man5/resolv.conf.5

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  16 .\"     @(#)resolver.5  5.9 (Berkeley) 12/14/89
  17 .\"     $Id: resolver.5,v 8.6 1999/05/21 00:01:02 vixie Exp $
  18 .\"
  19 .\" Added ndots remark by Bernhard R. Link - debian bug #182886
  20 .\"
  21 .TH RESOLV.CONF 5 2004-10-31
  22 .UC 4
  23 .SH NAME
  24 resolv.conf \- resolver configuration file
  25 .SH SYNOPSIS
  26 /etc/resolv.conf
  27 .SH DESCRIPTION
  28 The
  29 .I resolver
  30 is a set of routines in the C library
  31 that provide access to the Internet Domain Name System (DNS).
  32 The resolver configuration file contains information that is read
  33 by the resolver routines the first time they are invoked by a process.
  34 The file is designed to be human readable and contains a list of
  35 keywords with values that provide various types of resolver information.
  36 .LP
  37 On a normally configured system this file should not be necessary.
  38 The only name server to be queried will be on the local machine;
  39 the domain name is determined from the host name
  40 and the domain search path is constructed from the domain name.
  41 .LP
  42 The different configuration options are:
  43 .TP
  44 \fBnameserver\fP Name server IP address
  45 Internet address (in dot notation) of a name server
  46 that the resolver should query.
  47 Up to MAXNS (currently 3, see <resolv.h>) name servers may be listed,
  48 one per keyword.
  49 If there are multiple servers,
  50 the resolver library queries them in the order listed.
  51 If no \fBnameserver\fP entries are present,
  52 the default is to use the name server on the local machine.
  53 (The algorithm used is to try a name server, and if the query times out,
  54 try the next, until out of name servers,
  55 then repeat trying all the name servers
  56 until a maximum number of retries are made.)
  57 .TP
  58 \fBdomain\fP Local domain name.
  59 Most queries for names within this domain can use short names
  60 relative to the local domain.
  61 If no \fBdomain\fP entry is present, the domain is determined
  62 from the local host name returned by
  63 \fIgethostname\fP();
  64 the domain part is taken to be everything after the first `.'.
  65 Finally, if the host name does not contain a domain part, the root
  66 domain is assumed.
  67 .TP
  68 \fBsearch\fP Search list for host-name lookup.
  69 The search list is normally determined from the local domain name;
  70 by default, it contains only the local domain name.
  71 This may be changed by listing the desired domain search path
  72 following the \fIsearch\fP keyword with spaces or tabs separating
  73 the names.
  74 Resolver queries having fewer than
  75 .I ndots
  76 dots (default is 1) in them will be attempted using each component
  77 of the search path in turn until a match is found.
  78 For environments with multiple subdomains please read
  79 .BI "options ndots:" n
  80 below to avoid man-in-the-middle attacks and unnecessary
  81 traffic for the root-dns-servers.
  82 .\" When having a resolv.conv with a line
  83 .\"  search subdomain.domain.tld domain.tld
  84 .\" and doing a hostlookup, for example by
  85 .\"  ping host.anothersubdomain
  86 .\" it sends dns-requests for
  87 .\"  host.anothersubdomain.
  88 .\"  host.anothersubdomain.subdomain.domain.tld.
  89 .\"  host.anothersubdomain.domain.tld.
  90 .\" thus not only causing unnecessary traffic for the root-dns-servers
  91 .\" but broadcasting information to the outside and making man-in-the-middle
  92 .\" attacks possible.
  93 Note that this process may be slow and will generate a lot of network
  94 traffic if the servers for the listed domains are not local,
  95 and that queries will time out if no server is available
  96 for one of the domains.
  97 .IP
  98 The search list is currently limited to six domains
  99 with a total of 256 characters.
 100 .TP
 101 \fBsortlist\fP
 102 Sortlist allows addresses returned by gethostbyname to be sorted.
 103 A sortlist is specified by IP address netmask pairs.
 104 The netmask is
 105 optional and defaults to the natural netmask of the net.
 106 The IP address
 107 and optional network pairs are separated by slashes.
 108 Up to 10 pairs may
 109 be specified.
 110 E.g.,
 111 .br
 112 .in +2
 113 sortlist 130.155.160.0/255.255.240.0 130.155.0.0
 114 .in -2
 115 .br
 116 .TP
 117 \fBoptions\fP
 118 Options allows certain internal resolver variables to be modified.
 119 The syntax is
 120 .RS
 121 .IP
 122 \fBoptions\fP \fIoption\fP \fI...\fP
 123 .LP
 124 where \fIoption\fP is one of the following:
 125 .TP
 126 \fBdebug\fP
 127 sets RES_DEBUG in
 128 .IR _res.options .
 129 .TP
 130 .BI ndots: n
 131 sets a threshold for the number of dots which
 132 must appear in a name given to \fBres_query\fP() (see
 133 .BR resolver (3))
 134 before an \fIinitial absolute query\fP will be made.
 135 The default for
 136 \fIn\fP is ``1'', meaning that if there are any dots in a name, the name
 137 will be tried first as an absolute name before any \fIsearch list\fP
 138 elements are appended to it.
 139 .TP
 140 .BI timeout: n
 141 sets the amount of time the resolver will wait for a
 142 response from a remote name server before retrying the
 143 query via a different name server.
 144 Measured in seconds,
 145 the default is RES_TIMEOUT (currently 5, see <resolv.h>).
 146 .TP
 147 .BI attempts: n
 148 sets the number of times the resolver will send a
 149 query to its name servers before giving up and returning
 150 an error to the calling application.
 151 The default is RES_DFLRETRY (currently 2, see <resolv.h>).
 152 .TP
 153 .B rotate
 154 sets RES_ROTATE in
 155 .IR _res.options ,
 156 which causes round robin selection of nameservers from among those listed.
 157 This has the effect of spreading the query load among all listed servers,
 158 rather than having all clients try the first listed server first every time.
 159 .TP
 160 .B no-check-names
 161 sets RES_NOCHECKNAME in
 162 .IR _res.options ,
 163 which disables the modern BIND checking of incoming host names and
 164 mail names for invalid characters such as underscore (_), non-ASCII,
 165 or control characters.
 166 .TP
 167 .B inet6
 168 sets RES_USE_INET6 in
 169 .IR _res.options .
 170 This has the effect of trying a AAAA query before an A query inside the
 171 .BR gethostbyname ()
 172 function, and of mapping IPv4 responses in IPv6 ``tunnelled form''
 173 if no AAAA records are found but an A record set exists.
 174 .RE
 175 .LP
 176 The \fIdomain\fP and \fIsearch\fP keywords are mutually exclusive.
 177 If more than one instance of these keywords is present,
 178 the last instance wins.
 179 .LP
 180 The \fIsearch\fP keyword of a system's \fIresolv.conf\fP file can be
 181 overridden on a per-process basis by setting the environment variable
 182 ``\s-1LOCALDOMAIN\s+1'' to a space-separated list of search domains.
 183 .LP
 184 The \fIoptions\fP keyword of a system's \fIresolv.conf\fP file can be
 185 amended on a per-process basis by setting the environment variable
 186 ``\s-1RES_OPTIONS\s+1'' to a space-separated list of resolver options
 187 as explained above under \fBoptions\fP.
 188 .LP
 189 The keyword and value must appear on a single line, and the keyword
 190 (e.g. \fBnameserver\fP) must start the line.
 191 The value follows the keyword, separated by white space.
 192 .SH FILES
 193 .IR /etc/resolv.conf ,
 194 .I <resolv.h>
 195 .SH "SEE ALSO"
 196 .BR gethostbyname (3),
 197 .BR resolver (3),
 198 .BR hostname (7),
 199 .BR named (8)
 200 .br
 201 Name Server Operations Guide for BIND