If during first ‘vagrant up’ you see in log output:
Vagrant insecure key detected. Vagrant will automatically replace
this with a newly generated keypair for better security.Inserting generated public key within guest…
Removing insecure key from the guest if its present…
Key inserted! Disconnecting and reconnecting using new SSH key…
you can add this line:
config.ssh.insert_key = false
to your ‘Vagrantfile’, because since version 1.7.0 (December 9, 2014):
If the default insecure keypair is used, Vagrant will automatically replace it with a randomly generated keypair on first vagrant up. [GH-2608]
https://github.com/mitchellh/vagrant/blob/master/CHANGELOG.md