247 lines
6.8 KiB
Groff
247 lines
6.8 KiB
Groff
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.\" Copyright (c) 1983, 1987 The Regents of the University of California.
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms are permitted provided
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.\" that: (1) source distributions retain this entire copyright notice and
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.\" comment, and (2) distributions including binaries display the following
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.\" acknowledgement: ``This product includes software developed by the
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.\" University of California, Berkeley and its contributors'' in the
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.\" documentation or other materials provided with the distribution and in
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.\" all advertising materials mentioning features or use of this software.
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.\" Neither the name of the University nor the names of its contributors may
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.\" be used to endorse or promote products derived from this software without
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.\" specific prior written permission.
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.\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED
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.\" WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
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.\" MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
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.\"
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.\" @(#)gethostbyname.3 6.12 (Berkeley) 6/23/90
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.\"
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.Dd June 23, 1990
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.Dt GETHOSTBYNAME @LIB_NETWORK_EXT_U@
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.Os BSD 4
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.Sh NAME
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.Nm gethostbyname ,
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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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.Nd get network host entry
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.Sh SYNOPSIS
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.Fd #include <netdb.h>
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.Ft extern int
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.Fa h_errno;
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.Pp
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.Ft struct hostent *
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.Fn gethostbyname "char *name";
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.Ft struct hostent *
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.Fn gethostbyname2 "char *name" "int af";
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.Ft struct hostent *
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.Fn gethostbyaddr "char *addr" "int len, type";
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.Ft struct hostent *
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.Fn gethostent
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.Fn sethostent "int stayopen";
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.Fn endhostent
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.Fn herror "char *string";
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.Sh DESCRIPTION
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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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each return a pointer to a
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.Ft hostent
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structure (see below) describing an internet host
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referenced by name or by address, as the function names indicate.
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This structure contains either the information obtained from the name server,
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.Xr @INDOT@named @SYS_OPS_EXT@ ,
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or broken-out fields from a line in
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.Pa /etc/hosts .
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If the local name server is not running, these routines do a lookup in
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.Pa /etc/hosts .
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.Bd -literal -offset indent
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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 h_name
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Official name of the host.
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.It h_aliases
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A zero-terminated array of alternate names for the host.
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.It 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 h_length
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The length, in bytes, of the address.
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.It h_addr_list
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A zero-terminated array of network addresses for the host.
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Host addresses are returned in network byte order.
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.It h_addr
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The first address in
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.Li 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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will search for the named host in each parent domain given in the
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.Dq Li search
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directive of
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.Xr resolv.conf @FORMAT_EXT@
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unless the name contains a dot
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.Pq Dq \&. .
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If the name contains no dot, and if the environment variable
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.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 @DESC_EXT@
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for the domain search procedure and the alias file format.
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.Pp
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.Fn Gethostbyname2
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is an evolution of
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.Fn gethostbyname
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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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Currently, the
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.Fa af
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argument must be specified as
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.Dv AF_INET
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else the function will return
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.Dv NULL
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after having set
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.Ft h_errno
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to
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.Dv NETDB_INTERNAL .
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.Pp
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.Fn Sethostent
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may be used to request the use of a connected TCP 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 TCP
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and to retain the connection after each call to
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.Fn gethostbyname
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or
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.Fn gethostbyaddr .
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Otherwise, queries are performed using UDP datagrams.
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.Pp
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.Fn Endhostent
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closes the TCP connection.
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.Sh ENVIRONMENT
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.Bl -tag -width "HOSTALIASES " -compress
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.It Ev HOSTALIASES
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Name of file containing
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.Pq Ar host alias , full hostname
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pairs.
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.El
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.Sh FILES
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.Bl -tag -width "HOSTALIASES " -compress
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.It Pa /etc/hosts
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See
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.Xr hosts @FORMAT_EXT@ .
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.It Ev HOSTALIASES
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Name of file containing
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.Pq Ar host alias , full hostname
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pairs.
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.El
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.Sh DIAGNOSTICS
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.Pp
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Error return status from
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.Fn gethostbyname
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and
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.Fn gethostbyaddr
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is indicated by return of a null pointer.
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The external integer
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.Ft 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 non-NULL, 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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.Ft h_errno
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can have the following values:
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.Bl -tag -width "HOST_NOT_FOUND " -offset indent
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.It Dv NETDB_INTERNAL
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This indicates an internal error in the library, unrelated to the network
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or name service.
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.Ft errno
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will be valid in this case; see
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.Xr perror @SYSCALL_EXT@ .
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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, as one might expect.
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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 hosts @FORMAT_EXT@ ,
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.Xr hostname @DESC_EXT@ ,
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.Xr @INDOT@named @SYS_OPS_EXT@ ,
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.Xr resolver @LIB_NETWORK_EXT@ ,
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.Xr resolver @FORMAT_EXT@ .
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.Sh CAVEAT
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.Pp
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.Fn Gethostent
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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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.Pa 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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.Bd -filled -offset indent
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.Pp
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.Fn Gethostent
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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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.Fn Sethostent
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is redefined to open and rewind the file. If the
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.Fa stayopen
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argument is non-zero,
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the hosts data base will not be closed after each call to
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.Fn gethostbyname
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or
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.Fn gethostbyaddr .
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.Pp
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.Fn Endhostent
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is redefined to close the file.
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.Ed
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.Sh BUGS
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All information is contained in a static area so it must be copied if it is
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to be saved. Only the Internet address format is currently understood.
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