Why Can\'t I Access My Subdomain API from the Main Domain?

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

This issue arises when a web application hosted on a subdomain attempts to access an API server on the main domain, but encounters the "No 'Access-Control-Allow-Origin' header is present on the requested resource" error.

CORS Policy Enforcement:

The message indicates that the response from the API server lacks the "Access-Control-Allow-Origin" header, which is essential for enabling cross-origin resource sharing (CORS). CORS is a security mechanism that prevents web applications from making unauthorized requests to other domains.

Possible Causes:

1. Misconfigured CORS Settings: Ensure that the API server is correctly configured to allow cross-origin requests from the subdomain by adding the appropriate "Access-Control-Allow-Origin" header to its responses.
2. Reverse Proxy Issue: Verify that any reverse proxies or load balancers between the subdomain and the API server are not blocking the "Access-Control-Allow-Origin" header.

Troubleshooting:

1. Check Preflight Response: Use the Chrome DevTools or a curl command to examine the preflight request and response. Ensure that the preflight response contains the necessary "Access-Control-Allow-Origin" header.
2. Send Request Directly to API: Try sending the preflight request directly to the API server to eliminate the possibility of interference from other components.
3. Test CORS Configuration: Use online CORS testing tools or browser extensions to verify that the CORS settings are working correctly.
4. Inspect Response Headers: Use tools like curl to inspect the response headers from the API server and ensure that the "Access-Control-Allow-Origin" header is present.
5. Check AWS Load Balancer Settings: In the case of AWS, check if the target group associated with the load balancer is configured for HTTPS, as this can sometimes lead to CORS issues.

Additional Information:

• [Handling CORS in Go with Gin](https://www.digitalocean.com/community/tutorials/how-to-handle-cross-origin-resource-sharing-cors-in-go)
• [Using CORS with Gin-gonic](https://godoc.org/github.com/gin-gonic/gin#hdr-CORS)
• [Testing CORS with cURL](https://developer.mozilla.org/en-US/docs/Glossary/Preflight_request#Testing_CORS_with_cURL)

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