DDOS attacks against public DNSBLs typically focus on exposed DNS interfaces and the website. To counter these attacks, DNSBLs operate multiple mirrors, but they can still be overwhelmed.
Karmasphere offers server software that supports both replication and lookup operations. High-volume users of Karmasphere are invited to download a server node and run it locally. (In traditional terms, think of it as operating both as an rsync client and an rbldnsd server.) The node connects to upstream Karmasphere replication servers which are not publicly exposed, and download a full dataset which stays up to date using our replication technology. These hidden upstream servers do not serve public queries; they only support replication. This architecture insulates them from DDOS attacks.
We have a number of other techniques to deal with DDOS attacks, but we cannot reveal them on a public webpage. Commercial customers are invited to contact us for details.