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 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 an interface it will overwrite resolv.conf
whenever the interface becomes the default gateway. The information written
is generated from the DHCP options domain-name-servers, domain-name and
domain-search and the contents of the file
resolv.conf.tail
are appended to the generated
information. If dhclient has no domain-name-servers, no domain-name and no
domain-search information then it will not overwrite the existing
resolv.conf
, even if
resolv.conf.tail
exists.
resolv.conf.tail
is normally used to specify options
that are not available via DHCP e.g. lookup or family.
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
Up to ASR_MAXNS
(currently 5) 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.
To verify whether a server supports EDNS, query it using
the dig(1) query option
+edns=0
: the reply indicates compliance
(EDNS version 0) and whether a UDP packet larger than 512 bytes can
be used. Note that EDNS0 can cause the server to send packets large
enough to require fragmentation. Other factors such as packet
filters may impede these, particularly if there is a reduced MTU, as
is often the case with pppoe(4) or
with tunnels.
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), res_init(3), hosts(5), hostname(7), dhclient(8), nsd(8), rebound(8), unbound(8)
The resolv.conf
file format appeared in
4.3BSD.
August 30, 2019 | OpenBSD-current |