GETNAMEINFO
Section: Linux Programmer's Manual (3)
Updated: 2009-12-03
NAME
getnameinfo - address-to-name translation in protocol-independent manner
SYNOPSIS
#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);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
getnameinfo():
_POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_SOURCE
DESCRIPTION
The
getnameinfo()
function is the inverse of
getaddrinfo(3):
it converts a socket address to a corresponding host and service,
in a protocol-independent manner.
It combines the functionality of
gethostbyaddr(3)
and
getservbyport(3),
but unlike those functions,
getaddrinfo(3)
is reentrant and allows programs to eliminate
IPv4-versus-IPv6 dependencies.
The
sa
argument is a pointer to a generic socket address structure
(of type
sockaddr_in
or
sockaddr_in6)
of size
salen
that holds the input IP address and port number.
The arguments
host
and
serv
are pointers to caller-allocated buffers (of size
hostlen
and
servlen
respectively) into which
getnameinfo()
places null-terminated strings containing the host and
service names respectively.
The caller can specify that no hostname (or no service name)
is required by providing a NULL
host
(or
serv)
argument or a zero
hostlen
(or
servlen)
argument.
However, at least one of hostname or service name
must be requested.
The
flags
argument modifies the behavior of
getnameinfo()
as follows:
- NI_NAMEREQD
-
If set, then an error is returned if the hostname cannot be determined.
- NI_DGRAM
-
If set, then the service is datagram (UDP) based rather than
stream (TCP) based.
This is required for the few ports (512-514)
that have different services for UDP and TCP.
- NI_NOFQDN
-
If set, return only the hostname part of the fully qualified domain name
for local hosts.
- NI_NUMERICHOST
-
If set, then the numeric form of the hostname is returned.
(When not set, this will still happen in case the node's name
cannot be determined.)
- NI_NUMERICSERV
-
If set, then the numeric form of the service address is returned.
(When not set, this will still happen in case the service's name
cannot be determined.)
Extensions to getaddrinfo() for Internationalized Domain Names
Starting with glibc 2.3.4,
getnameinfo()
has been extended to selectively allow
hostnames to be transparently converted to and from the
Internationalized Domain Name (IDN) format (see RFC 3490,
Internationalizing Domain Names in Applications (IDNA)).
Three new flags are defined:
- NI_IDN
-
If this flag is used, then the name found in the lookup process is
converted from IDN format to the locale's encoding if necessary.
ASCII-only names are not affected by the conversion, which
makes this flag usable in existing programs and environments.
- NI_IDN_ALLOW_UNASSIGNED, NI_IDN_USE_STD3_ASCII_RULES
-
Setting these flags will enable the
IDNA_ALLOW_UNASSIGNED (allow unassigned Unicode code points) and
IDNA_USE_STD3_ASCII_RULES (check output to make sure it is a STD3
conforming hostname)
flags respectively to be used in the IDNA handling.
RETURN VALUE
On success 0 is returned, and node and service names, if requested,
are filled with null-terminated strings, possibly truncated to fit
the specified buffer lengths.
On error one of the following nonzero error codes is returned:
- EAI_AGAIN
-
The name could not be resolved at this time.
Try again later.
- EAI_BADFLAGS
-
The
flags
argument has an invalid value.
- EAI_FAIL
-
A nonrecoverable error occurred.
- EAI_FAMILY
-
The address family was not recognized,
or the address length was invalid for the specified family.
- EAI_MEMORY
-
Out of memory.
- EAI_NONAME
-
The name does not resolve for the supplied arguments.
NI_NAMEREQD
is set and the host's name cannot be located,
or neither hostname nor service name were requested.
- EAI_OVERFLOW
-
The buffer pointed to by
host
or
serv
was too small.
- EAI_SYSTEM
-
A system error occurred.
The error code can be found in
errno.
The
gai_strerror(3)
function translates these error codes to a human readable string,
suitable for error reporting.
FILES
/etc/hosts
/etc/nsswitch.conf
/etc/resolv.conf
VERSIONS
getnameinfo()
is provided in glibc since version 2.1.
CONFORMING TO
RFC 2553, POSIX.1-2001.
NOTES
In order to assist the programmer in choosing reasonable sizes
for the supplied buffers,
<netdb.h>
defines the constants
#define NI_MAXHOST 1025
#define NI_MAXSERV 32
Since glibc 2.8,
these definitions are exposed only if one of the feature test macros
_BSD_SOURCE,
_SVID_SOURCE,
or
_GNU_SOURCE
is defined.
The former is the constant
MAXDNAME
in recent versions of BIND's
<arpa/nameser.h>
header file.
The latter is a guess based on the services listed
in the current Assigned Numbers RFC.
EXAMPLE
The following code tries to get the numeric hostname and service name,
for a given socket address.
Note that there is no hardcoded reference to
a particular address family.
struct sockaddr *sa; /* input */
socklen_t len; /* input */
char hbuf[NI_MAXHOST], sbuf[NI_MAXSERV];
if (getnameinfo(sa, len, hbuf, sizeof(hbuf), sbuf,
sizeof(sbuf), NI_NUMERICHOST | NI_NUMERICSERV) == 0)
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 */
socklen_t len; /* input */
char hbuf[NI_MAXHOST];
if (getnameinfo(sa, len, hbuf, sizeof(hbuf),
NULL, 0, NI_NAMEREQD))
printf("could not resolve hostname");
else
printf("host=%s\n", hbuf);
An example program using
getnameinfo()
can be found in
getaddrinfo(3).
SEE ALSO
accept(2),
getpeername(2),
getsockname(2),
recvfrom(2),
socket(2),
getaddrinfo(3),
gethostbyaddr(3),
getservbyname(3),
getservbyport(3),
inet_ntop(3),
hosts(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.
Tatsuya Jinmei and Atsushi Onoe,
An Extension of Format for IPv6 Scoped Addresses,
internet draft, work in progress.
ftp://ftp.ietf.org/internet-drafts/draft-ietf-ipngwg-scopedaddr-format-02.txt
Craig Metz,
Protocol Independence Using the Sockets API,
Proceedings of the freenix track:
2000 USENIX annual technical conference, June 2000.
http://www.usenix.org/publications/library/proceedings/usenix2000/freenix/metzprotocol.html
COLOPHON
This page is part of release 3.27 of the Linux
man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- Extensions to getaddrinfo() for Internationalized Domain Names
-
- RETURN VALUE
-
- FILES
-
- VERSIONS
-
- CONFORMING TO
-
- NOTES
-
- EXAMPLE
-
- SEE ALSO
-
- COLOPHON
-
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Time: 07:35:13 GMT, March 26, 2013