Using Forwarders
Applies To: Windows Server 2008, Windows Server 2008 R2
To use forwarders to manage the Domain Name System (DNS) traffic between your network and the Internet, configure your network's firewall to allow only one DNS server to communicate with the Internet. When you configure the other DNS servers in your network to forward queries that they cannot resolve locally to that DNS server, it acts as your forwarder. For more information about forwarders, see Understanding Forwarders.
Forwarding sequence
The order of the IP addresses that are listed as forwarders on a DNS server determines the sequence in which the IP addresses are used. After the DNS server forwards the query to the forwarder with the first IP address, it waits a short period of time for an answer from that forwarder (according to the DNS server's forwarding time-out setting) before resuming the forwarding operation with the next IP address. It continues this process until it receives an affirmative answer from a forwarder.
For example, in the following illustration the DNS servers with the first and second forwarder IP addresses do not respond to the DNS server. The DNS server with the third forwarder IP address responds, and the query is forwarded to that DNS server.
Unlike conventional resolution, where a roundtrip time (RTT) is associated with each server, the IP addresses in the forwarders list are not ordered according to roundtrip time. You must reorder them manually to change the preference.
Conditional forwarders
Conditional forwarders are DNS servers that forward queries according to domain names. Rather than having a DNS server forward all queries it cannot resolve locally to a forwarder, you can configure DNS servers to forward queries to different forwarders according to the specific domain names that are contained in the queries. that is specified
When a DNS client or server performs a query operation against a DNS server, the DNS server checks to determine if the query can be resolved with its own zone data or the data stored in its cache. If the DNS server is configured to forward for the domain name that is designated in the query, the query is forwarded to the IP address of a forwarder that is associated with that domain name. For example, in the following illustration, each of the queries for the domain names is forwarded to a DNS server that is associated with the domain name.
If the DNS server has no forwarder listed for the name that is designated in the query, it attempts to resolve the query using standard recursion. For more information, see Configure a DNS Server to Use Forwarders.
You can use conditional forwarders to improve name resolution between internal (private) DNS namespaces that are not part of the DNS namespace of the Internet. Such DNS namespaces may be a result of a company merger. When you configure your DNS servers performing recursion to your internal root for different namespaces within your network.<<