If a name server cannot answer a query, it must query a remote server. You can configure primary or secondary name servers to act as forwarders. When you designate a server to be a forwarder, all off-site queries are first sent to the forwarder.
Forwarders that handle the off-site queries develop a robust cache of information. The forwarder probably can answer any given query with information from its cache, eliminating the need to make an outside query to a remote server.
When you decide to make a server a forwarder, configure the other servers in your zone to direct their queries through the forwarder. When a forwarder receives a query, it checks its cache for the information. If the information is unavailable, the forwarder issues a query to the root server.
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When you configure your name servers, you provide information about where to forward requests that the servers cannot answer. If you are configuring to use forwarders, you provide the names and IP addresses of servers above your location in your domain. Configure your other name servers to issue queries to the forwarders for queries they cannot answer.
Even if you are using forwarders, a name server that does not receive a timely response from its forwarder eventually attempts to query a root server directly.
If you have a primary name server with subdomains below it and the primary name server is not aware of the subdomains, the name server sends queries to external name servers.
You can configure your primary name server to not forward queries for specified internal subdomains to external name servers. Instead, the primary name server sends a negative response to any queries for the internal subdomains.