Projects should have OS Login enabled for SSH authentication

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Description

Enabling OS Login binds SSH certificates to IAM users and facilitates effective SSH certificate management.

Rationale

Enabling OS Login ensures that SSH keys used to connect to instances are mapped to IAM users. Revoking access to an IAM user will revoke all the SSH keys associated with that particular user. It facilitates centralized and automated SSH key pair management, which is useful in handling cases like compromised SSH key pairs and/or revocation of external, third-party, vendor users.

To use OS Login, the instance using Custom Images must have the latest version of the Linux Guest Environment installed. The following image families do not support OS Login:

  • Project cos-cloud (Container-Optimized OS) image family cos-stable.
  • All project coreos-cloud (CoreOS) image families
  • Project suse-cloud (SLES) image family sles-11
  • All Windows Server and SQL Server image families

The project’s enable-oslogin can be overridden by setting the enable-oslogin parameter to an instance metadata individually.

Impact

Enabling OS Login on a project disables metadata-based SSH key configurations on all instances of a project. Disabling OS Login restores SSH keys that you have configured in a project’s or an instance’s metadata.

Exception

VMs created by GKE should be excluded. These VMs have names that start with gke- and are labeled goog-gke-node.

Default value

By default, the parameter enable-oslogin is not set, which is equivalent to setting it to FALSE.

Remediation

From the console

  1. Go to the VM compute metadata page.
  2. Click Edit.
  3. Add a metadata entry for the key enable-oslogin with the value TRUE.
  4. Click Save to apply the changes.
  5. For every instance that overrides the project setting, go to the VM Instance’s page at https://console.cloud.google.com/compute/instances.
  6. Click the name of the instance from which you want to remove the metadata value.
  7. At the top of the instance’s details page, click Edit to edit the instance’s settings.
  8. Under Custom Metadata, remove any entries with the key enable-oslogin set to FALSE.
  9. At the bottom of the instance’s details page, click Save to apply your changes to the instance.

From the command line

  1. Configure OS Login for the project by running the following command:

    gcloud compute project-info add-metadata --metadata enable-oslogin=TRUE

  2. Use the following command to remove instance metadata that overrides the project setting:

    gcloud compute instances remove-metadata <INSTANCE_NAME> --keys=enable-oslogin

Optionally, you can enable two-factor authentication for OS Login. See Setting up OS Login with 2-step verification for more information.

References

  1. https://cloud.google.com/compute/docs/instances/managing-instance-access
  2. https://cloud.google.com/compute/docs/instances/managing-instance-access#enable_oslogin
  3. https://cloud.google.com/sdk/gcloud/reference/compute/instances/remove-metadata
  4. https://cloud.google.com/compute/docs/oslogin/setup-two-factor-authentication