Comment 1 for bug 1744479

Why did searches not find the upstream bug for me. It would have saved a lot of time:
https://github.com/hashicorp/vagrant/issues/8250

Apparently, the work-around is just to restart the network service inside each guest (no need to edit the file as I did):
$ sudo systemctl restart network

The issue was fixed upstream in vagrant 1.9.2.