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 ?