What are VPC Flow Logs Used For?

Last Updated : 11-Oct-2020

VPC Flow Logs is a feature that enables you to capture information about the IP traffic going to and from network interfaces in your VPC. Flow log data can be published to Amazon CloudWatch Logs or Amazon S3. After you've created a flow log, you can retrieve and view its data in the chosen destination.

Flow Log Use Cases

Flow logs can help you with a number of tasks, such as:

  • Diagnosing overly restrictive security group rules
  • Monitoring the traffic that is reaching your instance
  • Determining the direction of the traffic to and from the network interfaces

Flow log data is collected outside of the path of your network traffic, and therefore does not affect network throughput or latency. You can create or delete flow logs without any risk of impact to network performance.

Flow logs Configuration

You can create a flow log for a VPC, a subnet, or a network interface. If you create a flow log for a subnet or VPC, each network interface in that subnet or VPC is monitored.

Flow log data for a monitored network interface is recorded asĀ flow log records, which are log events consisting of fields that describe the traffic flow.

To create a flow log, you specify:

  • The resource for which to create the flow log
  • The type of traffic to capture (accepted traffic, rejected traffic, or all traffic)
  • The destinations to which you want to publish the flow log data

Publishing to CloudWatch Logs

low logs can publish flow log data directly to Amazon CloudWatch.

When publishing to CloudWatch Logs, flow log data is published to a log group, and each network interface has a unique log stream in the log group. Log streams contain flow log records. You can create multiple flow logs that publish data to the same log group. If the same network interface is present in one or more flow logs in the same log group, it has one combined log stream. If you've specified that one flow log should capture rejected traffic, and the other flow log should capture accepted traffic, then the combined log stream captures all traffic.

Publishing to Amazon S3

Flow logs can publish flow log data to Amazon S3.

When publishing to Amazon S3, flow log data is published to an existing Amazon S3 bucket that you specify. Flow log records for all of the monitored network interfaces are published to a series of log file objects that are stored in the bucket. If the flow log captures data for a VPC, the flow log publishes flow log records for all of the network interfaces in the selected VPC.

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