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.\"	@(#)resolver.5	5.9 (Berkeley) 12/14/89
.\"	$From: resolver.5,v 8.3 1995/12/06 20:34:35 vixie Exp $
.\"
.TH RESOLVER 5 "November 11, 1993"
.UC 4
.SH NAME
resolver \- resolver configuration file
.SH SYNOPSIS
/etc/resolv.conf
.SH DESCRIPTION
.LP
The
.I resolver
is a set of routines in the C library (\c
.IR resolv (3))
that provide access to the Internet Domain Name System.
The resolver configuration file contains information that is read
by the 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 resolver information.
.LP
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.
.LP
The different configuration options are:
.TP
\fBnameserver\fP
Internet address (in dot notation) of a name server
that the resolver should query.
Up to MAXNS (currently 3) name servers may be listed,
one per keyword.
If there are multiple servers,
the resolver library queries them in the order listed.
If no \fBnameserver\fP 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).
.TP
\fBdomain\fP
Local domain name.
Most queries for names within this domain can use short names
relative to the local domain.
If no \fBdomain\fP entry is present, the domain is determined
from the local host name returned by
\fIgethostname\fP\|(2);
the domain part is taken to be everything after the first `.'.
Finally, if the host name does not contain a domain part, the root
domain is assumed.
.TP
\fBsearch\fP
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 \fIsearch\fP keyword with spaces or tabs separating
the names.
Most 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.
.IP
The search list is currently limited to six domains
with a total of 256 characters.
.TP
\fBsortlist\fP
Sortlist allows addresses returned by gethostbyname to be sorted.
A 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.
.IP
e.g., sortlist 130.155.160.0/255.255.240.0 130.155.0.0
.TP
\fBoptions\fP
Options allows certain internal resolver variables to be modified.
The syntax is
.IP
\fBoptions\fP \fIoption\fP \fI...\fP
.IP
where \fIoption\fP is one of the following:
.IP
\fBdebug\fP \(em sets RES_DEBUG in _res.options.
.IP
\fBndots:\fP\fIn\fP \(em sets a threshold for the number of dots which
must appear in a name given to \fBres_query\fP (see \fIresolver\fP(3))
before an \fIinitial absolute query\fP will be made.  The default for
\fIn\fP is ``1'', meaning that if there are any dots in a name, the name
will be tried first as an absolute name before any \fIsearch list\fP
elements are appended to it.
.LP
The \fIdomain\fP and \fIsearch\fP keywords are mutually exclusive.
If more than one instance of these keywords is present,
the last instance wins.
.LP
The \fIsearch\fP keyword of a system's \fIresolv.conf\fP file can be
overridden on a per-process basis by setting the environment variable
``\s-1LOCALDOMAIN\s+1'' to a space-separated list of search domains.
.LP
The \fIoptions\fP keyword of a system's \fIresolv.conf\fP file can be
amended on a per-process basis by setting the environment variable
``\s-1RES_OPTIONS\s+1'' to a space-separated list of resolver options
as explained above under \fBoptions\fP.
.LP
The keyword and value must appear on a single line, and the keyword
(e.g., \fBnameserver\fP) must start the line.  The value follows
the keyword, separated by whitespace.
.SH FILES
.I /etc/resolv.conf
.SH SEE ALSO
gethostbyname(2), resolver(3), hostname(7), named(8)
.br
Name Server Operations Guide for BIND