The line for a stub server is similar to a secondary. (This feature is experimental as of 4.9.3.)
stub Berkeley.Edu 128.32.0.10 128.32.0.4 ucbhosts.bak
The first field specifies that the server is a stub server for the zone stated in the second field.
Stub zones are intended to ensure that a primary for a zone always has the correct NS records for children of that zone. If the primary is not a secondary for a child zone it should be configured with stub zones for all its children. Stub zones provide a mechanism to allow NS records for a zone to be specified in only one place.
primary CSIRO.AU csiro.dat
stub dms.CSIRO.AU 130.155.16.1 dms.stub
stub dap.CSIRO.AU 130.155.98.1 dap.stub