resolver - resolver configuration file
The resolver is a set of routines in the C library (see resolver(3N))
that provide access to the Internet Domain Name System. The resolver
configuration file contains information that is read by the resolver
routines the first time they are invoked by a process. The file is
designed to be human-readable, and contains a list of keywords with
values that provide various types of resolver information.
If the only name server to be queried is on the local machine, then
this file is not always necessary. The domain name could be determined
from the host name (see hostname(1)), if it has been set as a fully
qualified domain name.
Recognized configuration options include:
nameserver Internet (IP) address, in dot notation, of a name
server that the resolver should query. Up to
MAXNS (currently 3) name servers can be listed,
one per keyword. 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: Try a name
server; if the query times out, try the next and
continue until all name servers have been tried,
then repeat trying all the name servers until a
maximum number of retries have been made).
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() (see gethostname(2));
the domain part is interpreted as everything after
the first dot (.). Finally, if the host name does
not contain a domain part, the root domain is
search Search list for host-name lookup. If the search
option is not used the search list will contain
only the the local domain name. The search list
can 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
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component of the search path in turn until a match
is found. Note that this process may be slow and
generates a lot of network traffic if the servers
for the listed domains are not local, and that
queries 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 256 characters.
The first domain in the search list must be the
local domain for short names to work properly in
various files (such as .rhosts and inetd.sec)
sortlist Causes addresses returned by gethostbyname(3C) to
be sorted in accordance with network numbers
specified in the address list. This option allows
you to specify which subnets and networks for
gethostbyname() to prefer if it receives multiple
addresses as the result of a query from the domain
nameserver. The syntax is
A sortlist is specified by IP address netmask
pairs. The netmask is optional and defaults to
the netmask of the net. The IP address and
optional netmask pairs are separated by slashes.
Up to 10 pairs may be specified. The pairs are
separated by blank spaces.
The following sortlist directive sorts the address
on 128.32.42 subnet,
The argument after the slash is the subnet mask
for the subnet in question. To prefer an entire
network, you can omit the slash and the subnet
gethostbyname() sorts any addresses in a reply
that match these arguments into the order in which
they appear in the sortlist, and appends those
addresses that do not match to the end.
options Options allows certain internal resolver variables
to be modified. The syntax is
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options option ...
where currently the option supported is the
ndots:n Set a threshold for the number
of dots which must appear in a
name given to res_query (see
resolver(3N)) 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.
The domain and search keywords are mutually exclusive. If more than
one instance of these keywords is present, the last instance
The search keyword of a system's resolv.conf 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 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 under options.
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 white space.
Note that the resolver routine res_init() silently ignores errors when
reading this file (see resolver(3N)).
A typical resolv.conf file resembles the following:
In order to reduce situations that may cause connections to unintended
destinations, the administrator should carefully select which domains
are put in the search list in the resolv.conf file. HP recommends that
the possible domains for the search list be limited to those domains
administered within your trusted organization. For more information on
the security implications of search lists please see RFC 1535, located
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resolver was developed by the University of California, Berkeley.
/etc/resolv.conf Resolver configuration file.
named(1M), resolver(3N), gethostent(3N), hostname(5), RFC 1535
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