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.)
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 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.
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