Description

Compute instances should not be configured to have external IP addresses.

Rationale

To reduce your attack surface, Compute instances should not have public IP addresses. Instead, instances should be configured behind load balancers, to minimize the instance’s exposure to the internet.

You can connect to Linux VMs that do not have public IP addresses by using Identity-Aware Proxy for TCP forwarding. Learn more at https://cloud.google.com/compute/docs/instances/connecting-advanced#sshbetweeninstances.

For Windows VMs, see https://cloud.google.com/compute/docs/instances/connecting-to-instance.

Impact

Removing the external IP address from your Compute instance may cause some applications to stop working.

Prevention

You can configure the “Define allowed external IPs for VM instances” organization policy to prevent VMs from being configured with public IP addresses. Learn more at: https://cloud.google.com/compute/docs/ip-addresses/reserve-static-external-ip-address

Exception

Instances created by GKE should be excluded because some of them have external IP addresses and cannot be changed by editing the instance settings. Instances created by GKE should be excluded. These instances have names that start with “gke-” and are labeled “goog-gke-node”.

Remediation

From the console

1. Go to the VM instances page by visiting: https://console.cloud.google.com/compute/instances.
2. Click on the instance name to go the the Instance detail page.
3. Click Edit.
4. For each Network interface, ensure that External IP is set to None.
5. Click Done and then click Save.

From the command line

1. Describe the instance properties:

   gcloud compute instances describe <INSTANCE_NAME> --zone=<ZONE>
   

2. Identify the access config name that contains the external IP address. This access config appears in the following format:

   networkInterfaces:
   - accessConfigs:
    - kind: compute#accessConfig
      name: External NAT
      natIP: 130.211.181.55
      type: ONE_TO_ONE_NAT
   

3. Delete the access config:

   gcloud compute instances delete-access-config <INSTANCE_NAME> --zone=<ZONE> --access-config-name <ACCESS_CONFIG_NAME>
   

In the above example, the ACCESS_CONFIG_NAME is External NAT. The name of your access config might be different.

References

1. https://cloud.google.com/load-balancing/docs/backend-service#backends_and_external_ip_addresses
2. https://cloud.google.com/compute/docs/instances/connecting-advanced#sshbetweeninstances
3. https://cloud.google.com/compute/docs/instances/connecting-to-instance
4. https://cloud.google.com/compute/docs/ip-addresses/reserve-static-external-ip-address#unassign_ip
5. https://cloud.google.com/resource-manager/docs/organization-policy/org-policy-constraints