How to protect Auto Provision Server located at Reseller

[Tuanhtk]

Hello
I'm deploying auto provision. I was following: http://forum.yealink.com/forum/showthread.php?tid=1645.
It's successed.
But I have one question:
If I bring Auto Provision Server located at Reseller to public (use IP Public),
How to protect it ?
Because server can be FPT,TFTP,HTTP just protect by usename and password.
Attack it so easy by bruteforce or get username/password with wireshark in local. If we loss file cfg on this server, It's very dangerous.
Please help me.