+## Unsolved problems
+
+The biggest problem by far is generating meaningful `SOA` records.
+
+### SOA Version a.k.a. Generation Count
+
+DNS infrastructure (including PowerDNS implementation) relies on the
+"generation" field of the `SOA` RR when it makes decision to invalidate
+the cache. So, if there is zone data in the DNS cache, and a DNS server
+needs to respond to a request about an object from that zone, it first
+checks if the TTL has expired. If it has not, the server takes the data
+from the cache. If it has expired, the server asks the "authoritative
+source" (which is in our case the dnamecoin daemon) for the SOA record
+and compares the generation count in the received response with the
+number kept in the cache. If the "authoritative" SOA does not have a
+greater generation count than the cached SOA, DNS server **does not**
+refresh its cache, presuming that the data there is still valid.
+
+So, it is important that the generation count in the SOA record is
+incremented every time when the domain object, or any of the object that
+it "include"-s or to which it "delegate"-s is changed.
+
+At present, there is no machanism for that. In most cases, simply
+summing the number of entries in `name_history`-s of all domain object
+involved in resolution would work, but this approach would produce
+wrong result when an "import" entry is removed from a domain, because
+in such case the sum would decrease. It would also not notice the
+changes in an object "include"-ed in a subdomain, unless complete
+recursive resolution of the subdomain tree is enforced for when
+SOA record is requested. That would invalidate the reason to have
+caching in the first place.
+
+One possible workaround, currently implemented in `pdns-pipe-nmc`, is to
+use a derivative of absolute time, in our case the number of 10-munute
+intervals elapsed since Namecoin was concieved, as the SOA generation
+count.
+
+### Nameserver field
+
+There is no "reasonable" value that could be placed there. Except
+possibly the name of the host on which the PoweDNS instance is running,
+in the `.bit` zone. Currently, `pdns-pipe-nmc` just puts a dot "."
+there, and no problems where noticed so far.