upstream server:port variable ?

[Mansoor Peerbhoy]

Hello,

A quick question:
Does nginx have a variable which will give me the actual upstream server:port for the selected upstream server ?

For instance, if I have:

    upstream xxx
    {
        server  s1:7070;
        server  s2:7070;
        server  s3:7070;
        server  s4:7070;
    }

and if I have 

    server
    {
      ...
      location / 
      {
        proxy_pass http://xxx;
        proxy_set_header    Host    $variable_name; # <-- which variable should I use here, to get "s1:7070" or "s2:7070", etc. ?
      }
      ...
    }

$server_name  gives me the FQDN of the proxy server,
$proxy_host   gives me the name of the upstream block (in this case, "xxx")

Which variable should I use in order to get the precise name of the selected upstream server ?

Thanks
Mansoor Peerbhoy