1.\" $NetBSD: resolv.conf.5,v 1.21 2001/10/02 05:21:21 itojun 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 January 16, 1999 37.Dt RESOLV.CONF 5 38.Os 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 62IPv4 address 63.Pq in dot notation 64or IPv6 address 65.Pq in hex-and-colon notation 66of a name server that the resolver should query. 67Scoped IPv6 address notation is accepted as well 68.Po 69see 70.Xr inet6 4 71for details 72.Pc . 73Up to 74.Dv MAXNS 75(currently 3) name servers may be listed, 76one per keyword. 77If there are multiple servers, 78the resolver library queries them in the order listed. 79If no 80.Sy nameserver 81entries are present, 82the default is to use the name server on the local machine. 83(The algorithm used is to try a name server, and if the query times out, 84try the next, until out of name servers, 85then repeat trying all the name servers 86until a maximum number of retries are made). 87.It Sy domain 88Local domain name. 89Most queries for names within this domain can use short names 90relative to the local domain. 91If no 92.Sy domain 93entry is present, the domain is determined 94from the local host name returned by 95.Xr gethostname 3 ; 96the domain part is taken to be everything after the first `.'. 97Finally, if the host name does not contain a domain part, the root 98domain is assumed. 99.It Sy lookup 100This keyword is now ignored: its function has been superseded by 101features of 102.Xr nsswitch.conf 5 . 103.Pp 104.It Sy search 105Search list for host-name lookup. 106The search list is normally determined from the local domain name; 107by default, it begins with the local domain name, then successive 108parent domains that have at least two components in their names. 109This may be changed by listing the desired domain search path 110following the 111.Sy search 112keyword with spaces or tabs separating 113the names. 114Most resolver queries will be attempted using each component 115of the search path in turn until a match is found. 116Note that this process may be slow and will generate a lot of network 117traffic if the servers for the listed domains are not local, 118and that queries will time out if no server is available 119for one of the domains. 120.Pp 121The search list is currently limited to six domains 122with a total of 1024 characters. 123.It Sy sortlist 124Sortlist allows addresses returned by gethostbyname to 125be sorted. 126A sortlist is specified by IP address netmask pairs. 127The netmask is optional and defaults to the natural 128netmask of the net. 129The IP address and optional network pairs are separated by 130slashes. 131Up to 10 pairs may be specified, ie. 132.Pp 133.Sy sortlist 130.155.160.0/255.255.240.0 130.155.0.0 134.It Sy options 135Options allows certain internal resolver variables to be modified. 136The syntax is: 137.Pp 138.Sy options option ... 139.Pp 140where option is one of the following: 141.Bl -tag -width insecure1 142.It Sy debug 143enable debugging information, by setting RES_DEBUG in _res.options 144(see 145.Xr resolver 3 ) . 146.It Sy edns0 147attach OPT pseudo-RR for ENDS0 extension specified in RFC2671, 148to inform DNS server of our receive buffer size. 149The option will allow DNS servers to take advantage of non-default receive 150buffer size, and to send larger replies. 151DNS query packets with EDNS0 extension is not compatible with 152non-EDNS0 DNS servers. 153The option must be used only when all the DNS servers listed in 154.Sy nameserver 155lines are able to handle EDNS0 extension. 156.It Sy inet6 157enable support for IPv6-only applications, by setting RES_USE_INET6 in 158_res.options (see 159.Xr resolver 3 ) . 160The option is meaningful with certain kernel configuration only and 161use of this options is discouraged. 162.It Sy insecure1 163Do not require IP source address on the reply packet to be equal to the 164servers' address. 165.It Sy insecure2 166Do not check if the query section of the reply packet is equal 167to that of the query packet. 168For testing purposes only. 169.It Sy ndots:n 170sets a threshold for the number of dots which 171must appear in a name given to res_query (see 172.Xr resolver 3 ) 173before an initial absolute query will be made. 174The default for n is 1, meaning that if there are any 175dots in a name, the name will be tried first as an absolute 176name before any search list elements are appended to it. 177.El 178.El 179.Pp 180The 181.Sy domain 182and 183.Sy search 184keywords are mutually exclusive. 185If more than one instance of these keywords is present, 186the last instance will override. 187.Pp 188The 189.Sy search 190keyword of a system's 191.Pa resolv.conf 192file can be overridden on a per-process basis by setting the 193environment variable 194.Ev LOCALDOMAIN 195to a space-separated list of search domains. 196.Pp 197The 198.Sy options 199keyword of a system's 200.Pa resolv.conf 201file can be amended on a per-process basis by setting the 202environment variable 203.Ev RES_OPTIONS 204to a space-separated list of resolver options as explained above. 205.Pp 206The keyword and value must appear on a single line, and the keyword 207(e.g. 208.Sy nameserver ) 209must start the line. The value follows 210the keyword, separated by white space. 211.Sh FILES 212.Bl -tag -width /etc/resolv.conf -compact 213.It Pa /etc/resolv.conf 214The file 215.Nm resolv.conf 216resides in 217.Pa /etc . 218.El 219.Sh SEE ALSO 220.Xr gethostbyname 3 , 221.Xr resolver 3 , 222.Xr nsswitch.conf 5 , 223.Xr hostname 7 , 224.Xr named 8 225.Rs 226.%T "Name Server Operations Guide for BIND" 227.Re 228.Sh HISTORY 229The 230.Nm resolv.conf 231file format appeared in 232.Bx 4.3 . 233