Use VPC Flow Logs

VPC Flow Logs records a sample of network flows sent from and received by VM instances, including instances used as GKE nodes. These logs can be used for network monitoring, forensics, real-time security analysis, and expense optimization.

This page assumes you are familiar with the concepts described in VPC Flow Logs.

Enable VPC Flow Logs

When you enable VPC Flow Logs, you enable logging for all VMs in a subnet. However, you can reduce the amount of information written to logging. For details on the parameters you can control, see Log sampling and aggregation.

To customize metadata fields or configure log filtering, use the gcloud CLI or API.

Enable VPC Flow Logs when you create a subnet

Console

  1. In the Google Cloud console, go to the VPC networks page.

    Go to VPC networks

  2. Click the network where you want to add a subnet.

  3. Click Add subnet.

  4. For Flow logs, select On.

  5. If you want to adjust log sampling and aggregation, click Configure logs and adjust any of the following:

    • The Aggregation interval.
    • Whether to include metadata in the final log entries. By default, Include metadata includes all fields.
    • The Sample rate. 100% means that all entries are kept.

  6. Populate other fields as appropriate.

  7. Click Add.

gcloud

  1. In the Google Cloud console, activate Cloud Shell.

    Activate Cloud Shell

    At the bottom of the Google Cloud console, a Cloud Shell session starts and displays a command-line prompt. Cloud Shell is a shell environment with the Google Cloud CLI already installed and with values already set for your current project. It can take a few seconds for the session to initialize.

  2. Run the following command:

    gcloud compute networks subnets create SUBNET_NAME \
   --enable-flow-logs \
   [--logging-aggregation-interval=AGGREGATION_INTERVAL] \
   [--logging-flow-sampling=SAMPLE_RATE] \
   [--logging-filter-expr=FILTER_EXPRESSION] \
   [--logging-metadata=LOGGING_METADATA] \
   [--logging-metadata-fields=METADATA_FIELDS] \
   [other flags as needed]

    Replace the following:

    • AGGREGATION_INTERVAL: the aggregation interval for flow logs in that subnet. The interval can be set to any of the following: 5-sec (default), 30-sec, 1-min, 5-min, 10-min, or 15-min.
    • SAMPLE_RATE: the flow sampling rate. Flow sampling can be set from 0.0 (no sampling) to 1.0 (all logs). Default is 0.5.
    • FILTER_EXPRESSION: an expression that defines which logs you want to keep. The expression has a limit of 2,048 characters. For details, see Log filtering.

    • LOGGING_METADATA: the metadata annotations that you want to include in the logs:

      • include-all to include all metadata annotations
      • exclude-all to exclude all metadata annotations (default)
      • custom to include a custom list of metadata fields that you specify in METADATA_FIELDS.

    • METADATA_FIELDS: a comma-separated list of metadata fields you want to include in the logs. For example, src_instance,dst_instance. Can only be set if LOGGING_METADATA is set to custom.

API

Enable VPC Flow Logs when you create a new subnet.

POST https://compute.googleapis.com/compute/v1/projects/PROJECT_ID/regions/REGION/subnetworks
{
  "logConfig": {
    "aggregationInterval": "AGGREGATION_INTERVAL",
    "flowSampling": SAMPLING_RATE,
    "filterExpr": EXPRESSION,
    "metadata": METADATA_SETTING,
    "metadataFields": METADATA_FIELDS,
    "enable": true
  },
  "ipCidrRange": "IP_RANGE",
  "network": "NETWORK_URL",
  "name": "SUBNET_NAME"
}

Replace the placeholders with valid values:

  • PROJECT_ID is the ID of the project where the subnet will be created.
  • REGION is the region where the subnet will be created.
  • AGGREGATION_INTERVAL sets the aggregation interval for flow logs in the subnet. The interval can be set to any of the following: INTERVAL_5_SEC, INTERVAL_30_SEC, INTERVAL_1_MIN, INTERVAL_5_MIN, INTERVAL_10_MIN, or INTERVAL_15_MIN.
  • SAMPLING_RATE is the flow sampling rate. Flow sampling can be set from 0.0 (no sampling) to 1.0 (all logs). Default is .0.5.
  • EXPRESSION is the filter expression you use to filter which logs are actually written. The expression has a limit of 2,048 characters. For details, see Log filtering.

  • METADATA_SETTING specifies whether all metadata is logged (INCLUDE_ALL_METADATA), no metadata is logged (EXCLUDE_ALL_METADATA), or only specific metadata is logged (CUSTOM_METADATA). If this field is set to CUSTOM_METADATA, also populate the metadataFields field. Default is EXCLUDE_ALL_METADATA. Refer to metadata annotations for details.

  • METADATA_FIELDS are the metadata fields you wish to capture when you have set metadata: CUSTOM_METADATA. This is a comma-separated list of metadata fields, such as src_instance, src_vpc.project_id.

  • IP_RANGE is the primary internal IP address range of the subnet.

  • NETWORK_URL is the VPC network URL where the subnet will be created.

  • SUBNET_NAME is a name for the subnet.

For more information, refer to the subnetworks.insert method.

Terraform

You can use a Terraform module to create a custom mode VPC network and subnets.

The following example creates three subnets as follows:

  • subnet-01 has VPC Flow Logs disabled. When you create a subnet, VPC Flow Logs are disabled unless you explicitly enable them.
  • subnet-02 has VPC Flow Logs enabled with the default flow log settings.
  • subnet-03 has VPC Flow Logs enabled with some custom settings.
examples/simple_project/main.tf 
module "test-vpc-module" {
  source       = "terraform-google-modules/network/google"
  version      = "~> 7.0"
  project_id   = var.project_id # Replace this with your project ID in quotes
  network_name = "my-custom-mode-network"
  mtu          = 1460

  subnets = [
    {
      subnet_name   = "subnet-01"
      subnet_ip     = "10.10.10.0/24"
      subnet_region = "us-west1"
    },
    {
      subnet_name           = "subnet-02"
      subnet_ip             = "10.10.20.0/24"
      subnet_region         = "us-west1"
      subnet_private_access = "true"
      subnet_flow_logs      = "true"
    },
    {
      subnet_name               = "subnet-03"
      subnet_ip                 = "10.10.30.0/24"
      subnet_region             = "us-west1"
      subnet_flow_logs          = "true"
      subnet_flow_logs_interval = "INTERVAL_10_MIN"
      subnet_flow_logs_sampling = 0.7
      subnet_flow_logs_metadata = "INCLUDE_ALL_METADATA"
      subnet_flow_logs_filter   = "false"
    }
  ]
}

To learn how to apply or remove a Terraform configuration, see Basic Terraform commands.

Enable VPC Flow Logs for an existing subnet

Console

  1. In the Google Cloud console, go to the VPC networks page.

    Go to VPC networks

  2. Click the subnet that you want to update.

  3. Click Edit.

  4. For Flow logs, select On.

  5. If you want to adjust log sampling and aggregation, click Configure logs and adjust any of the following:

    • The Aggregation interval.
    • Whether to include metadata in the final log entries. By default, Include metadata includes all fields.
    • The Sample rate. 100% means that all entries are kept.

  6. Click Save.

gcloud

  1. In the Google Cloud console, activate Cloud Shell.

    Activate Cloud Shell

    At the bottom of the Google Cloud console, a Cloud Shell session starts and displays a command-line prompt. Cloud Shell is a shell environment with the Google Cloud CLI already installed and with values already set for your current project. It can take a few seconds for the session to initialize.

  2. Run the following command:

    gcloud compute networks subnets update SUBNET_NAME \
   --enable-flow-logs \
   [--logging-aggregation-interval=AGGREGATION_INTERVAL] \
   [--logging-flow-sampling=SAMPLE_RATE] \
   [--logging-filter-expr=FILTER_EXPRESSION] \
   [--logging-metadata=LOGGING_METADATA] \
   [--logging-metadata-fields=METADATA_FIELDS] \
   [other flags as needed]

    Replace the following:

    • AGGREGATION_INTERVAL: the aggregation interval for flow logs in that subnet. The interval can be set to any of the following: 5-sec (default), 30-sec, 1-min, 5-min, 10-min, or 15-min.
    • SAMPLE_RATE: the flow sampling rate. Flow sampling can be set from 0.0 (no sampling) to 1.0 (all logs). Default is 0.5.
    • FILTER_EXPRESSION is an expression that defines which logs you want to keep. The expression has a limit of 2,048 characters. For details, see Log filtering.

    • LOGGING_METADATA: the metadata annotations that you want to include in the logs:

      • include-all to include all metadata annotations
      • exclude-all to exclude all metadata annotations (default)
      • custom to include a custom list of metadata fields that you specify in METADATA_FIELDS.

    • METADATA_FIELDS: a comma-separated list of metadata fields you want to include in the logs. For example, src_instance,dst_instance. Can only be set if LOGGING_METADATA is set to custom.

API

Enable VPC Flow Logs for an existing subnet.

PATCH https://compute.googleapis.com/compute/v1/projects/PROJECT_ID/regions/REGION/subnetworks/SUBNET_NAME
{
  "logConfig": {
    "enable": true
    ...other logging fields.
  },
  "fingerprint": "SUBNETWORK_FINGERPRINT"
}

Replace the placeholders with valid values:

  • PROJECT_ID is the ID of the project where the subnet is located.
  • REGION is the region where the subnet is located.
  • SUBNET_NAME is the name of the existing subnet.
  • SUBNET_FINGERPRINT is the fingerprint ID for the existing subnet, which is provided when you describe a subnet.
  • For the other logging fields, see Enabling VPC Flow Logging when you create a subnet.

For more information, refer to the subnetworks.patch method.

View estimated log volume for existing subnets

The Google Cloud console provides an estimate of your log volume for existing subnets, which you can then use to estimate the cost of enabling flow logs. The estimate is based on flows captured at 5-second intervals for the subnet over the previous 7 days. Also, the size of each log depends on whether you enable metadata annotations.

Console

  1. In the Google Cloud console, go to the VPC networks page.

    Go to VPC networks

  2. Click the subnet that you want to estimate costs for.

  3. Click Edit.

  4. For Flow logs, select On.

  5. Click Configure logs.

  6. View Estimated logs generated per day to see the estimate.

  7. Click Cancel so that none of your changes are saved.

View which subnets have VPC Flow Logs enabled

You can check which subnets in a network have VPC Flow Logs enabled.

Console

  1. In the Google Cloud console, go to the VPC networks page.

    Go to VPC networks

  2. View the Flow logs column to see if logging is on or off.

gcloud

  1. In the Google Cloud console, activate Cloud Shell.

    Activate Cloud Shell

    At the bottom of the Google Cloud console, a Cloud Shell session starts and displays a command-line prompt. Cloud Shell is a shell environment with the Google Cloud CLI already installed and with values already set for your current project. It can take a few seconds for the session to initialize.

  2. Run the following command:

    gcloud compute networks subnets list \
   --project PROJECT_ID \
   --network="NETWORK" \
   --format="csv(name,region,logConfig.enable)"

    Replace the following:

    • PROJECT_ID: the ID of the project you are querying.
    • NETWORK: the name of the network containing the subnets.

Update VPC Flow Logs parameters

You can modify log sampling parameters. See Log sampling and aggregation for details on the parameters you can control.

To customize metadata fields or configure log filtering, use the gcloud CLI or API.

Console

  1. In the Google Cloud console, go to the VPC networks page.

    Go to VPC networks

  2. Click the subnet that you want to update.

  3. Click Edit.

  4. If you want to adjust log sampling and aggregation, click Configure logs and adjust any of the following:

    • The Aggregation interval.
    • Whether to include metadata in the final log entries. By default, Include metadata includes all fields.
    • The Sample rate. 100% means that all entries are kept.

  5. Click Save.

gcloud

  1. In the Google Cloud console, activate Cloud Shell.

    Activate Cloud Shell

    At the bottom of the Google Cloud console, a Cloud Shell session starts and displays a command-line prompt. Cloud Shell is a shell environment with the Google Cloud CLI already installed and with values already set for your current project. It can take a few seconds for the session to initialize.

  2. Run the following command:

    gcloud compute networks subnets update SUBNET_NAME \
   [--logging-aggregation-interval=AGGREGATION_INTERVAL] \
   [--logging-flow-sampling=SAMPLE_RATE] \
   [--logging-filter-expr=FILTER_EXPRESSION] \
   [--logging-metadata=LOGGING_METADATA] \
   [--logging-metadata-fields=METADATA_FIELDS] \

    Replace the following:

    • AGGREGATION_INTERVAL: the aggregation interval for flow logs in that subnet. The interval can be set to any of the following: 5-sec (default), 30-sec, 1-min, 5-min, 10-min, or 15-min.
    • SAMPLE_RATE: the flow sampling rate. Flow sampling can be set from 0.0 (no sampling) to 1.0 (all logs). Default is 0.5.
    • FILTER_EXPRESSION is an expression that defines what logs you want to keep. The expression has a limit of 2,048 characters. For details, see Log filtering.

    • LOGGING_METADATA: the metadata annotations that you want to include in the logs:

      • include-all to include all metadata annotations
      • exclude-all to exclude all metadata annotations (default)
      • custom to include a custom list of metadata fields that you specify in METADATA_FIELDS.

    • METADATA_FIELDS: a comma-separated list of metadata fields you want to include in the logs. For example, src_instance,dst_instance. Can only be set if LOGGING_METADATA is set to custom.

API

Modify the log sampling fields to update VPC Flow Logs behaviors.

PATCH https://compute.googleapis.com/compute/v1/projects/PROJECT_ID/regions/REGION/subnetworks/SUBNET_NAME
{
  "logConfig": {
    ...fields to modify
  },
  "fingerprint": "SUBNETWORK_FINGERPRINT"
}

Replace the placeholders with valid values:

  • PROJECT_ID is the ID of the project where the subnet is located.
  • REGION is the region where the subnet is located.
  • SUBNET_NAME is the name of the existing subnet.
  • SUBNET_FINGERPRINT is the fingerprint ID for the existing subnet, which is provided when you describe a subnet.
  • For the fields that you can modify, see Enabling VPC Flow Logging when you create a subnet.

For more information, refer to the subnetworks.patch method.

Disable VPC Flow Logs for a subnet

Console

  1. In the Google Cloud console, go to the VPC networks page.

    Go to VPC networks

  2. Click the subnet that you want to update.

  3. Click Edit.

  4. For Flow logs, select Off.

  5. Click Save.

gcloud

  1. In the Google Cloud console, activate Cloud Shell.

    Activate Cloud Shell

    At the bottom of the Google Cloud console, a Cloud Shell session starts and displays a command-line prompt. Cloud Shell is a shell environment with the Google Cloud CLI already installed and with values already set for your current project. It can take a few seconds for the session to initialize.

  2. Run the following command:

    gcloud compute networks subnets update SUBNET_NAME \
   --no-enable-flow-logs

API

Disable VPC Flow Logs on a subnet to stop collecting log records.

PATCH https://compute.googleapis.com/compute/v1/projects/PROJECT_ID/regions/REGION/subnetworks/SUBNET_NAME
{
  "logConfig": {
    "enable": false
  },
  "fingerprint": "SUBNETWORK_FINGERPRINT"
}

Replace the placeholders with valid values:

  • PROJECT_ID is the ID of the project where the subnet is located.
  • REGION is the region where the subnet is located.
  • SUBNET_NAME is the name of the existing subnet.
  • SUBNET_FINGERPRINT is the fingerprint ID for the existing subnet, which is provided when you describe a subnet.

For more information, refer to the subnetworks.patch method.

Access logs by using Logging

You can view VPC Flow Logs by using the Logs Explorer. To use the following queries, you need your project's project ID.

Configure IAM

To configure access control for logging, see the access control guide for Logging.

Access all flow logs

Console

  1. In the Google Cloud console, go to the Logs Explorer page.

    Go to Logs Explorer

  2. Click Resource.

  3. In the Select resource list, click Subnetwork, and then click Apply.

  4. Click Log name.

  5. In the Select log names list, click vpc_flows, and then click Apply.

Alternatively:

Console

  1. In the Google Cloud console, go to the Logs Explorer page.

    Go to Logs Explorer

  2. If you don't see the query editor field in the Query pane, click the Show query toggle.

  3. Paste the following into the query editor field. Replace PROJECT_ID with your project ID. 

    resource.type="gce_subnetwork"
logName="projects/PROJECT_ID/logs/compute.googleapis.com%2Fvpc_flows"

  4. Click Run query.

Access logs for a specific subnet

Console

  1. In the Google Cloud console, go to the Logs Explorer page.

    Go to Logs Explorer

  2. Click Resource.

  3. In the Select resource list, click Subnetwork.

  4. In the Subnetwork ID list, select the subnet, and then click Apply.

  5. In the Select log names list, click vpc_flows, and then click Apply.

Alternatively:

Console

  1. In the Google Cloud console, go to the Logs Explorer page.

    Go to Logs Explorer

  2. If you don't see the query editor field in the Query pane, click the Show query toggle.

  3. Paste the following into the query editor field. Replace PROJECT_ID with your project ID and SUBNET_NAME with your subnetwork. 

    resource.type="gce_subnetwork"
logName="projects/PROJECT_ID/logs/compute.googleapis.com%2Fvpc_flows"
resource.labels.subnetwork_name="SUBNET_NAME"

  4. Click Run query.

Access logs for a specific VM

Console

  1. In the Google Cloud console, go to the Logs Explorer page.

    Go to Logs Explorer

  2. If you don't see the query editor field in the Query pane, click the Show query toggle.

  3. Paste the following into the query editor field. Replace PROJECT_ID with your project ID and VM_NAME with the name of your VM. 

    resource.type="gce_subnetwork"
logName="projects/PROJECT_ID/logs/compute.googleapis.com%2Fvpc_flows"
jsonPayload.src_instance.vm_name="VM_NAME"

  4. Click Run query.

Access logs for traffic to a specific subnet range

Console

  1. In the Google Cloud console, go to the Logs Explorer page.

    Go to Logs Explorer

  2. If you don't see the query editor field in the Query pane, click the Show query toggle.

  3. Paste the following into the query editor field. Replace PROJECT_ID with your project ID and SUBNET_RANGE with a CIDR range such as 192.168.1.0/24. 

    resource.type="gce_subnetwork"
logName="projects/PROJECT_ID/logs/compute.googleapis.com%2Fvpc_flows"
ip_in_net(jsonPayload.connection.dest_ip, SUBNET_RANGE)

  4. Click Run query.

Access logs for a specific GKE cluster

Console

  1. In the Google Cloud console, go to the Logs Explorer page.

    Go to Logs Explorer

  2. If you don't see the query editor field in the Query pane, click the Show query toggle.

  3. Paste the following into the query editor field. Replace PROJECT_ID with your project ID and SUBNET_NAME with the name of your subnetwork. 

    resource.type="k8s_cluster"
logName="projects/PROJECT_ID/logs/vpc_flows"
resource.labels.cluster_name="CLUSTER_NAME"

  4. Click Run query.

Access logs for only egress traffic from a subnet

Console

  1. In the Google Cloud console, go to the Logs Explorer page.

    Go to Logs Explorer

  2. If you don't see the query editor field in the Query pane, click the Show query toggle.

  3. Paste the following into the query editor field. Replace PROJECT_ID with the ID of your project and SUBNET_NAME with the name of the subnet to view egress traffic from. 

    logName="projects/PROJECT_ID/logs/compute.googleapis.com%2Fvpc_flows" AND
jsonPayload.reporter="SRC" AND
jsonPayload.src_vpc.subnetwork_name="SUBNET_NAME" AND
(jsonPayload.dest_vpc.subnetwork_name!="SUBNET_NAME" OR NOT jsonPayload.dest_vpc.subnetwork_name:*)

  4. Click Run query.

Access logs for all egress traffic from a VPC network

Console

  1. In the Google Cloud console, go to the Logs Explorer page.

    Go to Logs Explorer

  2. If you don't see the query editor field in the Query pane, click the Show query toggle.

  3. Paste the following into the query editor field. Replace PROJECT_ID with the ID of your project and VPC_NAME with the name of your VPC network.

    logName="projects/PROJECT_ID/logs/compute.googleapis.com%2Fvpc_flows" AND
jsonPayload.reporter="SRC" AND
jsonPayload.src_vpc.vpc_name="VPC_NAME" AND
(jsonPayload.dest_vpc.vpc_name!="VPC_NAME" OR NOT jsonPayload.dest_vpc:*)

  4. Click Run query.

Access logs for specific ports and protocols

For an individual destination port

Console

  1. In the Google Cloud console, go to the Logs Explorer page.

    Go to Logs Explorer

  2. If you don't see the query editor field in the Query pane, click the Show query toggle.

  3. Paste the following into the query editor field. Replace PROJECT_ID with your project ID, PORT with the destination port, and PROTOCOL with the protocol. 

    resource.type="gce_subnetwork"
logName="projects/PROJECT_ID/logs/compute.googleapis.com%2Fvpc_flows"
jsonPayload.connection.dest_port=PORT
jsonPayload.connection.protocol=PROTOCOL

  4. Click Run query.

For more than one destination port

Console

  1. In the Google Cloud console, go to the Logs Explorer page.

    Go to Logs Explorer

  2. If you don't see the query editor field in the Query pane, click the Show query toggle.

  3. Paste the following into the query editor field. Replace PROJECT_ID with your project ID, PORT1 and PORT2 with the destination ports, and PROTOCOL with the protocol. 

    resource.type="gce_subnetwork"
logName="projects/PROJECT_ID/logs/compute.googleapis.com%2Fvpc_flows"
jsonPayload.connection.dest_port=(PORT1 OR PORT2)
jsonPayload.connection.protocol=PROTOCOL

  4. Click Run query.

Route logs to BigQuery, Pub/Sub, and custom targets

You can route flow logs from Logging to a destination of your choice as described in the Routing and storage overview in the Logging documentation. Refer to the previous section for example filters.

Troubleshooting

No vpc_flows appear in Logging for the gce_subnetwork resource

  • Confirm that logging is enabled for the given subnet.
  • VPC flows are only supported for VPC networks. If you have a legacy network, you will not see any logs.
  • In Shared VPC networks, logs only appear in the host project, not the service projects. Make sure you look for the logs in the host project.
  • Logging exclusion filters block specified logs. Make sure there are no exclusion rules that discard VPC Flow Logs.
    1. Go to Resource usage.
    2. Click the Exclusions tab.
    3. Make sure there are no exclusion rules that might discard VPC Flow Logs.

No RTT or byte values on some of the logs

  • RTT measurements may be missing if not enough packets were sampled to capture RTT. This is more likely to happen for low volume connections.
  • RTT values are available only for TCP flows.
  • Some packets are sent with no payload. If header-only packets were sampled, the bytes value will be 0.

Some flows are missing

  • Only TCP, UDP, ICMP, ESP, and GRE protocols are supported. VPC Flow Logs does not support any other protocols.
  • Logs are sampled. Some packets in very low volume flows might be missed.

Missing GKE annotations in some logs

Make sure that your GKE cluster is a supported version.

Missing logs for some GKE flows

Make sure Intranode visibility is enabled in the cluster. Otherwise, flows between Pods on the same node are not logged.

Flow logs appear to be disabled even though you enabled them

  • When you're configuring a proxy-only subnet for internal Application Load Balancers and you're using the gcloud compute networks subnets command to enable VPC Flow Logs, the command appears to succeed, but flow logs aren't actually enabled. The --enable-flow-logs flag doesn't take effect when you also include the --purpose=INTERNAL_HTTPS_LOAD_BALANCER flag.

    When you use the Google Cloud console or the API to enable flow logs, you see the error message: "Invalid value for field 'resource.enableFlowLogs': 'true'. Invalid field set in subnetwork with purpose INTERNAL_HTTPS_LOAD_BALANCER."

    Because proxy-only subnets have no VMs, VPC Flow Logs is not supported. This is intended behavior.

What's next