Why Does My API Server on a Subdomain Fail to Respond with the \'Access-Control-Allow-Origin\' Header Despite Proper CORS Configuration?

Failed to Access Subdomain from Main Domain: No 'Access-Control-Allow-Origin' Header

The problem arises when you attempt to access an API server hosted on a subdomain from a React app on the main domain. While using CORS (Cross-Origin Resource Sharing) for cross-origin communication, the server must send the 'Access-Control-Allow-Origin' header to specify which domains are allowed access.

Problem Definition

You've encountered a CORS policy error when trying to access the API from the React app, with the error message "No 'Access-Control-Allow-Origin' header is present on the requested resource." Despite using different approaches to set the CORS middleware, including using the gin-contrib/cors library and manually adding the header, the error persists.

Solution

It turns out that the issue lies not with the server-side CORS configuration but with the settings on your AWS Load Balancer Target Group. You had previously set the protocol to HTTPS in the target group, even though you had only provided ACM certificates to Route 53 and ALB. Once you replaced HTTPS with HTTP in the target group, the issue was resolved and the API became accessible from the React app.

Debugging CORS Issues

To diagnose CORS-related issues, it's crucial to examine the preflight request using the Chrome DevTools or a dedicated tool like cURL. Check the preflight response header to ensure that the expected 'Access-Control-Allow-Origin' header is present. If the response indicates a status code other than 204 No Content, it may signal an issue with the server-side response or its interpretation by the client.

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