60b27ebb25
On each resolver query, use stat(2) to see if the modification time of /etc/resolv.conf has changed. If so, reload the file and reinitialize the resolver library. However, only call stat(2) if at least two seconds have passed since the last call to stat(2), since calling it on every query could kill performance. This new behavior is enabled by default. Add a "reload-period" option to disable it or change the period of the test. Document this behavior and option in resolv.conf(5). Polish the man page just enough to appease igor. https://lists.freebsd.org/pipermail/freebsd-arch/2015-October/017342.html Reviewed by: kp, wblock Discussed with: jilles, imp, alfred MFC after: 1 month Relnotes: yes Sponsored by: Dell Inc. Differential Revision: https://reviews.freebsd.org/D3867
230 lines
7.0 KiB
Groff
230 lines
7.0 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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.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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