getnameinfo
GETNAMEINFO(3) Linux Programmer's Manual GETNAMEINFO(3)
NAME
getnameinfo - address-to-name translation in protocol-independent man-
ner
SYNOPSIS
#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
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, getname-
info() 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 re-
spectively) 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 re-
quired 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 de-
termined.
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 do-
main 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 re-
turned. (When not set, this will still happen in case the ser-
vice's name cannot be determined.)
Extensions to getnameinfo() 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 de-
fined:
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 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 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.
ATTRIBUTES
For an explanation of the terms used in this section, see at-
tributes(7).
+--------------+---------------+--------------------+
|Interface | Attribute | Value |
+--------------+---------------+--------------------+
|getnameinfo() | Thread safety | MT-Safe env locale |
+--------------+---------------+--------------------+
CONFORMING TO
POSIX.1-2001, POSIX.1-2008, RFC 2553.
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 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 ser-
vices listed in the current Assigned Numbers RFC.
Before glibc version 2.2, the hostlen and servlen arguments were typed
as size_t.
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 *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 ad-
dress 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).
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
/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
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