resolver configuration files
file specifies how the
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
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.
is used to
configure the network it will normally overwrite the
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
routines, the file
may be created manually. This
file will be appended to the generated
file by dhclient, ensuring options
remain. If no updated information is available to dhclient, and
is not present, then
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
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.
) 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:
- 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).
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.)
- 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
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
- This keyword is used by the library routines
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 is bind file. Furthermore, if the
system's resolv.conf file does not exist,
then the only database used is file.
- Query a domain name server.
- Search for entries in
- 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
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.
- Allows addresses returned by
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:
- 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:
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.
- Allows certain internal resolver variables to be modified.
The syntax is:
Where option is one of the following:
- Print debugging messages, if libc is compiled with
DEBUG. By default on
OpenBSD this option does nothing.
- 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. On OpenBSD this option does
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
- Enables support for IPv6-only applications, by setting
RES_USE_INET6 in _res.options (see
OpenBSD this option does nothing.
- Do not require IP source address on the reply packet to
be equal to the server's address.
- Do not check if the query section of the reply packet
is equal to that of the query packet. For testing purposes only.
- Sets a threshold for the number of dots which must
appear in a name given to
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.
- Forces the use of TCP for queries. Normal behaviour is
to query via UDP but fall back to TCP on failure.
keywords are mutually exclusive. If more than one instance of these keywords
is present, the last instance will override.
- A space-separated list of search domains, overriding the
search keyword of a system's
- A space-separated list of resolver options, overriding the
options keyword of a system's
file format appeared in