目前已在 Ubunutu 14.04、13.10 和 12.04 上进行了测试，我尝试使用 PPTP 连接到我的 VPN 提供商。我已遵循各种说明，包括提供商给我的说明，但这些说明都不起作用。
在全新安装的 ubuntu 上我运行以下命令（常规位）：
apt-get install network-manager-pptp pptp-linux
之后，我在网络管理器中填写了常用配置，包括启用 MPPE。我的 VPN 尝试连接，但最终失败。这是我尝试连接之前和之后的系统日志。为了确保我的 VPN 提供商和凭据正常工作，我在 Mac 上进行了测试，没有出现问题。我做错了什么？任何帮助都非常感谢。
May 13 18:53:28 user-VirtualBox NetworkManager[688]: <info> Starting VPN service 'pptp'...
May 13 18:53:28 user-VirtualBox NetworkManager[688]: <info> VPN service 'pptp' started (org.freedesktop.NetworkManager.pptp), PID 1530
May 13 18:53:28 user-VirtualBox NetworkManager[688]: <info> VPN service 'pptp' appeared; activating connections
May 13 18:53:28 user-VirtualBox NetworkManager[688]: <info> VPN plugin state changed: init (1)
May 13 18:53:28 user-VirtualBox NetworkManager[688]: <info> VPN plugin state changed: starting (3)
May 13 18:53:28 user-VirtualBox NetworkManager[688]: <info> VPN connection 'VPN connection 1' (Connect) reply received.
May 13 18:53:28 user-VirtualBox pppd[1532]: Plugin /usr/lib/pppd/2.4.5/nm-pptp-pppd-plugin.so loaded.
May 13 18:53:28 user-VirtualBox pppd[1532]: pppd 2.4.5 started by root, uid 0
May 13 18:53:28 user-VirtualBox pppd[1532]: Using interface ppp0
May 13 18:53:28 user-VirtualBox pppd[1532]: Connect: ppp0 <--> /dev/pts/3
May 13 18:53:28 user-VirtualBox pptp[1537]: nm-pptp-service-1530 log[main:pptp.c:314]: The synchronous pptp option is NOT activated
May 13 18:53:28 user-VirtualBox NetworkManager[688]: SCPlugin-Ifupdown: devices added (path: /sys/devices/virtual/net/ppp0, iface: ppp0)
May 13 18:53:28 user-VirtualBox NetworkManager[688]: SCPlugin-Ifupdown: device added (path: /sys/devices/virtual/net/ppp0, iface: ppp0): no ifupdown configuration found.
May 13 18:53:28 user-VirtualBox pptp[1543]: nm-pptp-service-1530 log[ctrlp_rep:pptp_ctrl.c:251]: Sent control packet type is 1 'Start-Control-Connection-Request'
May 13 18:53:28 user-VirtualBox pptp[1543]: nm-pptp-service-1530 log[ctrlp_disp:pptp_ctrl.c:739]: Received Start Control Connection Reply
May 13 18:53:28 user-VirtualBox pptp[1543]: nm-pptp-service-1530 log[ctrlp_disp:pptp_ctrl.c:773]: Client connection established.
May 13 18:53:29 user-VirtualBox pptp[1543]: nm-pptp-service-1530 log[ctrlp_rep:pptp_ctrl.c:251]: Sent control packet type is 7 'Outgoing-Call-Request'
May 13 18:53:29 user-VirtualBox pptp[1543]: nm-pptp-service-1530 log[ctrlp_disp:pptp_ctrl.c:858]: Received Outgoing Call Reply.
May 13 18:53:29 user-VirtualBox pptp[1543]: nm-pptp-service-1530 log[ctrlp_disp:pptp_ctrl.c:897]: Outgoing call established (call ID 0, peer's call ID 35968).
May 13 18:53:59 user-VirtualBox pppd[1532]: LCP: timeout sending Config-Requests
May 13 18:53:59 user-VirtualBox pppd[1532]: Connection terminated.
May 13 18:53:59 user-VirtualBox NetworkManager[688]: <warn> VPN plugin failed: 1
May 13 18:53:59 user-VirtualBox NetworkManager[688]: SCPlugin-Ifupdown: devices removed (path: /sys/devices/virtual/net/ppp0, iface: ppp0)
May 13 18:53:59 user-VirtualBox pppd[1532]: Modem hangup
May 13 18:53:59 user-VirtualBox pptp[1537]: nm-pptp-service-1530 warn[decaps_hdlc:pptp_gre.c:204]: short read (-1): Input/output error
May 13 18:53:59 user-VirtualBox pptp[1537]: nm-pptp-service-1530 warn[decaps_hdlc:pptp_gre.c:216]: pppd may have shutdown, see pppd log
May 13 18:53:59 user-VirtualBox NetworkManager[688]: <warn> VPN plugin failed: 1
May 13 18:53:59 user-VirtualBox pppd[1532]: Exit.
May 13 18:53:59 user-VirtualBox NetworkManager[688]: <warn> VPN plugin failed: 1
May 13 18:53:59 user-VirtualBox NetworkManager[688]: <info> VPN plugin state changed: stopped (6)
May 13 18:53:59 user-VirtualBox NetworkManager[688]: <info> VPN plugin state change reason: 0
May 13 18:53:59 user-VirtualBox pptp[1543]: nm-pptp-service-1530 log[callmgr_main:pptp_callmgr.c:234]: Closing connection (unhandled)
May 13 18:53:59 user-VirtualBox pptp[1543]: nm-pptp-service-1530 log[ctrlp_rep:pptp_ctrl.c:251]: Sent control packet type is 12 'Call-Clear-Request'
May 13 18:53:59 user-VirtualBox pptp[1543]: nm-pptp-service-1530 log[call_callback:pptp_callmgr.c:79]: Closing connection (call state)
May 13 18:53:59 user-VirtualBox NetworkManager[688]: <warn> error disconnecting VPN: Could not process the request because no VPN connection was active.
May 13 18:53:59 user-VirtualBox NetworkManager[688]: <info> Policy set 'Wired connection 1' (eth0) as default for IPv4 routing and DNS.
May 13 18:54:04 user-VirtualBox NetworkManager[688]: <info> VPN service 'pptp' disappeared