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3.8 Ensure that VPC Flow Logs is Enabled for Every Subnet in a VPC Network

Information

Flow Logs is a feature that enables users to capture information about the IP traffic going to and from network interfaces in the organization's VPC Subnets. Once a flow log is created, the user can view and retrieve its data in Stackdriver Logging. It is recommended that Flow Logs be enabled for every business-critical VPC subnet.

Rationale:

VPC networks and subnetworks not reserved for internal HTTP(S) load balancing provide logically isolated and secure network partitions where GCP resources can be launched. When Flow Logs are enabled for a subnet, VMs within that subnet start reporting on all Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) flows. Each VM samples the TCP and UDP flows it sees, inbound and outbound, whether the flow is to or from another VM, a host in the on-premises datacenter, a Google service, or a host on the Internet. If two GCP VMs are communicating, and both are in subnets that have VPC Flow Logs enabled, both VMs report the flows.

Flow Logs supports the following use cases:

Network monitoring

Understanding network usage and optimizing network traffic expenses

Network forensics

Real-time security analysis

Flow Logs provide visibility into network traffic for each VM inside the subnet and can be used to detect anomalous traffic or provide insight during security workflows.

The Flow Logs must be configured such that all network traffic is logged, the interval of logging is granular to provide detailed information on the connections, no logs are filtered, and metadata to facilitate investigations are included.

Note: Subnets reserved for use by internal HTTP(S) load balancers do not support VPC flow logs.

Impact:

Standard pricing for Stackdriver Logging, BigQuery, or Cloud Pub/Sub applies. VPC Flow Logs generation will be charged starting in GA as described in reference: https://cloud.google.com/vpc/

Solution

From Google Cloud Console

Go to the VPC network GCP Console visiting https://console.cloud.google.com/networking/networks/list

Click the name of a subnet, The Subnet details page displays.

Click the EDIT button.

Set Flow Logs to On.

Expand the Configure Logs section.

Set Aggregation Interval to 5 SEC.

Check the box beside Include metadata.

Set Sample rate to 100.

Click Save.

Note: It is not possible to configure a Log filter from the console.
From Google Cloud CLI
To enable VPC Flow Logs for a network subnet, run the following command:

gcloud compute networks subnets update [SUBNET_NAME] --region [REGION] --enable-flow-logs --logging-aggregation-interval=interval-5-sec --logging-flow-sampling=1 --logging-metadata=include-all

Default Value:

By default, Flow Logs is set to Off when a new VPC network subnet is created.

See Also

https://workbench.cisecurity.org/benchmarks/11843

Item Details

Category: AUDIT AND ACCOUNTABILITY, SYSTEM AND INFORMATION INTEGRITY

References: 800-53|AU-2, 800-53|AU-7, 800-53|AU-12, 800-53|SI-4, 800-53|SI-4(4), CSCv7|6.2, CSCv7|12.8

Plugin: GCP

Control ID: 984c6cd99a680d0709bf0ddf3e85df9e79b37188884a3cbb99af7237983f21d5