I noticed that the firewall on my router blocked incoming FritzBox connections on port 80. Since the router is behind my cable modem and I did not configure any port forwarding, this meant that the box itself initiates these connections.
In the process of verifying this issue, I saw, that these connections start when I connect to the web GUI of the box and they end a short while after I disconnect.
I tried to google this issue a few times but didn’t come up with anything useful at first.The solution came when I carefully searched the AVM documentation and it is a well documented behaviour of the box: Firewall meldet Angriffe an TCP-Port 80 bzw. 14013 oder unangeforderte Pakete vom Typ 0x88e1.
There seems no English version available for this artice, so here is at least the translated headline: Firewall reports attacks on TCP port 80 respectively 14013 or unrequested packets of type 0x88e1.
On port 80 the box tries to detect other devices which offer web services in your home network. These services would be made available on the FritzBox GUI in “Devices and Users” when found.
The traffic on this port is used for Windows clients to detect which user is logged on. This feature is necessary for the child protection feature of the FritzBox. The requests only show up if the feature is activated.
Packets of ether type 0x88e1 are used to detect any FritzPowerline adapters in the network and appear every 5 seconds. In case adapters are found, they will show up in the “Devices and Users” area of the GUI.