.\"	$OpenBSD: getnameinfo.3,v 1.27 2003/08/28 10:16:38 jmc 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 <sys/types.h>
.Fd #include <sys/socket.h>
.Fd #include <netdb.h>
.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 to
.Xr gethostbyaddr 3
and
.Xr getservbyport 3
in a 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 a string like
.Dq Li fe80::1%ne0 .
Refer to
.Xr getaddrinfo 3
for the notation.
.\"
.Sh EXAMPLES
The following code tries to get a numeric hostname, and service name,
for given socket address.
Observe that there is no hardcoded reference to a 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 RFC 2553
.%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 STANDARDS
The
.Fn getnameinfo
function is defined in IEEE POSIX 1003.1g draft specification,
and documented in
.Dq Basic Socket Interface Extensions for IPv6
.Pq RFC 2553 .
.\"
.Sh HISTORY
The implementation first appeared in WIDE Hydrangea IPv6 protocol stack kit.
.\"
.Sh CAVEATS
.Fn getnameinfo
returns both numeric and FQDN notation of the address specified in
.Fa sa .
There is no return value that indicates if the string returned in
.Fa host
is a result of binary to numeric-text translation (like
.Xr inet_ntop 3 ) ,
or the result of DNS reverse lookup.
Therefore, malicious parties could set up a PTR record as below:
.Bd -literal -offset indent
1.0.0.127.in-addr.arpa. IN PTR  10.1.1.1
.Ed
.Pp
and trick the caller of
.Fn getnameinfo
into believing that
.Fa sa
is
.Li 10.1.1.1
when it actually is
.Li 127.0.0.1 .
.Pp
To prevent such attacks, the use of
.Dv NI_NAMEREQD
is recommended when you use the result of
.Fn getnameinfo
for access control purposes:
.Bd -literal -offset indent
struct sockaddr *sa;
socklen_t salen;
char addr[NI_MAXHOST];
struct addrinfo hints, *res;
int error;

error = getnameinfo(sa, salen, addr, sizeof(addr),
    NULL, 0, NI_NAMEREQD);
if (error == 0) {
	memset(&hints, 0, sizeof(hints));
	hints.ai_socktype = SOCK_DGRAM;	/*dummy*/
	hints.ai_flags = AI_NUMERICHOST;
	if (getaddrinfo(addr, "0", &hints, &res) == 0) {
		/* malicious PTR record */
		freeaddrinfo(res);
		printf("bogus PTR record\\n");
		return -1;
	}
	/* addr is FQDN as a result of PTR lookup */
} else {
	/* addr is numeric string */
	error = getnameinfo(sa, salen, addr, sizeof(addr),
	    NULL, 0, NI_NUMERICHOST);
}
.Ed
.\"
.Sh BUGS
The current implementation is not thread-safe.
.Pp
The text was shamelessly copied from RFC 2553.
.Pp
.Ox
intentionally uses a different
.Dv NI_MAXHOST
value from what RFC 2553 suggests, to avoid buffer length handling mistakes.