231 lines
7.1 KiB
Groff
231 lines
7.1 KiB
Groff
.\" Copyright (c) 1986, 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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.\" @(#)resolver.5 8.1 (Berkeley) 6/5/93
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.\" $FreeBSD$
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.\"
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.Dd October 12, 2015
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.Dt RESOLVER 5
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.Os
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.Sh NAME
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.Nm resolver
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.Nd resolver configuration file
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.Sh SYNOPSIS
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.Nm resolv.conf
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.Sh DESCRIPTION
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The
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.Xr resolver 3
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is a set of routines in the C library
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which provide access to the Internet Domain Name System.
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The resolver configuration file contains information that is read
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by the resolver routines the first time they are invoked by a process.
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The file is designed to be human readable and contains a list of
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keywords with values that provide various types of resolver information.
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.Pp
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On a normally configured system this file should not be necessary.
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The only name server to be queried will be on the local machine,
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the domain name is determined from the host name,
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and the domain search path is constructed from the domain name.
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.Pp
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The different configuration options are:
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.Bl -tag -width nameserver
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.It Sy nameserver
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Internet address (in dot notation) of a name server
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that the resolver should query.
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Up to
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.Dv MAXNS
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(currently 3) name servers may be listed,
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one per keyword.
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If there are multiple servers,
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the resolver library queries them in the order listed.
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If no
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.Sy nameserver
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entries are present,
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the default is to use the name server on the local machine.
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(The algorithm used is to try a name server, and if the query times out,
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try the next, until out of name servers,
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then repeat trying all the name servers
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until a maximum number of retries are made).
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.It Sy domain
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Local domain name.
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Most queries for names within this domain can use short names
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relative to the local domain.
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If no
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.Sy domain
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entry is present, the domain is determined
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from the local host name returned by
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.Xr gethostname 3 ;
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the domain part is taken to be everything after the first
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.Ql \&. .
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Finally, if the host name does not contain a domain part, the root
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domain is assumed.
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.It Sy search
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Search list for host-name lookup.
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The search list is normally determined from the local domain name;
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by default, it contains only the local domain name.
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This may be changed by listing the desired domain search path
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following the
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.Sy search
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keyword with spaces or tabs separating
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the names.
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Most resolver queries will be attempted using each component
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of the search path in turn until a match is found.
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Note that this process may be slow and will generate a lot of network
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traffic if the servers for the listed domains are not local,
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and that queries will time out if no server is available
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for one of the domains.
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.Pp
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The search list is currently limited to six domains
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with a total of 256 characters.
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.It Sy sortlist
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Sortlist allows addresses returned by gethostbyname to be sorted.
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A sortlist is specified by IP address netmask pairs.
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The netmask is
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optional and defaults to the natural netmask of the net.
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The IP address
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and optional network pairs are separated by slashes.
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Up to 10 pairs may
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be specified.
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E.g.,
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.Pp
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.Dl "sortlist 130.155.160.0/255.255.240.0 130.155.0.0"
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.It Sy options
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Options allows certain internal resolver variables to be modified.
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The syntax is
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.Pp
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\fBoptions\fP \fIoption\fP \fI...\fP
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.Pp
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where
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.Sy option
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is one of the following:
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.Bl -tag -width no_tld_query
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.It Sy debug
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sets
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.Dv RES_DEBUG
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in _res.options.
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.It Sy ndots: Ns Ar n
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sets a threshold for the number of dots which must appear in a name given to
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.Fn res_query
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(see
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.Xr resolver 3 )
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before an
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.Em initial absolute query
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will be made.
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The default for
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.Em n
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is
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.Dq 1 ,
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meaning that if there are any dots in a name, the name
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will be tried first as an absolute name before any
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.Em search list
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elements are appended to it.
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.It Sy timeout: Ns Ar n
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sets the initial amount of time the resolver will wait
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for a response from a remote
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name server before retrying the query via a different name server.
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The resolver may wait longer during subsequent retries
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of the current query since an exponential back-off is applied to
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the timeout value.
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Measured in seconds, the default is
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.Dv RES_TIMEOUT ,
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the allowed maximum is
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.Dv RES_MAXRETRANS
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(see
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.In resolv.h ) .
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.It Sy attempts: Ns Ar n
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sets the number of times the resolver will send a query to each of
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its name servers
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before giving up and returning an error to the calling application.
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The default is
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.Dv RES_DFLRETRY ,
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the allowed maximum is
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.Dv RES_MAXRETRY
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(see
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.In resolv.h ) .
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.It Sy no_tld_query
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tells the resolver not to attempt to resolve a top level domain name, that
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is, a name that contains no dots.
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Use of this option does not prevent
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the resolver from obeying the standard
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.Sy domain
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and
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.Sy search
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rules with the given name.
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.It Sy reload-period: Ns Ar n
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The resolver checks the modification time of
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.Pa /etc/resolv.conf
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every
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.Ar n
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seconds.
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If
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.Pa /etc/resolv.conf
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has changed, it is automatically reloaded.
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The default for
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.Ar n
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is two seconds.
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Setting it to zero disables the file check.
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.El
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.Pp
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Options may also be specified as a space or tab separated list using the
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.Dv RES_OPTIONS
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environment variable.
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.El
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.Pp
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The
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.Sy domain
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and
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.Sy search
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keywords are mutually exclusive.
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If more than one instance of these keywords is present,
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the last instance will override.
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.Pp
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The keyword and value must appear on a single line, and the keyword
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.Pq for example, Sy nameserver
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must start the line.
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The value follows the keyword, separated by white space.
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.Sh FILES
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.Bl -tag -width /etc/resolv.conf -compact
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.It Pa /etc/resolv.conf
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The file
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.Nm resolv.conf
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resides in
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.Pa /etc .
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.El
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.Sh SEE ALSO
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.Xr gethostbyname 3 ,
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.Xr resolver 3 ,
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.Xr hostname 7 ,
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.Xr resolvconf 8
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.Rs
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.%T "Name Server Operations Guide for BIND"
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.Re
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.Sh HISTORY
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The
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.Nm resolv.conf
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file format appeared in
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.Bx 4.3 .
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