VBoxManage: error: Failed to create the host-only adapter
I had the same problem today. The reason was that I had another VM running in VirtualBox.
Solution:
- Open VirtualBox and shut down every VM running
- Go to System Preferences > Security & Privacy Then hit the "Allow" button to let Oracle (VirtualBox) load.
- Restart VirtualBox
sudo "/Library/Application Support/VirtualBox/LaunchDaemons/VirtualBoxStartup.sh" restart
You should now be able to run vagrant up or vagrant reload and have your new host configured.
As mentioned in this answer, recent versions of macOS can block VirtualBox.
Solution:
Go to System Preferences > Security & Privacy Then hit the "Allow" button to let Oracle (VirtualBox) load.
(thanks to @pazhyn, @lsimonetti & @dave-beauchesne for clarifications)
For Mac OS X 10.9.3 and Vagrant 1.6.3 and VirtualBox 4.3.6 this problem was fixed with restarting the VirtualBox
sudo "/Library/Application Support/VirtualBox/LaunchDaemons/VirtualBoxStartup.sh" restart