用RESTclient访问nginx 为什么返回的status code是508？

Question

如下是nginx.conf，很简单的一个配置文件。
为啥我用RESTClient不论向127.0.0.1:8011发送get，还是post请求，均返回508...
在浏览器上直接访问127.0.0.1:8011，什么也不返回
难道是因为upstream qq_backend中的server有问题吗？

#user  nobody;
worker_processes  1;

#error_log  logs/error.log;
#error_log  logs/error.log  notice;
#error_log  logs/error.log  info;

#pid        logs/nginx.pid;


events {
    worker_connections  1024;
}


http {

    lua_shared_dict _ups_zone 64m; # 定义upstream共享内存空间
    include       mime.types;
    default_type  application/octet-stream;

    #log_format  main  '$remote_addr - $remote_user [$time_local] "$request" '
    #                  '$status $body_bytes_sent "$http_referer" '
    #                  '"$http_user_agent" "$http_x_forwarded_for"';

    #access_log  logs/access.log  main;

    sendfile        on;
    #tcp_nopush     on;

    #keepalive_timeout  0;
    keepalive_timeout  65;

    #gzip  on;

 

  
    server {
        listen       8011;
        error_log /home/liycq/lua/nginx1.10.2/logs/error.log debug;
        server_name  somename  alias  another.alias;
        
         location = /redis {
            internal;
            set_unescape_uri $key $arg_key;
            #ngx.log(ngx.ERR, "redis returned empty body")
            redis2_query get $key;
            redis2_pass 127.0.0.1:6380;
        }
        
         location / {
            proxy_pass http://qq_backend;
            #proxy_pass http://$target;
        }
        
        location = /lua-version { 
            default_type 'text/plain';
            content_by_lua ' 
            if jit then 
                ngx.say(jit.version) 
            else 
                ngx.say(_VERSION) end 
            '; 
        }
    }
    upstream qq_backend{
        server 14.17.42.40;
        server 14.17.32.211;
        server 59.37.96.63;
    }
}

如下是error.log中内容：
2016/11/23 20:04:06 [notice] 29393#0: signal process started
2016/11/23 20:04:21 [info] 29394#0: *66 client closed connection while waiting for request, client: 10.129.30.62, server: 0.0.0.0:8011
2016/11/23 20:05:41 [info] 29394#0: *70 client closed connection while waiting for request, client: 10.129.30.62, server: 0.0.0.0:8011