Android Internet connectivity request not encrypted

[hbraam]

I have OpenVPN running on my Android device with the "Reconnect on Reboot" and "Seamless Tunnel" options turned on. So, all network traffic should pass through the VPN tunnel. This seems to be case for nearly all HTTP traffic, with the following exception:

Using Wireshark I noticed that Android sends a HTTP GET request to verify whether the device has access to the Internet as follows:

HTTP://connectivitycheck.android.com/generate_204

This request is send in the clear (revealing user agent info), outside the VPN tunnel, while other traffic is encrypted as expected. Anyone has any idea why this request is not encrypted?