BackUp DNS

If you setup a 2nd Backup DNS server for Interworx (not load balanced machine but just a DNS server), do you need an equivalent license? Do I need an unlimited license or is a 35 domain license sufficient?

Also, can 1 server be the backup for several Interworx servers?

I believe the 35 domain license works just fine for a DNS server (using DNS Sync), you just can’t build a cluster.