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Querying VPC Flow Logs

In the course of supporting a Modernisation Platform customer you may need to investigate traffic flows in and our of Elastic Network Interfaces. You can do so by querying VPC flow logs.

Our shared VPC approach

The Modernisation Platform provisions on a per-business unit and per-environment basis. This allows us to provision resources on a lean basis, shared between applications and service teams in the same business unit.

Viewing VPC flow logs

As a result of our consolidate VPC approach you will not find the relevant customer flow logs by querying the customer account, but will instead need to access the relevant core-vpc-$environment account where they will be stored in an AWS CloudWatch log group.

You can query the flow logs in a variety of ways; with Amazon Athena, though the AWS CLI, and in the AWS Console through a web browser.

Understanding log entries

AWS VPC Flow logs are stored in log streams for each elastic network interface.

If you are interested in traffic passing in and out of a particular EC2 instance or Elastic Load Balancer you will need the relevant Elastic Network Interface ID.

Inside the log stream you will see, by default, fields showing the source ENI ID, source and destination IP addresses, and source and destination ports as well as if the traffic was accepted or rejected by an ACL or Security Group rule.

Viewing log entries through a web browser

  1. Sign into the appropriate core-vpc-$environment account through
  2. Browse to CloudWatch > Logs > Log Groups > $business-unit-$environment-vpc-flow-logs.
  3. Select the ENI you wish to view logs from.
  4. Narrow down the timeframe of logs with logs you with to view, or apply filters as appropriate.

An Example of a VPC Flow Log entry

2 111111111111 eni-aaaaa111111111111 57442 443 6 7 311 1648219031 1648219031 ACCEPT OK

The above entry shows a Type 2 flow log entry, in account 111111111111 from eni-aaaaa111111111111. The source and destination addresses indicate a short HTTPS transaction that was accepted by network ACL and SG rules.

2 111111111111 eni-aaaaa111111111111 35535 22 6 3 180 1418530010 1418530070 REJECT OK

The above entry shows another Type 2 flow entry, from the same account and ENI. In this entry we see what looks like a failed attempt to make an SSH connection to a public IP address.


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