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RESOLV.CONF(5) NetBSD File Formats Manual RESOLV.CONF(5)
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resolv.conf -- resolver configuration file
The resolv.conf file specifies how the resolver(3) routines in the C
library (which provide access to the Internet Domain Name System) should
operate. 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 informa-
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 con-
structed from the domain name.
The different configuration options are:
nameserver IPv4 address (in dot notation) or IPv6 address (in
hex-and-colon notation) of a name server that the resolver
should query. Scoped IPv6 address notation is accepted as
well (see inet6(4) for details). Up to MAXNS (currently 3)
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
lookup This keyword is now ignored: its function has been superseded
by features of nsswitch.conf(5).
search Search list for host-name lookup. The search list is nor-
mally determined from the local domain name; by default, it
begins with the local domain name, then successive parent
domains that have at least two components in their names.
This may be changed by listing the desired domain search path
following the search keyword with spaces or tabs separating
the names. Most resolver queries will be attempted using
each component of the search path in turn until a match is
found. 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 1024 characters.
sortlist Sortlist allows addresses returned by gethostbyname 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, ie.
sortlist 188.8.131.52/255.255.240.0 184.108.40.206
options Options allows certain internal resolver variables to be mod-
ified. The syntax is:
options option ...
where option is one of the following:
debug enable debugging information, by setting
RES_DEBUG in _res.options (see resolver(3)).
ndots:n sets a threshold for the number of dots which
must appear in a name given to res_query (see
resolver(3)) before an initial absolute query
will be made. The default for n is 1, mean-
ing 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
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 (see <resolv.h>).
attempts:n sets the number of times the resolver will
send a query to its name servers before giv-
ing up and returning an error to the calling
application. The default is RES_DFLRETRY
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 charac-
ters such as underscore (`_'), non-ASCII, or
check-names clears RES_NOCHECKNAME in _res.options, which
enables the modern BIND checking of incoming
host names and mail names as described above.
This is the default.
edns0 attach OPT pseudo-RR for ENDS0 extension
specified in RFC 2671, to inform DNS server
of our receive buffer size. The option will
allow DNS servers to take advantage of non-
default receive buffer size, and to send
larger replies. DNS query packets with EDNS0
extension is not compatible with non-EDNS0
DNS servers. The option must be used only
when all the DNS servers listed in nameserver
lines are able to handle EDNS0 extension.
inet6 enable support for IPv6-only applications, by
setting RES_USE_INET6 in _res.options (see
resolver(3)). The option is meaningful with
certain kernel configuration only and use of
this option is discouraged.
insecure1 Do not require IP source address on the reply
packet to be equal to the servers' address.
insecure2 Do not check if the query section of the
reply packet is equal to that of the query
packet. For testing purposes only.
no-tld-query sets RES_NOTLDQUERY in _res.options. This
option causes res_nsearch() to not attempt to
resolve a unqualified name as if it were a
top level domain (TLD). This option can
cause problems if the site has ``localhost''
as a TLD rather than having localhost on one
or more elements of the search list. This
option has no effect if neither RES_DEFNAMES
or RES_DNSRCH is set.
The domain and search keywords are mutually exclusive. If more than one
instance of these keywords is present, the last instance will override.
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.
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, sepa-
rated by white space.
/etc/resolv.conf The file resolv.conf resides in /etc.
gethostbyname(3), resolver(3), nsswitch.conf(5), hostname(7), named(8),
Paul Vixie, Kevin J. Dunlap, and Michael J. Karels, Name Server
Operations Guide for BIND, CSRG, Department of Electrical Engineering and
Computer Sciences, University of California, Berkeley, Release 4.9.4,
http://www.dns.net/dnsrd/docs/bog/bog.html, July 16, 1996.
The resolv.conf file format appeared in 4.3BSD.
NetBSD 10.99 Arril 30, 2021 NetBSD 10.99