RESOLV.CONF(5) | File Formats Manual | RESOLV.CONF(5) |
resolv.conf
,
resolv.conf.tail
— resolver
configuration files
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. If
the resolv.conf
file does not exist, only the local
host file /etc/hosts will be consulted, i.e. the
Domain Name System will not be used to resolve hosts.
The file is designed to be human readable and contains a list of keywords with values that provide various types of resolver information. A resolv.conf file is not required for some setups, so this file is optional. It can be created manually, and is also created as part of the OpenBSD install process if use of the DHCP protocol is specified for any interface or if any DNS nameservers are configured.
If dhclient(8) is
used to configure the network it will normally overwrite the
resolv.conf
file with updated information such as
nameserver addresses, losing any previous values the file contained. In
order to force options to be passed to the
resolver(3) routines, the
file resolv.conf.tail
may be created manually. This
file will be appended to the generated resolv.conf
file by dhclient, ensuring options remain. If no updated information is
available to dhclient, and resolv.conf.tail
is not
present, then resolv.conf
will not be modified by
dhclient.
On a machine whose network connection does not change frequently
(such as a desktop machine on a local-area network), the
resolv.conf.tail
file should not be necessary.
However the resolv.conf.tail
file may be useful on
notebooks, to search multiple domains, to refer to hard-coded information in
local files, or otherwise override the defaults.
A keyword and its values must appear on a single line, and the
keyword (e.g. nameserver
) must start the line. The
value follows the keyword, separated by whitespace. A hash mark (#) or
semicolon (;) in the file indicates the beginning of a comment; subsequent
characters up to the end of the line are not interpreted by the routines
that read the file.
The configuration options (which may be placed in either file) are:
nameserver
nameserver [10.0.0.1]:5353 nameserver [::1]:5353
Up to MAXNS
(currently 3) name servers
may be listed, one per line. 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 name servers until a maximum
number of retries are performed.)
domain
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 dot.
Finally, if the host name does not contain a domain part, the root domain
is assumed.lookup
If the lookup
keyword is not used in
the system's resolv.conf file then the assumed
order is bind file
. Furthermore, if the system's
resolv.conf file does not exist, then the only
database used is file
.
search
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. Only one search
line
should appear; if more than one is present, the last one found
overwrites any values found in earlier lines. So if such a line appears
in the resolv.conf.tail
file, it should include
all the domains that need to be searched.
sortlist
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. For example:
sortlist
130.155.160.0/255.255.240.0 130.155.0.0
family
family
family [family]A maximum of two families can be specified, where family can be any of:
If only one family is specified, only that family is tried.
options
options
option ...Where option is one of the following:
debug
DEBUG
. By default on
OpenBSD this option does nothing.edns0
nameserver
lines are able to handle the
extension. By default on OpenBSD this option
does nothing.inet6
insecure1
insecure2
ndots
:ntcp
The domain
and
search
keywords are mutually exclusive. If more than
one instance of these keywords is present, the last instance will
override.
LOCALDOMAIN
search
keyword of a system's
resolv.conf
or
resolv.conf.tail
file.RES_OPTIONS
options
keyword of a system's
resolv.conf
or
resolv.conf.tail
file.gethostbyname(3), resolver(3), hosts(5), hostname(7), dhclient(8), nsd(8), unbound(8)
The resolv.conf
file format appeared in
4.3BSD.
November 5, 2014 | OpenBSD-5.8 |