title: HAProxy - Add response header to indicate the server that was chosen
body: <p>Consider the following HAProxy Config:</p>
  
  <pre><code>frontend front
          default_backend default
  
  backend default
          balance roundrobin
          http-response set-header X-RGN us-east-1
          server app-1a app.us-east-1a.example.com:443 ssl verify none check
          server app-1c app.us-east-1c.example.com:443 ssl verify none check
          server app-1b app.us-east-1b.example.com:443 ssl verify none check
  </code></pre>
  
  <p>I would like to return a response header the indicates the server that was chosen. For example, if the frontend receives a request, it will balance roundrobin and forward the request to a backend server, when it responds, I would like to see in my browser which server was used.</p>
  
  <p>The config might look something like this:</p>
  
  <pre><code>frontend front
          default_backend default
  
  backend default
          balance roundrobin
          http-response set-header X-RGN us-east-1
          server app-1a app.us-east-1a.example.com:443 ssl verify none check
          server app-1c app.us-east-1c.example.com:443 ssl verify none check
          server app-1b app.us-east-1b.example.com:443 ssl verify none check
          http-response set-header X-Server app-1a if server -i app-1a
          http-response set-header X-Server app-1b if server -i app-1b
          http-response set-header X-Server app-1c if server -i app-1c
  </code></pre>
  
  <p>Has anyone tried this before?</p>