GETNAMEINFO(3) FreeBSD Library Functions Manual GETNAMEINFO(3)
NAME
getnameinfo - socket address structure to hostname and service name
SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
int
getnameinfo(const struct sockaddr *sa, socklen_t salen, char *host,
size_t hostlen, char *serv, size_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.
If a link-layer address or UNIX-domain address is passed to
getnameinfo(), its ASCII representation will be stored in host. The
string pointed to by serv will be set to the empty string if non-NULL;
flags will always be ignored. For a link-layer address, this can be used
as a replacement of the legacy link_ntoa(3) function.
The sockaddr structure sa should point to either a sockaddr_in,
sockaddr_in6, sockaddr_dl, or sockaddr_un structure (for IPv4, IPv6,
link-layer, or UNIX-domain respectively) that is salen bytes long. If
salen is shorter than the length corresponding to the specified address
family or longer than sizeof(struct sockaddr_storage), it returns
EAI_FAMILY. Note that sa->sa_len should be consistent with salen though
the value of sa->sa_len is not directly used in this function.
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_NUMERICSERV The service name is returned as a digit string
representing the port number.
NI_NUMERICSCOPE The scope identifier is returned as a digit string.
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 RFC 4007. 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), link_ntoa(3), resolver(3), inet(4), inet6(4), unix(4),
hosts(5), resolv.conf(5), services(5), hostname(7), named(8)
R. Gilligan, S. Thomson, J. Bound, J. McCann, and W. Stevens, Basic
Socket Interface Extensions for IPv6, RFC 3493, February 2003.
S. Deering, B. Haberman, T. Jinmei, E. Nordmark, and B. Zill, IPv6 Scoped
Address Architecture, RFC 4007, March 2005.
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 1003.1-2004
("POSIX.1") specification and documented in RFC 3493, "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);
}
FreeBSD 13.1-RELEASE-p6 March 15, 2018 FreeBSD 13.1-RELEASE-p6
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