1.\" $NetBSD: resolv.conf.5,v 1.12 1999/03/30 09:01:58 ross 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 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 is now ignored: its function has been superseded by 92features of 93.Xr nsswitch.conf 5 . 94.Pp 95.It Sy search 96Search list for host-name lookup. 97The search list is normally determined from the local domain name; 98by default, it begins with the local domain name, then successive 99parent domains that have at least two components in their names. 100This may be changed by listing the desired domain search path 101following the 102.Sy search 103keyword with spaces or tabs separating 104the names. 105Most resolver queries will be attempted using each component 106of the search path in turn until a match is found. 107Note that this process may be slow and will generate a lot of network 108traffic if the servers for the listed domains are not local, 109and that queries will time out if no server is available 110for one of the domains. 111.Pp 112The search list is currently limited to six domains 113with a total of 1024 characters. 114.It Sy sortlist 115Sortlist allows addresses returned by gethostbyname to 116be sorted. 117A sortlist is specified by IP address netmask pairs. 118The netmask is optional and defaults to the natural 119netmask of the net. 120The IP address and optional network pairs are seperated by 121slashes. 122Up to 10 pairs may be specified, ie. 123.Pp 124.Sy sortlist 130.155.160.0/255.255.240.0 130.155.0.0 125.It Sy options 126Options allows certain internal resolver variables to be modified. 127The syntax is: 128.Pp 129.Sy options option ... 130.Pp 131where option is one of the following: 132.Bl -tag -width ndots:n 133.It Sy debug 134enable debugging information, by setting RES_DEBUG in _res.options 135(see 136.Xr resolver 3 ). 137.It Sy inet6 138enable support for IPv6 addresses, by setting RES_USE_INET6 in 139_res.options (see 140.Xr resolver 3 ). 141.It Sy ndots:n 142sets a threshold for the number of dots which 143must appear in a name given to res_query (see 144.Xr resolver 3 ) 145before an initial absolute query will be made. 146The default for n is 1, meaning that if there are any 147dots in a name, the name will be tried first as an absolute 148name before any search list elements are appended to it. 149.El 150.El 151.Pp 152The 153.Sy domain 154and 155.Sy search 156keywords are mutually exclusive. 157If more than one instance of these keywords is present, 158the last instance will override. 159.Pp 160The 161.Sy search 162keyword of a system's 163.Pa resolv.conf 164file can be overridden on a per-process basis by setting the 165environment variable 166.Ev LOCALDOMAIN 167to a space-separated list of search domains. 168.Pp 169The 170.Sy options 171keyword of a system's 172.Pa resolv.conf 173file can be amended on a per-process basis by setting the 174environment variable 175.Ev RES_OPTIONS 176to a space-separated list of resolver options as explained above. 177.Pp 178The keyword and value must appear on a single line, and the keyword 179(e.g. 180.Sy nameserver ) 181must start the line. The value follows 182the keyword, separated by white space. 183.Sh FILES 184.Bl -tag -width /etc/resolv.conf -compact 185.It Pa /etc/resolv.conf 186The file 187.Nm resolv.conf 188resides in 189.Pa /etc . 190.El 191.Sh SEE ALSO 192.Xr gethostbyname 3 , 193.Xr resolver 3 , 194.Xr nsswitch.conf 5 , 195.Xr hostname 7 , 196.Xr named 8 197.Rs 198.%T "Name Server Operations Guide for BIND" 199.Re 200.Sh HISTORY 201The 202.Nm resolv.conf 203file format appeared in 204.Bx 4.3 . 205