those misconfigured domains where the servers reported by the parent
are the only ones working, and servers reported by the child do not.
+o Failure of validation and probing.
+ Retries on a validation failure are now 5x to a different nameserver IP
+ (if possible), and then it gives up, for one name, type, class entry in
+ the message cache. If a DNSKEY or DS fails in the chain of trust in the
+ key cache additionally, after the probing, a bad key entry is created that
+ makes the entire zone bogus for 900 seconds. This is a fixed value at
+ this time and is conservative in sending probes. It makes the compound
+ effect of many resolvers less and easier to handle, but penalizes
+ individual resolvers by having less probes and a longer time before fixes
+ are picked up.
+