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Restrict Virtual Machine IP Forwarding

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Risk Level: Medium (should be achieved)

Ensure that the virtual machine (VM) instances allowed to use IP forwarding, that belong to your project, folder, or organization, are defined using the "Restrict VM IP Forwarding" policy. This constraint policy helps you improve security and achieve regulatory compliance by explicitly defining the resource name of the VM instances allowed to use IP forwarding, i.e. projects/<project-id>/zones/<instance-zone>/instances/<instance-name>. You can also define the list of allowed instances within an a project, folder, or organization, using the following format: under:organizations/<organization-id>, under:folders/<folder-id>, under:projects/<project-id>. The set of VM instances that can enable IP forwarding in order to act as routers/packet forwarders, must be configured in the conformity rule settings, on your Trend Cloud One™ – Conformity account console.

Security

By default, any virtual machine instance can enable IP forwarding in any VPC network. When IP forwarding is enabled on a virtual machine`s network interface (NIC), it allows the VM to act as a router and receive traffic addressed to other destinations. Because IP forwarding is rarely required, except when the virtual machine (VM) is used as a network virtual appliance, the feature must be enabled only on those VM instances that requires it. With "Restrict VM IP Forwarding" constraint policy, you can define the set of virtual machine instances that can enable IP forwarding within your GCP organization.

Audit

To determine if VM IP forwarding restriction is enabled at the GCP organization level, perform the following operations:

Using GCP Console

01 Sign in to Google Cloud Management Console with the organizational unit credentials.

02 Click on the deployment selector from the top navigation bar, select ALL to list all the existing deployments, then choose the Google Cloud organization that you want to examine.

03 Navigate to Cloud Identity and Access Management (IAM) dashboard at https://console.cloud.google.com/iam-admin/iam.

04 In the navigation panel, select Organization Policies to access the complete list of the cloud organization policies available for your GCP organization.

05 Click inside the Filter by policy name or ID filter box, select Name and Restrict VM IP Forwarding to return the "Restrict VM IP Forwarding" policy.

06 Click on the name of the GCP organization policy returned at the previous step.

07 On the Policy details page, under Effective policy, check the Allowed configuration attribute value. If the Allowed attribute value is set to All, the policy constraints are not enforced for the organization, therefore any virtual machine instance can enable IP forwarding in any virtual network available within your GCP organization.

08 Repeat steps no. 2 – 7 for each organization available in your Google Cloud account.

Using GCP CLI

01 Run organizations list command (Windows/macOS/Linux) using custom query filters to list the ID of each organization available within your Google Cloud account: 

gcloud organizations list
    --format="table(name)"

02 The command output should return the requested organization identifiers (IDs): 

ID
112233441122
123412341234

03 Run resource-manager org-policies describe command (Windows/macOS/Linux) using the ID of the GCP organization that you want to examine as identifier parameter, to describe the enforcement configuration of the "Restrict VM IP Forwarding" policy, available for the selected GCP organization: 

gcloud alpha resource-manager org-policies describe
"compute.vmCanIpForward"
    --effective
    --organization=112233441122
    --format="value(listPolicy.allValues)"

04 The command request should return the requested configuration information: 

ALLOW

If the resource-manager org-policies describe command output returns ALLOW, as shown in the example above, the "Restrict VM IP Forwarding" policy constraints are not enforced for the entire organization, therefore any virtual machine instance can enable IP forwarding in any virtual network available within your GCP organization.

05 Repeat step no. 3 and 4 for each organization created within your Google Cloud account.

Remediation / Resolution

To implement virtual machine IP forwarding restriction at the GCP organization level, enable and configure the "Restrict VM IP Forwarding" constraint policy by performing the following operations:

Using GCP Console

01 Sign in to your Trend Cloud One™ – Conformity account, access Restrict Virtual Machine IP Forwarding conformity rule settings and note the list of Google Cloud VM instances that are allowed to use IP forwarding, provisioned within your organization.

02 Sign in to Google Cloud Management Console with the organizational unit credentials.

03 Click on the deployment selector from the top navigation bar, select ALL to list all the existing deployments, then choose the Google Cloud organization that you want to reconfigure.

04 Navigate to Cloud Identity and Access Management (IAM) dashboard at https://console.cloud.google.com/iam-admin/iam.

05 In the navigation panel, select Organization Policies to access the list with the cloud organization policies available for your organization.

06 Click inside the Filter by policy name or ID filter box, select Name and Restrict VM IP Forwarding to return only the "Restrict VM IP Forwarding" policy.

07 Click on the name of the GCP organization policy returned at the previous step.

08 On the Policy details page, click on the EDIT button from the dashboard top menu to edit the selected policy.

09 On the Edit policy configuration page, perform the following:

  1. Under Applies to, select Customize to choose the type of the policy to apply (i.e. customized policy).
  2. To override the inherited policies completely, select Replace under Policy enforcement.
  3. To use explicit values, select Custom from the Policy values dropdown list.
  4. For Policy type, select Allow to specify that the listed values will be the only allowed values, and all other values will be denied.
  5. In the Custom values section, use the configuration controls to define the Google Cloud virtual machine instances that are allowed to enable IP forwarding, identified at step no. 1. Use the following format to define the allowed VM instances: projects/<project-id>/zones/<instance-zone>/instances/<instance-name>, where <project-id> is the ID of the instance's project, the <instance-zone> is the Google Cloud zone where the instance was deployed, and the <instance-name> is the name of the virtual machine instance allowed to use IP forwarding. You can also use the under: prefix if you need to allow IP forwarding for all the VM instances available in a particular project (under:projects/<project-id>), a folder (under:folders/<folder-id>), or within an entire organization (under:organizations/<organization-id>).
  6. (Optional) To set a recommendation for other users, click SET RECOMMENDATION, enter a string value into the Recommended value text box, and click SET to apply the recommendation. This string value will be displayed in the Google Cloud console to provide guidance to users about this organization policy. This is just a communication tool, and does not affect the policy configuration.
  7. Click SAVE to apply the changes and enforce the "Restrict VM IP Forwarding" policy constraints.

10 If required, repeat steps no. 3 – 9 to enable the required policy for other organizations available in your Google Cloud account.

Using GCP CLI

01 Sign in to your Trend Cloud One™ – Conformity account, access Restrict Virtual Machine IP Forwarding conformity rule settings and note the list of Google Cloud VM instances that are allowed to use IP forwarding, provisioned within your organization.

02 Define the "Restrict VM IP Forwarding" organization policy constraints and save the YAML policy document to a file named cc-restrict-vm-ip-forwarding.yaml. Use the list of allowed Google Cloud virtual machine instances identified at step no. 1 to configure the allowed_values list. The following policy configuration example allows enabling IP forwarding for a VM instance identified by projects/<project-id>/zones/<instance-zone>/instances/<instance-name>, where <project-id> is the ID of the instance's project, <instance-zone> is the Google Cloud zone where the instance was provisioned, and <instance-name> is the name of the instance allowed to use IP forwarding (e.g. projects/cc-project5/zones/us-central1-a/instances/cc-virtual-appliance-instance): 

constraint: constraints/compute.vmCanIpForward
listPolicy:
  allowed_values:
    projects/cc-project5/zones/us-central1-a/instances/cc-virtual-appliance-instance

03 Run resource-manager org-policies set-policy command (Windows/macOS/Linux) using the ID of the Google Cloud Platform (GCP) organization that you want to reconfigure as identifier parameter, to enforce the "Restrict VM IP Forwarding" policy, using the policy document defined at the previous step, for the selected organization: 

gcloud beta resource-manager org-policies set-policy cc-restrict-vm-ip-forwarding.yaml
    --organization=112233441122

04 The command request should return the enforced organization policy metadata: 

constraint: constraints/compute.vmCanIpForward
etag: abcdabcdabcd
listPolicy:
  allowedValues:
  - projects/cc-project5/zones/us-central1-a/instances/cc-virtual-appliance-instance
updateTime: '2020-09-14T18:00:00.000Z'

05 If required, repeat step no. 3 and 4 to enforce the necessary policy for other organizations created within your Google Cloud account.

References

Publication date May 10, 2021

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