mirror of https://github.com/mkerrisk/man-pages
219 lines
6.1 KiB
Groff
219 lines
6.1 KiB
Groff
.\" Copyright (c) 1997 Martin Schulze (joey@infodrom.north.de)
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.\" Much of the text is copied from the manpage of resolv+(8).
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.\"
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.\" %%%LICENSE_START(GPLv2+_DOC_FULL)
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.\" This is free documentation; you can redistribute it and/or
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.\" modify it under the terms of the GNU General Public License as
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.\" published by the Free Software Foundation; either version 2 of
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.\" the License, or (at your option) any later version.
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.\"
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.\" The GNU General Public License's references to "object code"
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.\" and "executables" are to be interpreted as the output of any
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.\" document formatting or typesetting system, including
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.\" intermediate and printed output.
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.\"
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.\" This manual is distributed in the hope that it will be useful,
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.\" but WITHOUT ANY WARRANTY; without even the implied warranty of
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.\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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.\" GNU General Public License for more details.
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.\"
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.\" You should have received a copy of the GNU General Public
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.\" License along with this manual; if not, see
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.\" <http://www.gnu.org/licenses/>.
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.\" %%%LICENSE_END
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.\"
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.\" 2003-08-23 Martin Schulze <joey@infodrom.org> Updated according to glibc 2.3.2
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.TH HOST.CONF 5 2019-03-06 "Linux" "Linux System Administration"
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.SH NAME
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host.conf \- resolver configuration file
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.SH DESCRIPTION
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The file
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.I /etc/host.conf
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contains configuration information specific to the resolver library.
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It should contain one configuration keyword per line, followed by
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appropriate configuration information.
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The following keywords are recognized:
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.TP
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.I trim
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This keyword may be listed more than once.
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Each time it should be
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followed by a list of domains, separated by colons (\(aq:\(aq), semicolons
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(\(aq;\(aq) or commas (\(aq,\(aq), with the leading dot.
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When set, the
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resolver library will automatically trim the given domain name from the
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end of any hostname resolved via DNS.
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This is intended for use with
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local hosts and domains.
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(Related note:
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.I trim
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will not affect hostnames gathered via NIS or the
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.BR hosts (5)
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file.
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Care should be taken to
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ensure that the first hostname for each entry in the hosts file is
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fully qualified or unqualified, as appropriate for the local
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installation.)
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.TP
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.I multi
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Valid values are
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.IR on " and " off .
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If set to
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.IR on ,
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the resolver library will return all valid addresses for a host that
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appears in the
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.I /etc/hosts
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file,
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instead of only the first.
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This is
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.I off
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by default, as it may cause a substantial performance loss at sites
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with large hosts files.
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.TP
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.I reorder
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Valid values are
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.IR on " and " off .
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If set to
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.IR on ,
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the resolver library
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will attempt to reorder host addresses so that local addresses
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(i.e., on the same subnet) are listed first when a
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.BR gethostbyname (3)
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is performed.
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Reordering is done for all lookup methods.
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The default value is
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.IR off .
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.SH ENVIRONMENT
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The following environment variables can be used to allow users to
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override the behavior which is configured in
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.IR /etc/host.conf :
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.TP
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.B RESOLV_HOST_CONF
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If set, this variable points to a file that should be read instead of
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.IR /etc/host.conf .
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.TP
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.B RESOLV_MULTI
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Overrides the
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.I multi
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command.
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.TP
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.B RESOLV_REORDER
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Overrides the
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.I reorder
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command.
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.TP
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.B RESOLV_ADD_TRIM_DOMAINS
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A list of domains, separated by colons (\(aq:\(aq), semicolons (\(aq;\(aq), or
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commas (\(aq,\(aq), with the leading dot, which will be added to the list of
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domains that should be trimmed.
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.TP
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.B RESOLV_OVERRIDE_TRIM_DOMAINS
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A list of domains, separated by colons (\(aq:\(aq), semicolons (\(aq;\(aq), or
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commas (\(aq,\(aq), with the leading dot, which will replace the list of
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domains that should be trimmed.
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Overrides the
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.I trim
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command.
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.SH FILES
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.TP
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.I /etc/host.conf
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Resolver configuration file
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.TP
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.I /etc/resolv.conf
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Resolver configuration file
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.TP
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.I /etc/hosts
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Local hosts database
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.SH NOTES
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The following differences exist compared to the original implementation.
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A new command
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.I spoof
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and a new environment variable
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.B RESOLV_SPOOF_CHECK
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can take arguments like
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.IR off ", " nowarn ", and " warn .
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Line comments can appear anywhere and not only at the beginning of a line.
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.SS Historical
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The
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.BR nsswitch.conf (5)
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file is the modern way of controlling the order of host lookups.
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.PP
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In glibc 2.4 and earlier, the following keyword is recognized:
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.TP
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.I order
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This keyword specifies how host lookups are to be performed.
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It should be followed by one or more lookup methods, separated by commas.
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Valid methods are
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.IR bind ", " hosts ", and " nis .
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.TP
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.B RESOLV_SERV_ORDER
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Overrides the
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.I order
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command.
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.PP
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.\" commit 7d68cdaa4f748e87ee921f587ee2d483db624b3d
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Since glibc 2.0.7, and up through glibc 2.24, the following keywords and environment variable have
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been recognized but never implemented:
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.TP
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.I nospoof
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Valid values are
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.IR on " and " off .
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If set to
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.IR on ,
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the resolver library will attempt to prevent hostname spoofing to
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enhance the security of
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.BR rlogin " and " rsh .
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It works as follows: after performing a host address lookup,
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the resolver library will perform a hostname lookup for that address.
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If the two hostnames
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do not match, the query fails.
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The default value is
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.IR off .
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.TP
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.I spoofalert
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Valid values are
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.IR on " and " off .
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If this option is set to
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.I on
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and the
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.I nospoof
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option is also set,
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the resolver library will log a warning of the error via the
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syslog facility.
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The default value is
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.IR off .
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.TP
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.I spoof
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Valid values are
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.IR off ", " nowarn ", and " warn .
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If this option is set to
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.IR off ,
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spoofed addresses are permitted and no warnings will be emitted
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via the syslog facility.
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If this option is set to
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.IR warn ,
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the resolver library will attempt to prevent hostname spoofing to
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enhance the security and log a warning of the error via the syslog
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facility.
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If this option is set to
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.IR nowarn ,
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the resolver library will attempt to prevent hostname spoofing to
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enhance the security but not emit warnings via the syslog facility.
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Setting this option to anything else is equal to setting it to
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.IR nowarn .
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.TP
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.B RESOLV_SPOOF_CHECK
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Overrides the
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.IR nospoof ", " spoofalert ", and " spoof
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commands in the same way as the
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.I spoof
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command is parsed.
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Valid values are
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.IR off ", " nowarn ", and " warn .
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.SH SEE ALSO
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.BR gethostbyname (3),
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.BR hosts (5),
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.BR nsswitch.conf (5),
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.BR resolv.conf (5),
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.BR hostname (7),
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.BR named (8)
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