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[thirdparty/man-pages.git] / man5 / resolv.conf.5

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.\"
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.\" provided that the above copyright notice and this paragraph are
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.\" 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 8.6 1999/05/21 00:01:02 vixie Exp $
.\"
.\" Added ndots remark by Bernhard R. Link - debian bug #182886
.\"
.TH RESOLV.CONF 5 2013-02-12 "" "Linux Programmer's Manual"
.UC 4
.SH NAME
resolv.conf \- resolver configuration file
.SH SYNOPSIS
.B /etc/resolv.conf
.SH DESCRIPTION
The
.I resolver
is a set of routines in the C library
that provide access to the Internet Domain Name System (DNS).
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
If this file does not exist,
only the name server on the local machine will be queried;
the domain name is determined from the hostname
and the domain search path is constructed from the domain name.
.LP
The different configuration options are:
.TP
\fBnameserver\fP Name server IP address
Internet address of a name server that the resolver should query,
either an IPv4 address (in dot notation),
or an IPv6 address in colon (and possibly dot) notation as per RFC 2373.
Up to
.B MAXNS
(currently 3, see \fI<resolv.h>\fP) 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 hostname returned by
.BR gethostname (2);
the domain part is taken to be everything after the first \(aq.\(aq.
Finally, if the hostname 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.
Resolver queries having fewer than
.I ndots
dots (default is 1) in them will be attempted using each component
of the search path in turn until a match is found.
For environments with multiple subdomains please read
.BI "options ndots:" n
below to avoid man-in-the-middle attacks and unnecessary
traffic for the root-dns-servers.
.\" When having a resolv.conv with a line
.\"  search subdomain.domain.tld domain.tld
.\" and doing a hostlookup, for example by
.\"  ping host.anothersubdomain
.\" it sends dns-requests for
.\"  host.anothersubdomain.
.\"  host.anothersubdomain.subdomain.domain.tld.
.\"  host.anothersubdomain.domain.tld.
.\" thus not only causing unnecessary traffic for the root-dns-servers
.\" but broadcasting information to the outside and making man-in-the-middle
.\" attacks possible.
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
This option allows addresses returned by
.BR gethostbyname (3)
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.
Here is an example:

.in +4n
sortlist 130.155.160.0/255.255.240.0 130.155.0.0
.in
.br
.TP
\fBoptions\fP
Options allows certain internal resolver variables to be modified.
The syntax is
.RS
.IP
\fBoptions\fP \fIoption\fP \fI...\fP
.LP
where \fIoption\fP is one of the following:
.TP
\fBdebug\fP
.\" Since glibc 2.2?
sets
.BR RES_DEBUG
in
.IR _res.options .
.TP
.BI ndots: n
.\" Since glibc 2.2
sets a threshold for the number of dots which
must appear in a name given to
.BR res_query (3)
(see
.BR resolver (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.