HTTP X-Forwarded-For Header
Here is the syntax
X-Forwarded-For: [client1], [proxy1], ... [proxy#]
The value of this HTTP header is a comma and space separated list of IP addresses. The left-most IP address belongs to the original client. Each successive IP address belongs to each proxy that passed the HTTP request. Every proxy server adds the IP address where it received the request from. The last proxy server before reaching the destination will not appear in this header value. The last proxy server appears as the remote address of the request.
Here is an example:
X-Forwarded-For: 18.104.22.168, 22.214.171.124