Hello,
I manage a website called site.com, which hosts various images. For instance, one of the images can be found at site.com/uploads/img.png. Additionally, I have another site, site.fr, where I attempt to serve the same images via cdn.site.fr/uploads/img.png, with cdn.site.fr configured as an alias for site.com.

However, I've encountered an issue: frequently, when accessing site.fr, the images fail to load, resulting in an ERR_CONNECTION_REFUSED error for cdn.site.fr/uploads/img.png. Interestingly, the original image at site.com/uploads/img.png is accessible and returns a 200 OK response.
Could you help me identify the root cause of this problem? Where should I focus my investigation in the logs to understand why ERR_CONNECTION_REFUSED is occurring?

Issue lies in the configuration of your CDN setup. The ERR_CONNECTION_REFUSED error typically indicates a problem with the server's ability to establish a connection. Given that the original image at site.com is accessible, I'd focus on investigating the DNS resolution and SSL certificate configuration for cdn.site.fr.
It's possible that the alias setup is not properly configured, or there's an issue with the SSL certificate not being recognized by the browser, resulting in a connection refusal.

Are there any provisions in place for mitigating Distributed Denial-of-Service (DDoS) attacks on the same content delivery network (CDN) for cdn.site.ru or site.fr? Furthermore, if we were to refresh the page (e.g., by pressing F5), would all assets load correctly, or are there any potential caching or propagation issues that could impact the user experience? In other words, if we were to simulate a page reload, would the CDN's edge servers be able to handle the request and deliver the content seamlessly, or would we encounter any latency or errors?

Have you considered that leveraging FastCGI for image processing might be the root cause of the issue? I'd recommend fine-tuning the nginx configuration to optimize performance and alleviate potential bottlenecks.
Perhaps a more efficient approach would be to utilize a dedicated image processing module, such as ImageMagick or GraphicsMagick, which could be integrated into your web application via a PHP extension or a server-side scripting language.