GETNAMEINFO(3) BSD Library Functions Manual GETNAMEINFO(3)
NAME
getnameinfo -- socket address structure to hostname and service name
SYNOPSIS
#include <sys/socket.h>
#include <netdb.h>
int
getnameinfo(const struct sockaddr *restrict sa, socklen_t salen, char *restrict node,
socklen_t nodelen, char *restrict service, socklen_t servicelen, 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 either a sockaddr_in or sockaddr_in6 structure (for IPv4 or
IPv6 respectively) that is salen bytes long.
The host and service names associated with sa are stored in node and service which have length parame-ters parameters
ters nodelen and servicelen. The maximum value for nodelen is NI_MAXHOST and the maximum value for
servicelen 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_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 chap-ter chapter
ter 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);
LEGACY SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
The include file <sys/types.h> is necessary.
int getnameinfo(const struct sockaddr *restrict sa, socklen_t salen, char *restrict node,
size_t nodelen, char *service, size_t servicelen, int flags);
The type of nodelen and servicelen has changed.
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.1'') 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);
}
BSD December 20, 2004 BSD
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