2004-11-03 13:51:07 +00:00
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.\" 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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2009-02-28 18:50:37 +00:00
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.TH RESOLV.CONF 5 2009-03-01 "" "Linux Programmer's Manual"
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2004-11-03 13:51:07 +00:00
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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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2007-12-23 08:20:39 +00:00
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.B /etc/resolv.conf
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2004-11-03 13:51:07 +00:00
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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 hostname
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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
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.B MAXNS
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(currently 3, see \fI<resolv.h>\fP) 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 hostname returned by
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.BR gethostname (2);
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the domain part is taken to be everything after the first \(aq.\(aq.
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Finally, if the hostname 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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This option allows addresses returned by
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.BR gethostbyname (3)
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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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Here is an example:
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.in +4n
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sortlist 130.155.160.0/255.255.240.0 130.155.0.0
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.in
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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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.\" Since glibc 2.2?
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sets
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.BR RES_DEBUG
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in
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.IR _res.options .
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.TP
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.BI ndots: n
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.\" Since glibc 2.2
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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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.BR res_query (3)
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(see
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.BR resolver (3))
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before an \fIinitial absolute query\fP will be made.
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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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The maximum value for this option is silently capped to 15.
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.TP
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.BI timeout: n
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.\" Since glibc 2.2
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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.
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Measured in seconds,
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the default is
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.BR RES_TIMEOUT
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(currently 5, see \fI<resolv.h>\fP).
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The maximum value for this option is silently capped to 30.
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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.
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The default is
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.BR RES_DFLRETRY
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(currently 2, see \fI<resolv.h>\fP).
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The maximum value for this option is silently capped to 5.
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.TP
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.B rotate
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.\" Since glibc 2.2
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sets
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.BR RES_ROTATE
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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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.\" since glibc 2.2
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sets
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.BR RES_NOCHECKNAME
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in
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.IR _res.options ,
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which disables the modern BIND checking of incoming hostnames 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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.\" Since glibc 2.2
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sets
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.BR RES_USE_INET6
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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 (3)
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function, and of mapping IPv4 responses in IPv6 "tunneled form"
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if no AAAA records are found but an A record set exists.
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.TP
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2009-02-28 21:04:52 +00:00
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.BR ip6-bytestring " (since glibc 2.3.4)"
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sets
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.BR RES_USE_BSTRING
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in
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.IR _res.options .
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This causes reverse IPv6 lookups to be made using the bit-label format
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described in RFC\ 2673;
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if this option is not set, then nibble format is used.
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.TP
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2009-02-28 21:23:53 +00:00
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.BR ip6-dotint / no-ip6-dotint " (since glibc 2.3.4)"
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Clear/set
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.BR RES_NOIP6DOTINT
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in
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.IR _res.options .
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When this option is clear
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.RB ( ip6-dotint ),
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reverse IPv6 lookups are made in the (deprecated)
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.I ip6.int
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zone;
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when this option is set
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.RB ( no-ip6-dotint ),
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reverse IPv6 lookups are made in the
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.I ip6.arpa
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zone by default.
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This option is set by default.
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.TP
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.BR edns0 " (since glibc 2.6)"
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sets
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.BR RES_USE_EDNSO
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in
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.IR _res.options .
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This enables support for the DNS extensions described in RFC\ 2671.
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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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.B LOCALDOMAIN
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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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.B RES_OPTIONS
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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.
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The value follows 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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