我想建立与学校的VPN（PPTP）连接，但总是失败。
这是我从 tail -f /var/log/syslog 获得的信息
Sep 29 20:25:56 vaioz NetworkManager[1981]: <info> Starting VPN service 'pptp'...
Sep 29 20:25:56 vaioz NetworkManager[1981]: <info> VPN service 'pptp' started (org.freedesktop.NetworkManager.pptp), PID 3754
Sep 29 20:25:56 vaioz NetworkManager[1981]: <info> VPN service 'pptp' appeared; activating connections
Sep 29 20:26:00 vaioz NetworkManager[1981]: <info> VPN plugin state changed: starting (3)
Sep 29 20:26:00 vaioz NetworkManager[1981]: <info> VPN connection 'VPN-HOWEST' (Connect) reply received.
Sep 29 20:26:00 vaioz pppd[3761]: Plugin /usr/lib/pppd/2.4.5/nm-pptp-pppd-plugin.so loaded.
Sep 29 20:26:00 vaioz pppd[3761]: pppd 2.4.5 started by root, uid 0
Sep 29 20:26:00 vaioz pppd[3761]: Using interface ppp0
Sep 29 20:26:00 vaioz pppd[3761]: Connect: ppp0 <--> /dev/pts/1
Sep 29 20:26:00 vaioz NetworkManager[1981]: SCPlugin-Ifupdown: devices added (path: /sys/devices/virtual/net/ppp0, iface: ppp0)
Sep 29 20:26:00 vaioz NetworkManager[1981]: SCPlugin-Ifupdown: device added (path: /sys/devices/virtual/net/ppp0, iface: ppp0): no ifupdown configuration found.
Sep 29 20:26:00 vaioz pptp[3764]: nm-pptp-service-3754 log[main:pptp.c:314]: The synchronous pptp option is NOT activated
Sep 29 20:26:01 vaioz pptp[3772]: nm-pptp-service-3754 log[ctrlp_rep:pptp_ctrl.c:251]: Sent control packet type is 1 'Start-Control-Connection-Request'
Sep 29 20:26:01 vaioz pptp[3772]: nm-pptp-service-3754 log[ctrlp_disp:pptp_ctrl.c:739]: Received Start Control Connection Reply
Sep 29 20:26:01 vaioz pptp[3772]: nm-pptp-service-3754 log[ctrlp_disp:pptp_ctrl.c:773]: Client connection established.
Sep 29 20:26:02 vaioz pptp[3772]: nm-pptp-service-3754 log[ctrlp_rep:pptp_ctrl.c:251]: Sent control packet type is 7 'Outgoing-Call-Request'
Sep 29 20:26:02 vaioz pptp[3772]: nm-pptp-service-3754 log[ctrlp_disp:pptp_ctrl.c:858]: Received Outgoing Call Reply.
Sep 29 20:26:02 vaioz pptp[3772]: nm-pptp-service-3754 log[ctrlp_disp:pptp_ctrl.c:897]: Outgoing call established (call ID 0, peer's call ID 34132).
Sep 29 20:26:32 vaioz pppd[3761]: LCP: timeout sending Config-Requests
Sep 29 20:26:32 vaioz pppd[3761]: Connection terminated.
Sep 29 20:26:32 vaioz avahi-daemon[1952]: Withdrawing workstation service for ppp0.
Sep 29 20:26:32 vaioz NetworkManager[1981]: <warn> VPN plugin failed: 1
Sep 29 20:26:32 vaioz NetworkManager[1981]: SCPlugin-Ifupdown: devices removed (path: /sys/devices/virtual/net/ppp0, iface: ppp0)
Sep 29 20:26:32 vaioz pppd[3761]: Modem hangup
Sep 29 20:26:32 vaioz pptp[3764]: nm-pptp-service-3754 warn[decaps_hdlc:pptp_gre.c:204]: short read (-1): Input/output error
Sep 29 20:26:32 vaioz pptp[3764]: nm-pptp-service-3754 warn[decaps_hdlc:pptp_gre.c:216]: pppd may have shutdown, see pppd log
Sep 29 20:26:32 vaioz pptp[3772]: nm-pptp-service-3754 log[callmgr_main:pptp_callmgr.c:234]: Closing connection (unhandled)
Sep 29 20:26:32 vaioz pptp[3772]: nm-pptp-service-3754 log[ctrlp_rep:pptp_ctrl.c:251]: Sent control packet type is 12 'Call-Clear-Request'
Sep 29 20:26:32 vaioz pppd[3761]: Exit.
我使用网络管理器进行了配置，并按照本教程手动尝试：https://wiki.ubuntu.com/VPN但我总是遇到同样的“LCP：发送配置请求超时”问题。