Any server can make use of forwarders. A forwarder is another server capable of processing recursive queries that is willing to try resolving queries on behalf of other systems. The forwarders command specifies forwarders by internet address as follows:
forwarders 128.32.0.10 128.32.0.4
There are two main reasons for wanting to do so. First, some systems may not have full network access and may be prevented from sending any IP packets into the rest of the Internet and therefore must rely on a forwarder which does have access to the full net. The second reason is that the forwarder sees a union of all queries as they pass through its server and therefore it builds up a very rich cache of data compared to the cache in a typical workstation name server. In effect, the forwarder becomes a meta-cache that all hosts can benefit from, thereby reducing the total number of queries from that site to the rest of the net.
The effect of ``forwarders'' is to prepend some fixed addresses to the list of name servers to be tried for every query. Normally that list is made up only of higher-authority servers discovered via NS record lookups for the relevant domain. If the forwarders do not answer, then unless the slave directive was given, the appropriate servers for the domains will be queried directly.