When you enable VPC flow logs, records will be created in CloudWatch Logs (or S3 if you choose so, but CloudWatch Logs is definitely better for debugging). There will be one log stream per network interface, and the format of the logs is like so:
version account-id interface-id srcaddr dstaddr srcport dstport protocol packets bytes start end action log-status
`action` will usually by ACCEPT or REJECT
Check out the AWS documentation for more information.
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