1.\" $NetBSD: resolv.conf.5,v 1.9 1997/07/14 01:49:25 mikel 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 May 10, 1991 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 is used by the library routines 92.Xr gethostbyname 3 93and 94.Xr gethostbyaddr 3 . 95It specifies which databases should be searched, 96and the order to do so. 97The legal space-separated values are 98.Bl -tag -width bind 99.It Sy bind 100use the Domain Name server by querying the 101.Xr named 8 102.It Sy file 103search for entries in 104.Nm /etc/hosts 105.It Sy yp 106talk to the YP system if 107.Xr ypbind 8 108is running 109.El 110.Pp 111If the 112.Sy lookup 113keyword is not used in the system's 114.Pa resolv.conf 115file then the assumed order is 116.Sy bind file . 117Furthermore, if the system's 118.Pa resolv.conf 119file does not exist, then the only database used is 120.Sy file . 121.Pp 122.It Sy search 123Search list for host-name lookup. 124The search list is normally determined from the local domain name; 125by default, it begins with the local domain name, then successive 126parent domains that have at least two components in their names. 127This may be changed by listing the desired domain search path 128following the 129.Sy search 130keyword with spaces or tabs separating 131the names. 132Most resolver queries will be attempted using each component 133of the search path in turn until a match is found. 134Note that this process may be slow and will generate a lot of network 135traffic if the servers for the listed domains are not local, 136and that queries will time out if no server is available 137for one of the domains. 138.Pp 139The search list is currently limited to six domains 140with a total of 1024 characters. 141.It Sy sortlist 142Sortlist allows addresses returned by gethostbyname to 143be sorted. 144A sortlist is specified by IP address netmask pairs. 145The netmask is optional and defaults to the natural 146netmask of the net. 147The IP address and optional network pairs are seperated by 148slashes. 149Up to 10 pairs may be specified, ie. 150.Pp 151.Sy sortlist 130.155.160.0/255.255.240.0 130.155.0.0 152.It Sy options 153Options allows certain internal resolver variables to be modified. 154The syntax is: 155.Pp 156.Sy options option ... 157.Pp 158where option is one of the following: 159.Bl -tag -width ndots:n 160.It Sy debug 161enable debugging information, by setting RES_DEBUG in _res.options 162(see 163.Xr resolver 3 ). 164.It Sy inet6 165enable support for IPv6 addresses, by setting RES_USE_INET6 in 166_res.options (see 167.Xr resolver 3 ). 168.It Sy ndots:n 169sets a threshold for the number of dots which 170must appear in a name given to res_query (see 171.Xr resolver 3 ) 172before an initial absolute query will be made. 173The default for n is 1, meaning that if there are any 174dots in a name, the name will be tried first as an absolute 175name before any search list elements are appended to it. 176.El 177.El 178.Pp 179The 180.Sy domain 181and 182.Sy search 183keywords are mutually exclusive. 184If more than one instance of these keywords is present, 185the last instance will override. 186.Pp 187The 188.Sy search 189keyword of a system's 190.Pa resolv.conf 191file can be overridden on a per-process basis by setting the 192environment variable 193.Ev LOCALDOMAIN 194to a space-separated list of search domains. 195.Pp 196The 197.Sy options 198keyword of a system's 199.Pa resolv.conf 200file can be amended on a per-process basis by setting the 201environment variable 202.Ev RES_OPTIONS 203to a space-separated list of resolver options as explained above. 204.Pp 205The keyword and value must appear on a single line, and the keyword 206(e.g. 207.Sy nameserver ) 208must start the line. The value follows 209the keyword, separated by white space. 210.Sh FILES 211.Bl -tag -width /etc/resolv.conf -compact 212.It Pa /etc/resolv.conf 213The file 214.Nm resolv.conf 215resides in 216.Pa /etc . 217.El 218.Sh SEE ALSO 219.Xr gethostbyname 3 , 220.Xr resolver 3 , 221.Xr hostname 7 , 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