1.\" $OpenBSD: resolv.conf.5,v 1.5 1998/05/11 23:08:24 deraadt Exp $ 2.\" $NetBSD: resolv.conf.5,v 1.7 1996/03/06 18:22:16 scottr Exp $ 3.\" 4.\" Copyright (c) 1986, 1991 The Regents of the University of California. 5.\" All rights reserved. 6.\" 7.\" Redistribution and use in source and binary forms, with or without 8.\" modification, are permitted provided that the following conditions 9.\" are met: 10.\" 1. Redistributions of source code must retain the above copyright 11.\" notice, this list of conditions and the following disclaimer. 12.\" 2. Redistributions in binary form must reproduce the above copyright 13.\" notice, this list of conditions and the following disclaimer in the 14.\" documentation and/or other materials provided with the distribution. 15.\" 3. All advertising materials mentioning features or use of this software 16.\" must display the following acknowledgement: 17.\" This product includes software developed by the University of 18.\" California, Berkeley and its contributors. 19.\" 4. Neither the name of the University nor the names of its contributors 20.\" may be used to endorse or promote products derived from this software 21.\" without specific prior written permission. 22.\" 23.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 24.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 25.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 26.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 27.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 28.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 29.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 30.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 31.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 32.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 33.\" SUCH DAMAGE. 34.\" 35.\" @(#)resolver.5 5.12 (Berkeley) 5/10/91 36.\" 37.Dd May 10, 1991 38.Dt RESOLV.CONF 5 39.Os BSD 4 40.Sh NAME 41.Nm resolv.conf 42.Nd resolver configuration file 43.Sh DESCRIPTION 44The 45.Nm resolv.conf 46file specifies how the 47.Xr resolver 3 48routines in the C library 49(which provide access to the Internet Domain Name System) should operate. 50The resolver configuration file contains information that is read 51by the resolver routines the first time they are invoked by a process. 52The file is designed to be human readable and contains a list of 53keywords with values that provide various types of resolver information. 54.Pp 55On a normally configured system this file should not be necessary. 56The only name server to be queried will be on the local machine, 57the domain name is determined from the host name, 58and the domain search path is constructed from the domain name. 59.Pp 60The different configuration options are: 61.Bl -tag -width nameserver 62.It Sy nameserver 63Internet address (in dot notation) of a name server 64that the resolver should query. 65Up to 66.Dv MAXNS 67(currently 3) name servers may be listed, 68one per keyword. 69If there are multiple servers, 70the resolver library queries them in the order listed. 71If no 72.Sy nameserver 73entries are present, 74the default is to use the name server on the local machine. 75(The algorithm used is to try a name server, and if the query times out, 76try the next, until out of name servers, 77then repeat trying all the name servers 78until a maximum number of retries are made). 79.It Sy domain 80Local domain name. 81Most queries for names within this domain can use short names 82relative to the local domain. 83If no 84.Sy domain 85entry is present, the domain is determined 86from the local host name returned by 87.Xr gethostname 3 ; 88the domain part is taken to be everything after the first `.'. 89Finally, if the host name does not contain a domain part, the root 90domain is assumed. 91.It Sy lookup 92This keyword is used by the library routines 93.Xr gethostbyname 3 94and 95.Xr gethostbyaddr 3 . 96It specifies which databases should be searched, 97and the order to do so. 98The legal space-separated values are 99.Bl -tag -width bind 100.It Sy bind 101use the Domain Name server by querying the 102.Xr named 8 103.It Sy file 104search for entries in 105.Nm /etc/hosts 106.It Sy yp 107talk to the YP system if 108.Xr ypbind 8 109is running 110.El 111.Pp 112If the 113.Sy lookup 114keyword is not used in the system's 115.Pa resolv.conf 116file then the assumed order is 117.Sy bind file . 118Furthermore, if the system's 119.Pa resolv.conf 120file does not exist, then the only database used is 121.Sy file . 122.Pp 123.It Sy search 124Search list for host-name lookup. 125The search list is normally determined from the local domain name; 126by default, it begins with the local domain name, then successive 127parent domains that have at least two components in their names. 128This may be changed by listing the desired domain search path 129following the 130.Sy search 131keyword with spaces or tabs separating 132the names. 133Most resolver queries will be attempted using each component 134of the search path in turn until a match is found. 135Note that this process may be slow and will generate a lot of network 136traffic if the servers for the listed domains are not local, 137and that queries will time out if no server is available 138for one of the domains. 139.Pp 140The search list is currently limited to six domains 141with a total of 1024 characters. 142.It Sy sortlist 143Sortlist allows addresses returned by gethostbyname to 144be sorted. 145A sortlist is specified by IP address netmask pairs. 146The netmask is optional and defaults to the natural 147netmask of the net. 148The IP address and optional network pairs are seperated by 149slashes. 150Up to 10 pairs may be specified, ie. 151.Pp 152.Sy sortlist 130.155.160.0/255.255.240.0 130.155.0.0 153.It Sy options 154Options allows certain internal resolver variables to be modified. 155The syntax is: 156.Pp 157.Sy options option ... 158.Pp 159where option is one of the following: 160.Bl -tag -width ndots:n 161.It Sy debug 162sets RES_DEBUG in _res.options. 163.It Sy inet6 164enable support for IPv6 addresses, by setting RES_USE_INET6 in 165_res.options (see 166.Xr resolver 3 ). 167.It Sy ndots:n 168sets a threshold for the number of dots which 169must appear in a name given to res_query (see 170.Xr resolver 3 ) 171before an initial absolute query will be made. 172The default for n is 1, meaning that if there are any 173dots in a name, the name will be tried first as an absolute 174name before any search list elements are appended to it. 175.El 176.El 177.Pp 178The 179.Sy domain 180and 181.Sy search 182keywords are mutually exclusive. 183If more than one instance of these keywords is present, 184the last instance will override. 185.Pp 186The 187.Sy search 188keyword of a system's 189.Pa resolv.conf 190file can be overridden on a per-process basis by setting the 191environment variable 192.Ev LOCALDOMAIN 193to a space-separated list of search domains. 194.Pp 195The 196.Sy options 197keyword of a system's 198.Pa resolv.conf 199file can be amended on a per-process basis by setting the 200environment variable 201.Ev RES_OPTIONS 202to a space-separated list of resolver options as explained above. 203.Pp 204The keyword and value must appear on a single line, and the keyword 205(e.g. 206.Sy nameserver ) 207must start the line. The value follows 208the keyword, separated by white space. 209.Sh FILES 210.Bl -tag -width /etc/resolv.conf -compact 211.It Pa /etc/resolv.conf 212The file 213.Nm resolv.conf 214resides in 215.Pa /etc . 216.El 217.Sh SEE ALSO 218.Xr gethostbyname 3 , 219.Xr resolver 3 , 220.Xr hostname 7 , 221.Xr hosts 5 , 222.Xr named 8 223.Rs 224.%T "Name Server Operations Guide for BIND" 225.Re 226.Sh HISTORY 227The 228.Nm resolv.conf 229file format appeared in 230.Bx 4.3 . 231