resolv.conf — resolver configuration file
/etc/resolv.conf
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
addressInternet 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(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.
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.
sortlistSortlist 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
optionsOptions allows certain internal resolver variables to be modified. The syntax is
- options option ...
where
optionis one of the following:
debugsets
RES_DEBUGin_res.options.ndots:nsets 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
nis 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:nsets 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:nsets 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_ROTATEin_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_NOCHECKNAMEin_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_INET6in_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 "tunneled 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.
This page is part of release 2.79 of the Linux man-pages project. A
description of the project, and information about reporting
bugs, can be found at
http://www.kernel.org/doc/man-pages/.
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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 8.6 1999/05/21 00:01:02 vixie Exp $ Added ndots remark by Bernhard R. Link - debian bug #182886 |