Calculate Web ACL Capacity Unit (WCU) in AWS WAF

Base unit and text transformation

As part of my routine review of my company’s AWS WAF access control lists (ACLs), I also check the WCU of existing web ACLs to see if an ACL still can fit in more rules until it reaches the 1,500 WCU quota. While checking an ACL’s WCU, I also find it useful to also check WCU of individual rules to locate any rule with larger-than-usual WCU.

While individual and total WCU are shown during ACL creation/modification on the management console, a read-only role could only check the total WCU. It may be possible to use CheckCapacity CLI or API by separating each rule as an ACL, but that’ll involve excessive (online) API calls.

I further improved my script by implementing offline WCU calculation. While the AWS docs has a complete list of WCU of each match statement, I find the text transformation part is not clear enough.

For each Text transformation that you apply, add 10 WCUs.

It implies that any time you use text transformation, you gotta add 10 units. When I used this assumption, my calculation was off by a mile. It is more accurate to say:

For each unique Text transformation that you apply, add 10 WCUs.

For the purpose of WCU, a text transformation is actually made up of two components: request component and transformation action.

For example, URI path (request component) is transformed to lowercase (a transformation action), (URI path + lowercase) are considered as one unique text transformation. (URI path + lowercase) can be applied multiple times within a rule (through nested statements) and even within an ACL, it will still be counted one transformation only.

This means I need to account for repeated text transformation within a rule, so that it’s calculated only once. This is easily achieved through the use of Python Sets. The same applies when calculating total WCU of an ACL. When a unique text transformation is applied across different rules in an ACL, the sum of all rules’ WCU will be less than the ACL’s WCU.

When transforming a Header, it’s counted based on a specific header. For example, rule A has (Header(User-Agent) + lowercase) and rule B has (Header(Cookies) + lowercase), these are counted as two transformations, so they’ll use 20 WCUs.