NAME
getnameinfo —
socket address structure
to hostname and service name
SYNOPSIS
#include <netdb.h>
int
getnameinfo(
const
struct sockaddr * restrict sa,
socklen_t salen,
char * restrict host,
socklen_t hostlen,
char * restrict serv,
socklen_t servlen,
int flags);
DESCRIPTION
The
getnameinfo() function is used to convert a
sockaddr
structure to a pair of host name and service
strings. It is a replacement for and provides more flexibility than the
gethostbyaddr(3) and
getservbyport(3)
functions and is the converse of the
getaddrinfo(3) function.
The
sockaddr
structure
sa should
point to a
sockaddr_in
(for IPv4),
sockaddr_in6
(for IPv6),
sockaddr_atalk
(for AppleTalk),
sockaddr_link
(for link layer), or
sockaddr_local
(for local/unix) structures that are
salen bytes long.
The host and service names associated with
sa are stored
in
host and
serv which have length
parameters
hostlen and
servlen.
The maximum value for
hostlen is
NI_MAXHOST
and the maximum value for
servlen is
NI_MAXSERV
, as
defined by
<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.
The
flags argument is formed by
OR'ing
the following values:
-
-
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.
-
-
NI_NUMERICHOST
- Return the address in numeric form, as if calling
inet_ntop(3), instead of
a host name.
-
-
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.
-
-
- NI_NUMERICSCOPE
- For IPv6 addresses the numeric form of the IPv6 scope
identifier is returned. This flag is ignored for non-IPv6 addresses.
-
-
- NI_NUMERICSERV
- The service name is returned as a digit string representing
the port number.
-
-
- NI_DGRAM
- Specifies that the service being looked up is a datagram
service, and causes
getservbyport(3) to
be called with a second argument of “udp” instead of its
default of “tcp”. This is required for the few ports (512-514)
that have different services for UDP and TCP.
This implementation allows numeric IPv6 address notation with scope identifier,
as documented in chapter 11 of draft-ietf-ipv6-scoping-arch-02.txt. IPv6
link-local address will appear as a string like
“
fe80::1%ne0
”. Refer to
getaddrinfo(3) for more
information.
RETURN VALUES
getnameinfo() returns zero on success or one of the error
codes listed in
gai_strerror(3) if an
error occurs.
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.
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\n", hbuf, sbuf);
The following version checks if the socket address has a reverse address
mapping:
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\n", hbuf);
SEE ALSO
gai_strerror(3),
getaddrinfo(3),
gethostbyaddr(3),
getservbyport(3),
inet_ntop(3),
resolver(3),
hosts(5),
resolv.conf(5),
services(5),
hostname(7),
named(8)
R. Gilligan, S.
Thomson, J. Bound, and W.
Stevens, Basic Socket Interface Extensions for
IPv6, RFC 2553, March
1999.
S. Deering, B.
Haberman, T. Jinmei, E.
Nordmark, and B. Zill, IPv6
Scoped Address Architecture, internet draft,
draft-ietf-ipv6-scoping-arch-02.txt,
work in progress material.
Craig Metz, Protocol
Independence Using the Sockets API, Proceedings of the
FREENIX track: 2000 USENIX annual technical conference,
June 2000.
STANDARDS
The
getnameinfo() function is defined by the
IEEE Std 1003.1g-2000 (“POSIX.1g”) draft
specification and documented in
RFC 2553, “Basic
Socket Interface Extensions for IPv6”.
CAVEATS
getnameinfo() can return both numeric and FQDN forms of the
address specified in
sa. There is no return value that
indicates whether the string returned in
host is a
result of binary to numeric-text translation (like
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:
1.0.0.127.in-addr.arpa. IN PTR 10.1.1.1
and trick the caller of
getnameinfo() into believing that
sa is
10.1.1.1
when it is
actually
127.0.0.1
.
To prevent such attacks, the use of
NI_NAMEREQD
is
recommended when the result of
getnameinfo() is used for
access control purposes:
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);
}