我正在尝试在 Ubuntu 14.04 上通过 IPSec 连接到 VPN。我正在通过小程序进行设置：
一切正常，我可以连接到 VPN。但是，30 秒后它总是断开连接。日志如下：
Sep 21 21:16:52.638 xl2tpd[3778]: Connecting to host some.domain.com, port 1701
Sep 21 21:16:52.653 xl2tpd[3778]: Connection established to xx.xxx.xxx.xxx, 1701. Local: 9798, Remote: 61236 (ref=0/0).
Sep 21 21:16:52.654 xl2tpd[3778]: Calling on tunnel 9798
Sep 21 21:16:52.673 xl2tpd[3778]: Call established with xx.xxx.xxx.xxx, Local: 30953, Remote: 59837, Serial: 1 (ref=0/0)
Sep 21 21:16:52.679 xl2tpd[3778]: start_pppd: I'm running:
Sep 21 21:16:52.680 xl2tpd[3778]: "/usr/sbin/pppd"
Sep 21 21:16:52.681 xl2tpd[3778]: "passive"
Sep 21 21:16:52.681 xl2tpd[3778]: "nodetach"
Sep 21 21:16:52.682 xl2tpd[3778]: ":"
Sep 21 21:16:52.682 xl2tpd[3778]: "file"
Sep 21 21:16:52.682 xl2tpd[3778]: "/etc/ppp/VPN.options.xl2tpd"
Sep 21 21:16:52.683 xl2tpd[3778]: "/dev/pts/3"
Sep 21 21:16:52.701 pppd[3837]: Plugin passprompt.so loaded.
Sep 21 21:16:52.701 pppd[3837]: pppd 2.4.5 started by root, uid 0
Sep 21 21:16:52.702 pppd[3837]: Using interface ppp0
Sep 21 21:16:52.702 pppd[3837]: Connect: ppp0 <--> /dev/pts/3
Sep 21 21:16:52.757 pppd[3837]: CHAP authentication succeeded
Sep 21 21:16:52.815 pppd[3837]: local IP address xxx.xxxx.xx.xxx
Sep 21 21:16:52.818 pppd[3837]: remote IP address xxx.xxx.xx.xxx
Sep 21 21:17:41.580 Stopping xl2tpd: xl2tpd.
Sep 21 21:17:41.580 xl2tpd[3778]: death_handler: Fatal signal 15 received
Sep 21 21:17:41.580 pppd[3837]: Modem hangup
Sep 21 21:17:41.584 pppd[3837]: Connect time 0.9 minutes.
Sep 21 21:17:41.584 pppd[3837]: Sent 400 bytes, received 164 bytes.
Sep 21 21:17:41.585 pppd[3837]: Connection terminated.
Sep 21 21:17:41.602 ipsec_setup: Stopping Openswan IPsec...
Sep 21 21:17:41.623 pppd[3837]: Exit.
我在 Linux 机器和 Ubuntu 机器上都试过了，每次都是这样。这可能是什么原因？