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.\"     $OpenBSD: getnameinfo.3,v 1.16 2001/08/06 10:42:26 mpech Exp $

.\"     $KAME: getnameinfo.3,v 1.20 2001/01/05 13:37:37 itojun Exp $

.\"

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

.\"

.Dd May 25, 1995

.Dt GETNAMEINFO 3

.Os

.\"

.Sh NAME

.Nm getnameinfo

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

.\"

.Sh SYNOPSIS

.Fd #include 

.Fd #include 

.Fd #include 

.Ft int

.Fn getnameinfo "const struct sockaddr *sa" "socklen_t salen" \

"char *host" "size_t hostlen" "char *serv" "size_t servlen" "int flags"

.\"

.Sh DESCRIPTION

The

.Fn getnameinfo

function is defined for protocol-independent address-to-nodename translation.

Its functionality is a reverse conversion of

.Xr getaddrinfo 3 ,

and implements similar functionality with

.Xr gethostbyaddr 3

and

.Xr getservbyport 3

in more sophisticated manner.

.Pp

This function looks up an IP address and port number provided by the

caller in the DNS and system-specific database, and returns text

strings for both in buffers provided by the caller.

The function indicates successful completion by a zero return value;

a non-zero return value indicates failure.

.Pp

The first argument,

.Fa sa ,

points to either a

.Li sockaddr_in

structure (for IPv4) or a

.Li sockaddr_in6

structure (for IPv6) that holds the IP address and port number.

The

.Fa salen

argument gives the length of the

.Li sockaddr_in

or

.Li sockaddr_in6

structure.

.Pp

The function returns the nodename associated with the IP address in

the buffer pointed to by the

.Fa host

argument.

The caller provides the size of this buffer via the

.Fa hostlen

argument.

The service name associated with the port number is returned in the buffer

pointed to by

.Fa serv ,

and the

.Fa servlen

argument gives the length of this buffer.

The caller specifies not to return either string by providing a zero

value for the

.Fa hostlen

or

.Fa servlen

arguments.

Otherwise, the caller must provide buffers large enough to hold the

nodename and the service name, including the terminating null characters.

.Pp

Unfortunately most systems do not provide constants that specify the

maximum size of either a fully-qualified domain name or a service name.

Therefore to aid the application in allocating buffers for these two

returned strings the following constants are defined in

.Aq Pa netdb.h :

.Bd -literal -offset

#define NI_MAXHOST    MAXHOSTNAMELEN

#define NI_MAXSERV    32

.Ed

.Pp

The first value is actually defined as the constant

.Dv MAXDNAME

in recent versions of BIND's

.Aq Pa arpa/nameser.h

header (older versions of BIND define this constant to be 256)

and the second is a guess based on the services listed in the current

Assigned Numbers RFC.

.Pp

The final argument is a

.Fa flag

that changes the default actions of this function.

By default the fully-qualified domain name (FQDN) for the host is

looked up in the DNS and returned.

If the flag bit

.Dv NI_NOFQDN

is set, only the nodename portion of the FQDN is returned for local hosts.

.Pp

If the

.Fa flag

bit

.Dv NI_NUMERICHOST

is set, or if the host's name cannot be located in the DNS,

the numeric form of the host's address is returned instead of its name

.Po

e.g., by calling

.Fn inet_ntop

instead of

.Fn gethostbyaddr

.Pc .

If the

.Fa flag

bit

.Dv NI_NAMEREQD

is set, an error is returned if the host's name cannot be located in the DNS.

.Pp

If the flag bit

.Dv NI_NUMERICSERV

is set, the numeric form of the service address is returned

.Pq e.g., its port number

instead of its name.

The two

.Dv NI_NUMERICxxx

flags are required to support the

.Fl n

flag that many commands provide.

.Pp

A fifth flag bit,

.Dv NI_DGRAM ,

specifies that the service is a datagram service, and causes

.Fn getservbyport

to be called with a second argument of

.Qq udp

instead of its default of

.Qq tcp .

This is required for the few ports (512-514)

that have different services for UDP and TCP.

.Pp

These

.Dv NI_xxx

flags are defined in

.Aq Pa netdb.h .

.\"

.Ss Extension for scoped IPv6 address

The implementation allows experimental numeric IPv6 address notation with

scope identifier.

IPv6 link-local address will appear as string like

.Dq Li fe80::1%ne0 ,

if

.Dv NI_WITHSCOPEID

bit is enabled in

.Ar flags

argument.

Refer to

.Xr getaddrinfo 3

for the notation.

.\"

.Sh EXAMPLES

The following code tries to get numeric hostname, and service name,

for given socket address.

Observe that there is no hardcoded reference to particular address family.

.Bd -literal -offset indent

struct sockaddr *sa;    /* input */

char hbuf[NI_MAXHOST], sbuf[NI_MAXSERV];

if (getnameinfo(sa, sa->sa_len, hbuf, sizeof(hbuf), sbuf,

    sizeof(sbuf), NI_NUMERICHOST | NI_NUMERICSERV)) {

        errx(1, "could not get numeric hostname");

        /*NOTREACHED*/

}

printf("host=%s, serv=%s\en", hbuf, sbuf);

.Ed

.Pp

The following version checks if the socket address has reverse address mapping.

.Bd -literal -offset indent

struct sockaddr *sa;    /* input */

char hbuf[NI_MAXHOST];

if (getnameinfo(sa, sa->sa_len, hbuf, sizeof(hbuf), NULL, 0,

    NI_NAMEREQD)) {

        errx(1, "could not resolve hostname");

        /*NOTREACHED*/

}

printf("host=%s\en", hbuf);

.Ed

.\"

.Sh DIAGNOSTICS

The function indicates successful completion by a zero return value;

a non-zero return value indicates failure.

Error codes are as below:

.Bl -tag -width Er

.It Dv EAI_AGAIN

The name could not be resolved at this time.

Future attempts may succeed.

.It Dv EAI_BADFLAGS

The flags had an invalid value.

.It Dv EAI_FAIL

A non-recoverable error occurred.

.It Dv EAI_FAMILY

The address family was not recognized or the address length was invalid

for the specified family.

.It Dv EAI_MEMORY

There was a memory allocation failure.

.It Dv EAI_NONAME

The name does not resolve for the supplied parameters.

.Dv NI_NAMEREQD

is set and the host's name cannot be located,

or both nodename and servname were null.

.It Dv EAI_SYSTEM

A system error occurred.

The error code can be found in errno.

.El

.\"

.Sh SEE ALSO

.Xr getaddrinfo 3 ,

.Xr gethostbyaddr 3 ,

.Xr getservbyport 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 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.

.Pp

.Ox

intentionally uses different

.Dv NI_MAXHOST

value from what RFC2553 suggests, to avoid buffer length handling mistakes.