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.\"     $OpenBSD: getaddrinfo.3,v 1.19 2001/08/06 10:42:26 mpech Exp $

.\"     $KAME: getaddrinfo.3,v 1.29 2001/02/12 09:24:45 itojun Exp $

.\"

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

.\"

.Dd May 25, 1995

.Dt GETADDRINFO 3

.Os

.\"

.Sh NAME

.Nm getaddrinfo ,

.Nm freeaddrinfo ,

.Nm gai_strerror

.Nd nodename-to-address translation in protocol-independent manner

.\"

.Sh SYNOPSIS

.Fd #include 

.Fd #include 

.Fd #include 

.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.

.Pp

The

.Li addrinfo

structure is defined as a result of including the

.Aq Pa netdb.h

header:

.Bd -literal -offset

struct addrinfo {                                                  *

     int     ai_flags;     /* AI_PASSIVE, AI_CANONNAME, AI_NUMERICHOST */

     int     ai_family;    /* PF_xxx */

     int     ai_socktype;  /* SOCK_xxx */

     int     ai_protocol;  /* 0 or IPPROTO_xxx for IPv4 and IPv6 */

     size_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

.Pp

The

.Fa nodename

and

.Fa servname

arguments are pointers to NUL-terminated strings or

.Dv NULL .

One or both of these two arguments must be a non-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 non-null

.Fa nodename

string can be either a node name or a numeric host address string

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

A non-null

.Fa servname

string can be either a service name or a decimal port number.

.Pp

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 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 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 .

.Pp

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 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.

.Pp

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 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 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

.Pq for a connection-oriented protocol

or either

.Fn connect ,

.Fn sendto ,

or

.Fn sendmsg

.Pq for a connectionless protocol .

In this case, if the

.Fa nodename

argument is a null pointer, then the IP address portion of the

socket address structure will be set to the loopback address.

.Pp

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 NUL-terminated string

containing the canonical name of the specified

.Fa nodename .

.Pp

If the

.Dv AI_NUMERICHOST

bit is set in the

.Fa ai_flags

member of the

.Fa hints

structure, then a non-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.

.Pp

The arguments to

.Fn getaddrinfo

must sufficiently be consistent and unambiguous.

Here are pitfall cases you may encounter:

.Bl -bullet

.It

.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 .

.It

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 .

.It

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.

.It

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

.Pp

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.

.Pp

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

.Li sin6_scope_id

field for addresses.

This would make management of scoped address easier,

and allows cut-and-paste input of scoped address.

.Pp

At this moment the code supports only link-local addresses with the format.

Scope identifier is hardcoded to name of hardware interface associated

with the link.

.Po

such as

.Li ne0

.Pc .

Example would be like

.Dq Li fe80::1%ne0 ,

which means

.Do

.Li fe80::1

on the link associated with

.Li ne0

interface

.Dc .

.Pp

The implementation is still very experimental and non-standard.

The current implementation assumes one-by-one relationship between

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 from address family.

If the destination resolves to 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 particular address family.

The code works even if

.Nm 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 = PF_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, 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 = PF_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, 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:

.Pp

.Bl -tag -width EAI_ADDRFAMILY -compact

.It Dv EAI_ADDRFAMILY

Address family for

.Fa nodename

not supported.

.It Dv EAI_AGAIN

Temporary failure in name resolution.

.It Dv EAI_BADFLAGS

Invalid value for

.Fa ai_flags .

.It Dv EAI_FAIL

Non-recoverable failure in name resolution.

.It Dv EAI_FAMILY

.Fa ai_family

not supported.

.It Dv EAI_MEMORY

Memory allocation failure.

.It Dv EAI_NODATA

No address associated with

.Fa nodename .

.It Dv EAI_NONAME

.Fa nodename

nor

.Fa servname

provided, or not known.

.It Dv EAI_SERVICE

.Fa servname

not supported for

.Fa ai_socktype .

.It Dv EAI_SOCKTYPE

.Fa ai_socktype

not supported.

.It Dv EAI_SYSTEM

System error returned in

.Va errno .

.El

.Pp

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 getnameinfo 3 ,

.Xr gethostbyname 3 ,

.Xr getservbyname 3 ,

.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 W. Stevens

.%T Basic Socket Interface Extensions for IPv6

.%R RFC2553

.%D March 1999

.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 HISTORY

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

.\"

.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

.Pq RFC2553 .

.\"

.Sh BUGS

The current implementation is not thread-safe.

.Pp

The text was shamelessly copied from RFC2553.