How to load test an API behind a firewall

APIs protected by source-IP firewalls can return 403 errors during cloud load tests unless the load generator’s addresses are allowlisted, making stable static IP ranges important for testing publicly accessible but filtered staging environments. The described Postman workflow reuses existing functional-test collections and assertions, beginning with a small local smoke test to identify configuration problems before running larger cloud tests from Postman’s managed static IPs. Teams can configure ramped virtual-user loads, latency and error thresholds such as p90 latency below 500 ms, and real-time monitoring, then use the same cloud test as a CI/CD release gate that blocks promotion when performance targets fail. Successful setup is indicated by the absence of firewall errors, comparable low-load local and cloud results, identification of capacity limits, and performance trends across releases. Static-IP cloud testing is intended for public endpoints restricted by IP allowlists, while private-only APIs require a self-hosted runner inside the network; IP allowlisting also complements rather than replaces application authentication.

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