服务器搭建L2
1.先看看你的主机是否支持pptp，返回结果为yes就表示通过
2.是否开启了TUN
有的虚拟机主机需要开启，返回结果为cat: /dev/net/tun: File descriptor in bad state。就表示通过。
3.更新一下再安装
4.安装EPEL源
5.安装xl2tpd和libreswan
6.编辑xl2tpd配置文件
修改内容如下：
[global]
[lns default]
ip range = 172.100.1.100-172.100.1.150 #分配给客户端的地址池
local ip = 172.100.1.1
require chap = yes
refuse pap = yes
require authentication = yes
name = LinuxVPNserver
ppp debug = yes
pppoptfile = /etc/ppp/options.xl2tpd
length bit = yes</pre>
7.编辑pppoptfile文件
vim /etc/ppp/options.xl2tpd</pre>
修改内容如下：
ipcp-accept-local
ipcp-accept-remote
ms-dns 8.8.8.8
ms-dns 209.244.0.3
ms-dns 208.67.222.222
name xl2tpd
noccp
auth
crtscts
idle 1800
mtu 1410 #第一次配置不建议设置mtu，mru，否则可能789错误
mru 1410
nodefaultroute
debug
lock
proxyarp
connect-delay 5000
refuse-pap
refuse-chap
refuse-mschap
require-mschap-v2
persist
logfile /var/log/xl2tpd.log</pre>
8.编辑ipsec配置文件
vim /etc/ipsec.conf</pre>
config setup
protostack=netkey
dumpdir=/var/run/pluto/
virtual_private=%v4:10.0.0.0/8,%v4:172.100.0.0/12,%v4:25.0.0.0/8,%v4:100.64.0.0/10,%v6:fd00::/8,%v6:fe80::/10
include /etc/ipsec.d/*.conf</pre>
9.编辑include的conn文件
vim /etc/ipsec.d/l2tp-ipsec.conf</pre>
修改内容如下：
conn L2TP-PSK-NAT
rightsubnet=0.0.0.0/0
dpddelay=10
dpdtimeout=20
dpdaction=clear
forceencaps=yes
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=192.168.0.17 #service/VPS的外网地址，某些vps只有eth0一块网卡的，
#就填内网地址，内核开启nat转发就可以了，
#CentOS7以下的用iptables定义转发规则
leftprotoport=17/1701
right=%any
rightprotoport=17/%any</pre>
10.设置用户名密码
vim /etc/ppp/chap-secrets</pre>
修改内容：
vpnuser * pass *
说明：用户名[空格]service[空格]密码[空格]指定IP</pre>
11.设置PSK
vim /etc/ipsec.d/default.secrets</pre>
PSK "testvpn"</pre>
12.CentOS7防火墙设置
firewall-cmd --permanent --add-service=ipsec
firewall-cmd --permanent --add-port=1701/udp
firewall-cmd --permanent --add-port=4500/udp
firewall-cmd --permanent --add-masquerade
firewall-cmd --reload</pre>
13.IP_FORWARD 设置
vim /etc/sysctl.d/60-sysctl_ipsec.conf</pre>
net.ipv4.ip_forward = 1
net.ipv4.conf.all.accept_redirects = 0
net.ipv4.conf.all.rp_filter = 0
net.ipv4.conf.all.send_redirects = 0
net.ipv4.conf.default.accept_redirects = 0
net.ipv4.conf.default.rp_filter = 0
net.ipv4.conf.default.send_redirects = 0
net.ipv4.conf.eth0.accept_redirects = 0
net.ipv4.conf.eth0.rp_filter = 0
net.ipv4.conf.eth0.send_redirects = 0
net.ipv4.conf.eth1.accept_redirects = 0
net.ipv4.conf.eth1.rp_filter = 0
net.ipv4.conf.eth1.send_redirects = 0
net.ipv4.conf.eth2.accept_redirects = 0
net.ipv4.conf.eth2.rp_filter = 0
net.ipv4.conf.eth2.send_redirects = 0
net.ipv4.conf.ip_vti0.accept_redirects = 0
net.ipv4.conf.ip_vti0.rp_filter = 0
net.ipv4.conf.ip_vti0.send_redirects = 0
net.ipv4.conf.lo.accept_redirects = 0
net.ipv4.conf.lo.rp_filter = 0
net.ipv4.conf.lo.send_redirects = 0
net.ipv4.conf.ppp0.accept_redirects = 0
net.ipv4.conf.ppp0.rp_filter = 0
net.ipv4.conf.ppp0.send_redirects = 0</pre>
重启生效
systemctl restart network</pre>
13.ipsec启动&检查
systemctl enable ipsec
systemctl restart ipsec</pre>
检查：ipsec verify
正常输出：
Verifying installed system and configuration files
Version check and ipsec on-path [OK]
Libreswan 3.15 (netkey) on 3.10.0-123.13.2.el7.x86_64
Checking for IPsec support in kernel [OK]
NETKEY: Testing XFRM related proc values
ICMP default/send_redirects [OK]
ICMP default/accept_redirects [OK]
XFRM larval drop [OK]
Pluto ipsec.conf syntax [OK]
Hardware random device [N/A]
Two or more interfaces found, checking IP forwarding [OK]
Checking rp_filter [OK]
Checking that pluto is running [OK]
Pluto listening for IKE on udp 500 [OK]
Pluto listening for IKE/NAT-T on udp 4500 [OK]
Pluto ipsec.secret syntax [OK]
Checking 'ip' command [OK]
Checking 'iptables' command [OK]
Checking 'prelink' command does not interfere with FIPSChecking for obsolete ipsec.conf options [OK]
Opportunistic Encryption [DISABLED]</pre>
14.xl2tpd启动
systemctl enable xl2tpd
systemctl restart xl2tpd</pre>
15.Windows连接
Windows连接，需要修改注册表键值（据说可以不用修改，但是我的不修改的话，一直789，log无显示）
