument.
The caller can process each
.Li addrinfo
structure in this list by following the
.Fa ai_next
pointer, until a
.Dv NULL
pointer is encountered.
In each returned
.Li addrinfo
structure the three members
.Fa ai_family ,
.Fa ai_socktype ,
and
.Fa ai_protocol
are the corresponding arguments for a call to the
.Fn socket
function.
In each
.Li addrinfo
structure the
.Fa ai_addr
member points to a filled-in socket address structure whose length is
specified by the
.Fa ai_addrlen
member.

p
If the
.Dv AI_PASSIVE
bit is set in the
.Fa ai_flags
member of the
.Fa hints
structure, then the caller plans to use the returned socket address
structure in a call to
.Fn bind .
In this case, if the
.Fa nodename
argument is a
.Dv 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.

p
If the
.Dv AI_PASSIVE
bit is not set in the
.Fa ai_flags
member of the
.Fa hints
structure, then the returned socket address structure will be ready for a
call to
.Fn connect

q for a connection-oriented protocol
or either
.Fn connect ,
.Fn sendto ,
or
.Fn sendmsg

q for a connectionless protocol .
In this case, if the
.Fa nodename
argument is a
.Dv NULL
pointer, then the IP address portion of the
socket address structure will be set to the loopback address.

p
If the
.Dv AI_CANONNAME
bit is set in the
.Fa ai_flags
member of the
.Fa hints
structure, then upon successful return the
.Fa ai_canonname
member of the first
.Li addrinfo
structure in the linked list will point to a null-terminated string
containing the canonical name of the specified
.Fa nodename .

p
If the
.Dv AI_NUMERICHOST
bit is set in the
.Fa ai_flags
member of the
.Fa hints
structure, then a

f non Dv -NULL
.Fa nodename
string must be a numeric host address string.
Otherwise an error of
.Dv EAI_NONAME
is returned.
This flag prevents any type of name resolution service (e.g., the DNS)
from being called.

p
If the
.Dv AI_NUMERICSERV
bit is set in the
.Fa ai_flags
member of the
.Fa hints
structure, then a

f non Dv NULL
.Fa servname
string supplied shall be a numeric port string.
Otherwise, an
.Dv EAI_NONAME
error shall be returned.
This flag shall prevent any
type of name resolution service (for example, NIS) from being
invoked.

p
The arguments to
.Fn getaddrinfo
must sufficiently be consistent and unambiguous.
Here are pitfall cases you may encounter:
l -bullet t .Fn getaddrinfo
will raise an error if members of the
.Fa hints
structure are not consistent.
For example, for internet address families,
.Fn getaddrinfo
will raise an error if you specify
.Dv SOCK_STREAM
to
.Fa ai_socktype
while you specify
.Dv IPPROTO_UDP
to
.Fa ai_protocol .
t If you specify a
.Fa servname
which is defined only for certain
.Fa ai_socktype ,
.Fn getaddrinfo
will raise an error because the arguments are not consistent.
For example,
.Fn getaddrinfo
will raise an error if you ask for
.Dq Li tftp
service on
.Dv SOCK_STREAM .
t For internet address families, if you specify
.Fa servname
while you set
.Fa ai_socktype
to
.Dv SOCK_RAW ,
.Fn getaddrinfo
will raise an error, because service names are not defined for the internet
.Dv SOCK_RAW
space.
t If you specify a numeric
.Fa servname ,
while leaving
.Fa ai_socktype
and
.Fa ai_protocol
unspecified,
.Fn getaddrinfo
will raise an error.
This is because the numeric
.Fa servname
does not identify any socket type, and
.Fn getaddrinfo
is not allowed to glob the argument in such case.
.El

p
All of the information returned by
.Fn getaddrinfo
is dynamically allocated:
the
.Li addrinfo
structures, the socket address structures, and canonical node name
strings pointed to by the addrinfo structures.
To return this information to the system the function
.Fn freeaddrinfo
is called.
The
.Fa addrinfo
structure pointed to by the
.Fa ai argument
is freed, along with any dynamic storage pointed to by the structure.
This operation is repeated until a
.Dv NULL
.Fa ai_next
pointer is encountered.

p
To aid applications in printing error messages based on the
.Dv EAI_xxx
codes returned by
.Fn getaddrinfo ,
.Fn gai_strerror
is defined.
The argument is one of the
.Dv EAI_xxx
values defined earlier and the return value points to a string describing
the error.
If the argument is not one of the
.Dv EAI_xxx
values, the function still returns a pointer to a string whose contents
indicate an unknown error.

.Ss Extension for scoped IPv6 address
The implementation allows experimental numeric IPv6 address notation with
scope identifier.
By appending the percent character and scope identifier to addresses,
you can fill the
.Li sin6_scope_id
field for addresses.
This would make management of scoped address easier,
and allows cut-and-paste input of scoped address.

p
At this moment the code supports only link-local addresses with the format.
Scope identifier is hardcoded to the name of the hardware interface associated
with the link.

o
such as
.Li ne0

c .
An example is
.Dq Li fe80::1%ne0 ,
which means
.Do
.Li fe80::1
on the link associated with the
.Li ne0
interface
.Dc .

p
The implementation is still very experimental and non-standard.
The current implementation assumes a one-to-one relationship between
the interface and link, which is not necessarily true from the specification.

.Sh EXAMPLES
The following code tries to connect to
.Dq Li www.kame.net
service
.Dq Li http .
via stream socket.
It loops through all the addresses available, regardless of address family.
If the destination resolves to an IPv4 address, it will use
.Dv AF_INET
socket.
Similarly, if it resolves to IPv6,
.Dv AF_INET6
socket is used.
Observe that there is no hardcoded reference to a particular address family.
The code works even if
.Nm getaddrinfo
returns addresses that are not IPv4/v6.
d -literal -offset indent struct addrinfo hints, *res, *res0;
int error;
int s;
const char *cause = NULL;

hints.ai_family = PF_UNSPEC;
hints.ai_socktype = SOCK_STREAM;
hints.ai_flags = 0;
hints.ai_protocol = 0;
hints.ai_addrlen = 0;
hints.ai_canonname = NULL;
hints.ai_addr = NULL;
hints.ai_next = NULL;
error = getaddrinfo("www.kame.net", "http", \*[Am]hints, \*[Am]res0);
if (error) {
 errx(1, "%s", gai_strerror(error));
 /*NOTREACHED*/
}
s = -1;
for (res = res0; res; res = res-\*[Gt]ai_next) {
 s = socket(res-\*[Gt]ai_family, res-\*[Gt]ai_socktype,
 res-\*[Gt]ai_protocol);
 if (s \*[Lt] 0) {
 cause = "socket";
 continue;
 }

 if (connect(s, res-\*[Gt]ai_addr, res-\*[Gt]ai_addrlen) \*[Lt] 0) {
 cause = "connect";
 close(s);
 s = -1;
 continue;
 }

 break; /* okay we got one */
}
if (s \*[Lt] 0) {
 err(1, "%s", cause);
 /*NOTREACHED*/
}
freeaddrinfo(res0);
.Ed

p
The following example tries to open a wildcard listening socket onto service
.Dq Li http ,
for all the address families available.
d -literal -offset indent struct addrinfo hints, *res, *res0;
int error;
int s[MAXSOCK];
int nsock;
const char *cause = NULL;

hints.ai_family = PF_UNSPEC;
hints.ai_socktype = SOCK_STREAM;
hints.ai_flags = AI_PASSIVE;
hints.ai_protocol = 0;
hints.ai_addrlen = 0;
hints.ai_canonname = NULL;
hints.ai_addr = NULL;
hints.ai_next = NULL;
error = getaddrinfo(NULL, "http", \*[Am]hints, \*[Am]res0);
if (error) {
 errx(1, "%s", gai_strerror(error));
 /*NOTREACHED*/
}
nsock = 0;
for (res = res0; res \*[Am]\*[Am] nsock \*[Lt] MAXSOCK; res = res-\*[Gt]ai_next) {
 s[nsock] = socket(res-\*[Gt]ai_family, res-\*[Gt]ai_socktype,
 res-\*[Gt]ai_protocol);
 if (s[nsock] \*[Lt] 0) {
 cause = "socket";
 continue;
 }

 if (bind(s[nsock], res-\*[Gt]ai_addr, res-\*[Gt]ai_addrlen) \*[Lt] 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 DIAGNOSTICS
Error return status from
.Fn getaddrinfo
is zero on success and non-zero on errors.
Non-zero error codes are defined in
.Aq Pa netdb.h ,
and as follows:

p
l -tag -width EAI_ADDRFAMILY -compact t Dv EAI_ADDRFAMILY Address family for
.Fa nodename
not supported.
t Dv EAI_AGAIN Temporary failure in name resolution.
t Dv EAI_BADFLAGS Invalid value for
.Fa ai_flags .
t Dv EAI_FAIL Non-recoverable failure in name resolution.
t Dv EAI_FAMILY .Fa ai_family
not supported.
t Dv EAI_MEMORY Memory allocation failure.
t Dv EAI_NODATA No address associated with
.Fa nodename .
t Dv EAI_NONAME .Fa nodename
nor
.Fa servname
provided, or not known.
t Dv EAI_SERVICE .Fa servname
not supported for
.Fa ai_socktype .
t Dv EAI_SOCKTYPE .Fa ai_socktype
not supported.
t Dv EAI_SYSTEM System error returned in
.Va errno .
.El

p
If called with proper argument,
.Fn gai_strerror
returns a pointer to a string describing the given error code.
If the argument is not one of the
.Dv EAI_xxx
values, the function still returns a pointer to a string whose contents
indicate an unknown error.

.Sh SEE ALSO
.Xr gethostbyname 3 ,
.Xr getnameinfo 3 ,
.Xr getservbyname 3 ,
.Xr hosts 5 ,
.Xr nsswitch.conf 5 ,
.Xr resolv.conf 5 ,
.Xr services 5 ,
.Xr hostname 7 ,
.Xr named 8

p
.Rs
.%A R. Gilligan
.%A S. Thomson
.%A J. Bound
.%A W. Stevens
.%T Basic Socket Interface Extensions for IPv6
.%R RFC 2553
.%D March 1999
.Re
.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 Tatsuya Jinmei
.%A Atsushi Onoe
.%T "An Extension of Format for IPv6 Scoped Addresses"
.%R internet draft
.%N draft-ietf-ipngwg-scopedaddr-format-02.txt
.%O work in progress material
.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 in IEEE POSIX 1003.1g draft specification,
and documented in
.Dq Basic Socket Interface Extensions for IPv6

q RFC 2553 .

.Sh HISTORY
The implementation first appeared in WIDE Hydrangea IPv6 protocol stack kit.

.Sh BUGS
The current implementation is not thread-safe.

p
The text was shamelessly copied from RFC 2553.