I've searched around but found no satisfying answer yet. I have all modules in sites/all/modules, so no site specific modules are present. I'm working on a VM.

First problem - creating a dump file fails:

drush @sites sql-dump --result-file

Second problem - updating the modules:

drush @sites update targets all sites, which is nice. But the modules are in all/modules, individual sites do not have any modules

Side question: Is there a way to update only the core?