Name
getnameinfo — address-to-name translation in protocol-independent manner
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); |
![[Note]](../stylesheet/note.png) |
Note | ||||
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DESCRIPTION
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 sa 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.
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
flagsargument 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_NAMEREQDis set and the host's name cannot be located, or neither hostname nor service name were requested. EAI_OVERFLOW-
The buffer pointed to by
hostorservwas 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.
ATTRIBUTES
For an explanation of the terms used in this section, see attributes(7).
| Interface | Attribute | Value |
getnameinfo() |
Thread safety | MT-Safe env locale |
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 services 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 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).
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 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
COLOPHON
This page is part of release 4.07 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/.
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%%%LICENSE_START(PUBLIC_DOMAIN) This page is in the public domain. %%%LICENSE_END Almost all details are from RFC 2553. 2004-12-14, mtk, Added EAI_OVERFLOW error 2004-12-14 Fixed description of error return |