[nginx] 对UA为空的请求返回403

nginx blocking blank user agent .
sometime apps’ backgroud request always visit a url, and these requests’ user agent is black, so I want block them.

针对一个location的nginx配置

       location / {
            if ($http_user_agent = ""){
                access_log off;
                return403;
            }

            default_type   text/html;
            content_by_lua '
                ngx.say("
你好 baby 
")
            ';
        }

模拟请求

lzz@ubuntu:conf$ curl 'http://localhost'
你好 baby /h1>