libc/net/getnameinfo.3

.\"	$KAME: getnameinfo.3,v 1.37 2005/01/05 03:23:05 itojun Exp $
.\"	$OpenBSD: getnameinfo.3,v 1.36 2004/12/21 09:48:20 jmc Exp $
.\"
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.\" $FreeBSD$
.\"
.Dd June 27, 2022
.Dt GETNAMEINFO 3
.Os
.Sh NAME
.Nm getnameinfo
.Nd socket address structure to hostname and service name
.Sh SYNOPSIS
.In sys/types.h
.In sys/socket.h
.In netdb.h
.Ft int
.Fo getnameinfo
.Fa "const struct sockaddr *sa" "socklen_t salen" "char *host"
.Fa "size_t hostlen" "char *serv" "size_t servlen" "int flags"
.Fc
.Sh DESCRIPTION
The
.Fn getnameinfo
function is used to convert a
.Li sockaddr
structure to a pair of host name and service strings.
It is a replacement for and provides more flexibility than the
.Xr gethostbyaddr 3
and
.Xr getservbyport 3
functions and is the converse of the
.Xr getaddrinfo 3
function.
.Pp
If a link-layer address or UNIX-domain address is passed to
.Fn getnameinfo ,
its ASCII representation will be stored in
.Fa host .
The string pointed to by
.Fa serv
will be set to the empty string if non-NULL;
.Fa flags
will always be ignored.
For a link-layer address,
this can be used as a replacement of the legacy
.Xr link_ntoa 3
function.
.Pp
The
.Li sockaddr
structure
.Fa sa
should point to either a
.Li sockaddr_in ,
.Li sockaddr_in6 ,
.Li sockaddr_dl ,
or
.Li sockaddr_un
structure
.Po for IPv4 ,
IPv6,
link-layer,
or UNIX-domain respectively
.Pc
that is
.Fa salen
bytes long.
If
.Fa salen
is shorter than the length corresponding to the specified
address family or longer than
.Fn sizeof "struct sockaddr_storage" ,
it returns
.Er EAI_FAMILY .
Note that
.Va sa->sa_len
should be consistent with
.Fa salen
though the value of
.Va sa->sa_len
is not directly used in this function.
.Pp
The host and service names associated with
.Fa sa
are stored in
.Fa host
and
.Fa serv
which have length parameters
.Fa hostlen
and
.Fa servlen .
The maximum value for
.Fa hostlen
is
.Dv NI_MAXHOST
and
the maximum value for
.Fa servlen
is
.Dv NI_MAXSERV ,
as defined by
.Aq Pa netdb.h .
If a length parameter is zero, no string will be stored.
Otherwise, enough space must be provided to store the
host name or service string plus a byte for the NUL terminator.
.Pp
The
.Fa flags
argument is formed by
.Tn OR Ns 'ing
the following values:
.Bl -tag -width "NI_NUMERICSCOPEXX"
.It Dv NI_NOFQDN
A fully qualified domain name is not required for local hosts.
The local part of the fully qualified domain name is returned instead.
.It Dv NI_NUMERICHOST
Return the address in numeric form, as if calling
.Xr inet_ntop 3 ,
instead of a host name.
.It Dv NI_NAMEREQD
A name is required.
If the host name cannot be found in DNS and this flag is set,
a non-zero error code is returned.
If the host name is not found and the flag is not set, the
address is returned in numeric form.
.It NI_NUMERICSERV
The service name is returned as a digit string representing the port number.
.It NI_NUMERICSCOPE
The scope identifier is returned as a digit string.
.It NI_DGRAM
Specifies that the service being looked up is a datagram
service, and causes
.Xr getservbyport 3
to be called with a second argument of
.Dq udp
instead of its default of
.Dq tcp .
This is required for the few ports (512\-514) that have different services
for
.Tn UDP
and
.Tn TCP .
.El
.Pp
This implementation allows numeric IPv6 address notation with scope identifier,
as documented in chapter 11 of RFC 4007.
IPv6 link-local address will appear as a string like
.Dq Li fe80::1%ne0 .
Refer to
.Xr getaddrinfo 3
for more information.
.Sh RETURN VALUES
.Fn getnameinfo
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 get a numeric host name, and service name,
for a 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 a 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 SEE ALSO
.Xr gai_strerror 3 ,
.Xr getaddrinfo 3 ,
.Xr gethostbyaddr 3 ,
.Xr getservbyport 3 ,
.Xr inet_ntop 3 ,
.Xr link_ntoa 3 ,
.Xr resolver 3 ,
.Xr inet 4 ,
.Xr inet6 4 ,
.Xr unix 4 ,
.Xr hosts 5 ,
.Xr resolv.conf 5 ,
.Xr services 5 ,
.Xr hostname 7
.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 RFC 4007
.%D March 2005
.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 by the
.St -p1003.1-2004
specification and documented in
.Tn "RFC 3493" ,
.Dq Basic Socket Interface Extensions for IPv6 .
.Sh CAVEATS
.Fn getnameinfo
can return both numeric and FQDN forms of the address specified in
.Fa sa .
There is no return value that indicates whether the string returned in
.Fa host
is a result of binary to numeric-text translation (like
.Xr inet_ntop 3 ) ,
or is the result of a DNS reverse lookup.
Because of this, malicious parties could set up a PTR record as follows:
.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 is actually
.Li 127.0.0.1 .
.Pp
To prevent such attacks, the use of
.Dv NI_NAMEREQD
is recommended when the result of
.Fn getnameinfo
is used
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\en");
		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
.\".Pp
.\".Ox
.\"intentionally uses a different
.\".Dv NI_MAXHOST
.\"value from what
.\".Tn "RFC 2553"
.\"suggests, to avoid buffer length handling mistakes.