Ansible and Vagrant SSH Keys

I recently came across the question how to handle SSH keys with Vagrant and Ansible.

Vagrant

Traditionally all Vagrant boxes and VMs require a fixed login (vagrant) with a fixed SSH key. This was very convenient in a development context, but could raise security issues in case Vagrant VMs were used for anything important and users were not aware of the insecure key.

In current Vagrant versions only boxes (i. e. base images) use the insecure key. When Vagrant starts a new VM it generates a new individual SSH key for this instance. Vagrant keeps these custom keys in the .vagrant/machines subdirectory; so host to guest logins (like vagrant ssh) are still possible.

A vagrant up shows the multi-step procedure:

    workstation: Vagrant insecure key detected. Vagrant will automatically replace
    workstation: this with a newly generated keypair for better security.
    workstation: 
    workstation: Inserting generated public key within guest...
    workstation: Removing insecure key from the guest if it's present...
    workstation: Key inserted! Disconnecting and reconnecting using new SSH key...
==> workstation: Machine booted and ready!

To prevent this mechanism one can configure the VM with config.ssh.insert_key = False. This is necessary when modifying a box. Say you want to take a bento base box, install your development tools, and then repackage the VM as your own development box in order to distribute it to a development team. — With the default behaviour (config.ssh.insert_key = True) every repackage build would generate a new basebox with an individual ssh keypair; this makes the new boxes practically unusable, because every user would have to get and configure the custom SSH key for each box. With config.ssh.insert_key = False the box will retain the previous key; that means you users have the same experience as with a normal box.

Note: As a compromise between the two modes one can set a custom ssh key for all baseboxes with config.ssh.private_key_path. This might be useful for companies with many internal boxes. In this case one distribute one SSH key to use with all Vagrant boxes but not use the publicly known insecure standard key.

Ansible

If all VMs use the same SSH key the setup is straightforward: Configure all Vagrant VMs with config.ssh.insert_key = False or config.ssh.private_key_path and use that key for the Ansible login.

With Vagrant’s default behaviour there is no common SSH key for all VMs. In this case one has to configure Ansible to use the right keys for every VM, but the setup is still simple as long as you have the default /vagrant mount with the .vagrant subdirectory.

This subdirectory contains all Vagrant state and contains these files, right now the private_key is the important one:

$ find .vagrant/
.vagrant/
.vagrant/rgloader
.vagrant/rgloader/loader.rb
.vagrant/machines
.vagrant/machines/workstation
.vagrant/machines/workstation/virtualbox
.vagrant/machines/workstation/virtualbox/synced_folders
.vagrant/machines/workstation/virtualbox/vagrant_cwd
.vagrant/machines/workstation/virtualbox/index_uuid
.vagrant/machines/workstation/virtualbox/action_set_name
.vagrant/machines/workstation/virtualbox/private_key
.vagrant/machines/workstation/virtualbox/id
.vagrant/machines/workstation/virtualbox/box_meta
.vagrant/machines/workstation/virtualbox/action_provision
.vagrant/machines/workstation/virtualbox/creator_uid

I usually run one Vagrant VM as an Ansible controller (with node.vm.provision :ansible_local) and then 1 to N other VMs as installation targets. To enable access from the controller to the other VMs I include this line in the Ansible inventory: ansible_ssh_private_key_file=/vagrant/.vagrant/machines/{{ inventory_hostname }}/virtualbox/private_key

This should work just the same for running Ansible on your host machine and use it to provision the guest VMs. In this case the path would be a relative one: .vagrant/machines/....

The only important detail is the consistent naming of the VM in Vagrant and Ansible. With the ansible_ssh_private_key_file as above Vagrant’s VM name (set with config.vm.define) and Ansible’s inventory_hostname have to be the same. They are not required to match the guest’s hostname (config.vm.hostname), but I strongly recommend to either use the same value or use an obvious mapping. I usually try to use an FQDN for the hostname, and then use the short hostname (the first component) as the VM name and inventory name.

The only important detail is a consistent naming between Vagrant’s VM name (as set with config.vm.define), the VM’s hostname

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