title: How to get IP of backend selected?
body: <p>I am using HAProxy with multiple backends and some ACLs to select a backend, I want my client (for testing purposes and etc) to know the ip of the backend that was selected. </p>
  
  <p>For example, Client sends request to HAProxy which loadbalances between ip A and ip B. In the response header, I want the ip of A if A was selected by HAProxy and I want the ip of B if B was selected. I know of %[dst] but it returns the ip of the HAProxy server instead.</p>
  
  <p>Right now I'm putting <code>http-response set-header X-Forwarded-Host %[dst]:%[dst_port]</code> in the frontend and it is returning the ip and port of the HAProxy</p>