GETNAMEINFO(3) BSD Programmer's Manual GETNAMEINFO(3)
getnameinfo - socket address structure to hostname and service name
#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);
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 pro-
vides 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 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 re-
turned. 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 da-
tagram 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 iden-
tifier, 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.
getnameinfo() returns zero on success or one of the error codes listed in
gai_strerror(3) if an error occurs.
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);
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.
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".
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);
}
The implementation of getnameinfo() is not thread-safe.
OpenBSD intentionally uses a different NI_MAXHOST value from what RFC
2553 suggests, to avoid buffer length handling mistakes.
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