Thanks for all the hints. krfb does what I need. One can use control center to change the behaviour. Here you can configure, that it starts automatically and that the user doesn't have to accept the session. That's what I did and it is great and easy (I think). The only problem was that I had to restart before the feature with the acceptance did work. The user sees that somebody is watching ... but I find this is good. Greetings Dirk