GETNAMEINFO(3) Linux Programmers Manual GETNAMEINFO(3)
NAME
getnameinfo - address-to-name translation in protocol-independent man
ner
SYNOPSIS
#include
#include
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 the inverse of getaddrinfo(3): it con
verts a socket address to a corresponding host and service, in a proto
col-independent manner. It combines the functionality of gethost
byaddr(3) and getservbyport(3), but unlike those functions, getad
drinfo(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 nodes 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
services name cannot be determined.)
Extensions to getaddrinfo() for Internationalized Domain Names
Starting with glibc 2.3.4, getnameinfo() has been extended to selec
tively allow hostnames to be transparently converted to and from the
Internationalized Domain Name (IDN) format (see RFC 3490, Internation
alizing 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 locales encoding if neces
sary. ASCII-only names are not affected by the conversion,
which makes this flag usable in existing programs and environ
ments.
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 non-zero 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 non-recoverable 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 hosts 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, defines the constants
#define NI_MAXHOST 1025
#define NI_MAXSERV 32
The former is the constant MAXDNAME in recent versions of BINDs
header file. The latter is a guess based on the ser
vices 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 */
socklent_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 */
socklent_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 Inter
face 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/inter
net-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/proceed
ings/usenix2000/freenix/metzprotocol.html
COLOPHON
This page is part of release 3.05 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/.
GNU 2008-06-18 GETNAMEINFO(3)
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