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Someone asked for VPC Flow Logs and we don't know where to start

Flow Logs capture accept/reject of network traffic per ENI. Indispensable for security investigation and cost troubleshooting (NAT). Setup is a checkbox in every cloud, just don't dump them in the same region or you'll pile on cost.

Try this first

  1. 1AWS: enable VPC Flow Logs at VPC level, output to S3 or CloudWatch Logs. S3 is cheaper for archive, CloudWatch better for live query.
  2. 2Activate only REJECT traffic if volume is high, or both for 14 days retention. ALL is complete but can cost TBs per month in a busy VPC.
  3. 3Azure: NSG Flow Logs to a storage account, query via Traffic Analytics in Log Analytics. Same pattern.
  4. 4GCP: VPC Flow Logs in the subnet config, sample rate 0.5 or 1.0. Output to BigQuery or Cloud Logging.
  5. 5Build a few standard queries: top talkers, rejected-internal, untagged traffic. Otherwise they sit unused.

When to bring us in

For an active security investigation or pen test, Flow Logs alone aren't enough. That's where mirror traffic or packet capture is the right level, with help from someone with network forensics experience.

See also

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