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.\" 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
|
