上面两篇文章介绍了PPTP和openvpn的搭建方式，接下来我们开始来搭建L2TP VPN的server的环境，本次搭建的Linux环境是centos7，其操作步骤如下：
1、一些预先准备的环境
yum install -y make gcc gmp-devel xmlto bison flex xmlto libpcap-devel lsof vim-enhanced man
yum install openswan ppp xl2tpd
2、配置
（1）vi /etc/ipsec.conf
替换为如下内容，把下面0.0.0.0换成VPN server的IP（注意一定要有字符缩进，距离不要改变）
config setup
nat_traversal=yes
virtual_private=%v4:10.0.0.0/8,%v4:192.168.0.0/16,%v4:172.16.0.0/12
oe=off
protostack=netkey
conn L2TP-PSK-NAT
rightsubnet=vhost:%priv
also=L2TP-PSK-noNAT
conn L2TP-PSK-noNAT
authby=secret
pfs=no
auto=add
keyingtries=3
rekey=no
ikelifetime=8h
keylife=1h
type=transport
left=0.0.0.0
leftprotoport=17/1701
right=%any
rightprotoport=17/%any
（2）编辑/etc/ipsec.secrets
替换内容如下：（0.0.0.0换成VPN server的IP）
include /etc/ipsec.d/*.secrets
0.0.0.0 %any: PSK "jelly"
（3）修改/添加 /etc/sysctl.conf并生效
在/etc/sysctl.conf的末尾加上如下内容
net.ipv4.ip_forward = 1
net.ipv4.conf.default.rp_filter = 0
net.ipv4.conf.all.send_redirects = 0
net.ipv4.conf.default.send_redirects = 0
net.ipv4.conf.all.log_martians = 0
net.ipv4.conf.default.log_martians = 0
net.ipv4.conf.default.accept_source_route = 0
net.ipv4.conf.all.accept_redirects = 0
net.ipv4.conf.default.accept_redirects = 0
net.ipv4.icmp_ignore_bogus_error_responses = 1
生效上面的修改使用如下命令
sysctl -p
（4）验证ipsec运行状态
ipsec restart
ipsec verify
如果出现如下内容，说明已经成功：
Checking your system to see if IPsec got installed and started correctly:
Version check and ipsec on-path [OK]
Linux Openswan U2.6.32/K2.6.32-431.20.3.0.1.el6.centos.plus.x86_64 (netkey)
Checking for IPsec support in kernel [OK]
SAref kernel support [N/A]
NETKEY: Testing for disabled ICMP send_redirects [OK]
NETKEY detected, testing for disabled ICMP accept_redirects [OK]
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 [OK]
Checking for 'iptables' command [OK]
Opportunistic Encryption Support [DISABLED]
（5） 编辑 /etc/xl2tpd/xl2tpd.conf 这一步可以跳过
ip range 写客户端的局域网IP段，local ip写客户端局域网IP
[lns default]
ip range = 10.10.0.2-10.10.0.100
local ip = 10.10.0.1
require chap = yes
refuse pap = yes
require authentication = yes
name = LinuxVPNserver
ppp debug = yes
pppoptfile = /etc/ppp/options.xl2tpd
length bit = yes
bps = 1000000
（6）配置用户名,密码:编辑 /etc/ppp/chap-secrets
# Secrets for authentication using CHAP
# client server secret IP addresses
username * "userpass" *
注：username换成你要登录的用户名，userpass换成密码
（7）重启xl2tp
service xl2tpd restart
（8）添加自启动
chkconfig xl2tpd on
chkconfig iptables on
chkconfig ipsec on
（9）转发规则（需要设置，否则无法上网）
yum install iptables-services -y
iptables -t nat -A POSTROUTING -s 10.10.0.0/24 -o eth0 -j MASQUERADE
service iptables save
注：10.10.0.0/24跟/etc/xl2tpd/xl2tpd.conf的设置相对应，eth0要改成你局域网的网络名字
（10）关闭防火墙
systemctl stop firewalld
接下来就可以用电脑连接：
首先，需要修改注册表regedit
[HKEY_LOCAL_MACHINE/SYSTEM/CurrentControlSet/Services/RasMan/Parameters]
ProhibitIPSec的值修改为1
如果没有ProhibitIPSec项，需要点击右键新建dword，重命名为ProhibitIPSec，值设置为1
其次，重启你的本地电脑（必须重启哦）
连接方式和pptp的vpn连接方式相同！！！