osx的vpn每48分钟就断线，要用户自己连，从10.6到10.9 apple压根就没想过去修。
The built-in Cisco VPN client introduced in Mac OS X 10.6 Snow Leopard has
the habit of disconnecting itself when re-keying should be performed but
fails after about 48 minutes up to one hour. https://discussions.apple.com/thread/3275811?start=0&tstart=0
