Wikipedia
Dbndns
dbndns is a fork of the djbdns software package, maintained by the Debian Project, made possible by the release of djbdns to the public domain.
The fork was created so as to add many common patches to djbdns. Most notably, this now includes IPv6 support.
Previously, it was necessary to get a special 'djbdns-installer' package that downloaded the djbdns source from the author's site and apply a patch, but the free software status means this is no longer necessary, and Debian can directly carry the source and patches, producing a redistributable binary deb package.
This package has filtered through into Ubuntu.