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. Neither the name of the University nor the names of its contributors 13.\" may be used to endorse or promote products derived from this software 14.\" without specific prior written permission. 15.\" 16.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 17.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 20.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26.\" SUCH DAMAGE. 27.\" 28.\" @(#)resolver.5 8.1 (Berkeley) 6/5/93 29.\" $FreeBSD: src/share/man/man5/resolver.5,v 1.7.2.1 2001/08/17 13:08:47 ru Exp $ 30.\" $DragonFly: src/share/man/man5/resolver.5,v 1.4 2008/05/02 02:05:06 swildner Exp $ 31.\" 32.Dd November 11, 1993 33.Dt RESOLVER 5 34.Os 35.Sh NAME 36.Nm resolver 37.Nd resolver configuration file 38.Sh SYNOPSIS 39.Nm resolv.conf 40.Sh DESCRIPTION 41The 42.Xr resolver 3 43is a set of routines in the C library 44which provide access to the Internet Domain Name System. 45The resolver configuration file contains information that is read 46by the resolver routines the first time they are invoked by a process. 47The file is designed to be human readable and contains a list of 48keywords with values that provide various types of resolver information. 49.Pp 50On a normally configured system this file should not be necessary. 51The only name server to be queried will be on the local machine, 52the domain name is determined from the host name, 53and the domain search path is constructed from the domain name. 54.Pp 55The different configuration options are: 56.Bl -tag -width nameserver 57.It Sy nameserver 58Internet address (in dot notation) of a name server 59that the resolver should query. 60Up to 61.Dv MAXNS 62(currently 3) name servers may be listed, 63one per keyword. 64If there are multiple servers, 65the resolver library queries them in the order listed. 66If no 67.Sy nameserver 68entries are present, 69the default is to use the name server on the local machine. 70(The algorithm used is to try a name server, and if the query times out, 71try the next, until out of name servers, 72then repeat trying all the name servers 73until a maximum number of retries are made). 74.It Sy domain 75Local domain name. 76Most queries for names within this domain can use short names 77relative to the local domain. 78If no 79.Sy domain 80entry is present, the domain is determined 81from the local host name returned by 82.Xr gethostname 3 ; 83the domain part is taken to be everything after the first `.'. 84Finally, if the host name does not contain a domain part, the root 85domain is assumed. 86.It Sy search 87Search list for host-name lookup. 88The search list is normally determined from the local domain name; 89by default, it contains only the local domain name. 90This may be changed by listing the desired domain search path 91following the 92.Sy search 93keyword with spaces or tabs separating 94the names. 95Most resolver queries will be attempted using each component 96of the search path in turn until a match is found. 97Note that this process may be slow and will generate a lot of network 98traffic if the servers for the listed domains are not local, 99and that queries will time out if no server is available 100for one of the domains. 101.Pp 102The search list is currently limited to six domains 103with a total of 256 characters. 104.It Sy sortlist 105Sortlist allows addresses returned by gethostbyname to be sorted. 106A sortlist is specified by IP address netmask pairs. 107The netmask is 108optional and defaults to the natural netmask of the net. 109The IP address 110and optional network pairs are separated by slashes. 111Up 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.Bl -tag -width no_tld_query 125.It Sy debug 126sets 127.Dv RES_DEBUG 128in _res.options. 129.It Sy ndots:n 130sets a threshold for the number of dots which must appear in a name given to 131.Fn res_query 132(see 133.Xr resolver 3 ) 134before an 135.Em initial absolute query 136will be made. 137The 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.Ev 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