path: root/lib/libc/net/getnameinfo.3

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.\"     From: @(#)gethostbyname.3	8.4 (Berkeley) 5/25/95
.\"     $Id: getnameinfo.3,v 1.2 1999/07/05 04:40:59 aaron Exp $
.\"
.Dd May 25, 1995
.Dt GETNAMEINFO 3
.Os
.Sh NAME
.Nm getnameinfo
.Nd address-to-nodename translation in protocol-independent manner
.Sh SYNOPSIS
.Fd #include <sys/socket.h>
.Fd #include <netdb.h>
.Ft int
.Fn getnameinfo "const struct sockaddr *sa" "socklen_t salen" \
"char *host" "size_t hostlen" "char *serv" "size_t servlen" "int flags"
.Sh DESCRIPTION
The
.Fn getnameinfo
function is defined for protocol-independent address-to-nodename translation.
Its functionality is a reverse conversion of
.Xr getaddrinfo 3 ,
and implements similar functionality with
.Xr gethostbyaddr 3 and
.Xr getservbyport 3
in more sophisticated manner.
.Pp
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 successful completion by a zero return value;
a non-zero return value indicates failure.
.Pp
The first argument,
.Fa sa ,
points to either a
.Li sockaddr_in
structure (for IPv4) or a
.Li sockaddr_in6
structure (for IPv6) that holds the IP address and port number.
The
.Fa salen
argument gives the length of the
.Li sockaddr_in
or
.Li sockaddr_in6
structure.
.Pp
The function returns the nodename associated with the IP address in
the buffer pointed to by the
.Fa host
argument.
The caller provides the size of this buffer via the
.Fa hostlen
argument.
The service name associated with the port number is returned in the buffer
pointed to by
.Fa serv ,
and the
.Fa servlen
argument gives the length of this buffer.
The caller specifies not to return either string by providing a zero
value for the
.Fa hostlen
or
.Fa servlen
arguments.
Otherwise, the caller must provide buffers large enough to hold the
nodename and the  service name, including the terminating null characters.
.Pp
Unfortunately most systems do not provide constants that specify the
maximum size of either a fully-qualified domain name or a service name.
Therefore to aid the application in allocating buffers for these two
returned strings the following constants are defined in
.Aq Pa netdb.h :
.Bd -literal -offset
#define NI_MAXHOST  1025
#define NI_MAXSERV    32
.Ed
.Pp
The first value is actually defined as the constant
.Dv MAXDNAME
in recent versions of BIND's
.Aq Pa 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.
.Pp
The final argument is a
.Fa 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
.Dv NI_NOFQDN
is set, only the nodename portion of the FQDN is returned for local hosts.
.Pp
If the
.Fa flag
bit
.Dv 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
.Po
e.g., by calling
.Fn inet_ntop
instead of
.Fn getnodebyaddr
.Pc .
If the
.Fa flag
bit
.Dv NI_NAMEREQD
is set, an error is returned if the host's name cannot be located in the DNS.
.Pp
If the flag bit
.Dv NI_NUMERICSERV
is set, the numeric form of the service address is returned
.Pq e.g., its port number
instead of its name.
The two
.Dv NI_NUMERICxxx
flags are required to support the
.Fl n
flag that many commands provide.
.Pp
A fifth flag bit,
.Dv NI_DGRAM ,
specifies that the service is a datagram service, and causes
.Fn getservbyport
to be called with a second argument of
.Qq udp
instead of its default of
.Qq tcp .
This is required for the few ports (512-514)
that have different services for UDP and TCP.
.Pp
These
.Dv NI_xxx
flags are defined in
.Aq Pa netdb.h .
.Sh FILES
.Bl -tag -width /etc/resolv.conf -compact
.It Pa /etc/hosts
.It Pa /etc/host.conf
.It Pa /etc/resolv.conf
.El
.Sh DIAGNOSTICS
The function indicates successful completion by a zero return value;
a non-zero return value indicates failure.
.Sh SEE ALSO
.Xr getaddrinfo 3 ,
.Xr gethostbyaddr 3 ,
.Xr getservbyport 3 ,
.Xr hosts 5 ,
.Xr services 5 ,
.Xr hostname 7 ,
.Xr named 8
.Pp
R. Gilligan, S.  Thomson, J. Bound, and W. Stevens,
``Basic Socket Interface Extensions for IPv6,'' RFC2553, March 1999.
.Sh STANDARDS
The
.Fn getaddrinfo
function is defined IEEE POSIX 1003.1g draft specification,
and documented in
.Dq Basic Socket Interface Extensions for IPv6
.Pq RFC2533 .
.Sh BUGS
The text was shamelessly copied from RFC2553.