Forwarders and Slaves
When a DNS server receives a query, it attempts to locate the requested information within its local zones and from the cache. If it cannot locate the requested information and is not authoritative for the requested information, it must communicate with other servers to resolve the request. However, in some cases network administrators might not want the server to communicate directly with other servers. For example, if your organization were connected to the Internet by means of a slow wide area link, you might not want every DNS server in your organization to connect directly to DNS servers on the Internet.
To solve this problem, DNS allows for the use of forwarders . Forwarders are DNS servers that are designated to provide forwarding of off-site queries for other DNS servers. For example, you could designate one DNS server as a forwarder for names of computers on the Internet, and then configure your other servers to use that forwarder to resolve names for which they are not authoritative.
You do not need to perform any special configuration on the computer designated as a forwarder. You must configure the DNS server that needs to forward queries by providing the IP address of the forwarders.
A server can use a forwarder in a nonexclusive or exclusive mode. In a nonexclusive mode, when a server receives a DNS query for which it is not authoritative and cannot resolve through its own zones or cache, it passes the query to one of the designated forwarders. The forwarder then carries out whatever communication is necessary to resolve the query and returns the results to the requesting server, which returns the results to the original requester. If the forwarder cannot resolve the query, the server that received the original query attempts to resolve the query on its own.
In an exclusive mode, servers rely completely on the name-resolving ability of the forwarders. Servers using forwarders in an exclusive mode are known as slaves . When a slave receives a DNS query that it cannot resolve through its own zones, it passes the query to one of the designated forwarders. The forwarder then carries out whatever communication is necessary to resolve the query and returns the results to the slave, which returns the results to the original requester. If the forwarder cannot resolve the request, the slave returns a query failure to the original requestor. Slaves make no attempt to resolve the query on their own if the forwarder cannot satisfy the request.