Buffer file auto purse chunk event can not reach the hosts

[itlabvn]

Hello. I am new with fluentd. I use td-agent v0.12 with ubuntu
Here is my td-agent.conf. My problem is , i want config fluent to buffer data if can not reach graylog in buffer_path and re-transfer to host when reached. But it auto purse chunk event can not reach my hosts. Plz help. Thank you

<source>
  @type forward
</source>
<match docker>
   @type gelf
   host 172.17.0.4
   port 12201
   protocol tcp
   flush_interval 10s
   buffer_type file
   buffer_path /var/log/td-agent/buffer/td.*.buffer
</match>