So on the aternos.org site you have to start your server manually. But I soon wanted to do a small FTB server with some people and there's the problem that I do not always want to start the server myself if you want to join someone. But I do not want to give them access to the config that can start it himself. Can I let the server start somehow automatically clock time, or is there also a free alternative to aternos?
Nope. These are the disadvantages if everything should always be free. Get your NEN server for a few euro then you do not have the problem.
You do not come around to hosting your own server.
Normally, you would create a cron job, which automatically executes a start script automatically. This start script could be located on an external server and start the gameserver via API.
The prerequisite is that there's a public Aternos API.
This is the theoretical, not the most economical way. In the end, it would be wiser to rent your own gameserver. At best, it will provide its own cron configuration.
Do not think he can do anything with Cronjob.
But if you already get a free server you have to live with the disadvantages.
