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376 lines
8.8 KiB
Groff
376 lines
8.8 KiB
Groff
.\" 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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.\" 4. 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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.\" $FreeBSD$
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.\"
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.Dd May 12, 2006
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.Dt GETHOSTBYNAME 3
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.Os
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.Sh NAME
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.Nm gethostbyname ,
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.Nm gethostbyname2 ,
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.Nm gethostbyaddr ,
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.Nm gethostent ,
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.Nm sethostent ,
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.Nm endhostent ,
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.Nm herror ,
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.Nm hstrerror
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.Nd get network host entry
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In netdb.h
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.Vt int h_errno ;
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.Ft struct hostent *
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.Fn gethostbyname "const char *name"
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.Ft struct hostent *
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.Fn gethostbyname2 "const char *name" "int af"
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.Ft struct hostent *
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.Fn gethostbyaddr "const void *addr" "socklen_t len" "int type"
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.Ft struct hostent *
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.Fn gethostent void
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.Ft void
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.Fn sethostent "int stayopen"
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.Ft void
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.Fn endhostent void
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.Ft void
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.Fn herror "const char *string"
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.Ft const char *
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.Fn hstrerror "int err"
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.Sh DESCRIPTION
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.Bf -symbolic
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The
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.Xr getaddrinfo 3
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and
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.Xr getnameinfo 3
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functions are preferred over the
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.Fn gethostbyname ,
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.Fn gethostbyname2 ,
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and
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.Fn gethostbyaddr
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functions.
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.Ef
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.Pp
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The
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.Fn gethostbyname ,
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.Fn gethostbyname2
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and
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.Fn gethostbyaddr
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functions
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each return a pointer to an object with the
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following structure describing an internet host
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referenced by name or by address, respectively.
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.Pp
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The
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.Fa name
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argument passed to
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.Fn gethostbyname
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or
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.Fn gethostbyname2
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should point to a
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.Dv NUL Ns -terminated
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hostname.
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The
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.Fa addr
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argument passed to
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.Fn gethostbyaddr
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should point to an address which is
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.Fa len
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bytes long,
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in binary form
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(i.e., not an IP address in human readable
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.Tn ASCII
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form).
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The
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.Fa type
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argument specifies the address family
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(e.g.\&
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.Dv AF_INET , AF_INET6 ,
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etc.) of this address.
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.Pp
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The structure returned contains either the information obtained from the name
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server,
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.Xr named 8 ,
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broken-out fields from a line in
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.Pa /etc/hosts ,
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or database entries supplied by the
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.Xr yp 8
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system.
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The order of the lookups is controlled by the
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.Sq hosts
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entry in
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.Xr nsswitch.conf 5 .
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.Bd -literal
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struct hostent {
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char *h_name; /* official name of host */
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char **h_aliases; /* alias list */
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int h_addrtype; /* host address type */
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int h_length; /* length of address */
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char **h_addr_list; /* list of addresses from name server */
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};
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#define h_addr h_addr_list[0] /* address, for backward compatibility */
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.Ed
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.Pp
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The members of this structure are:
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.Bl -tag -width h_addr_list
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.It Va h_name
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Official name of the host.
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.It Va h_aliases
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A
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.Dv NULL Ns -terminated
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array of alternate names for the host.
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.It Va h_addrtype
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The type of address being returned; usually
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.Dv AF_INET .
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.It Va h_length
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The length, in bytes, of the address.
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.It Va h_addr_list
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A
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.Dv NULL Ns -terminated
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array of network addresses for the host.
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Host addresses are returned in network byte order.
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.It Va h_addr
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The first address in
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.Va h_addr_list ;
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this is for backward compatibility.
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.El
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.Pp
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When using the nameserver,
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.Fn gethostbyname
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and
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.Fn gethostbyname2
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will search for the named host in the current domain and its parents
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unless the name ends in a dot.
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If the name contains no dot, and if the environment variable
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.Dq Ev HOSTALIASES
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contains the name of an alias file, the alias file will first be searched
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for an alias matching the input name.
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See
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.Xr hostname 7
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for the domain search procedure and the alias file format.
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.Pp
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The
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.Fn gethostbyname2
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function is an evolution of
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.Fn gethostbyname
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which is intended to allow lookups in address families other than
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.Dv AF_INET ,
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for example
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.Dv AF_INET6 .
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.Pp
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The
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.Fn sethostent
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function
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may be used to request the use of a connected
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.Tn TCP
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socket for queries.
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If the
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.Fa stayopen
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flag is non-zero,
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this sets the option to send all queries to the name server using
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.Tn TCP
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and to retain the connection after each call to
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.Fn gethostbyname ,
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.Fn gethostbyname2
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or
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.Fn gethostbyaddr .
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Otherwise, queries are performed using
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.Tn UDP
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datagrams.
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.Pp
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The
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.Fn endhostent
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function
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closes the
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.Tn TCP
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connection.
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.Pp
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The
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.Fn herror
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function writes a message to the diagnostic output consisting of the
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string argument
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.Fa string ,
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the constant string
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.Qq Li ":\ " ,
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and a message corresponding to the value of
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.Va h_errno .
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.Pp
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The
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.Fn hstrerror
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function returns a string which is the message text corresponding to the
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value of the
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.Fa err
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argument.
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.Sh FILES
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.Bl -tag -width /etc/nsswitch.conf -compact
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.It Pa /etc/hosts
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.It Pa /etc/nsswitch.conf
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.It Pa /etc/resolv.conf
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.El
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.Sh EXAMPLES
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Print out the hostname associated with a specific IP address:
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.Bd -literal -offset indent
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const char *ipstr = "127.0.0.1";
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struct in_addr ip;
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struct hostent *hp;
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if (!inet_aton(ipstr, &ip))
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errx(1, "can't parse IP address %s", ipstr);
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if ((hp = gethostbyaddr((const void *)&ip,
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sizeof ip, AF_INET)) == NULL)
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errx(1, "no name associated with %s", ipstr);
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printf("name associated with %s is %s\en", ipstr, hp->h_name);
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.Ed
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.Sh DIAGNOSTICS
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Error return status from
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.Fn gethostbyname ,
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.Fn gethostbyname2
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and
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.Fn gethostbyaddr
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is indicated by return of a
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.Dv NULL
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pointer.
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The integer
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.Va h_errno
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may then be checked to see whether this is a temporary failure
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or an invalid or unknown host.
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The routine
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.Fn herror
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can be used to print an error message describing the failure.
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If its argument
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.Fa string
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is
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.Pf non- Dv NULL ,
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it is printed, followed by a colon and a space.
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The error message is printed with a trailing newline.
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.Pp
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The variable
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.Va h_errno
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can have the following values:
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.Bl -tag -width HOST_NOT_FOUND
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.It Dv HOST_NOT_FOUND
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No such host is known.
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.It Dv TRY_AGAIN
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This is usually a temporary error
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and means that the local server did not receive
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a response from an authoritative server.
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A retry at some later time may succeed.
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.It Dv NO_RECOVERY
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Some unexpected server failure was encountered.
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This is a non-recoverable error.
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.It Dv NO_DATA
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The requested name is valid but does not have an IP address;
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this is not a temporary error.
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This means that the name is known to the name server but there is no address
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associated with this name.
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Another type of request to the name server using this domain name
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will result in an answer;
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for example, a mail-forwarder may be registered for this domain.
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.El
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.Sh SEE ALSO
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.Xr getaddrinfo 3 ,
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.Xr getnameinfo 3 ,
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.Xr inet_aton 3 ,
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.Xr resolver 3 ,
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.Xr hosts 5 ,
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.Xr hostname 7 ,
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.Xr named 8
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.Sh CAVEAT
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The
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.Fn gethostent
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function
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is defined, and
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.Fn sethostent
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and
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.Fn endhostent
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are redefined,
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when
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.Lb libc
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is built to use only the routines to lookup in
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.Pa /etc/hosts
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and not the name server.
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.Pp
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The
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.Fn gethostent
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function
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reads the next line of
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.Pa /etc/hosts ,
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opening the file if necessary.
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.Pp
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The
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.Fn sethostent
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function
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opens and/or rewinds the file
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.Pa /etc/hosts .
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If the
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.Fa stayopen
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argument is non-zero,
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the file will not be closed after each call to
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.Fn gethostbyname ,
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.Fn gethostbyname2
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or
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.Fn gethostbyaddr .
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.Pp
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The
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.Fn endhostent
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function
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closes the file.
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.Sh HISTORY
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The
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.Fn herror
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function appeared in
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.Bx 4.3 .
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The
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.Fn endhostent ,
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.Fn gethostbyaddr ,
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.Fn gethostbyname ,
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.Fn gethostent ,
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and
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.Fn sethostent
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functions appeared in
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.Bx 4.2 .
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The
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.Fn gethostbyname2
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function first appeared in
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.Tn BIND
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version 4.9.4.
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.Sh BUGS
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These functions use a thread-specific data storage;
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if the data is needed for future use, it should be
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copied before any subsequent calls overwrite it.
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.Pp
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Though these functions are thread-safe,
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still it is recommended to use the
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.Xr getaddrinfo 3
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family of functions, instead.
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.Pp
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Only the Internet
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address format is currently understood.
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