运行环境：ubuntu 14.04 64位
配置推荐网址:
https://linux.cn/article-3409-1.html
首先，官方教程是必须的：
https://help.ubuntu.com/community/L2TPServer
没有安照这个做过了就先不要到处百度“如何部署L2TP”之类的了。先参阅Official Ubuntu Documentation。
主要的问题出在：
sudo ipsec verify
这一步。因为官方要求是得到一个没有任何错误的反馈。
在第一次测试的时候我出现了2个[FAILED]，具体情况如下：
Checking your system to see if IPsec got installed and started correctly:
Version check and ipsec on-path [OK]
Linux Openswan U2.6.38/K3.13.0-52-generic (netkey)
Checking for IPsec support in kernel [OK]
SAref kernel support [N/A]
NETKEY: Testing XFRM related proc values [FAILED]
Please disable /proc/sys/net/ipv4/conf/*/send_redirects
or NETKEY will cause the sending of bogus ICMP redirects!
[FAILED]
Please disable /proc/sys/net/ipv4/conf/*/accept_redirects
or NETKEY will accept bogus ICMP redirects!
[OK]
Hardware RNG detected, testing if used properly [FAILED]
Hardware RNG is present but 'rngd' or 'clrngd' is not running.
No harware random used!
Checking that pluto is running [OK]
Pluto listening for IKE on udp 500 [OK]
Pluto listening for NAT-T on udp 4500 [OK]
Checking for 'ip' command [OK]
Checking /bin/sh is not /bin/dash [WARNING]
Checking for 'iptables' command [OK]
Opportunistic Encryption Support [DISABLED]
我们先解决：
NETKEY: Testing XFRM related proc values [FAILED]
解决方案：在命令行里黏贴以下命令
for each in /proc/sys/net/ipv4/conf/*
> do
> echo 0 > $each/accept_redirects
> echo 0 > $each/send_redirects
> done
然后解决：
Hardware RNG detected, testing if used properly [FAILED]
Hardware RNG is present but 'rngd' or 'clrngd' is not running.
No harware random used!
解决方案：
sudo apt-get install rng-tools