Table of Contents

Introduction: The Frustrating 405 Error
Understanding the 405 Method Not Allowed Error
- What is a 405 Error Exactly?
- Why It Happens with Nginx and ASP.NET Core
Pre-flight Check: Diagnosing the Root Cause
- Check Nginx Access and Error Logs
- Test the API Directly with cURL
The 3-Step Fix for the 405 Nginx Error
Putting It All Together: A Complete Nginx Configuration
FAQs and Advanced Troubleshooting

The "405 Method Not Allowed" error in an Nginx/ASP.NET Core setup almost always points to a reverse proxy misconfiguration.
The core of the fix is ensuring your Nginx `location` block properly uses `proxy_pass` to forward requests to the Kestrel server.
You must forward critical HTTP headers like `Host`, `X-Real-IP`, and `X-Forwarded-For` so the ASP.NET Core application knows the original request details.
Never use the `try_files` directive inside a location block that is meant for proxying API requests, as it will interfere with non-GET/HEAD methods.
Always validate your Nginx configuration with `sudo nginx -t` before reloading with `sudo systemctl reload nginx` to avoid downtime.