getnameinfo - address-to-nodename translation in protocol-independent
manner
Standard C Library (libc, -lc)
#include <netdb.h>
int
getnameinfo(const struct sockaddr *sa, socklen_t salen, char *host,
socklen_t hostlen, char *serv, socklen_t servlen, int flags);
The getnameinfo() function is defined for protocol-independent addressto-nodename
translation. Its functionality is a reverse conversion of
getaddrinfo(3), and implements similar functionality with
gethostbyaddr(3) and getservbyport(3) in more sophisticated manner.
This function looks up an IP address and port number provided by the
caller in the DNS and system-specific database, and returns text strings
for both in buffers provided by the caller. The function indicates successful
completion by a zero return value; a non-zero return value indicates
failure.
The first argument, sa, points to either a sockaddr_in structure (for
IPv4) or a sockaddr_in6 structure (for IPv6) that holds the IP address
and port number. The salen argument gives the length of the sockaddr_in
or sockaddr_in6 structure.
The function returns the nodename associated with the IP address in the
buffer pointed to by the host argument. The caller provides the size of
this buffer via the hostlen argument. The service name associated with
the port number is returned in the buffer pointed to by serv, and the
servlen argument gives the length of this buffer. The caller specifies
not to return either string by providing a zero value for the hostlen or
servlen arguments. Otherwise, the caller must provide buffers large
enough to hold the nodename and the service name, including the terminating
null characters.
Unfortunately most systems do not provide constants that specify the maximum
size of either a fully-qualified domain name or a service name.
Therefore to aid the application in allocating buffers for these two
returned strings the following constants are defined in <netdb.h>:
#define NI_MAXHOST 1025
#define NI_MAXSERV 32
The first value is actually defined as the constant MAXDNAME in recent
versions of BIND's <arpa/nameser.h> header (older versions of BIND define
this constant to be 256) and the second is a guess based on the services
listed in the current Assigned Numbers RFC.
The final argument is a flag that changes the default actions of this
function. By default the fully-qualified domain name (FQDN) for the host
is looked up in the DNS and returned. If the flag bit NI_NOFQDN is set,
only the nodename portion of the FQDN is returned for local hosts.
If the flag bit NI_NUMERICHOST is set, or if the host's name cannot be
located in the DNS, the numeric form of the host's address is returned
instead of its name (e.g., by calling inet_ntop() instead of
gethostbyaddr()). If the flag bit NI_NAMEREQD is set, an error is
returned if the host's name cannot be located in the DNS.
If the flag bit NI_NUMERICSERV is set, the numeric form of the service
address is returned (e.g., its port number) instead of its name. The two
NI_NUMERICxxx flags are required to support the -n flag that many commands
provide.
A fifth flag bit, NI_DGRAM, specifies that the service is a datagram service,
and causes getservbyport() 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.
These NI_xxx flags are defined in <netdb.h>.
Extension for scoped IPv6 address [Toc] [Back]
The implementation allows experimental numeric IPv6 address notation with
scope identifier. IPv6 link-local address will appear as string like
``fe80::1%ne0''. Refer to getaddrinfo(3) for the notation.
The following code tries to get numeric hostname, and service name, for
given socket address. Observe that there is no hardcoded reference to
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 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);
The function indicates successful completion by a zero return value; a
non-zero return value indicates failure. Error codes are as below:
EAI_AGAIN The name could not be resolved at this time. Future
attempts may succeed.
EAI_BADFLAGS The flags had an invalid value.
EAI_FAIL A non-recoverable error occurred.
EAI_FAMILY The address family was not recognized or the address
length was invalid for the specified family.
EAI_MEMORY There was a memory allocation failure.
EAI_NONAME The name does not resolve for the supplied parameters.
NI_NAMEREQD is set and the host's name cannot be
located, or both nodename and servname were null.
EAI_SYSTEM A system error occurred. The error code can be found
in errno.
getaddrinfo(3), gethostbyaddr(3), getservbyport(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, RFC2553, March 1999.
Tatsuya Jinmei and Atsushi Onoe, An Extension of Format for IPv6 Scoped
Addresses, internet draft, draft-ietf-ipngwg-scopedaddr-format-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 IEEE POSIX 1003.1g draft specification,
and documented in ``Basic Socket Interface Extensions for IPv6''
(RFC2553).
The implementation first appeared in WIDE Hydrangea IPv6 protocol stack
kit.
The current implementation is not thread-safe.
The text was shamelessly copied from RFC2553.
BSD May 25, 1995 BSD
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