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Configuring by :interface doesn't work very well because Vagrant has no idea about what interfaces are present in the VM, for example if the image has 'docker' installed but not biosdevname, then interface_names[0] = "docker0" which is usually not what you want mapped to the first network from the Vagrantfile. So if the plugins (like vagrant-libvirt) or the Vagrantfile has given us a network with a MAC address, use that to find the interface name for the network. Otherwise use slot numbers as before.
Vagrant Core Plugins
These are plugins that ship with Vagrant. Vagrant core uses its own plugin system to power a lot of the core pieces that ship with Vagrant. Each plugin will have its own README which explains its specific role.