connect() failed (111: Connection refused) while connecting to upstream

Question

出错误的nginx.conf

#..........................................
    upstream auth_server_pool {
        server 10.10.100.251:21000;
    }
#..........................................
    server {
      listen       80;
      server_name  auth.imt.cool;

      proxy_set_header X-Forwarded-Host $host;
      proxy_set_header X-Forwarded-Server $host;
      proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;

      location / {
        proxy_pass http://auth_server_pool;
        proxy_connect_timeout 600;
        proxy_read_timeout 600;
      } 
    }
#..........................................

访问地址：http://auth.imt.cool/uaa/login/captcha
访问提示：

==========================================================================================
改成不使用upstream的nginx.conf，访问正常

#..........................................
    server {
      listen       80;
      server_name  auth.imt.cool;

      proxy_set_header X-Forwarded-Host $host;
      proxy_set_header X-Forwarded-Server $host;
      proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;

      location / {
        proxy_pass http://10.10.100.251:21000;
        proxy_connect_timeout 600;
        proxy_read_timeout 600;
      } 
    }
#..........................................

正常获取到

不知原因在哪里的？