Comment 4 for bug 997200

As a workaround, since the connectivity check feature is already available, just not activated by default, you may wish to activate it with what will likely be the additional configuration we will ship. To do this, you can add the following lines to /etc/NetworkManager/NetworkManager.conf

[connectivity]
uri=http://start.ubuntu.com/connectivity-check.html
interval=300
response=<!DOCTYPE HTML PUBLIC

This changes the behavior of NM to use a different "connected" state when the checks fail, so the exact behavior of applications may change (and will need to be tested). At least, this currently appears to work properly on my computer; pending further testing once my portal session expires.