我的 pptp 连接无法保持连接，它会在不到一分钟的时间内断开连接
这是一些相关的日志条目
May 31 13:32:31 localhost NetworkManager[931]: <info> Starting VPN service 'pptp'...
May 31 13:32:31 localhost NetworkManager[931]: <info> VPN service 'pptp' started (org.freedesktop.NetworkManager.pptp), PID 15216
May 31 13:32:31 localhost NetworkManager[931]: <info> VPN service 'pptp' appeared; activating connections
May 31 13:32:31 localhost NetworkManager[931]: <info> VPN plugin state changed: init (1)
May 31 13:32:31 localhost NetworkManager[931]: <info> VPN plugin state changed: starting (3)
May 31 13:32:31 localhost NetworkManager[931]: <info> VPN connection 'Dynalabs' (Connect) reply received.
May 31 13:32:31 localhost pppd[15221]: …