RESOLVER(5)						      BSD File Formats Manual						       RESOLVER(5)

NAME

     resolver -- resolver configuration file

SYNOPSIS

     /etc/resolv.conf

DESCRIPTION

     The resolver is a set of routines in the C library (resolve(3)) that provide access to the Internet Domain Name System.  The resolver config-
     uration 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.

     On a normally configured system, this file should not be necessary.  The only name server to be queried will be on the local machine, the
     domain name is determined from the host name, and the domain search path is constructed from the domain name.

     The different configuration directives are:

     nameserver
	      Internet address (in dot notation) of a name server that the resolver should query.  Up to MAXNS (see <resolv.h>) name servers may
	      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 to try a name server, and
	      if the query times out, try the next, until out of name servers, then repeat trying all the name servers until a maximum number of
	      retries are 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(2); 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.

     search   Search list for host-name lookup.  The search list is normally determined from the local domain name; by default, it contains only
	      the local domain name.  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 256 characters.

     sortlist
	      Allows addresses returned by gethostbyname 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

     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.

	      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.

	      timeout:n
			sets the amount of time the resolver will wait for a response from a remote name server before retrying the query via a
			different name server.	Measured in seconds, the default is RES_TIMEOUT (see <resolv.h> ).

	      attempts:n
			sets the number of times the resolver will send a query to its name servers before giving up and returning an error to the
			calling application.  The default is RES_DFLRETRY (see <resolv.h> ).

	      rotate	sets RES_ROTATE in _res.options, which causes round robin selection of nameservers from among those listed.  This has the
			effect of spreading the query load among all listed servers, rather than having all clients try the first listed server
			first every time.

	      no-check-names
			sets RES_NOCHECKNAME in _res.options, which disables the modern BIND checking of incoming host names and mail names for
			invalid characters such as underscore (_), non-ASCII, or control characters.

	      inet6	sets RES_USE_INET6 in _res.options.  This has the effect of trying a AAAA query before an A query inside the gethostbyname
			function, and of mapping IPv4 responses in IPv6 ``tunnelled form'' if no AAAA records are found but an A record set
			exists.

     The domain and search keywords are mutually exclusive.  If more than one instance of these keywords is present, the last instance wins.

     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.

FILES

     /etc/resolv.conf <resolv.h>

SEE ALSO

     gethostbyname(3), hostname(7), named(8), resolver(3), resolver(5).  ``Name Server Operations Guide for BIND''

4th Berkeley Distribution					 November 11, 1993					 4th Berkeley Distribution