Preferred fallback network interfaces with Vagrant?

Question

My Vagrant boxes use public networking so they can advertise themselves over zeroconf/Bonjour. The Vagrantfile explicitly sets the bridged network interface:

  config.vm.network :public_network, :bridge => 'en2: USB Ethernet'

Most of the time everything just works, but if I'm connected via a different network and the specified interface doesn't exist, vagrant up will prompt me to pick from the available network interfaces:

[default] Specific bridge 'en2: USB Ethernet' not found. You may be asked to specify
which network to bridge to.
[default] Available bridged network interfaces:
1) en0: Wi-Fi (AirPort)
2) p2p0
What interface should the network bridge to?

Is there a way to tell Vagrant to choose from a list of preferred network interfaces? What I want is a graceful fallback if the primary network isn't available.

Answer 1

Here's a solution I came up with that seems to be working well so far:

In Vagrantfile, add the following to the top of the file:

pref_interface = ['en2: USB Ethernet', 'en0: Wi-Fi (AirPort)']
vm_interfaces = %x( VBoxManage list bridgedifs | grep ^Name ).gsub(/Name:\s+/, '').split("\n")
pref_interface = pref_interface.map {|n| n if vm_interfaces.include?(n)}.compact
$network_interface = pref_interface[0]

Then, inside Vagrant.configure, use $network_interface to specify the bridge:

config.vm.network :public_network, :bridge => $network_interface