Sometimes, ifdown is not enough.

Changing network configuration in the Vagrantfile doesn't change network on the Debian virtualbox. `/sbin/ifrequest eth1` show the right configuration, but `/sbin/ifconfig` an old one.

The hidden error is : `RTNETLINK answers: File exists`

A simple provisioning shell can fix this bug :
`config.vm.provision :shell, inline: "sudo /sbin/ifdown eth1 && sudo /sbin/ifup eth1"`

This patch flush each network interfaces.