241 lines
7.2 KiB
Groff
241 lines
7.2 KiB
Groff
.\" Copyright (c) 1986 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
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.\" provided that the above copyright notice and this paragraph are
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.\" duplicated in all such forms and that any documentation,
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.\" advertising materials, and other materials related to such
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.\" distribution and use acknowledge that the software was developed
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.\" by the University of California, Berkeley. The name of the
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.\" University may not be used to endorse or promote products derived
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.\" from this software without specific prior written permission.
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.\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR
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.\" IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED
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.\" WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
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.\"
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.\" @(#)resolver.5 5.9 (Berkeley) 12/14/89
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.\" $Id: resolver.5,v 8.9 2001/12/28 04:24:21 marka Exp $
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.\"
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.Dd November 11, 1993
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.Dt RESOLVER @FORMAT_EXT_U@
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.Os BSD 4
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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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.Pa /etc/resolv.conf
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.Sh DESCRIPTION
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The
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.Nm resolver
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is a set of routines in the C library
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.Pq Xr resolve @LIB_NETWORK_EXT@
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that provide access to the Internet Domain Name System.
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The
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.Nm resolver
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configuration file contains information that is read
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by the
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.Nm resolver
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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
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.Nm resolver
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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 directives are:
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.Bl -tag -width "nameser"
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.It Li nameserver
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Internet address (in dot notation) of a name server that the
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.Nm resolver
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should query. Up to
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.Dv MAXNS
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(see
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.Pa <resolv.h> )
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name servers may be listed, one per keyword.
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If there are multiple servers, the
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.Nm resolver
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library queries them in the order listed.
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If no
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.Li nameserver
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entries are present, 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 Li 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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.Li domain
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entry is present, the domain is determined from the local host name returned by
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.Xr gethostname @BSD_SYSCALL_EXT@ ;
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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 Li 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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.Li search
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keyword with spaces or tabs separating the names.
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Most
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.Nm resolver
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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 Li sortlist
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Allows addresses returned by gethostbyname to be sorted.
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A
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.Li sortlist
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is specified by IP address netmask pairs. The netmask is
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optional and defaults to the natural netmask of the net. The IP address
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and optional network pairs are separated by slashes. Up to 10 pairs may
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be specified. For example:
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.Bd -literal -offset indent
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sortlist 130.155.160.0/255.255.240.0 130.155.0.0
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.Ed
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.It Li options
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Allows certain internal
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.Nm resolver
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variables to be modified.
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The syntax is
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.D1 Li options Ar option ...
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where
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.Ar option
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is one of the following:
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.Bl -tag -width "ndots:n "
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.It Li debug
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sets
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.Dv RES_DEBUG
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in
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.Ft _res.options .
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.It Li ndots: Ns Ar 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
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.Fn res_query
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(see
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.Xr resolver @LIB_NETWORK_EXT@ )
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before an
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.Em initial absolute query
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will be made. The default for
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.Ar n
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is
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.Dq 1 ,
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meaning that if there are
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.Em any
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dots in a name, the name 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 Li timeout: Ns Ar 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. Measured in
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seconds, the default is
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.Dv RES_TIMEOUT
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(see
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.Pa <resolv.h> ) .
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.It Li attempts: Ns Ar 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. The
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default is
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.Dv RES_DFLRETRY
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(see
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.Pa <resolv.h> ) .
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.It Li 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 Li 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 (_), non-ASCII, or control characters.
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.It Li inet6
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sets
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.Dv RES_USE_INET6
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in
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.Ft _res.options .
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This has the effect of trying a AAAA query before an A query inside the
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.Ft gethostbyname
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function, and of mapping IPv4 responses in IPv6 ``tunnelled form'' if no
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AAAA records are found but an A record set exists.
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.It Li 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 "localhost" 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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.Li domain
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and
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.Li 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 wins.
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.Pp
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The
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.Li search
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keyword of a system's
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.Pa resolv.conf
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file can be
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overridden on a per-process basis by setting the environment variable
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.Dq 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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.Li 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 environment variable
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.Dq Ev RES_OPTIONS to a space-separated list of
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.Nm resolver
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options as explained above under
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.Li options .
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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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.Li nameserver )
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must start the line. The value follows the keyword, separated by white space.
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.Sh FILES
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.Pa /etc/resolv.conf
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.Pa <resolv.h>
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.Sh SEE ALSO
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.Xr gethostbyname @LIB_NETWORK_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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.Dq Name Server Operations Guide for Sy BIND
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