NAME
resolv.conf
,
resolv.conf.tail
—
resolver configuration files
DESCRIPTION
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
- 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 line. If there are multiple servers, the resolver library queries them in the order listed. If nonameserver
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 is assumed. 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 values are:
If the
lookup
keyword is not used in the system's resolv.conf file then the assumed order isbind file
. Furthermore, if the system's resolv.conf file does not exist, then the only database used isfile
. 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
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. 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 theresolv.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 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
- 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 nothing. 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,
allowing 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 nothing. 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 absolute name before any search list elements are appended to it.
tcp
- Forces the use of TCP for queries. Normal behaviour is to query via UDP but fall back to TCP on failure.
The domain
and
search
keywords are mutually exclusive. If more than
one instance of these keywords is present, the last instance will
override.
ENVIRONMENT
LOCALDOMAIN
- A space-separated list of search domains, overriding the
search
keyword of a system'sresolv.conf
orresolv.conf.tail
file. RES_OPTIONS
- A space-separated list of resolver options, overriding the
options
keyword of a system'sresolv.conf
orresolv.conf.tail
file.
FILES
- /etc/resolv.conf
- /etc/resolv.conf.tail
SEE ALSO
gethostbyname(3), resolver(3), hosts(5), hostname(7), dhclient(8), nsd(8), unbound(8)
HISTORY
The resolv.conf
file format appeared in
4.3BSD.