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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | FILES | ATTRIBUTES | STANDARDS | HISTORY | NOTES | EXAMPLES | SEE ALSO | COLOPHON |
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getnameinfo(3) Library Functions Manual getnameinfo(3)
getnameinfo - address-to-name translation in protocol-independent
manner
Standard C library (libc, -lc)
#include <sys/socket.h>
#include <netdb.h>
int getnameinfo(const struct sockaddr *restrict addr, socklen_t addrlen,
char host[_Nullable restrict .hostlen],
socklen_t hostlen,
char serv[_Nullable restrict .servlen],
socklen_t servlen,
int flags);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
getnameinfo():
Since glibc 2.22:
_POSIX_C_SOURCE >= 200112L
glibc 2.21 and earlier:
_POSIX_C_SOURCE
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, getnameinfo() is reentrant and allows programs to
eliminate IPv4-versus-IPv6 dependencies.
The addr argument is a pointer to a generic socket address
structure (of type sockaddr_in or sockaddr_in6) of size addrlen
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 getnameinfo() 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.
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.
/etc/hosts
/etc/nsswitch.conf
/etc/resolv.conf
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────────────────┬───────────────┬────────────────────┐
│ Interface │ Attribute │ Value │
├──────────────────────────┼───────────────┼────────────────────┤
│ getnameinfo() │ Thread safety │ MT-Safe env locale │
└──────────────────────────┴───────────────┴────────────────────┘
POSIX.1-2008. RFC 2553.
glibc 2.1. POSIX.1-2001.
Before glibc 2.2, the hostlen and servlen arguments were typed as
size_t.
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 suitable
feature test macros are defined, namely: _GNU_SOURCE,
_DEFAULT_SOURCE (since glibc 2.19), or (in glibc versions up to
and including 2.19) _BSD_SOURCE or _SVID_SOURCE.
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.
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 *addr; /* input */
socklen_t addrlen; /* input */
char hbuf[NI_MAXHOST], sbuf[NI_MAXSERV];
if (getnameinfo(addr, addrlen, 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 *addr; /* input */
socklen_t addrlen; /* input */
char hbuf[NI_MAXHOST];
if (getnameinfo(addr, addrlen, 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).
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⟩.
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
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Linux man-pages 6.9.1 2024-06-15 getnameinfo(3)
Pages that refer to this page: getaddrinfo(3), gethostbyname(3), getifaddrs(3), getipnodebyname(3), inet(3), inet_ntop(3), sockaddr(3type)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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