Slave mode is used if the use of forwarders is the only possible way to resolve queries due to lack of full net access or if you wish to prevent the name server from using other than the listed forwarders. Slave mode is activated by placing the simple command
options forward-only
in the bootfile. If this option is used, then you must specify forwarders. When in slave mode, the server will forward each query to each of the forwarders until an answer is found or the list of forwarders is exhausted. The server will not try to contact any remote name server other than those named in the forwarders list.
So while forwarders prepends addresses to the ``server list'' for each query, options forward-only causes the ``server list'' to contain only those addresses listed in the forwarders declarations. Careless use of the options forward-only directive can cause really horrible forwarding loops, since you could end up forwarding queries only to some set of hosts which are also slaves, and one or several of them could be forwarding queries back to you.
Use of the options forward-only directive should be considered very carefully. Note that this same behaviour can be achieved using the deprecated directive, slave.