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GETNAMEINFO(3) Linux Programmer's Manual GETNAMEINFO(3)
getnameinfo - address-to-name translation in protocol-independent
manner
#include <sys/socket.h>
#include <netdb.h>
int getnameinfo(const struct sockaddr *addr, socklen_t addrlen,
char *host, socklen_t hostlen,
char *serv, socklen_t servlen, int flags);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
getnameinfo():
Since glibc 2.22: _POSIX_C_SOURCE >= 201112L
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
getnameinfo() is provided in glibc since version 2.1.
For an explanation of the terms used in this section, see
attributes(7).
┌──────────────┬───────────────┬────────────────────┐
│Interface │ Attribute │ Value │
├──────────────┼───────────────┼────────────────────┤
│getnameinfo() │ Thread safety │ MT-Safe env locale │
└──────────────┴───────────────┴────────────────────┘
POSIX.1-2001, POSIX.1-2008, RFC 2553.
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 fea‐
ture 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.
Before glibc version 2.2, the hostlen and servlen arguments were
typed as size_t.
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 release 4.15 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
GNU 2017-09-15 GETNAMEINFO(3)
Pages that refer to this page: getaddrinfo(3), gethostbyname(3), getifaddrs(3), getipnodebyname(3), inet(3), inet_ntop(3)
Copyright and license for this manual page
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