Recently, my Google App Engine (GAE) platform started returning 403 Forbidden errors to all incoming requests even if the IP is allowed access. After investigating, I discovered that I had 1002 active firewall rules in place. Interestingly, as soon as I manually deleted some rules and the count dropped below 1000 rules, the platform resumed normal operation, and the 403 errors disappeared.
The problem raises two major concerns:
Why did GAE give blanket 403 errors for all incoming requests (even from IPs not blocked by the firewall) after the firewall rule count exceeded 1000?
Why did the API allow the rule count to exceed 1000, when in the past it consistently rejected any attempts to go beyond this limit with the following error message:
{āerrorā: {ācodeā: 400, āmessageā: āCannot add rule. Total rule count may not exceed 1000 rulesā, āstatusā: āINVALID_ARGUMENTā}}
Additional Details:
Platform: Google App Engine Standard Environment
Firewall Rules: Mixture of IP blocks, both specific IPs and CIDR ranges (subnets)
Is there an internal limit or behavior in GAE that causes a platform-wide 403 error if the number of firewall rules exceeds 1000?
This seems to be an edge case or a potential bug but is very concerning as all of my end users were unable to use the platform which reduces their trust on the application. Any insights or documentation around this behavior would be greatly appreciated.
In App Engine, you can create a firewall with up to 1000 prioritized individual rules
That implies the maximum # of rules you can have is 1000. There are 2 possible ways to implement this
a) You get an error when you try to exceed this limit (that would be my preferred choice and it seems like thatās what you prefer/expect)
b) You donāt get an error but gcloud simply ignores rules beyond this limit.
If your default rule is āALLOWā, it would be the last rule. This means that if gcloud went with option b, then it would never see that rule and so would deny everything else (hence the 403 errors)
Based on the Google Cloud documentation, the 1000 firewall rule limit is a documented limitation of Google App Engineās standard environment. The document doesnāt explicitly state that exceeding this limit will cause a blanket 403 error for all requests, but it strongly implies it.
Hereās an explanation addressing your concerns:
1. Why did GAE give blanket 403 errors for all incoming requests (even from IPs not blocked by the firewall) after the firewall rule count exceeded 1000?
The document emphasizes the importance of rule priority and the sequential evaluation of rules. With 1002 rules, the processing and evaluation of the rules could be significantly slower or error-prone. Itās plausible that with a very large number of rules, the system either:
Times out: The firewall rule evaluation process takes longer than the request timeout, resulting in a 403 error before the request is properly evaluated.
Resource Exhaustion: Processing a huge number of rules could exhaust available system resources, leading to errors and the 403 response.
Internal Error: The sheer volume of rules might trigger an internal error within the GAE firewall processing mechanism. The 403 error could be a generic catch-all response for such an internal failure.
2. Why did the API allow the rule count to exceed 1000, when in the past it consistently rejected any attempts to go beyond this limit?
This points to an inconsistency or potential flaw in the APIās enforcement of the 1000-rule limit. Itās possible thereās a race condition in the APIās counter mechanism that allows you to exceed the limit under specific circumstances. Itās also possible that the error message was previously more strict and has since become less so. This points to a need for better error handling and monitoring within the App Engine service.
Here are some workarounds to avoid the Problem:
Consolidation: Analyze your existing 1000+ rules. Many likely overlap or can be grouped into broader CIDR ranges.
Prioritization: Ensure your rules are appropriately prioritized. The highest priority rule that matches a request determines the outcome.
Deny by Default (and Whitelist): Instead of explicitly allowing every IP, start with a ādeny allā default rule and add only explicit āallowā rules for trusted IP ranges or networks.
Regular Audits: Implement a system to regularly review and audit your firewall rules. This prevents the accumulation of unnecessary rules over time.
Consider Cloud Armor: For more advanced security needs and management of large numbers of IP addresses, explore Google Cloud Armor. It provides a more sophisticated and scalable solution for web application firewall (WAF) rules.
In summary: While the documentation specifies a limit of 1000 rules, it doesnāt detail the specific behavior of exceeding that limit. Your experience suggests unexpected consequences of exceeding the documented limit, leading to resource exhaustion or internal errors that manifest as 403 errors for all requests. Googleās support should be contacted to report this issue, as itās a significant operational vulnerability.