1.\" Copyright (c) 1986, 1991, 1993 2.\" The Regents of the University of California. All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 3. All advertising materials mentioning features or use of this software 13.\" must display the following acknowledgement: 14.\" This product includes software developed by the University of 15.\" California, Berkeley and its contributors. 16.\" 4. Neither the name of the University nor the names of its contributors 17.\" may be used to endorse or promote products derived from this software 18.\" without specific prior written permission. 19.\" 20.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 21.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 22.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 23.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 24.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 25.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 26.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 27.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 28.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 29.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 30.\" SUCH DAMAGE. 31.\" 32.\" @(#)resolver.5 8.1 (Berkeley) 6/5/93 33.\" $FreeBSD$ 34.\" 35.Dd November 11, 1993 36.Dt RESOLVER 5 37.Os BSD 4 38.Sh NAME 39.Nm resolver 40.Nd resolver configuration file 41.Sh SYNOPSIS 42.Nm resolv.conf 43.Sh DESCRIPTION 44The 45.Xr resolver 3 46is a set of routines in the C library 47which provide access to the Internet Domain Name System. 48The resolver configuration file contains information that is read 49by the resolver routines the first time they are invoked by a process. 50The file is designed to be human readable and contains a list of 51keywords with values that provide various types of resolver information. 52.Pp 53On a normally configured system this file should not be necessary. 54The only name server to be queried will be on the local machine, 55the domain name is determined from the host name, 56and the domain search path is constructed from the domain name. 57.Pp 58The different configuration options are: 59.Bl -tag -width nameserver 60.It Sy nameserver 61Internet address (in dot notation) of a name server 62that the resolver should query. 63Up to 64.Dv MAXNS 65(currently 3) name servers may be listed, 66one per keyword. 67If there are multiple servers, 68the resolver library queries them in the order listed. 69If no 70.Sy nameserver 71entries are present, 72the default is to use the name server on the local machine. 73(The algorithm used is to try a name server, and if the query times out, 74try the next, until out of name servers, 75then repeat trying all the name servers 76until a maximum number of retries are made). 77.It Sy domain 78Local domain name. 79Most queries for names within this domain can use short names 80relative to the local domain. 81If no 82.Sy domain 83entry is present, the domain is determined 84from the local host name returned by 85.Xr gethostname 3 ; 86the domain part is taken to be everything after the first `.'. 87Finally, if the host name does not contain a domain part, the root 88domain is assumed. 89.It Sy search 90Search list for host-name lookup. 91The search list is normally determined from the local domain name; 92by default, it contains only the local domain name. 93This may be changed by listing the desired domain search path 94following the 95.Sy search 96keyword with spaces or tabs separating 97the names. 98Most resolver queries will be attempted using each component 99of the search path in turn until a match is found. 100Note that this process may be slow and will generate a lot of network 101traffic if the servers for the listed domains are not local, 102and that queries will time out if no server is available 103for one of the domains. 104.Pp 105The search list is currently limited to six domains 106with a total of 256 characters. 107.It Sy sortlist 108Sortlist allows addresses returned by gethostbyname to be sorted. 109A sortlist is specified by IP address netmask pairs. The netmask is 110optional and defaults to the natural netmask of the net. The IP address 111and optional network pairs are separated by slashes. Up to 10 pairs may 112be specified. 113.Pp 114e.g. sortlist 130.155.160.0/255.255.240.0 130.155.0.0 115.It Sy options 116Options allows certain internal resolver variables to be modified. 117The syntax is 118.Pp 119\fBoptions\fP \fIoption\fP \fI...\fP 120.Pp 121where 122.Sy option 123is one of the following: 124.Pp 125.Bl -tag -width no_tld_query 126.It Sy debug 127sets 128.Dv RES_DEBUG 129in _res.options. 130.It Sy ndots:n 131sets a threshold for the number of dots which must appear in a name given to 132.Fn res_query 133(see 134.Xr resolver 3 ) 135before an 136.Em initial absolute query 137will be made. The default for 138.Em n 139is 140.Dq 1 , 141meaning that if there are any dots in a name, the name 142will be tried first as an absolute name before any 143.Em search list 144elements are appended to it. 145.It Sy no_tld_query 146tells the resolver not to attempt to resolve a top level domain name, that 147is, a name that contains no dots. Use of this option doesn't prevent 148the resolver from obeying the standard 149.Sy domain 150and 151.Sy search 152rules with the given name. 153.El 154.Pp 155Options may also be specified as a space or tab separated list using the 156.Dv RES_OPTIONS 157environment variable. 158.El 159.Pp 160The 161.Sy domain 162and 163.Sy search 164keywords are mutually exclusive. 165If more than one instance of these keywords is present, 166the last instance will override. 167.Pp 168The keyword and value must appear on a single line, and the keyword 169(e.g. 170.Sy nameserver ) 171must start the line. The value follows 172the keyword, separated by white space. 173.Sh FILES 174.Bl -tag -width /etc/resolv.conf -compact 175.It Pa /etc/resolv.conf 176The file 177.Nm resolv.conf 178resides in 179.Pa /etc . 180.El 181.Sh SEE ALSO 182.Xr gethostbyname 3 , 183.Xr resolver 3 , 184.Xr hostname 7 , 185.Xr named 8 186.Rs 187.%T "Name Server Operations Guide for BIND" 188.Re 189.Sh HISTORY 190The 191.Nm resolv.conf 192file format appeared in 193.Bx 4.3 . 194