搭建L2TP over IPSec VPN
1.服务器安装软件
yum install openswan xl2tpd ppp1
2.服务器配置文件/etc/ipsec.conf修改内容如下，VPN客户端内网地址网段自定义，公网地址为服务器公网地址
config setup
protostack=netkey
dumpdir=/var/run/pluto/
nat_traversal=yes
virtual_private=%v4:VPN客户端内网地址网段/24
conn L2TP-PSK-NAT
rightsubnet=vhost:%priv
also=L2TP-PSK-noNAT
conn L2TP-PSK-noNAT
authby=secret
pfs=no
auto=add
keyingtries=3
dpddelay=30
dpdtimeout=120
dpdaction=clear
rekey=no
ikelifetime=8h
keylife=1h
type=transport
left=公网地址
leftprotoport=17/1701
right=%any
rightprotoport=17/%any
3.设置VPN共享密钥，服务器配置文件/etc/ipsec.secrets修改内容如下，密钥自定义
%any: PSK "密钥"1
4.修改服务器内核参数，服务器配置文件/etc/sysctl.conf修改或者添加内容如下
net.ipv4.ip_forward = 1
net.ipv4.conf.default.rp_filter = 0
net.ipv4.conf.all.rp_filter = 0
net.ipv4.conf.all.send_redirects = 0
net.ipv4.conf.default.send_redirects = 0
net.ipv4.conf.all.accept_redirects = 0
net.ipv4.conf.default.accept_redirects = 0
net.core.xfrm_larval_drop = 1
5.服务器执行命令，修改内核参数立即生效
sysctl -p1
6.执行脚本
for each in /proc/sys/net/ipv4/conf/*
do
echo 0 > $each/accept_redirects
echo 0 > $each/send_redirects
done
7.防火墙添加规则
iptables -t nat -A POSTROUTING ! -s 127.0.0.1/8 -j MASQUERADE
iptables -I FORWARD -p tcp -m tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu
8.服务器启动ipsec服务
* RHEL/CentOS 6系列
service ipsec start1
- RHEL/CentOS 7系列
systemctl start ipsec1
9.服务器检查ipsec服务
ipsec verify1
运行结果显示类似如下，除两行不是[OK]表明正常：
Verifying installed system and configuration files
Version check and ipsec on-path [OK]
Libreswan 3.15 (netkey) on 3.10.0-123.9.3.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]
10.服务器配置文件/etc/xl2tpd/xl2tpd.conf修改内容如下
[global]
ipsec saref = yes
[lns default]
ip range = VPN客户端内网起始IP地址-VPN客户端内网结束IP地址
local ip = VPN服务器内网IP地址
require chap = yes
refuse pap = yes
require authentication = yes
name = VPN Server
ppp debug = yes
pppoptfile = /etc/ppp/options.xl2tpd
length bit = yes
11.服务器配置文件/etc/ppp/options.xl2tpd修改内容如下
ipcp-accept-local
ipcp-accept-remote
require-mschap-v2
ms-dns DNS服务器地址1
ms-dns DNS服务器地址2
noccp
noipv6
auth
crtscts
idle 1800
nodefaultroute
debug
lock
noproxyarp
connect-delay 5000
lcp-echo-interval 30
lcp-echo-failure 3
logfile /var/log/xl2tpd.log
12.服务器启动xl2tpd服务
* RHEL/CentOS 6系列
service xl2tpd start1
- RHEL/CentOS 7系列
systemctl start xl2tpd1
13.服务器配置文件/etc/ppp/chap-secrets添加账号，格式如下
账号 * 密码 *1
Windows 7及以上系统连接L2TP over IPSec VPN不稳定解决方法：
