getnameinfo(3) UNIX Programmer's Manual getnameinfo(3)
NAME
getnameinfo - address-to-name translation in protocol-independent man-
ner
SYNOPSIS
#include<sys/socket.h>
#include<netdb.h>
intgetnameinfo(conststructsockaddr*sa,socklen_t salen,
char*host,size_t hostlen,
char*serv,size_t servlen,int flags);
DESCRIPTION
The getnameinfo(3) 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 getad-
drinfo(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 behaviour of getnameinfo(3) 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 a 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.
RETURNVALUE
On success 0 is returned, and node and service names, if requested, are
filled with NUL-terminated strings, possibly truncated to fit the spec-
ified buffer lengths. On error a nonzero value is returned, and errno
is set appropriately.
ERRORS
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_SYSTEM
A system error occurred. The error code can be found in errno.
FILES
/etc/hosts
/etc/nsswitch.conf
/etc/resolv.conf
NOTE
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 ser-
vices listed in the current Assigned Numbers RFC.
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 *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);
CONFORMINGTO
RFC 2553. (See also XNS, issue 5.2.)
SEEALSO
getaddrinfo(3), gethostbyaddr(3), getservbyname(3), getservbyport(3),
inet_ntop(3), socket(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-for-
mat-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/proceed-
ings/usenix2000/freenix/metzprotocol.html
Linux Man Page 2000-12-11 getnameinfo(3)
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