vpn客户端连接不上

配置好vpn客户端后，使用pppd call connectvpn后，使用ps -ef|grep pppd，能看到相应的进程，使用ifconfig 看不到网络连接。过一会进程退出。
查看 /var/log/messages 信息为

Using interface ppp0
Sep  7 10:22:09 iZ23so71psiZ pppd[30764]: Connect: ppp0 <--> /dev/pts/1
Sep  7 10:22:09 iZ23so71psiZ pptp[30765]: anon log[main:pptp.c:314]: The synchronous pptp option is NOT activated
Sep  7 10:22:09 iZ23so71psiZ pptp[30773]: anon log[ctrlp_rep:pptp_ctrl.c:251]: Sent control packet type is 1 'Start-Control-Connection-Request'
Sep  7 10:22:09 iZ23so71psiZ pptp[30773]: anon log[ctrlp_disp:pptp_ctrl.c:739]: Received Start Control Connection Reply
Sep  7 10:22:09 iZ23so71psiZ pptp[30773]: anon log[ctrlp_disp:pptp_ctrl.c:773]: Client connection established.
Sep  7 10:22:10 iZ23so71psiZ pptp[30773]: anon log[ctrlp_rep:pptp_ctrl.c:251]: Sent control packet type is 7 'Outgoing-Call-Request'
Sep  7 10:22:10 iZ23so71psiZ pptp[30773]: anon log[ctrlp_disp:pptp_ctrl.c:858]: Received Outgoing Call Reply.
Sep  7 10:22:10 iZ23so71psiZ pptp[30773]: anon log[ctrlp_disp:pptp_ctrl.c:897]: Outgoing call established (call ID 0, peer's call ID 11398).
Sep  7 10:22:40 iZ23so71psiZ pppd[30764]: LCP: timeout sending Config-Requests
Sep  7 10:22:40 iZ23so71psiZ pppd[30764]: Connection terminated.
Sep  7 10:22:40 iZ23so71psiZ pppd[30764]: Modem hangup
Sep  7 10:22:40 iZ23so71psiZ pptp[30765]: anon warn[decaps_hdlc:pptp_gre.c:204]: short read (-1): Input/output error
Sep  7 10:22:40 iZ23so71psiZ pptp[30765]: anon warn[decaps_hdlc:pptp_gre.c:216]: pppd may have shutdown, see pppd log
Sep  7 10:22:40 iZ23so71psiZ pptp[30773]: anon log[callmgr_main:pptp_callmgr.c:234]: Closing connection (unhandled)
Sep  7 10:22:40 iZ23so71psiZ pppd[30764]: Exit.
Sep  7 10:22:40 iZ23so71psiZ pptp[30773]: anon log[ctrlp_rep:pptp_ctrl.c:251]: Sent control packet type is 12 'Call-Clear-Request'
Sep  7 10:22:40 iZ23so71psiZ pptp[30773]: anon log[call_callback:pptp_callmgr.c:79]: Closing connection (call state)