Yesterday I got this message and later found out it has some thing to do with my internet provider Telekom.
Can someone explain this to me? This is not normal right?
It was probably not Telekom, but one of its subscribers.
Is port 8123 forwarded in your router to your HA instance?
Yes, port forwarding for port 8123
There you have your answer.
The kind people on this forum are not the only ones who know that port 8123 is used by Home Assistant.
For external access to your setup, it might be a good idea to subscribe to Nabu Casa - and support the development of Home Assistant.
Or to set up a VPN or Wireguard connection to your HA server.
Or at least use a different port from the Internet side of your router (NOT recommended).
It’s as easy as typing nmap -p 8123 192.168.1.0/24 into your Linux CLI to check, if port 8123 is open on a machine on your subnet - or for that matter: Telekom’s
Is the IP address in the log the same as your home’s external public IP address? If yes, then your router probably has loopback mode capability. Meaning a browser device inside your home is setup to reach HA by your external own public IP address but when your router gets a packet from inside your home address destined to your own external public IP address, the router instead forwards back inside your home via the port forwarding rule (and thus to your HA server). So it is possible that one of your internal browsing devices is causing this.