RESOLV.CONF(5) OpenBSD Programmer's Manual RESOLV.CONF(5)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, the DHCP client back-end
dhclient-script(8) 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-script(8), ensuring options remain.
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 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 be specified
using [host]:port syntax. When a non-standard port is
specified the host address must be enclosed in square
brackets. For example:
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 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 ``.''. 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:
bind Use the Domain Name server by querying named(8).
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
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 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
separated by slashes. Up to 10 pairs may be specified, e.g.:
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 family1 [family2]
A maximum of two families can be specified, where family can
be any of:
inet4 IPv4 queries.
inet6 IPv6 queries.
options Allows certain internal resolver variables to be modified.
The syntax is:
options option ...
where option is one of the following:
debug Sets RES_DEBUG in _res.options.
edns0 Attach OPT pseudo-RR for EDNS0 extension specified
in RFC 2671, to inform DNS server of our receive
buffer size. The option will allow DNS servers to
take advantage of non-default receive buffer size,
and to send larger replies. DNS query packets
with EDNS0 extension are not compatible with non-
EDNS0 DNS servers. The option must be used only
when all the DNS servers listed in nameserver
lines are able to handle EDNS0 extension.
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 (see
resolver(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.
The search keyword of a system's resolv.conf or resolv.conf.tail file can
be overridden on a per-process basis by setting the environment variable
LOCALDOMAIN to a space-separated list of search domains.
The options keyword of a system's resolv.conf or resolv.conf.tail file
can be amended on a per-process basis by setting the environment variable
RES_OPTIONS to a space-separated list of resolver options as explained
above.
The keyword and value must appear on a single line, and the keyword (e.g.
nameserver) must start the line. The value follows the keyword,
separated by whitespace.
FILES
/etc/resolv.conf
/etc/resolv.conf.tail
SEE ALSOgethostbyname(3), resolver(3), hosts(5), hostname(7), dhclient-script(8),
dhcp(8), named(8)
Name Server Operations Guide for BIND.
HISTORY
The resolv.conf file format appeared in 4.3BSD.
BUGS
Due to resolver internal issues, getaddrinfo(3) may not behave as lookup
suggests. Consequently, userland programs that use getaddrinfo(3) may
behave differently from what lookup says.
OpenBSD 4.9 April 15, 2010 OpenBSD 4.9
