NAME | SYNOPSIS | DESCRIPTION | FILES | SEE ALSO | COLOPHON
RESOLV.CONF(5) Linux Programmer's Manual RESOLV.CONF(5)
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 hostname 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 hostname returned by gethostname(2); the
domain part is taken to be everything after the first '.'. Finally, if
the hostname 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
This option 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. Here is an example:
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
options option ...
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. The value for this option is silently capped to
15.
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>). The value for this
option is silently capped to 30.
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>). The value for this option is silently capped
to 5.
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 hostnames 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
"tunneled form" if no AAAA records are found but an A record set
exists.
ip6-bytestring (since glibc 2.3.4)
sets RES_USE_BSTRING in _res.options. This causes reverse IPv6
lookups to be made using the bit-label format described in
RFC 2673; if this option is not set, then nibble format is used.
ip6-dotint/no-ip6-dotint (since glibc 2.3.4)
Clear/set RES_NOIP6DOTINT in _res.options. When this option is
clear (ip6-dotint), reverse IPv6 lookups are made in the
(deprecated) ip6.int zone; when this option is set (no-
ip6-dotint), reverse IPv6 lookups are made in the ip6.arpa zone
by default. This option is set by default.
edns0 (since glibc 2.6)
sets RES_USE_EDNSO in _res.options. This enables support for
the DNS extensions described in RFC 2671.
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.
/etc/resolv.conf, <resolv.h>
gethostbyname(3), resolver(3), hostname(7), named(8)
Name Server Operations Guide for BIND
This page is part of release 3.23 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/.
4th Berkeley Distribution 2009-03-01 RESOLV.CONF(5)