Description

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

Rationale

Enabling osLogin ensures that SSH keys used to connect to instances are mapped with IAM users. Revoking access to 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 response to compromised SSH key pairs and/or revocation of external/third-party/Vendor users.

Impact

Enabling OS Login on project disables metadata-based SSH key configurations on all instances from a project. Disabling OS Login restores SSH keys that you have configured in project or instance meta-data.

Audit

From Google Cloud Console

1. Go to the VM compute metadata page by visiting https://console.cloud.google.com/compute/metadata.
2. Ensure that key enable-oslogin is present with value set to TRUE.
3. Because instances can override project settings, ensure that no instance has custom metadata with key enable-oslogin and value FALSE.

From Google Cloud CLI

1. List the instances in your project and get details on each instance:

        gcloud compute instances list --format=json

2. Verify that the section commonInstanceMetadata has a key enable-oslogin set to value TRUE.

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

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

Additional Information

1. In order to use osLogin, instance using Custom Images must have the latest version of the Linux Guest Environment installed. The following image families do not yet 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

2. Project enable-oslogin can be over-ridden by setting enable-oslogin parameter to an instance metadata individually.