Might be a bit of a strange request but here goes.
“Default Value” use current value
In the case where a Lookup/Reverse DNS/etc can’t find anything it returns undefined, is it possible default to the existing Lookup Field value?
Where a lookup fails to return anything, is there a way to drop through to another lookup and so on until something is returned?
Context here is resolving IP addresses and either replacing and/or adding it to an event. If it can’t resolve it by either lookups or Reverse DNS lookups then simply return the IP.