man-pages/man5/resolv.conf.5

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.\"	@(#)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 2004-10-31 "" "Linux Programmer's Manual"
.UC 4
.SH NAME
resolv.conf \- resolver configuration file
.SH SYNOPSIS
/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
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 Name server IP address
Internet address (in dot notation) of a name server
that the resolver should query.
Up to 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 host name returned by
.BR gethostname (3);
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.
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
Sortlist 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.
E.g.,
.br
.in +2
sortlist 130.155.160.0/255.255.240.0 130.155.0.0
.in -2
.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
sets RES_DEBUG in
.IR _res.options .
.TP
.BI ndots: n
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.
.TP
.BI timeout: n
sets the amount of time the resolver will wait for a
response from a remote name server before retrying the
query via a different name server.
Measured in seconds,
the default is RES_TIMEOUT (currently 5, see \fI<resolv.h>\fP).
.TP
.BI attempts: n
sets the number of times the resolver will send a
query to its name servers before giving up and returning
an error to the calling application.
The default is RES_DFLRETRY (currently 2, see \fI<resolv.h>\fP).
.TP
.B rotate
sets RES_ROTATE in
.IR _res.options ,
which causes round robin selection of nameservers from among those listed.
This has the effect of spreading the query load among all listed servers,
rather than having all clients try the first listed server first every time.
.TP
.B no-check-names
sets RES_NOCHECKNAME in
.IR _res.options ,
which disables the modern BIND checking of incoming host names and
mail names for invalid characters such as underscore (_), non-ASCII,
or control characters.
.TP
.B inet6
sets RES_USE_INET6 in
.IR _res.options .
This has the effect of trying a AAAA query before an A query inside the
.BR gethostbyname (3)
function, and of mapping IPv4 responses in IPv6 ``tunneled form''
if no AAAA records are found but an A record set exists.
.RE
.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 white space.
.SH FILES
.IR /etc/resolv.conf ,
.I <resolv.h>
.SH "SEE ALSO"
.BR gethostbyname (3),
.BR resolver (3),
.BR hostname (7),
.BR named (8)
.br
Name Server Operations Guide for BIND