1.\" $NetBSD: resolv.conf.5,v 1.10 1999/01/15 13:04:10 lukem Exp $ 2.\" 3.\" Copyright (c) 1986, 1991 The Regents of the University of California. 4.\" All rights reserved. 5.\" 6.\" Redistribution and use in source and binary forms, with or without 7.\" modification, are permitted provided that the following conditions 8.\" are met: 9.\" 1. Redistributions of source code must retain the above copyright 10.\" notice, this list of conditions and the following disclaimer. 11.\" 2. Redistributions in binary form must reproduce the above copyright 12.\" notice, this list of conditions and the following disclaimer in the 13.\" documentation and/or other materials provided with the distribution. 14.\" 3. All advertising materials mentioning features or use of this software 15.\" must display the following acknowledgement: 16.\" This product includes software developed by the University of 17.\" California, Berkeley and its contributors. 18.\" 4. Neither the name of the University nor the names of its contributors 19.\" may be used to endorse or promote products derived from this software 20.\" without specific prior written permission. 21.\" 22.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 23.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 24.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 25.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 26.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 27.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 28.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 29.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 30.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 31.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 32.\" SUCH DAMAGE. 33.\" 34.\" @(#)resolver.5 5.12 (Berkeley) 5/10/91 35.\" 36.Dd January 16, 1999 37.Dt RESOLV.CONF 5 38.Os BSD 4 39.Sh NAME 40.Nm resolv.conf 41.Nd resolver configuration file 42.Sh DESCRIPTION 43The 44.Nm resolv.conf 45file specifies how the 46.Xr resolver 3 47routines in the C library 48(which provide access to the Internet Domain Name System) should operate. 49The resolver configuration file contains information that is read 50by the resolver routines the first time they are invoked by a process. 51The file is designed to be human readable and contains a list of 52keywords with values that provide various types of resolver information. 53.Pp 54On a normally configured system this file should not be necessary. 55The only name server to be queried will be on the local machine, 56the domain name is determined from the host name, 57and the domain search path is constructed from the domain name. 58.Pp 59The different configuration options are: 60.Bl -tag -width nameserver 61.It Sy nameserver 62Internet address (in dot notation) of a name server 63that the resolver should query. 64Up to 65.Dv MAXNS 66(currently 3) name servers may be listed, 67one per keyword. 68If there are multiple servers, 69the resolver library queries them in the order listed. 70If no 71.Sy nameserver 72entries are present, 73the default is to use the name server on the local machine. 74(The algorithm used is to try a name server, and if the query times out, 75try the next, until out of name servers, 76then repeat trying all the name servers 77until a maximum number of retries are made). 78.It Sy domain 79Local domain name. 80Most queries for names within this domain can use short names 81relative to the local domain. 82If no 83.Sy domain 84entry is present, the domain is determined 85from the local host name returned by 86.Xr gethostname 3 ; 87the domain part is taken to be everything after the first `.'. 88Finally, if the host name does not contain a domain part, the root 89domain is assumed. 90.It Sy lookup 91This keyword has been deprecated by the functionality described in 92.Xr nsswitch.conf 5 . 93.Pp 94.It Sy search 95Search list for host-name lookup. 96The search list is normally determined from the local domain name; 97by default, it begins with the local domain name, then successive 98parent domains that have at least two components in their names. 99This may be changed by listing the desired domain search path 100following the 101.Sy search 102keyword with spaces or tabs separating 103the names. 104Most resolver queries will be attempted using each component 105of the search path in turn until a match is found. 106Note that this process may be slow and will generate a lot of network 107traffic if the servers for the listed domains are not local, 108and that queries will time out if no server is available 109for one of the domains. 110.Pp 111The search list is currently limited to six domains 112with a total of 1024 characters. 113.It Sy sortlist 114Sortlist allows addresses returned by gethostbyname to 115be sorted. 116A sortlist is specified by IP address netmask pairs. 117The netmask is optional and defaults to the natural 118netmask of the net. 119The IP address and optional network pairs are seperated by 120slashes. 121Up to 10 pairs may be specified, ie. 122.Pp 123.Sy sortlist 130.155.160.0/255.255.240.0 130.155.0.0 124.It Sy options 125Options allows certain internal resolver variables to be modified. 126The syntax is: 127.Pp 128.Sy options option ... 129.Pp 130where option is one of the following: 131.Bl -tag -width ndots:n 132.It Sy debug 133enable debugging information, by setting RES_DEBUG in _res.options 134(see 135.Xr resolver 3 ). 136.It Sy inet6 137enable support for IPv6 addresses, by setting RES_USE_INET6 in 138_res.options (see 139.Xr resolver 3 ). 140.It Sy ndots:n 141sets a threshold for the number of dots which 142must appear in a name given to res_query (see 143.Xr resolver 3 ) 144before an initial absolute query will be made. 145The default for n is 1, meaning that if there are any 146dots in a name, the name will be tried first as an absolute 147name before any search list elements are appended to it. 148.El 149.El 150.Pp 151The 152.Sy domain 153and 154.Sy search 155keywords are mutually exclusive. 156If more than one instance of these keywords is present, 157the last instance will override. 158.Pp 159The 160.Sy search 161keyword of a system's 162.Pa resolv.conf 163file can be overridden on a per-process basis by setting the 164environment variable 165.Ev LOCALDOMAIN 166to a space-separated list of search domains. 167.Pp 168The 169.Sy options 170keyword of a system's 171.Pa resolv.conf 172file can be amended on a per-process basis by setting the 173environment variable 174.Ev RES_OPTIONS 175to a space-separated list of resolver options as explained above. 176.Pp 177The keyword and value must appear on a single line, and the keyword 178(e.g. 179.Sy nameserver ) 180must start the line. The value follows 181the keyword, separated by white space. 182.Sh FILES 183.Bl -tag -width /etc/resolv.conf -compact 184.It Pa /etc/resolv.conf 185The file 186.Nm resolv.conf 187resides in 188.Pa /etc . 189.El 190.Sh SEE ALSO 191.Xr gethostbyname 3 , 192.Xr resolver 3 , 193.Xr nsswitch.conf 5 , 194.Xr hostname 7 , 195.Xr named 8 196.Rs 197.%T "Name Server Operations Guide for BIND" 198.Re 199.Sh HISTORY 200The 201.Nm resolv.conf 202file format appeared in 203.Bx 4.3 . 204