mirror of https://github.com/mkerrisk/man-pages
196 lines
7.3 KiB
Groff
196 lines
7.3 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 8.6 1999/05/21 00:01:02 vixie Exp $
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.\"
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.\" Added ndots remark by Bernhard R. Link - debian bug #182886
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.\"
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.TH RESOLV.CONF 5 2004-10-31
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.UC 4
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.SH NAME
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resolv.conf \- resolver configuration file
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.SH SYNOPSIS
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/etc/resolv.conf
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.SH DESCRIPTION
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The
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.I resolver
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is a set of routines in the C library
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that provide access to the Internet Domain Name System (DNS).
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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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.LP
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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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.LP
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The different configuration options are:
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.TP
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\fBnameserver\fP Name server IP address
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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 MAXNS (currently 3, see <resolv.h>) 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 \fBnameserver\fP 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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.TP
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\fBdomain\fP 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 \fBdomain\fP entry is present, the domain is determined
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from the local host name returned by
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\fIgethostname\fP();
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the domain part is taken to be everything after the first `.'.
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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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.TP
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\fBsearch\fP 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 \fIsearch\fP keyword with spaces or tabs separating
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the names.
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Resolver queries having fewer than
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.I ndots
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dots (default is 1) in them 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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For environments with multiple subdomains please read
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.BI "options ndots:" n
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below to avoid man-in-the-middle attacks and unnecessary
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traffic for the root-dns-servers.
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.\" When having a resolv.conv with a line
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.\" search subdomain.domain.tld domain.tld
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.\" and doing a hostlookup, for example by
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.\" ping host.anothersubdomain
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.\" it sends dns-requests for
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.\" host.anothersubdomain.
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.\" host.anothersubdomain.subdomain.domain.tld.
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.\" host.anothersubdomain.domain.tld.
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.\" thus not only causing unnecessary traffic for the root-dns-servers
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.\" but broadcasting information to the outside and making man-in-the-middle
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.\" attacks possible.
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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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.IP
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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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.TP
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\fBsortlist\fP
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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. 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. E.g.,
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.br
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.in +2
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sortlist 130.155.160.0/255.255.240.0 130.155.0.0
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.in -2
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.br
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.TP
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\fBoptions\fP
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Options allows certain internal resolver variables to be modified.
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The syntax is
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.RS
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.IP
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\fBoptions\fP \fIoption\fP \fI...\fP
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.LP
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where \fIoption\fP is one of the following:
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.TP
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\fBdebug\fP
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sets RES_DEBUG in
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.IR _res.options .
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.TP
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.BI 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 \fBres_query\fP() (see
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.BR resolver (3))
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before an \fIinitial absolute query\fP will be made. The default for
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\fIn\fP is ``1'', 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 \fIsearch list\fP
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elements are appended to it.
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.TP
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.BI timeout: n
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sets the amount of time the resolver will wait for a
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response from a remote name server before retrying the
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query via a different name server. Measured in seconds,
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the default is RES_TIMEOUT (currently 5, see <resolv.h>).
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.TP
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.BI attempts: n
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sets the number of times the resolver will send a
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query to its name servers before giving up and returning
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an error to the calling application. The default
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is RES_DFLRETRY (currently 2, see <resolv.h>).
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.TP
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.B rotate
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sets RES_ROTATE in
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.IR _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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.TP
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.B no-check-names
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sets RES_NOCHECKNAME in
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.IR _res.options ,
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which disables the modern BIND checking of incoming host names and
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mail names for invalid characters such as underscore (_), non-ASCII,
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or control characters.
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.TP
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.B inet6
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sets RES_USE_INET6 in
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.IR _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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.BR gethostbyname ()
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function, and of mapping IPv4 responses in IPv6 ``tunnelled form''
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if no AAAA records are found but an A record set exists.
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.RE
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.LP
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The \fIdomain\fP and \fIsearch\fP 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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.LP
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The \fIsearch\fP keyword of a system's \fIresolv.conf\fP file can be
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overridden on a per-process basis by setting the environment variable
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``\s-1LOCALDOMAIN\s+1'' to a space-separated list of search domains.
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.LP
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The \fIoptions\fP keyword of a system's \fIresolv.conf\fP file can be
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amended on a per-process basis by setting the environment variable
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``\s-1RES_OPTIONS\s+1'' to a space-separated list of resolver options
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as explained above under \fBoptions\fP.
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.LP
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The keyword and value must appear on a single line, and the keyword
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(e.g. \fBnameserver\fP) must start the line. The value follows
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the keyword, separated by white space.
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.SH FILES
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.IR /etc/resolv.conf ,
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.I <resolv.h>
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.SH "SEE ALSO"
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.BR gethostbyname (3),
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.BR resolver (3),
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.BR hostname (7),
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.BR named (8)
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.br
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Name Server Operations Guide for BIND
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