There is another type of server, called a forwarder server. The characteristics of this server are not connected with whether it is a primary or secondary server for any zone, but with the way in which the translation of DNS questions is performed.
So far we have said that the resolver transfers the request for the translation to a name server, i.e., it sends a query to a name server and waits for the final answer (the client sends a recursive query and waits for a final answer). If the name server is not able to answer itself, it performs a recursive translation via non-recursive queries. First it contacts the root name server. The root name server tells the resolver which name servers it must ask for answers to its query. Then it contacts the recommended name server. This name server sends many packets into the Internet.
If a company network is connected to the Internet by a slow line, then the name server loads the line by its translations. In such a case, it is advantageous to configure some of the name servers as forwarder servers.
Figure 1.11: Communication of a local name server with a forwarder server
The local name server transmits the queries to the forwarder server. However, the local name server marks these queries as recursive. The forwarder server takes the request from the local name server and performs translation via non-recursive queries on the Internet by itself. It then returns only the final result to our name server.
The local name server waits for the answer from the forwarder server for the final result. If the local name server does not get the answer in the set time out limit, then it contacts the root name servers and tries to solve the case by iteration.
If the local name server is not supposed to contact the root name servers, but is supposed to only wait for the answer, then it is necessary to indicate such a server in its configuration as a forwarder-only. In BIND version 4.x such a server is called slave. Forwarder-only (slave) servers are used on intranets (behind the firewall) where contact with root name servers is not possible. The forwarder server then contacts a name server, which is part of the firewall.
The forwarder server can work as a caching-only server in both variants, and it can also be the primary or secondary name server for some zones.
It is also possible to configure forwarder servers in Windows 2003 Server as shown in the figure below:
Figure 1.12: Forwarders configuration in Windows 2003
Run the DNS from the Administrative Tools. Right-click to your DNS server and choose Properties. Select the Forwarders tab. Click New and enter the name of the domain you want to resolve by forwarders. Insert the IP addresses of the forwarder servers below. You can insert into the Number of seconds before forward queries time out box a time limit during which the server waits for an answer from a forwarder server. We can establish a slave server by clicking the Do not use recursion for this domain option.