getnameinfo(3) Library Functions Manual getnameinfo(3)
NAME
getnameinfo - address-to-name translation in protocol-independent man-
ner
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#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
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 since glibc 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 │
└─────────────────────────────────┴───────────────┴────────────────────┘
STANDARDS
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 2.2, the hostlen and servlen arguments were typed as
size_t.
EXAMPLES
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⟩.
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