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GETNAMEINFO(3) OpenBSD Programmer's Manual GETNAMEINFO(3)
getnameinfo - address-to-nodename translation in protocol-independent
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. Its functionality is a reverse conversion of
getaddrinfo(3), and implements similar functionality with
gethostbyaddr(3)and getservbyport(3) in more sophisticated manner.
This function looks up an IP address and port number provided by the
caller in the DNS and system-specific database, and returns text strings
for both in buffers provided by the caller. The function indicates suc-
cessful completion by a zero return value; a non-zero return value indi-
The first argument, sa, points to either a sockaddr_in structure (for
IPv4) or a sockaddr_in6 structure (for IPv6) that holds the IP address
and port number. The salen argument gives the length of the sockaddr_in
or sockaddr_in6 structure.
The function returns the nodename associated with the IP address in the
buffer pointed to by the host argument. The caller provides the size of
this buffer via the hostlen argument. The service name associated with
the port number is returned in the buffer pointed to by serv, and the
servlen argument gives the length of this buffer. The caller specifies
not to return either string by providing a zero value for the hostlen or
servlen arguments. Otherwise, the caller must provide buffers large
enough to hold the nodename and the service name, including the termi-
nating null characters.
Unfortunately most systems do not provide constants that specify the max-
imum size of either a fully-qualified domain name or a service name.
Therefore to aid the application in allocating buffers for these two re-
turned strings the following constants are defined in <netdb.h>:
#define NI_MAXHOST 1025
#define NI_MAXSERV 32
The first value is actually defined as the constant MAXDNAME in recent
versions of BIND's <arpa/nameser.h> header (older versions of BIND define
this constant to be 256) and the second is a guess based on the services
listed in the current Assigned Numbers RFC.
The final argument is a flag that changes the default actions of this
function. By default the fully-qualified domain name (FQDN) for the host
is looked up in the DNS and returned. If the flag bit NI_NOFQDN is set,
only the nodename portion of the FQDN is returned for local hosts.
If the flag bit NI_NUMERICHOST is set, or if the host's name cannot be
located in the DNS, the numeric form of the host's address is returned
instead of its name (e.g., by calling inet_ntop() instead of
getnodebyaddr()). If the flag bit NI_NAMEREQD is set, an error is re-
turned if the host's name cannot be located in the DNS.
If the flag bit NI_NUMERICSERV is set, the numeric form of the service
address is returned (e.g., its port number) instead of its name. The two
NI_NUMERICxxx flags are required to support the -n flag that many com-
A fifth flag bit, NI_DGRAM, specifies that the service is a datagram ser-
vice, and causes getservbyport() to be called with a second argument of
"udp" instead of its default of "tcp". This is required for the few ports
(512-514) that have different services for UDP and TCP.
These NI_xxx flags are defined in <netdb.h>.
The function indicates successful completion by a zero return value; a
non-zero return value indicates failure.
getaddrinfo(3), gethostbyaddr(3), getservbyport(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,'' RFC2553, March 1999.
The getaddrinfo() function is defined IEEE POSIX 1003.1g draft specifica-
tion, and documented in ``Basic Socket Interface Extensions for IPv6''
The text was shamelessly copied from RFC2553.
OpenBSD 2.6 May 25, 1995 2
Source: OpenBSD 2.6 man pages. Copyright: Portions are copyrighted by BERKELEY
SOFTWARE DESIGN, INC., The Regents of the University of California, Massachusetts
Institute of Technology, Free Software Foundation, FreeBSD Inc., and others.
(Corrections, notes, and links courtesy of RocketAware.com)
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