.\" Copyright (c) 1986 The Regents of the University of California. .\" All rights reserved. .\" .\" Redistribution and use in source and binary forms are permitted .\" provided that the above copyright notice and this paragraph are .\" duplicated in all such forms and that any documentation, .\" advertising materials, and other materials related to such .\" distribution and use acknowledge that the software was developed .\" by the University of California, Berkeley. The name of the .\" University may not be used to endorse or promote products derived .\" from this software without specific prior written permission. .\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR .\" IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED .\" WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. .\" .\" @(#)resolver.5 5.9 (Berkeley) 12/14/89 .\" $Id: resolver.5,v 8.4 1997/03/14 02:29:49 vixie Exp $ .\" .Dd November 11, 1993 .Dt RESOLVER @FORMAT_EXT_U@ .Os BSD 4 .Sh NAME .Nm resolver .Nd resolver configuration file .Sh SYNOPSIS .Pa /etc/resolv.conf .Sh DESCRIPTION The .Nm resolver is a set of routines in the C library .Pq Xr resolve @LIB_NETWORK_EXT@ that provide access to the Internet Domain Name System. The .Nm resolver configuration file contains information that is read by the .Nm resolver routines the first time they are invoked by a process. The file is designed to be human readable and contains a list of keywords with values that provide various types of .Nm resolver information. .Pp On a normally configured system, this file should not be necessary. The only name server to be queried will be on the local machine, the domain name is determined from the host name, and the domain search path is constructed from the domain name. .Pp The different configuration options are: .Bl -tag -width "nameser" .It Li nameserver Internet address (in dot notation) of a name server that the .Nm resolver should query. Up to .Dv MAXNS (currently 3) name servers may be listed, one per keyword. If there are multiple servers, the .Nm resolver library queries them in the order listed. If no .Li nameserver entries are present, the default is to use the name server on the local machine. (The algorithm used is to try a name server, and if the query times out, try the next, until out of name servers, then repeat trying all the name servers until a maximum number of retries are made). .It Li domain Local domain name. Most queries for names within this domain can use short names relative to the local domain. If no .Li domain entry is present, the domain is determined from the local host name returned by .Xr gethostname @BSD_SYSCALL_EXT@ ; the domain part is taken to be everything after the first .Sq \&. . Finally, if the host name does not contain a domain part, the root domain is assumed. .It Li search Search list for host-name lookup. The search list is normally determined from the local domain name; by default, it contains only the local domain name. This may be changed by listing the desired domain search path following the .Li search keyword with spaces or tabs separating the names. Most .Nm resolver queries will be attempted using each component of the search path in turn until a match is found. Note that this process may be slow and will generate a lot of network traffic if the servers for the listed domains are not local, and that queries will time out if no server is available for one of the domains. .Pp The search list is currently limited to six domains with a total of 256 characters. .It Li sortlist Allows addresses returned by gethostbyname to be sorted. A .Li sortlist is specified by IP address netmask pairs. The netmask is optional and defaults to the natural netmask of the net. The IP address and optional network pairs are separated by slashes. Up to 10 pairs may be specified. For example: .Bd -literal -offset indent sortlist 130.155.160.0/255.255.240.0 130.155.0.0 .Ed .It Li options Allows certain internal .Nm resolver variables to be modified. The syntax is .D1 Li options Ar option ... where .Ar option is one of the following: .Bl -tag -width "ndots:n " .It Li debug sets .Dv RES_DEBUG in .Ft _res.options . .It Li ndots: Ns Ar n sets a threshold for the number of dots which must appear in a name given to .Fn res_query (see .Xr resolver @LIB_NETWORK_EXT@ ) before an .Em initial absolute query will be made. The default for .Ar n is .Dq 1 , meaning that if there are .Em any dots in a name, the name will be tried first as an absolute name before any .Em search list elements are appended to it. .El .El .Pp The .Li domain and .Li search keywords are mutually exclusive. If more than one instance of these keywords is present, the last instance wins. .Pp The .Li search keyword of a system's .Pa resolv.conf file can be overridden on a per-process basis by setting the environment variable .Dq Ev LOCALDOMAIN to a space-separated list of search domains. .Pp The .Li options keyword of a system's .Pa resolv.conf file can be amended on a per-process basis by setting the environment variable .Dq Ev RES_OPTIONS to a space-separated list of .Nm resolver options as explained above under .Li options . .Pp The keyword and value must appear on a single line, and the keyword (e.g., .Li nameserver ) must start the line. The value follows the keyword, separated by white space. .Sh FILES .Pa /etc/resolv.conf .Sh SEE ALSO .Xr gethostbyname @LIB_NETWORK_EXT@ , .Xr hostname @DESC_EXT@ , .Xr @INDOT@named @SYS_OPS_EXT@ , .Xr resolver @LIB_NETWORK_EXT@ , .Xr resolver @FORMAT_EXT@ . .Dq Name Server Operations Guide for Sy BIND