我正在使用 ubuntu 15.10，是 Linux/GNU 系统的初学者。
我对 VPN 的细节也几乎一无所知。所以关于 VPN 的术语可能是错误的。
我和我的同事使用带 VPN 的服务器。但是网络管理员将协议更改为 IPsec/L2TP。我们被告知了服务器的名称、DNS 服务器、预共享密钥、用户名和密码。
在阅读了该网站上的一些问题和答案后，我猜测自 14.10 以来，预共享密钥的 IPSec/L2TP 变得困难了。
然而，阅读这个问题和答案，我决定尝试网络管理器 L2TP 插件
但是...安装始终失败。我很困惑，不知道该怎么做。
我按照如下方式进行操作：
$ apt-get install strongswan network-manager-strongswan
$ LANG=C sudo add-apt-repository ppa:seriy-pr/network-manager-l2tp
WARNING:root:could not open file '/etc/apt/sources.list'
WARNING:root:could not open file '/etc/apt/sources.list.d/ubuntu-ja.list'
WARNING:root:could not open file '/etc/apt/sources.list'
WARNING:root:could not open file '/etc/apt/sources.list.d/ubuntu-ja.list'
Packages for https://github.com/seriyps/NetworkManager-l2tp
Screenshot: http://dl.seriyps.ru/img/network-manager-l2tp.png
Full description (in russian) http://seriyps.ru/blog/2012/01/31/plagin-l2tp-dlya-networkmanager/
Installation:
sudo apt-add-repository ppa:seriy-pr/network-manager-l2tp
sudo apt-get update
sudo apt-get install network-manager-l2tp-gnome
!!!!!!!!!!!!!!!!!!
sudo service xl2tpd stop
sudo update-rc.d xl2tpd disable
!!!!!!!!!!!!!!!!!!
詳しい情報: https://launchpad.net/~seriy-pr/+archive/ubuntu/network-manager-l2tp
[ENTER] を押すと続行します。ctrl-c で追加をキャンセルできます
WARNING:root:could not open file '/etc/apt/sources.list'
WARNING:root:could not open file '/etc/apt/sources.list.d/ubuntu-ja.list'
gpg: keyring `/tmp/tmpdb17vvm8/secring.gpg' created
gpg: keyring `/tmp/tmpdb17vvm8/pubring.gpg' created
gpg: requesting key B11E44D2 from hkp server keyserver.ubuntu.com
gpg: /tmp/tmpdb17vvm8/trustdb.gpg: trustdb created
gpg: key B11E44D2: public key "Launchpad PPA for Sergey Prochorov" imported
gpg: Total number processed: 1
gpg: imported: 1 (RSA: 1)
OK
$ LANG=C sudo apt-get update
...
...
Ign http://ppa.launchpad.net wily Release.gpg
Ign http://ppa.launchpad.net wily Release
...
...
Err http://ppa.launchpad.net wily/main amd64 Packages
404 Not Found
...
...
Err http://ppa.launchpad.net wily/main i386 Packages
404 Not Found
Ign http://ppa.launchpad.net wily/main Translation-en
Ign http://ppa.launchpad.net wily/main Translation-ja
Fetched 840 kB in 12s (65.3 kB/s)
W: Failed to fetch http://ppa.launchpad.net/seriy-pr/network-manager-l2tp/ubuntu/dists/wily/main/binary-amd64/Packages 404 Not Found
W: Failed to fetch http://ppa.launchpad.net/seriy-pr/network-manager-l2tp/ubuntu/dists/wily/main/binary-i386/Packages 404 Not Found
E: Some index files failed to download. They have been ignored, or old ones used instead.
$ LANG=C sudo apt-get install network-manager-l2tp
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Unable to locate package network-manager-l2tp
正如输出所示，我无法安装network-manager-l2tp。我无法准确理解这些输出。
我该如何解决这个问题？
这是我安装的网络管理器模块。
$ LANG=C apt list network-manager-*
Listing... Done
network-manager-dbg/wily-updates 1.0.4-0ubuntu5.2 amd64
network-manager-dev/wily-updates 1.0.4-0ubuntu5.2 amd64
network-manager-gnome/wily,now 0.9.10.1-0ubuntu7 amd64 [installed]
network-manager-iodine/wily 0.0.5-1ubuntu1 amd64
network-manager-iodine-gnome/wily 0.0.5-1ubuntu1 amd64
network-manager-openconnect/wily,now 1.0.2-1build1 amd64 [installed,automatic]
network-manager-openconnect-gnome/wily,now 1.0.2-1build1 amd64 [installed]
network-manager-openvpn/wily,now 0.9.10.0-1ubuntu2 amd64 [installed,automatic]
network-manager-openvpn-gnome/wily,now 0.9.10.0-1ubuntu2 amd64 [installed]
network-manager-pptp/wily,now 0.9.10.0-1ubuntu1 amd64 [installed]
network-manager-pptp-gnome/wily,now 0.9.10.0-1ubuntu1 amd64 [installed]
network-manager-strongswan/wily,now 1.3.1-1ubuntu1 amd64 [installed]
network-manager-vpnc/wily,now 0.9.10.0-1ubuntu1 amd64 [installed,automatic]
network-manager-vpnc-gnome/wily,now 0.9.10.0-1ubuntu1 amd64 [installed]
附言：我知道有些答案建议使用 openswan，但我尽量不想这么做。我担心安全问题。他们说 openswan 因安全风险而被移除。我使用 ubuntu 作为工作桌面，所以我不能故意忽略安全风险。
答案1
Sergey Prokhorov 尚未为 Wily 创建版本（尚未？）。您可以在您发布的链接中看到这一点。该插件适用于 Ubuntu 14.X（在 14.04 上测试）。顺便问一下，您是否需要 L2TP over IPSec？我认为 Strongswan 适用于前者，而 Prokhorovs 插件适用于后者。但我不是专家。
干杯，奥利弗