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.\" $FreeBSD$ 30.\" 31.Dd October 12, 2015 32.Dt RESOLVER 5 33.Os 34.Sh NAME 35.Nm resolver 36.Nd resolver configuration file 37.Sh SYNOPSIS 38.Nm resolv.conf 39.Sh DESCRIPTION 40The 41.Xr resolver 3 42is a set of routines in the C library 43which provide access to the Internet Domain Name System. 44The resolver configuration file contains information that is read 45by the resolver routines the first time they are invoked by a process. 46The file is designed to be human readable and contains a list of 47keywords with values that provide various types of resolver information. 48.Pp 49On a normally configured system this file should not be necessary. 50The only name server to be queried will be on the local machine, 51the domain name is determined from the host name, 52and the domain search path is constructed from the domain name. 53.Pp 54The different configuration options are: 55.Bl -tag -width nameserver 56.It Sy nameserver 57Internet address (in dot notation) of a name server 58that the resolver should query. 59Up to 60.Dv MAXNS 61(currently 3) name servers may be listed, 62one per keyword. 63If there are multiple servers, 64the resolver library queries them in the order listed. 65If no 66.Sy nameserver 67entries are present, 68the default is to use the name server on the local machine. 69(The algorithm used is to try a name server, and if the query times out, 70try the next, until out of name servers, 71then repeat trying all the name servers 72until a maximum number of retries are made). 73.It Sy domain 74Local domain name. 75Most queries for names within this domain can use short names 76relative to the local domain. 77If no 78.Sy domain 79entry is present, the domain is determined 80from the local host name returned by 81.Xr gethostname 3 ; 82the domain part is taken to be everything after the first 83.Ql \&. . 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. 113E.g., 114.Pp 115.Dl "sortlist 130.155.160.0/255.255.240.0 130.155.0.0" 116.It Sy options 117Options allows certain internal resolver variables to be modified. 118The syntax is 119.Pp 120\fBoptions\fP \fIoption\fP \fI...\fP 121.Pp 122where 123.Sy option 124is one of the following: 125.Bl -tag -width no_tld_query 126.It Sy debug 127sets 128.Dv RES_DEBUG 129in _res.options. 130.It Sy ndots: Ns Ar 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. 138The default for 139.Em n 140is 141.Dq 1 , 142meaning that if there are any dots in a name, the name 143will be tried first as an absolute name before any 144.Em search list 145elements are appended to it. 146.It Sy timeout: Ns Ar n 147sets the initial amount of time the resolver will wait 148for a response from a remote 149name server before retrying the query via a different name server. 150The resolver may wait longer during subsequent retries 151of the current query since an exponential back-off is applied to 152the timeout value. 153Measured in seconds, the default is 154.Dv RES_TIMEOUT , 155the allowed maximum is 156.Dv RES_MAXRETRANS 157(see 158.In resolv.h ) . 159.It Sy attempts: Ns Ar n 160sets the number of times the resolver will send a query to each of 161its name servers 162before giving up and returning an error to the calling application. 163The default is 164.Dv RES_DFLRETRY , 165the allowed maximum is 166.Dv RES_MAXRETRY 167(see 168.In resolv.h ) . 169.It Sy no_tld_query 170tells the resolver not to attempt to resolve a top level domain name, that 171is, a name that contains no dots. 172Use of this option does not prevent 173the resolver from obeying the standard 174.Sy domain 175and 176.Sy search 177rules with the given name. 178.It Sy reload-period: Ns Ar n 179The resolver checks the modification time of 180.Pa /etc/resolv.conf 181every 182.Ar n 183seconds. 184If 185.Pa /etc/resolv.conf 186has changed, it is automatically reloaded. 187The default for 188.Ar n 189is two seconds. 190Setting it to zero disables the file check. 191.El 192.Pp 193Options may also be specified as a space or tab separated list using the 194.Dv RES_OPTIONS 195environment variable. 196.El 197.Pp 198The 199.Sy domain 200and 201.Sy search 202keywords are mutually exclusive. 203If more than one instance of these keywords is present, 204the last instance will override. 205.Pp 206The keyword and value must appear on a single line, and the keyword 207.Pq for example, Sy nameserver 208must start the line. 209The value follows the 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.Rs 223.%T "Name Server Operations Guide for BIND" 224.Re 225.Sh HISTORY 226The 227.Nm resolv.conf 228file format appeared in 229.Bx 4.3 . 230