OK, I've had a look at what is going on here and I have to admin that I can't understand what Astaro have done, or more to the point why!

On V6 of ASL the ez-ipupdate rpm is version 3.0-12 (I'm guessing packaged up by Astaro) and it contains version 3.0.10 of ez-ipupdate. This version is from the year 2000 and before ez-ipupdate supported dyndns-custom. This explains why the old hacks don't work in V6.
I had a look at the package on the V5 CD I have and it contains a package version of 3.0-5, but this contains the latest version of ez-ipupdate (3.0.11b8). There has been no development work on ez-ipupdate since 2002.

I have also seen a vulnerability advisory on ez-ipupdate that exists in all versions. It would probably be advisable for Astaro to look into replacing it with something newer. Preferably something that supports the use of SSL so that usernames and passwords are kept secure.

In the meantime I am experimenting with removing the V6 package with the package from V5 to see if this at least overcomes the immediate problem.

Simon