297 lines
9.3 KiB
Groff
297 lines
9.3 KiB
Groff
.\" $NetBSD: resolv.conf.5,v 1.31 2021/04/30 16:12:18 christos Exp $
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.\"
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.\" Copyright (c) 1986, 1991 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, 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 5.12 (Berkeley) 5/10/91
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.\"
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.Dd Arril 30, 2021
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.Dt RESOLV.CONF 5
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.Os
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.Sh NAME
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.Nm resolv.conf
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.Nd resolver configuration file
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.Sh DESCRIPTION
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The
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.Nm resolv.conf
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file specifies how the
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.Xr resolver 3
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routines in the C library
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(which provide access to the Internet Domain Name System) should operate.
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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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IPv4 address
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.Pq in dot notation
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or IPv6 address
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.Pq in hex-and-colon notation
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of a name server that the resolver should query.
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Scoped IPv6 address notation is accepted as well
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.Po
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see
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.Xr inet6 4
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for details
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.Pc .
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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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.Sq \&. .
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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 lookup
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This keyword is now ignored: its function has been superseded by
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features of
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.Xr nsswitch.conf 5 .
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.Pp
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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 begins with the local domain name, then successive
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parent domains that have at least two components in their names.
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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 1024 characters.
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.It Sy sortlist
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Sortlist allows addresses returned by gethostbyname to
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be sorted.
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A sortlist is specified by IP address netmask pairs.
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The netmask is optional and defaults to the natural
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netmask of the net.
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The IP address and optional network pairs are separated by
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slashes.
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Up to 10 pairs may be specified, ie.
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.Pp
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.Sy 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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.Sy options option ...
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.Pp
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where option is one of the following:
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.Bl -tag -width no-check-names
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.It Sy debug
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enable debugging information, by setting RES_DEBUG in _res.options
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(see
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.Xr resolver 3 ) .
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.It Sy ndots:n
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sets a threshold for the number of dots which
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must appear in a name given to res_query (see
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.Xr resolver 3 )
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before an initial absolute query will be made.
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The default for n is 1, meaning that if there are any
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dots in a name, the name will be tried first as an absolute
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name before any search list elements are appended to it.
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.It Sy timeout:n
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sets the amount of time the resolver will wait 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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Measured in seconds, the default is
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.Dv RES_TIMEOUT
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(see
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.Aq Pa resolv.h ) .
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.It Sy attempts:n
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sets the number of times the resolver will send a query to 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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(see
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.Aq Pa resolv.h ) .
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.It Sy rotate
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sets
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.Dv RES_ROTATE
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in
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.Ft _res.options ,
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which causes round robin selection of nameservers from among those listed.
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This has the effect of spreading the query load among all listed servers,
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rather than having all clients try the first listed server first every time.
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.It Sy no-check-names
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sets
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.Dv RES_NOCHECKNAME
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in
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.Ft _res.options ,
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which disables the modern BIND checking of incoming host names and mail names
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for invalid characters such as underscore
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.Pq Sq _ ,
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non-ASCII, or control characters.
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.It Sy check-names
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clears
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.Dv RES_NOCHECKNAME
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in
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.Ft _res.options ,
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which enables the modern BIND checking of incoming host names and mail names
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as described above.
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This is the default.
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.It Sy edns0
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attach OPT pseudo-RR for ENDS0 extension specified in RFC 2671,
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to inform DNS server of our receive buffer size.
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The option will allow DNS servers to take advantage of non-default receive
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buffer size, and to send larger replies.
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DNS query packets with EDNS0 extension is not compatible with
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non-EDNS0 DNS servers.
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The option must be used only when all the DNS servers listed in
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.Sy nameserver
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lines are able to handle EDNS0 extension.
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.It Sy inet6
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enable support for IPv6-only applications, by setting RES_USE_INET6 in
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_res.options (see
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.Xr resolver 3 ) .
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The option is meaningful with certain kernel configuration only and
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use of this option is discouraged.
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.It Sy insecure1
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Do not require IP source address on the reply packet to be equal to the
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servers' address.
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.It Sy insecure2
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Do not check if the query section of the reply packet is equal
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to that of the query packet.
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For testing purposes only.
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.It Sy no-tld-query
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sets
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.Dv RES_NOTLDQUERY
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in
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.Ft _res.options .
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This option causes
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.Fn res_nsearch
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to not attempt to resolve a unqualified name as if it were a top level
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domain (TLD).
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This option can cause problems if the site has
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.Dq localhost
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as a TLD rather
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than having localhost on one or more elements of the search list.
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This option has no effect if neither
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.Dv RES_DEFNAMES
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or
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.Dv RES_DNSRCH
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is set.
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.El
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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
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.Sy search
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keyword of a system's
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.Pa resolv.conf
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file can be overridden on a per-process basis by setting the
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environment variable
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.Ev LOCALDOMAIN
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to a space-separated list of search domains.
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.Pp
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The
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.Sy options
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keyword of a system's
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.Pa resolv.conf
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file can be amended on a per-process basis by setting the
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environment variable
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.Ev RES_OPTIONS
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to a space-separated list of resolver options as explained above.
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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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(e.g.
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.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 nsswitch.conf 5 ,
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.Xr hostname 7 ,
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.Xr named 8 ,
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.Xr resolvconf 8
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.Rs
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.%A Paul Vixie
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.%A Kevin J. Dunlap
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.%A Michael J. Karels
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.%T "Name Server Operations Guide for BIND"
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.%N Release 4.9.4
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.%I CSRG ,
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.%I Department of Electrical Engineering and Computer Sciences ,
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.%I University of California, Berkeley
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.%D July 16, 1996
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.%U http://www.dns.net/dnsrd/docs/bog/bog.html
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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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