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1. Why doing this

If your DNS is behind a firewall, it can't reach the Internet. It can't fulfill one of its main task, which is to provide name resolution (its a DNS where S stand for server :-) ). By pointing to a forwarder (often a caching only DNS running on the firewall itself), the problem is solved.

Another reason appear when a DNS is located at the end of a slow link. Instead of trying to solve itself all the query it will send the request to a forwarder at the other end of the link. Often the second DNS has a higher usage than the first, thus already has a greater knowledge. It is in position to answer the query right away using its large cache.


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