RESOLV.CONF(5) BSD 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 over-
write 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 informa-
tion 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 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). A non-standard port may also be spec-
ified, with the host address enclosed in square brackets.
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 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 dot. Finally, if
the host name does not contain a domain part, the root domain
lookup This keyword is used by the library routines gethostbyname(3)
and gethostbyaddr(3). It specifies which databases should be
searched, and the order to do so. The legal space-separated
bind Query a domain name server.
file Search for entries in /etc/hosts.
yp Talk to the YP system if ypbind(8) is running.
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 list for hostname lookup. The search list is normally
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 follow-
ing 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 avail-
able 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 over-
writes 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 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 sepa-
rated by slashes. Up to 10 pairs may be specified. For
sortlist 126.96.36.199/255.255.240.0 188.8.131.52
family Specify which type of Internet protocol family to prefer, if
a host is reachable using different address families. By
default IPv4 addresses are queried first, and then IPv6
addresses. The syntax is:
family family [family]
A maximum of two families can be specified, where family can
be any of:
inet4 IPv4 queries.
inet6 IPv6 queries.
If only one family is specified, only that family is tried.
options Allows certain internal resolver variables to be modified.
The syntax is:
options option ...
Where option is one of the following:
debug Print debugging messages, if libc is compiled with
DEBUG. By default on OpenBSD this option does
edns0 Attach an OPT pseudo-RR for the EDNS0 extension,
as specified in RFC 2671. This informs DNS
servers of a client's receive buffer size, allow-
ing them to take advantage of a non-default
receive buffer size, and thus send larger replies.
DNS query packets with the EDNS0 extension are not
compatible with non-EDNS0 DNS servers, so the
option must be used only when all the servers
listed in nameserver lines are able to handle the
extension. By default on OpenBSD this option does
inet6 Enables support for IPv6-only applications, by
setting RES_USE_INET6 in _res.options (see
resolver(3)). Use of this option is discouraged,
and meaningless on OpenBSD.
insecure1 Do not require IP source address on the reply
packet to be equal to the server's 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.
ndots:n Sets a threshold for the number of dots which must
appear in a name given to res_query(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 abso-
lute name before any search list elements are
appended to it.
tcp Forces the use of TCP for queries. Normal behav-
iour is to query via UDP but fall back to TCP on
The domain and search keywords are mutually exclusive. If more than one
instance of these keywords is present, the last instance will override.
LOCALDOMAIN A space-separated list of search domains, overriding the
search keyword of a system's resolv.conf or
RES_OPTIONS A space-separated list of resolver options, overriding
the options keyword of a system's resolv.conf or
gethostbyname(3), resolver(3), hosts(5), hostname(7), dhclient(8),
The resolv.conf file format appeared in 4.3BSD.
BSD November 5, 2014 BSD