To ensure that all NIS servers in the same NIS domain are using
the same set of maps, slave or copy-only servers should retrieve maps
from the master server as follows:
Whenever a slave or copy-only server is initialized.
Whenever a slave or copy-only server returns online after being unavailable.
Copy-only servers only: when polling of the master shows updated maps available.
Copy-only servers only: whenever new maps are added to the master server
because the master cannot push new maps to copy-only servers.