Here is my config:

-----
upstream http_backend {
server 127.0.0.1:81;
server 10.1.1.2 backup;
}

server {
...

location /http/ {
proxy_pass http://http_backend;
...
}
}
-----

As long as everything is working, $upstream_addr returns the IP (127.0.0.1:81). But when any backend returns HTTP/502, $upstream_addr returns the upstream name (http_backend) as the $upstream_addr.

Shouldn't this return the lastest server IP tried before failure?