Rate-based rule caveats - Amazon WAF, Amazon Firewall Manager, and Amazon Shield Advanced
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Rate-based rule caveats

Amazon WAF rate limiting is designed to control high request rates and protect your application's availability in the most efficient and effective way possible. It's not intended for precise request-rate limiting.

  • Amazon WAF estimates the current request rate using an algorithm that gives more importance to more recent requests. Because of this, Amazon WAF will apply rate limiting near the limit that you set, but does not guarantee an exact limit match.

  • Each time that Amazon WAF estimates the rate of requests, Amazon WAF looks back at the number of requests that came in during the configured evaluation window. Due to this and other factors such as propagation delays, it's possible for requests to be coming in at too high a rate for up to several minutes before Amazon WAF detects and rate limits them. Similarly. the request rate can be below the limit for a period of time before Amazon WAF detects the decrease and discontinues the rate limiting action. Usually, this delay is below 30 seconds.

  • If you change any of the rate limit settings in a rule that's in use, the change resets the rule's rate limiting counts. This can pause the rule's rate limiting activities for up to a minute. The rate limit settings are the evaluation window, rate limit, request aggregation settings, forwarded IP configuration, and scope of inspection.