How to access SSH keys for a Google Cloud Platform Compute Engine VM instance

gcloud-computegoogle-compute-enginesshssh-keysvirtual machine

I created a new instance via the Google Cloud web console from a CentOS 6.x image. I saw a blank on the creation form where I could paste in an existing SSH key; since this was my first instance, I didn't have one yet. I assumed it would take me through the key creation process like Amazon EC2 does. It didn't.

The instance appears to be created, but I can't figure out how to get the SSH key for it. The instance web page has a button that says "SSH" and it let me log in briefly via a pop-up web browser window that simulates an SSH session. However, it only let me into a user-level account, not root. The pop-up had a menu item to change the user and changing it to root does nothing but generate connection errors. Now I can't log into my instance at all!

I've searched but can't find any straight-forward documentation that explains this aspect of Google Compute instances.

Do I have to create my own SSH keys manually and paste them into the form during instance creation? Is there an obvious step I'm missing?

Best Answer

By default, a new Google Compute Engine (GCE) VM instance does not have SSH keys pre-assigned to it, so you cannot "retrieve" them as they don't exist—it's up to you to create them, or use a tool like gcloud (see below) which will prompt you to create them if you don't have SSH keys yet.

You have several options for connecting to your newly-created GCE VM.

One option is to connect using the "SSH" button in the Developer Console GUI next to the instance in the list of instances, which will open a browser window and a terminal session to the instance.

If you would like to connect via SSH client on the command-line, you can use gcloud tool (part of the Google Cloud SDK):

gcloud compute ssh example-instance

You can see the full set of flags and options on the gcloud compute ssh help page, along with several examples.

If you don't already have SSH keys, it will prompt you to create them and then connect to the instance. If you already have keys, you can use existing SSH keys, which it will transfer to the instance.

By default, gcloud expects keys to be located at the following paths:

  • $HOME/.ssh/google_compute_engine – private key
  • $HOME/.ssh/google_compute_engine.pub – public key

If you want to reuse keys from a different location with gcloud, consider either making symlinks or pointing gcloud there using the --ssh-key-file flag.

Note: if you don't use gcloud at all, you have to manually add the SSH keys to the instance's metadata as described in Setting up ssh keys at the instance level which you can do via gcloud or manually via Google Cloud console.

You can also create your own keys using ssh-keygen which is what gcloud will also use under the covers. You can connect to the instance using ssh directly instead of gcloud but you will need to specify extra parameters to do so:

ssh -i KEY_FILE -o UserKnownHostsFile=/dev/null \
    -o CheckHostIP=no -o StrictHostKeyChecking=no \
    USER@IP_ADDRESS

which will require the following parameters:

  • KEY_FILE – [Required] The file where the keys are stored on the computer, e.g., ~/.ssh/google_compute_engine.

  • USER – [Required] The username to log in that instance. Typically, this is the username of the local user running gcloud compute.

  • IP_ADDRESS – [Required] The external IP address of the instance.

For more details, see the SSH docs.