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.\"	@(#)resolver.5	5.9 (Berkeley) 12/14/89
.\"	$From: resolver.5,v 8.3 1995/12/06 20:34:35 vixie Exp $
.\"
.Dd June 6, 2003
.Dt resolver 5
.Os "Mac OS X"
.Sh NAME
.Nm resolver
.Nd resolver configuration file format
.Sh DESCRIPTION
The
.Nm
is a set of routines in the C library 
.Xr resolv(3)
that provide access to the Internet Domain Name System (DNS).
A resolver configuration file contains information used to specify parameters
for a DNS resolver client.
The file contains a list of keywords with values that provide various types
of resolver information.
.Pp
Mac OS X supports a DNS search strategy that may involve multiple
DNS resolver clients.
See the
.Sx SEARCH STRATEGY
section below for an overview of multi-client DNS
search.
.Pp
Each DNS client is configured using the contents of a single configuration
file of the format described below, or from a property list supplied from
some other system configuration database.
Note that the 
.Pa /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.
.Pp
The different configuration options are given below.
.Ss nameserver
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 optionally have a trailing dot followed by a port number.
For example, 
.Li 10.0.0.17.55
specifies that the nameserver at 10.0.0.17
uses port 55.
.Pp
Up to
.Va 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.
.Ss port
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 
.Sx nameserver 
above) to override the default 
or the port number specified as a value for this keyword.
.Ss domain
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
.Pa /etc/resolver
directory.
In that case the file name is used as the domain name.
However, 
.Sx 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 
.Sx SEARCH STRATEGY
section for more details.
.Ss search
Search list for host-name lookup.
This parameter is only used by the "Super" DNS resolver, which
manages the DNS search strategy amongst multiple 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.
.Pp
The search list is currently limited to six domains
with a total of 256 characters.
.Ss search_order
Only required for those clients that share a domain name with other clients.
Queries will be sent to these clients in order by ascending
.Sx search_order
value.
For example, this allows two clients for the ".local"
domain, which is used by Apple's multicast DNS, but which may
also be used at some sites as private DNS domain name.
.Ss sortlist
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: 
.Bd -literal -offset indent
 sortlist 130.155.160.0/255.255.240.0 130.155.0.0
.Ed
.Ss timeout
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.
.Ss options
Options allows certain internal resolver variables to be modified.
The syntax is:
.Pp
options
.Ar option Li "..."
.Pp
where 
.Ar option
is one of the following:
.Bl -tag -width -indent
.It Ar debug
sets
.Va RES_DEBUG
in the resolver options.
.It Ar timeout:n
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
.Sx timeout
keyword (see above).
.It Ar ndots:n
Sets a threshold for the number of dots which
must appear in a name given to
.Sx res_query
(see 
.Xr resolver(3))
before an initial absolute query will be made.  The default for
.Ar 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 
.Sx search
list elements are appended to it.
.Pp
The keyword and value must appear on a single line,
and the keyword must start the line.
The value follows the keyword, separated by white space.
.El
.Sh SEARCH STRATEGY
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. "apple.com".
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.
.Pp
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 "apple.com", a search for
"www.apple.com" 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
.Pa /etc/resolv.conf
file or to the "primary" DNS
configuration on the system are used for the query.
.Pp
If multiple clients are available for the same domain name, the clients ordered
according to a
.Sx search_order
value (see above).
Queries are sent to these resolvers in sequence by ascending value of
search_order.
.Pp
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 
.Pa /etc/resolv.conf
file and
in the files found in the 
.Pa /etc/resolver
directory.
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.
.Sh FILES
/etc/resolv.conf, /etc/resolver/*
.Sh SEE ALSO
gethostbyname(2), getaddrinfo(3), resolver(3)