What header stores a visitor's real IP address when using Alibaba Cloud CDN?

The header that stores a visitor's real IP address when using Alibaba Cloud CDN is the X-Forwarded-For header. This header is commonly used in proxy setups and CDN environments to pass the original client's IP address through to the server, which might be behind one or more layers of proxy servers or a load balancer.

When a client makes a request to a server through a CDN, the CDN acts as an intermediary. By default, the server would only see the IP address of the CDN, not the actual client's IP. The X-Forwarded-For header remedies this by appending the client's original IP address to the request. This allows web applications and services to recognize the true source of the traffic, which is particularly important for logging, security, and analytics purposes.

While other headers like HTTP-Client-IP and Proxy-Client-IP may also carry similar information, they are less commonly used in the context of CDN configurations, and their usage may vary across different systems and setups. The Real-IP-Address is not a standard HTTP header, further emphasizing that X-Forwarded-For is the de facto standard for transmitting the client's real IP address in a CDN context.