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resolver(5)		    BSD	File Formats Manual		   resolver(5)

     resolver -- resolver configuration	file format

     The resolver is a set of routines in the C	library	resolv(3) that provide
     access to the Internet Domain Name	System (DNS).  A resolver configura-
     tion file contains	information used to specify parameters for a DNS re-
     solver client.  The file contains a list of keywords with values that
     provide various types of resolver information.

     Mac OS X supports a DNS search strategy that may involve multiple DNS re-
     solver clients.  See the SEARCH STRATEGY section below for	an overview of
     multi-client DNS search.

     Each DNS client is	configured using the contents of a single configura-
     tion file of the format described below, or from a	property list supplied
     from some other system configuration database.  Note that the
     /etc/resolv.conf file, which contains configuration for the default (or
     "primary")	DNS resolver client, is	maintained automatically by Mac	OS X
     and should	not be edited manually.	 Changes to the	DNS configuration
     should be made by using the Network Preferences panel.

     The different configuration options are given below.

     Internet address (in dot notation for IPv4	or in colon notation for IPv6)
     of	a name server that the resolver	should query.  The address may option-
     ally have a trailing dot followed by a port number.  For example, specifies that the nameserver	at uses port 55.

     Up	to MAXNS (currently 3) name servers may	be listed, one per keyword.
     If	there are multiple servers, the	resolver library queries them in the
     order listed.  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.

     IP	port number to be used for this	resolver.  The default port is 53.
     The port number for an individual nameserver may be specified as part of
     the nameserver address (see nameserver above) to override the default or
     the port number specified as a value for this keyword.

     Domain name associated with this resolver configuration.  This option is
     normally not required by the Mac OS X DNS search system when the resolver
     configuration is read from	a file in the /etc/resolver directroy.	In
     that case the file	name is	used as	the domain name.  However, domain must
     be	provided when there are	multiple resolver clients for the same domain
     name, since multiple files	may not	exist having the same name.  See the
     SEARCH STRATEGY section for more details.

     Search list for host-name lookup.	This parameter is only used by the
     "Super" DNS resolver, which manages the DNS search	strategy amongst mul-
     tiple DNS resolver	clients.  Unqualified queries will be attempted	using
     each component of the search list 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

     Only required for those clients that share	a domain name with other
     clients.  Queries will be sent to these clients in	order by ascending
     search_order value.  For example, this allows two clients for the ".lo-
     cal" domain, which	is used	by Apple's multicast DNS, but which may	also
     be	used at	some sites as private DNS domain name.

     Sortlist allows addresses returned	by gethostbyname to be sorted.	A
     sortlist is specified by IP address netmask pairs.	The netmask is op-
     tional 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:


     Specifies the total amount	of time	allowed	for a name resolution.	This
     time interval is divided by the number of nameservers and the number of
     retries allowed for each nameserver.

     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	the resolver options.

	      sets the per-retry timeout for resolver queries.	The total
	      timeout allowed for a query depends on the number	of retries and
	      the number of nameservers.  This value is	ignored	if a total
	      timeout is specified using the timeout keyword (see above).

     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 ab-
	      solute 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 keyword and value must appear	on a single line, and the key-
	      word must	start the line.	 The value follows the keyword,	sepa-
	      rated by white space.

     Mac OS X uses a DNS search	strategy that supports multiple	DNS client
     configurations.  Each DNS client has its own set of nameserver addresses
     and its own set of	operational parameters.	 Each client can perform DNS
     queries and searches independent of other clients.	 Each client has a
     symbolic name which is of the same	format as a domain name, e.g. "ap-".	A special meta-client, known as	the "Super" DNS	client acts as
     a router for DNS queries.	The Super client chooses among all available
     clients by	finding	a best match between the domain	name given in a	query
     and the names of all known	clients.

     Queries for qualified names are sent using	a client configuration that
     best matches the domain name given	in the query.  For example, if there
     is	a client named "", a search for "" would use the
     resolver configuration specified for that client.	The matching algorithm
     chooses the client	with the maximum number	of matching domain components.
     For example, if there are clients named "a.b.c", and "b.c", a search for
     "x.a.b.c" would use the "a.b.c" resolver configuration, while a search
     for "x.y.b.c" would use the "b.c" client.	If there are no	matches, the
     configuration settings in the default client, generally corresponding to
     the /etc/resolv.conf file or to the "primary" DNS configuration on	the
     system are	used for the query.

     If	multiple clients are available for the same domain name, the clients
     ordered according to a search_order value (see above).  Queries are sent
     to	these resolvers	in sequence by ascending value of search_order.

     The configuration for a particular	client may be read from	a file having
     the format	described in this man page.  These are at present located by
     the system	in the /etc/resolv.conf	file and in the	files found in the
     /etc/resolver directroy.  However,	client configurations are not limited
     to	file storage.  The implementation of the DNS multi-client search
     strategy may also locate client configuratins in other data sources, such
     as	the System Configuration Database.  Users of the DNS system should
     make no assumptions about the source of the configuration data.

     /etc/resolv.conf, /etc/resolver/*

     gethostbyname(2), getaddrinfo(3), resolver(3)

Mac OS				 June 6, 2003				Mac OS


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