1.\" $NetBSD: resolv.conf.5,v 1.25 2010/05/15 16:20:03 jruoho 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. Neither the name of the University nor the names of its contributors 15.\" may be used to endorse or promote products derived from this software 16.\" without specific prior written permission. 17.\" 18.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 19.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 20.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 21.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 22.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 23.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 24.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 25.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 26.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 27.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 28.\" SUCH DAMAGE. 29.\" 30.\" @(#)resolver.5 5.12 (Berkeley) 5/10/91 31.\" 32.Dd May 15, 2010 33.Dt RESOLV.CONF 5 34.Os 35.Sh NAME 36.Nm resolv.conf 37.Nd resolver configuration file 38.Sh DESCRIPTION 39The 40.Nm resolv.conf 41file specifies how the 42.Xr resolver 3 43routines in the C library 44(which provide access to the Internet Domain Name System) should operate. 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 58IPv4 address 59.Pq in dot notation 60or IPv6 address 61.Pq in hex-and-colon notation 62of a name server that the resolver should query. 63Scoped IPv6 address notation is accepted as well 64.Po 65see 66.Xr inet6 4 67for details 68.Pc . 69Up to 70.Dv MAXNS 71(currently 3) name servers may be listed, 72one per keyword. 73If there are multiple servers, 74the resolver library queries them in the order listed. 75If no 76.Sy nameserver 77entries are present, 78the default is to use the name server on the local machine. 79(The algorithm used is to try a name server, and if the query times out, 80try the next, until out of name servers, 81then repeat trying all the name servers 82until a maximum number of retries are made). 83.It Sy domain 84Local domain name. 85Most queries for names within this domain can use short names 86relative to the local domain. 87If no 88.Sy domain 89entry is present, the domain is determined 90from the local host name returned by 91.Xr gethostname 3 ; 92the domain part is taken to be everything after the first 93.Sq \&. . 94Finally, if the host name does not contain a domain part, the root 95domain is assumed. 96.It Sy lookup 97This keyword is now ignored: its function has been superseded by 98features of 99.Xr nsswitch.conf 5 . 100.Pp 101.It Sy search 102Search list for host-name lookup. 103The search list is normally determined from the local domain name; 104by default, it begins with the local domain name, then successive 105parent domains that have at least two components in their names. 106This may be changed by listing the desired domain search path 107following the 108.Sy search 109keyword with spaces or tabs separating 110the names. 111Most resolver queries will be attempted using each component 112of the search path in turn until a match is found. 113Note that this process may be slow and will generate a lot of network 114traffic if the servers for the listed domains are not local, 115and that queries will time out if no server is available 116for one of the domains. 117.Pp 118The search list is currently limited to six domains 119with a total of 1024 characters. 120.It Sy sortlist 121Sortlist allows addresses returned by gethostbyname to 122be sorted. 123A sortlist is specified by IP address netmask pairs. 124The netmask is optional and defaults to the natural 125netmask of the net. 126The IP address and optional network pairs are separated by 127slashes. 128Up to 10 pairs may be specified, ie. 129.Pp 130.Sy sortlist 130.155.160.0/255.255.240.0 130.155.0.0 131.It Sy options 132Options allows certain internal resolver variables to be modified. 133The syntax is: 134.Pp 135.Sy options option ... 136.Pp 137where option is one of the following: 138.Bl -tag -width insecure1 139.It Sy debug 140enable debugging information, by setting RES_DEBUG in _res.options 141(see 142.Xr resolver 3 ) . 143.It Sy edns0 144attach OPT pseudo-RR for ENDS0 extension specified in RFC 2671, 145to inform DNS server of our receive buffer size. 146The option will allow DNS servers to take advantage of non-default receive 147buffer size, and to send larger replies. 148DNS query packets with EDNS0 extension is not compatible with 149non-EDNS0 DNS servers. 150The option must be used only when all the DNS servers listed in 151.Sy nameserver 152lines are able to handle EDNS0 extension. 153.It Sy inet6 154enable support for IPv6-only applications, by setting RES_USE_INET6 in 155_res.options (see 156.Xr resolver 3 ) . 157The option is meaningful with certain kernel configuration only and 158use of this option is discouraged. 159.It Sy insecure1 160Do not require IP source address on the reply packet to be equal to the 161servers' address. 162.It Sy insecure2 163Do not check if the query section of the reply packet is equal 164to that of the query packet. 165For testing purposes only. 166.It Sy ndots:n 167sets a threshold for the number of dots which 168must appear in a name given to res_query (see 169.Xr resolver 3 ) 170before an initial absolute query will be made. 171The default for n is 1, meaning that if there are any 172dots in a name, the name will be tried first as an absolute 173name before any search list elements are appended to it. 174.El 175.El 176.Pp 177The 178.Sy domain 179and 180.Sy search 181keywords are mutually exclusive. 182If more than one instance of these keywords is present, 183the last instance will override. 184.Pp 185The 186.Sy search 187keyword of a system's 188.Pa resolv.conf 189file can be overridden on a per-process basis by setting the 190environment variable 191.Ev LOCALDOMAIN 192to a space-separated list of search domains. 193.Pp 194The 195.Sy options 196keyword of a system's 197.Pa resolv.conf 198file can be amended on a per-process basis by setting the 199environment variable 200.Ev RES_OPTIONS 201to a space-separated list of resolver options as explained above. 202.Pp 203The keyword and value must appear on a single line, and the keyword 204(e.g. 205.Sy nameserver ) 206must start the line. The value follows 207the keyword, separated by white space. 208.Sh FILES 209.Bl -tag -width /etc/resolv.conf -compact 210.It Pa /etc/resolv.conf 211The file 212.Nm resolv.conf 213resides in 214.Pa /etc . 215.El 216.Sh SEE ALSO 217.Xr gethostbyname 3 , 218.Xr resolver 3 , 219.Xr nsswitch.conf 5 , 220.Xr hostname 7 , 221.Xr named 8 222.Rs 223.%A Paul Vixie 224.%A Kevin J. Dunlap 225.%A Michael J. Karels 226.%T "Name Server Operations Guide for BIND" 227.%N Release 4.9.4 228.%I CSRG, 229.%I Department of Electrical Engineering and Computer Sciences, 230.%I University of California, Berkeley 231.%D July 16, 1996 232.%U http://www.dns.net/dnsrd/docs/bog/bog.html 233.Re 234.Sh HISTORY 235The 236.Nm resolv.conf 237file format appeared in 238.Bx 4.3 . 239