我已使用本指南成功为 Ubuntu 16.04 安装了 network-manager-l2tp：https://askubuntu.com/a/797764/578622
我可以输入所有需要的信息，然后启用连接，但过了一会儿它只是显示“连接失败”。
我查看了一下/var/log/syslog，发现以下内容：
Aug 6 16:43:21 pi-laptop NetworkManager[908]: initiating Main Mode IKE_SA nm-ipsec-l2tp-10981[1] to 193.170.109.20
Aug 6 16:43:21 pi-laptop NetworkManager[908]: generating ID_PROT request 0 [ SA V V V V ]
Aug 6 16:43:21 pi-laptop NetworkManager[908]: sending packet: from 192.168.0.11[500] to 193.170.109.20[500] (280 bytes)
Aug 6 16:43:21 pi-laptop NetworkManager[908]: received packet: from 193.170.109.20[500] to 192.168.0.11[500] (124 bytes)
Aug 6 16:43:21 pi-laptop NetworkManager[908]: parsed ID_PROT response 0 [ SA V V ]
Aug 6 16:43:21 pi-laptop NetworkManager[908]: received NAT-T (RFC 3947) vendor ID
Aug 6 16:43:21 pi-laptop NetworkManager[908]: received FRAGMENTATION vendor ID
Aug 6 16:43:21 pi-laptop NetworkManager[908]: generating ID_PROT request 0 [ KE No NAT-D NAT-D ]
Aug 6 16:43:21 pi-laptop NetworkManager[908]: sending packet: from 192.168.0.11[500] to 193.170.109.20[500] (244 bytes)
Aug 6 16:43:21 pi-laptop NetworkManager[908]: received packet: from 193.170.109.20[500] to 192.168.0.11[500] (304 bytes)
Aug 6 16:43:21 pi-laptop NetworkManager[908]: parsed ID_PROT response 0 [ KE No V V V V NAT-D NAT-D ]
Aug 6 16:43:21 pi-laptop NetworkManager[908]: received Cisco Unity vendor ID
Aug 6 16:43:21 pi-laptop NetworkManager[908]: received XAuth vendor ID
Aug 6 16:43:21 pi-laptop NetworkManager[908]: received unknown vendor ID: 00:99:c1:e5:06:c4:f7:b0:f0:62:e0:82:4f:8e:1a:aa
Aug 6 16:43:21 pi-laptop NetworkManager[908]: received unknown vendor ID: 1f:07:f7:0e:aa:65:14:d3:b0:fa:96:54:2a:50:01:00
Aug 6 16:43:21 pi-laptop NetworkManager[908]: local host is behind NAT, sending keep alives
Aug 6 16:43:21 pi-laptop NetworkManager[908]: generating ID_PROT request 0 [ ID HASH N(INITIAL_CONTACT) ]
Aug 6 16:43:21 pi-laptop NetworkManager[908]: sending packet: from 192.168.0.11[4500] to 193.170.109.20[4500] (100 bytes)
Aug 6 16:43:21 pi-laptop NetworkManager[908]: received packet: from 193.170.109.20[4500] to 192.168.0.11[4500] (84 bytes)
Aug 6 16:43:21 pi-laptop NetworkManager[908]: parsed ID_PROT response 0 [ ID HASH V ]
Aug 6 16:43:21 pi-laptop NetworkManager[908]: received DPD vendor ID
Aug 6 16:43:21 pi-laptop NetworkManager[908]: IKE_SA nm-ipsec-l2tp-10981[1] established between 192.168.0.11[192.168.0.11]...193.170.109.20[193.170.109.20]
Aug 6 16:43:21 pi-laptop NetworkManager[908]: scheduling reauthentication in 10080s
Aug 6 16:43:21 pi-laptop NetworkManager[908]: maximum IKE_SA lifetime 10620s
Aug 6 16:43:21 pi-laptop NetworkManager[908]: generating QUICK_MODE request 2613055945 [ HASH SA No ID ID NAT-OA NAT-OA ]
Aug 6 16:43:21 pi-laptop NetworkManager[908]: sending packet: from 192.168.0.11[4500] to 193.170.109.20[4500] (244 bytes)
Aug 6 16:43:21 pi-laptop NetworkManager[908]: received packet: from 193.170.109.20[4500] to 192.168.0.11[4500] (204 bytes)
Aug 6 16:43:21 pi-laptop NetworkManager[908]: parsed QUICK_MODE response 2613055945 [ HASH SA No ID ID N((24576)) NAT-OA NAT-OA ]
Aug 6 16:43:21 pi-laptop NetworkManager[908]: no acceptable traffic selectors found
Aug 6 16:43:21 pi-laptop NetworkManager[908]: establishing connection 'nm-ipsec-l2tp-10981' failed
Aug 6 16:43:21 pi-laptop NetworkManager[908]: ** Message: strongSwan ready for action.
此后日志继续并显示有关 xl2tp 尝试连接的一些信息，这很有趣，因为连接已失败no acceptable traffic selectors found？
谷歌刚刚提到一些人说这是配置文件语法中的错误，但我不认为这是这里的问题，因为所有配置都是由前面提到的网络管理器插件创建的，并且日志不会抱怨配置错误。
在我的 Windows 系统上，连接工作正常，尽管我必须在 EAP 设置中输入一个 radius 地址（按照服务器管理员给出的说明）。这会是个问题吗？例如，在 Android 上，它开箱即用……
有人能向我解释一下这里发生了什么以及如何解决它吗？我真的需要这个 VPN 连接才能在 Ubuntu 下工作。