That stuff don't come for free? Yes it does! It is a part of the standard BIND (Or at least it was in 1997 whin I last looked at it.) Do a dejanews-search for my name in the *mail* and *bind* newsgroups. I wrote a quick HOWTO summer of 1997 (I think it was)
http://groups.google.com/groups?q=jan+marius+evang+bind+dynamic+dns&hl=no&selm=xml3eqdxsb3.fsf%40hrotti.ifi.uio.no&rnum=1 >
http://groups.google.com/groups?q=jan+marius+evang+bind+dynamic+dns&hl=no&selm=xml3eqdxsb3.fsf%40hrotti.ifi.uio.no&rnum=1