我正在尝试在虚拟机中使用 xubuntu 14.04 中的 PPTP VPN。以前使用效果很好，在我的主机上也能用。但这次不行...有人知道吗？
系统日志：
Nov 15 18:53:03 molecule NetworkManager[820]: VPN service 'pptp' started (org.freedesktop.NetworkManager.pptp), PID 2862
Nov 15 18:53:03 molecule NetworkManager[820]: VPN service 'pptp' appeared; activating connections
Nov 15 18:53:03 molecule NetworkManager[820]: VPN plugin state changed: init (1)
Nov 15 18:53:03 molecule NetworkManager[820]: VPN plugin state changed: starting (3)
Nov 15 18:53:03 molecule NetworkManager[820]: VPN connection 'Frankfurt' (Connect) reply received.
Nov 15 18:53:03 molecule pppd[2866]: Plugin /usr/lib/pppd/2.4.5/nm-pptp-pppd-plugin.so loaded.
Nov 15 18:53:03 molecule pppd[2866]: pppd 2.4.5 started by root, uid 0
Nov 15 18:53:03 molecule pptp[2869]: nm-pptp-service-2862 log[main:pptp.c:314]: The synchronous pptp option is NOT activated
Nov 15 18:53:03 molecule pppd[2866]: Using interface ppp0
Nov 15 18:53:03 molecule pppd[2866]: Connect: ppp0 /dev/pts/2
Nov 15 18:53:03 molecule NetworkManager[820]: SCPlugin-Ifupdown: devices added (path: /sys/devices/virtual/net/ppp0, iface: ppp0)
Nov 15 18:53:03 molecule NetworkManager[820]: SCPlugin-Ifupdown: device added (path: /sys/devices/virtual/net/ppp0, iface: ppp0): no ifupdown configuration found.
Nov 15 18:53:03 molecule NetworkManager[820]: /sys/devices/virtual/net/ppp0: couldn't determine device driver; ignoring...
Nov 15 18:53:03 molecule pptp[2885]: nm-pptp-service-2862 log[ctrlp_rep:pptp_ctrl.c:251]: Sent control packet type is 1 'Start-Control-Connection-Request'
Nov 15 18:53:03 molecule pptp[2885]: nm-pptp-service-2862 log[ctrlp_disp:pptp_ctrl.c:739]: Received Start Control Connection Reply
Nov 15 18:53:03 molecule pptp[2885]: nm-pptp-service-2862 log[ctrlp_disp:pptp_ctrl.c:773]: Client connection established.
Nov 15 18:53:04 molecule pptp[2885]: nm-pptp-service-2862 log[ctrlp_rep:pptp_ctrl.c:251]: Sent control packet type is 7 'Outgoing-Call-Request'
Nov 15 18:53:04 molecule pptp[2885]: nm-pptp-service-2862 log[ctrlp_disp:pptp_ctrl.c:858]: Received Outgoing Call Reply.
Nov 15 18:53:04 molecule pptp[2885]: nm-pptp-service-2862 log[ctrlp_disp:pptp_ctrl.c:897]: Outgoing call established (call ID 0, peer's call ID 55680).
Nov 15 18:53:34 molecule pppd[2866]: LCP: timeout sending Config-Requests
Nov 15 18:53:34 molecule pppd[2866]: Connection terminated.
Nov 15 18:53:34 molecule avahi-daemon[660]: Withdrawing workstation service for ppp0.
Nov 15 18:53:34 molecule NetworkManager[820]: VPN plugin failed: 1
Nov 15 18:53:34 molecule NetworkManager[820]: SCPlugin-Ifupdown: devices removed (path: /sys/devices/virtual/net/ppp0, iface: ppp0)
Nov 15 18:53:34 molecule pppd[2866]: Modem hangup
Nov 15 18:53:34 molecule pptp[2869]: nm-pptp-service-2862 warn[decaps_hdlc:pptp_gre.c:204]: short read (-1): Input/output error
Nov 15 18:53:34 molecule pptp[2869]: nm-pptp-service-2862 warn[decaps_hdlc:pptp_gre.c:216]: pppd may have shutdown, see pppd log
Nov 15 18:53:34 molecule pptp[2885]: nm-pptp-service-2862 log[callmgr_main:pptp_callmgr.c:234]: Closing connection (unhandled)
Nov 15 18:53:34 molecule pptp[2885]: nm-pptp-service-2862 log[ctrlp_rep:pptp_ctrl.c:251]: Sent control packet type is 12 'Call-Clear-Request'
Nov 15 18:53:34 molecule pptp[2885]: nm-pptp-service-2862 log[call_callback:pptp_callmgr.c:79]: Closing connection (call state)
Nov 15 18:53:34 molecule pppd[2866]: Exit.
Nov 15 18:53:34 molecule NetworkManager[820]: VPN plugin failed: 1
Nov 15 18:53:34 molecule NetworkManager[820]: VPN plugin failed: 1
Nov 15 18:53:34 molecule NetworkManager[820]: VPN plugin state changed: stopped (6)
Nov 15 18:53:34 molecule NetworkManager[820]: VPN plugin state change reason: 0
Nov 15 18:53:34 molecule NetworkManager[820]: Policy set 'Wired connection 1' (eth0) as default for IPv4 routing and DNS.
Nov 15 18:53:34 molecule NetworkManager[820]: error disconnecting VPN: Could not process the request because no VPN connection was active.
Nov 15 18:53:40 molecule NetworkManager[820]: VPN service 'pptp' disappeared