resolver(5) File Formats Manual resolver(5)

resolverresolver 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 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.

Mac OS X supports a DNS search strategy that may involve multiple DNS resolver 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 configuration 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 optionally 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 directory. 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 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.

The search list is currently limited to six domains with a total of 256 characters.

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 ".local" 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 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:


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:

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).
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.

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.

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. "". 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 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.

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

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

June 6, 2003 Mac OS X