As we'll see below, it is made up of a collection of small, independent, mutually-distrusting utilities that even run under separate user accounts. Another common aspect of DJB code is program-friendly configuration files. Finally, of course, there is DJB's licensing for his software. What you can not do, however, is distribute a version that is changed in any way. There are those who are not happy with that restriction, and it does tend to frustrate the free software development dynamic.
Nobody can distribute a value-added version of djbdns in that way.
What people can do is distribute patches. Russ Nelson has set up a site at djbdns.org that contains various patches that people have contributed, pointers to commercial djbdns support providers, and more. As has been mentioned, djbdns is a collection of small programs. It possesses no authoritative data of its own. It takes great care to get authoritative answers starting from the root servers, and, thus, defends itself from cache-poisoning attacks.