Name
resolv.conf — resolver configuration file
DESCRIPTION
The 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.
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.
The different configuration options are:
nameserver Name server IP
address-
Internet address (in dot notation) of a name server
that the resolver should query. Up to MAXNS (currently
3, see <resolv.h>) name servers may be listed,
one per keyword. If there are multiple servers, the
resolver library queries them in the order listed. If
no 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.)
domain Local domain
name.-
Most queries for names within this domain can use
short names relative to the local domain. If no
domain entry
is present, the domain is determined from the local
host name returned by gethostname(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.
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 search keyword with
spaces or tabs separating the names. Resolver queries
having fewer than 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
options
ndots:n below to avoid
man-in-the-middle attacks and unnecessary traffic for
the root-dns-servers. 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.
The search list is currently limited to six domains
with a total of 256 characters.
sortlist-
Sortlist allows addresses returned by 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.,
sortlist 130.155.160.0/255.255.240.0 130.155.0.0
options-
Options allows certain internal resolver variables
to be modified. The syntax is
where option is one of
the following:
debug-
sets RES_DEBUG in _res.options.
ndots:n-
sets a threshold for the number of dots
which must appear in a name given to res_query(3)
(see resolver(3))
before an initial
absolute query will be made. The
default for n is ``1'',
meaning that if there are any dots in a name,
the name will be tried first as an absolute
name before any search list
elements are appended to it.
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
<resolv.h>).
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
<resolv.h>).
- rotate
-
sets RES_ROTATE in _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.
- no-check-names
-
sets RES_NOCHECKNAME in _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.
- inet6
-
sets RES_USE_INET6 in _res.options.
This has the effect of trying a AAAA query
before an A query inside the gethostbyname(3)
function, and of mapping IPv4 responses in
IPv6 ``tunnelled form'' if no AAAA records
are found but an A record set exists.
The domain
and search
keywords are mutually exclusive. If more than one
instance of these keywords is present, the last
instance wins.
The search
keyword of a system's resolv.conf file can be
overridden on a per-process basis by setting the environment
variable ``LOCALDOMAIN'' to a space-separated list of search
domains.
The options
keyword of a system's resolv.conf file can be
amended on a per-process basis by setting the environment
variable ``RES_OPTIONS'' to a space-separated list of
resolver options as explained above under options.
The keyword and value must appear on a single line, and
the keyword (e.g. nameserver) must start the
line. The value follows the keyword, separated by white
space.
FILES
/etc/resolv.conf, <resolv.h>
SEE ALSO
gethostbyname(3), resolver(3), hostname(7), named(8)
Name Server Operations Guide for BIND
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 8.6 1999/05/21 00:01:02 vixie Exp $
Added ndots remark by Bernhard R. Link - debian bug #182886
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