nginx upstreamで502 Bad Gateway発生時に切り替える設定

upstream app {
  server バックエンドのIP1 max_fails=3 fail_timeout=30s;
　server バックエンドのIP2 max_fails=3 fail_timeout=30s;
}

server {
    listen 80;
    server_name リバースプロキシ側のIP;    

    location / {
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header Host $http_host;
        proxy_redirect off;
        proxy_pass http://app;
        proxy_next_upstream error timeout http_502;
        proxy_next_upstream_tries 0;
        proxy_next_upstream_timeout 10;
        proxy_connect_timeout 5s;
        proxy_send_timeout 5s;
        proxy_read_timeout 5s;
    }
}

proxy_next_upstream error timeout http_500 http_502 http_503 http_504;
proxy_next_upstream_tries 3;

upstream backend {
    server app01:8080 max_fails=3 fail_timeout=30s;
    server app02:8080 max_fails=3 fail_timeout=30s;
}