Process Transit Gateway Flow Logs records in Amazon CloudWatch Logs

You can work with flow log records as you would with any other log events collected by CloudWatch Logs. For more information about monitoring log data and metric filters, see Creating metrics from log events using filters in the Amazon CloudWatch User Guide.

Example: Create a CloudWatch metric filter and alarm for a flow log

In this example, you have a flow log for tgw-123abc456bca. You want to create an alarm that alerts you if there have been 10 or more rejected attempts to connect to your instance over TCP port 22 (SSH) within a 1-hour time period. First, you must create a metric filter that matches the pattern of the traffic for which to create the alarm. Then, you can create an alarm for the metric filter.

To create a metric filter for rejected SSH traffic and create an alarm for the filter

1. Open the CloudWatch console at https://console.aws.amazon.com/cloudwatch/.

2. In the navigation pane, choose Logs, Log groups.

3. Select the checkbox for the log group, and then choose Actions, Create metric filter.

4. For Filter Pattern, enter the following.

   [version, resource_type, account_id,tgw_id="tgw-123abc456bca”, tgw_attachment_id, tgw_src_vpc_account_id, tgw_dst_vpc_account_id, tgw_src_vpc_id, tgw_dst_vpc_id, tgw_src_subnet_id, tgw_dst_subnet_id, tgw_src_eni, tgw_dst_eni, tgw_src_az_id, tgw_dst_az_id, tgw_pair_attachment_id, srcaddr= "10.0.0.1", dstaddr, srcport=“80”, dstport, protocol=“6”, packets, bytes,start,end, log_status, type,packets_lost_no_route, packets_lost_blackhole, packets_lost_mtu_exceeded, packets_lost_ttl_expired, tcp_flags,region, flow_direction, pkt_src_aws_service, pkt_dst_aws_service]

5. For Select log data to test, select the log stream for your transit gateway. (Optional) To view the lines of log data that match the filter pattern, choose Test pattern. When you're ready, choose Next.

6. Enter a filter name, metric namespace, and metric name. Set the metric value to 1. When you're done, choose Next and then choose Create metric filter.

7. In the navigation pane, choose Alarms, All alarms.

8. Choose Create alarm.

9. Choose the namespace for the metric filter that you created.

   It can take a few minutes for a new metric to display in the console.

10. Select the metric name that you created, and then choose Select metric.

11. Configure the alarm as follows, and then choose Next:

    • For Statistic, choose Sum. This ensure that you capture the total number of data points for the specified time period.

    • For Period, choose 1 hour.

    • For Whenever, choose Greater/Equal and enter 10 for the threshold.

    • For Additional configuration, Datapoints to alarm, leave the default of 1.

12. For Notification, select an existing SNS topic, or choose Create new topic to create a new one. Choose Next.

13. Enter a name and description for the alarm and choose Next.

14. When you are done configuring the alarm, choose Create alarm.