getnameinfo — address-to-name translation in protocol-independent manner
#include <sys/socket.h> #include <netdb.h>
int
getnameinfo( |
const struct sockaddr * | sa, |
socklen_t | salen, | |
char * | host, | |
size_t | hostlen, | |
char * | serv, | |
size_t | servlen, | |
int | flags) ; |
The getnameinfo
() function
is defined for protocol-independent address-to-nodename
translation. It combines the functionality of gethostbyaddr(3) and
getservbyport(3) and is the
inverse of getaddrinfo(3). The
sa
argument is a
pointer to a generic socket address structure (of type
sockaddr_in or sockaddr_in6) of size salen
that holds the input IP
address and port number. The arguments host
and serv
are pointers to buffers
(of size hostlen
and
servlen
respectively)
to hold the return values.
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
) parameter.
However, at least one of hostname or service name must be
requested.
The flags
argument
modifies the behavior of getnameinfo
() as follows:
NI_NOFQDN
If set, return only the hostname part of the FQDN 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 looked up.)
NI_NAMEREQD
If set, then an error is returned if the hostname cannot be looked up.
NI_NUMERICSERV
If set, then the service address is returned in numeric form, for example by its port number.
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.
Starting with glibc 2.3.4, getnameinfo
() has been extended to
selectively allow host names 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 host name) 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
parameter has an invalid value.
EAI_FAIL
A non-recoverable 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
parameters. 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.
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
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.
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 *sa; /* input */ char hbuf[NI_MAXHOST], sbuf[NI_MAXSERV]; if (getnameinfo(sa, sa−>sa_len, 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 *sa; /* input */ char hbuf[NI_MAXHOST]; if (getnameinfo(sa, sa−>sa_len, 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).
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 2.79 of the Linux man-pages
project. A
description of the project, and information about reporting
bugs, can be found at
http://www.kernel.org/doc/man-pages/.
This page is in the public domain. Almost all details are from RFC 2553. 2004-12-14, mtk, Added EAI_OVERFLOW error 2004-12-14 Fixed description of error return |