搞定了 是nginx的1.9.8默认是支持ipv6的dns查询，然而目标网站没有ipv6的记录所以无法拿到解析结果. Resolver x.x.x.x ipv6=off; 添加这个指令即可
By default, nginx will look up both IPv4 and IPv6 addresses while resolving. If looking up of IPv6 addresses is not desired, the ipv6=off parameter can be specified. http://nginx.org/en/docs/http/ngx_http_core_module.html#resolver