Log upstream service in nginx load balancer log

Question

I have 2 nginx webservers with a nginx load balancer spliting the load.
I am trying to log to which webserver was a request sent into the log file /var/log/nginx/access.log but I can't find such option in ngx_http_log_module docs.

Any idea if this is possible and how to do it?

Answer 1

Variable $upstream_addr contains IP:port pairs for all visited upstreams.