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Vagrant >1.7 is unable to login to the default boot2docker host if it has been halted and brought back up again. This is because the insecure SSH keypair is replaced but boot2docker doesn't persist filesystem changes between reboots. So when next brought up Vagrant tries to use the new keypair which is no longer valid and subsequently isn't able to login. Prevent this from happening by disabling the `ssh.insert_key` option on versions where it is available.
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.