Hey
A nice feature for the Update Client would be that that the User can select which domain should be Updated. That would be very userfull if you have more than only one Domain.
Or is there a way that i can Specifie the Domain on Linux?

You can specify the domain name to update in the configuration file. However, you can also create a group (location) and add a single domain name to the group (location) and specify the group in the configuration file.

You can refer to 'location' in the documentation below:

https://www.dynu.com/DynamicDNS/IP-Update-Protocol