350 lines
9.7 KiB
Groff
350 lines
9.7 KiB
Groff
.\" $NetBSD: getnameinfo.3,v 1.30 2003/09/07 16:22:21 wiz Exp $
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.\" $KAME: getnameinfo.3,v 1.27 2003/04/30 06:06:42 itojun Exp $
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.\"
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.\" Copyright (c) 1983, 1987, 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" From: @(#)gethostbyname.3 8.4 (Berkeley) 5/25/95
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.\"
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.Dd August 28, 2003
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.Dt GETNAMEINFO 3
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.Os
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.\"
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.Sh NAME
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.Nm getnameinfo
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.Nd address-to-nodename translation in protocol-independent manner
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.\"
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.Sh LIBRARY
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.Lb libc
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.\"
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.Sh SYNOPSIS
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.In netdb.h
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.Ft int
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.Fn getnameinfo "const struct sockaddr *sa" "socklen_t salen" \
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"char *host" "socklen_t hostlen" "char *serv" "socklen_t servlen" \
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"int flags"
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.\"
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.Sh DESCRIPTION
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The
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.Fn getnameinfo
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function is defined for protocol-independent address-to-nodename translation.
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Its functionality is a reverse conversion of
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.Xr getaddrinfo 3 ,
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and implements similar functionality to
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.Xr gethostbyaddr 3
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and
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.Xr getservbyport 3
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in a more sophisticated manner.
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.Pp
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This function looks up an IP address and port number provided by the
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caller in the DNS and system-specific database, and returns text
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strings for both in buffers provided by the caller.
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The function indicates successful completion by a zero return value;
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a non-zero return value indicates failure.
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.Pp
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The first argument,
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.Fa sa ,
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points to either a
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.Li sockaddr_in
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structure (for IPv4) or a
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.Li sockaddr_in6
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structure (for IPv6) that holds the IP address and port number.
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The
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.Fa salen
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argument gives the length of the
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.Li sockaddr_in
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or
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.Li sockaddr_in6
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structure.
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.Pp
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The function returns the nodename associated with the IP address in
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the buffer pointed to by the
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.Fa host
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argument.
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The caller provides the size of this buffer via the
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.Fa hostlen
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argument.
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The service name associated with the port number is returned in the buffer
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pointed to by
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.Fa serv ,
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and the
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.Fa servlen
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argument gives the length of this buffer.
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The caller specifies not to return either string by providing a zero
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value for the
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.Fa hostlen
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or
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.Fa servlen
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arguments.
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Otherwise, the caller must provide buffers large enough to hold the
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nodename and the service name, including the terminating null characters.
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.Pp
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Unfortunately most systems do not provide constants that specify the
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maximum size of either a fully-qualified domain name or a service name.
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Therefore to aid the application in allocating buffers for these two
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returned strings the following constants are defined in
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.Aq Pa netdb.h :
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.Bd -literal -offset
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#define NI_MAXHOST 1025
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#define NI_MAXSERV 32
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.Ed
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.Pp
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The first value is actually defined as the constant
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.Dv MAXDNAME
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in recent versions of BIND's
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.Aq Pa arpa/nameser.h
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header
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.Po
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older versions of BIND define this constant to be 256
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.Pc
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and the second is a guess based on the services listed in the current
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Assigned Numbers RFC.
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.Pp
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The final argument is a
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.Fa flag
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that changes the default actions of this function.
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By default the fully-qualified domain name (FQDN) for the host is
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looked up in the DNS and returned.
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If the flag bit
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.Dv NI_NOFQDN
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is set, only the nodename portion of the FQDN is returned for local hosts.
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.Pp
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If the
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.Fa flag
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bit
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.Dv NI_NUMERICHOST
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is set, or if the host's name cannot be located in the DNS,
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the numeric form of the host's address is returned instead of its name
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.Po
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e.g., by calling
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.Fn inet_ntop
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instead of
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.Fn gethostbyaddr
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.Pc .
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If the
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.Fa flag
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bit
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.Dv NI_NAMEREQD
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is set, an error is returned if the host's name cannot be located in the DNS.
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.Pp
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If the flag bit
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.Dv NI_NUMERICSERV
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is set, the numeric form of the service address is returned
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.Pq e.g., its port number
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instead of its name.
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The two
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.Dv NI_NUMERICxxx
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flags are required to support the
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.Fl n
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flag that many commands provide.
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.Pp
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A fifth flag bit,
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.Dv NI_DGRAM ,
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specifies that the service is a datagram service, and causes
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.Fn getservbyport
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to be called with a second argument of
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.Dq udp
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instead of its default of
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.Dq tcp .
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This is required for the few ports (512-514)
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that have different services for UDP and TCP.
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.Pp
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These
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.Dv NI_xxx
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flags are defined in
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.Aq Pa netdb.h .
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.\"
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.Ss Extension for scoped IPv6 address
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The implementation allows experimental numeric IPv6 address notation with
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scope identifier.
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IPv6 link-local address will appear as a string like
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.Dq Li fe80::1%ne0 .
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Refer to
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.Xr getaddrinfo 3
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for the notation.
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.\"
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.Sh EXAMPLES
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The following code tries to get a numeric hostname, and service name,
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for given socket address.
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Observe that there is no hardcoded reference to a particular address family.
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.Bd -literal -offset indent
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struct sockaddr *sa; /* input */
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char hbuf[NI_MAXHOST], sbuf[NI_MAXSERV];
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if (getnameinfo(sa, sa-\*[Gt]sa_len, hbuf, sizeof(hbuf), sbuf,
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sizeof(sbuf), NI_NUMERICHOST | NI_NUMERICSERV)) {
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errx(1, "could not get numeric hostname");
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/*NOTREACHED*/
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}
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printf("host=%s, serv=%s\\n", hbuf, sbuf);
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.Ed
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.Pp
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The following version checks if the socket address has reverse address mapping.
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.Bd -literal -offset indent
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struct sockaddr *sa; /* input */
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char hbuf[NI_MAXHOST];
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if (getnameinfo(sa, sa-\*[Gt]sa_len, hbuf, sizeof(hbuf), NULL, 0,
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NI_NAMEREQD)) {
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errx(1, "could not resolve hostname");
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/*NOTREACHED*/
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}
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printf("host=%s\\n", hbuf);
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.Ed
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.\"
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.Sh DIAGNOSTICS
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The function indicates successful completion by a zero return value;
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a non-zero return value indicates failure.
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Error codes are as below:
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.Bl -tag -width Er
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.It Dv EAI_AGAIN
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The name could not be resolved at this time.
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Future attempts may succeed.
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.It Dv EAI_BADFLAGS
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The flags had an invalid value.
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.It Dv EAI_FAIL
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A non-recoverable error occurred.
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.It Dv EAI_FAMILY
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The address family was not recognized or the address length was invalid
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for the specified family.
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.It Dv EAI_MEMORY
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There was a memory allocation failure.
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.It Dv EAI_NONAME
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The name does not resolve for the supplied parameters.
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.Dv NI_NAMEREQD
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is set and the host's name cannot be located,
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or both nodename and servname were null.
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.It Dv EAI_SYSTEM
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A system error occurred.
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The error code can be found in errno.
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.El
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.\"
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.Sh SEE ALSO
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.Xr getaddrinfo 3 ,
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.Xr gethostbyaddr 3 ,
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.Xr getservbyport 3 ,
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.Xr hosts 5 ,
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.Xr resolv.conf 5 ,
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.Xr services 5 ,
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.Xr hostname 7 ,
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.Xr named 8
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.Pp
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.Rs
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.%A R. Gilligan
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.%A S. Thomson
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.%A J. Bound
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.%A W. Stevens
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.%T Basic Socket Interface Extensions for IPv6
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.%R RFC 2553
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.%D March 1999
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.Re
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.Rs
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.%A Tatsuya Jinmei
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.%A Atsushi Onoe
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.%T "An Extension of Format for IPv6 Scoped Addresses"
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.%R internet draft
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.%N draft-ietf-ipngwg-scopedaddr-format-02.txt
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.%O work in progress material
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.Re
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.Rs
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.%A Craig Metz
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.%T Protocol Independence Using the Sockets API
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.%B "Proceedings of the freenix track: 2000 USENIX annual technical conference"
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.%D June 2000
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.Re
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.\"
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.Sh STANDARDS
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The
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.Fn getnameinfo
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function is defined in IEEE POSIX 1003.1g draft specification,
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and documented in
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.Dq Basic Socket Interface Extensions for IPv6
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.Pq RFC 2553 .
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.\"
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.Sh HISTORY
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The implementation first appeared in WIDE Hydrangea IPv6 protocol stack kit.
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.\"
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.Sh CAVEATS
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.Fn getnameinfo
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returns both numeric and FQDN notation of the address specified in
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.Fa sa .
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There is no return value that indicates if the string returned in
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.Fa host
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is a result of binary to numeric-text translation (like
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.Xr inet_ntop 3 ) ,
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or the result of DNS reverse lookup.
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Therefore, malicious parties could set up a PTR record like below:
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.Bd -literal -offset indent
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1.0.0.127.in-addr.arpa. IN PTR 10.1.1.1
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.Ed
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.Pp
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and trick the caller of
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.Fn getnameinfo
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into believing that
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.Fa sa
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is
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.Li 10.1.1.1
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when it actually is
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.Li 127.0.0.1 .
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.Pp
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To prevent such attacks, the use of
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.Dv NI_NAMEREQD
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is recommended when you use the result of
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.Fn getnameinfo
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for access control purposes:
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.Bd -literal -offset indent
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struct sockaddr *sa;
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socklen_t salen;
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char addr[NI_MAXHOST];
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struct addrinfo hints, *res;
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int error;
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error = getnameinfo(sa, salen, addr, sizeof(addr),
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NULL, 0, NI_NAMEREQD);
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if (error == 0) {
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memset(\*[Am]hints, 0, sizeof(hints));
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hints.ai_socktype = SOCK_DGRAM; /*dummy*/
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hints.ai_flags = AI_NUMERICHOST;
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if (getaddrinfo(addr, "0", \*[Am]hints, \*[Am]res) == 0) {
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/* malicious PTR record */
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freeaddrinfo(res);
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printf("bogus PTR record\\n");
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return -1;
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}
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/* addr is FQDN as a result of PTR lookup */
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} else {
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/* addr is numeric string */
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error = getnameinfo(sa, salen, addr, sizeof(addr),
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NULL, 0, NI_NUMERICHOST);
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}
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.Ed
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.\"
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.Sh BUGS
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The current implementation is not thread-safe.
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.Pp
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The text was shamelessly copied from RFC 2553.
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