Alibaba Cloud CDN provides services externally through the HTTP and HTTPS protocols. Clients provide services externally through the universal ports of Alibaba Cloud CDN (Port 80 for HTTP and Port 443 for HTTPS).
All the back-to-source requests of Alibaba Cloud CDN are sent to the origin site server through Port 80 or Port 443. Currently, Alibaba Cloud CDN does not support defining other ports as back-to-source ports. An important point to be noted is that when the back-to-source requests of Alibaba Cloud CDN are sent to the Server Load Balancer, the front-end protocol must be HTTP or HTTPS and the front-end port must be Port 80 or Port 443, and the back-end protocol and back-end port must be defined based on the origin site configuration.
If the problem persists, contact Alibaba Cloud After-Sales Technical Support.