Remarks
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DNS is essential for a proper operation of the network;
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If DNS fails, connections to any remote host are no more
possible;
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To allow network operation in case of failures of a nameserver,
each nameserver (called primary nameserver should have at least
another nameserver (called secondary nameserver) which holds a copy
of the zone file;
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The secondary nameserver periodically queries the primary
nameserver whether the zone was changed. In case of the change, it
is transferred into its cache and also stored in a local file,
which will be used next time at starting up as initial data;
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When data about a new host should be added to the DNS database,
or a change of address/name should be done, the entry is done only
on the primary nameserver; the secondary nameserver will get it
automatically.