update-policy restricting to a subnet

[Nicholas F Miller]

Is it possible to restrict user machines to only be able to update  
their 'A' records on a specific subnet? We would like to allow DDNS  
but restrict it to specific subnets and only allow the machines to  
update their 'A' records. Allow-updates will not get us the record  
restrictions we would need to implement this and it doesn't appear  
that update-policy has any understanding of subnet scoping.
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Nicholas Miller, ITS, University of Colorado at Boulder