Understanding CORS Errors: 'failed to access subdomain from main domain: No 'Access-Control-Allow-Origin''

Cross-Origin Resource Sharing (CORS) is a mechanism that allows different domains to interact with one another. When an API server runs on a subdomain and a React app operates on the main domain, CORS errors can occur, such as the "failed to access subdomain from main domain: No 'Access-Control-Allow-Origin'" error.

Investigating the Issue

Inspect the preflight request in the Chrome DevTools to check for cached responses and preflight requests. Execute the preflight request using a tool like curl, remembering to add the -i option to output the response headers.

Case Analysis

• Case 1: This approach utilizes the default CORS configuration, which enables a wildcard origin. However, testing reveals that it functions properly.
• Case 2: Customizing the allowed origins, methods, and headers also proves successful.
• Case 3: Manually handling CORS headers is another viable solution.

Resolving the Issue

Misconfiguration in AWS Load Balancer Target Group (Based on OP's Solution)

The root cause of this particular issue was a misconfiguration in the AWS Load Balancer Target Group. The target group's protocol was set to HTTPS even though appropriate SSL certificates were not provided. Correcting the protocol resolved the problem.

Debugging Tips

• Disable caching in Chrome DevTools to avoid cached preflight responses.
• Directly send preflight requests to your service to eliminate potential proxy interference.
• Print the request buffer (e.g., using httputil.DumpRequest) for debug purposes.