domain:        [mandatory]  [single]     [primary/look-up key]
descr:         [mandatory]  [multiple]   [ ]
admin-c:       [mandatory]  [multiple]   [inverse key]
tech-c:        [mandatory]  [multiple]   [inverse key]
zone-c:        [mandatory]  [multiple]   [inverse key]
nserver:       [optional]   [multiple]   [inverse key]
sub-dom:       [optional]   [multiple]   [inverse key]
dom-net:       [optional]   [multiple]   [ ]
remarks:       [optional]   [multiple]   [ ]
notify:        [optional]   [multiple]   [inverse key]
mnt-by:        [optional]   [multiple]   [inverse key]
mnt-lower:     [optional]   [multiple]   [inverse key]
refer:         [optional]   [single]     [ ]
changed:       [mandatory]  [multiple]   [ ]
source:        [mandatory]  [single]     [ ]