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4.6 Ensure That IP Forwarding Is Not Enabled on Instances (Automated)​

Profile Applicability​

• Level 1

Description​

Compute Engine instance cannot forward a packet unless the source IP address of the packet matches the IP address of the instance. Similarly, GCP won't deliver a packet whose destination IP address is different than the IP address of the instance receiving the packet. However, both capabilities are required if you want to use instances to help route packets.

Forwarding of data packets should be disabled to prevent data loss or information disclosure.

Rationale​

Compute Engine instance cannot forward a packet unless the source IP address of the packet matches the IP address of the instance. Similarly, GCP won't deliver a packet whose destination IP address is different than the IP address of the instance receiving the packet. However, both capabilities are required if you want to use instances to help route packets. To enable this source and destination IP check, disable the canIpForward field, which allows an instance to send and receive packets with non-matching destination or source IPs.

Impact​

Deleting instance(s) acting as routers/packet forwarders may break the network connectivity.

Audit​

From Console:

  1. Go to the VM Instances page by visiting: https://console.cloud.google.com/compute/instances.
  2. For every instance, click on its name to go to the VM instance details page.
  3. Under the Network interfaces section, ensure that IP forwarding is set to Off for every network interface.

From Command Line:

  1. List all instances:
gcloud compute instances list --format='table(name,canIpForward)'
  1. Ensure that CAN_IP_FORWARD column in the output of above command does not contain True for any VM instance.

Exception: Instances created by GKE should be excluded because they need to have IP forwarding enabled and cannot be changed. Instances created by GKE have names that start with "gke-".

Remediation​

You only edit the canIpForward setting at instance creation time. Therefore, you need to delete the instance and create a new one where canIpForward is set to false.

From Console:

  1. Go to the VM Instances page by visiting: https://pantheon.corp.google.com/compute/instances.
  2. Select the VM Instance you want to remediate.
  3. Click the Delete button.
  4. On the 'VM Instances' page, click `CREATE INSTANCE'.
  5. Create a new instance with the desired configuration. By default, the instance is configured to not allow IP forwarding.

From Command Line:

  1. Delete the instance:
gcloud compute instances delete INSTANCE_NAME
  1. Create a new instance to replace it, with IP forwarding set to Off
gcloud compute instances create

References​

https://cloud.google.com/vpc/docs/using-routes#canipforward

Additional Information​

You can only set the canIpForward field at instance creation time. After an instance is created, the field becomes read-only.