安装软件：
yum install epel-release
yum install openswan xl2tpd ppp lsof

1./etc/ipsec.conf

config setup
protostack=netkey
dumpdir=/var/run/pluto/
nat_traversal=yes
virtual_private=%v4:10.0.0.0/8,%v4:192.168.0.0/16,%v4:172.16.0.0/12,%v4:25.0.0.0/8,%v4:100.64.0.0/10,%v6:fd00::/8,%v6:fe80::/10
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=xxx.xxx.xxx.xxx
leftprotoport=17/1701
right=%any
rightprotoport=17/%any

2./etc/ipsec.d/my.secrets  #密钥

xxx.xxx.xxx.xxx %any: PSK "fucktest"

3./etc/sysctl.conf  #转发

net.ipv4.ip_forward = 1
net.ipv4.conf.default.accept_redirects = 0
net.ipv4.conf.default.send_redirects = 0
net.ipv4.conf.eth0.rp_filter = 0
net.ipv4.conf.default.rp_filter = 0

sysctl -p

4.打开ipsec服务，并检查状态。正常状态如下

[root@server ~]# ipsec verify
Verifying installed system and configuration files

Version check and ipsec on-path [OK]
Libreswan 3.8 (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 NAT and MASQUERADEing [TEST INCOMPLETE]
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]

5./etc/xl2tpd/xl2tpd.conf  #xl2tpd配置

[global]
ipsec saref = yes
listen-addr = xxx.xxx.xxx.xxx ;这里是你的主机外网ip地址,;号是注释,和一般的配置文件不同
; Use refinfo of 22 if using an SAref kernel patch based on openswan 2.6.35 or
; when using any of the SAref kernel patches for kernels up to 2.6.35.
; saref refinfo = 30
;
force userspace = yes
;
; debug tunnel = yes
[lns default]
ip range = 10.0.10.2-10.0.10.100 ;这里是VPN client的内网ip地址范围
local ip = 10.0.10.1 ;这里是VPN server的内网地址
refuse chap = yes
refuse pap = yes
require authentication = yes
name = LinuxVPNserver
ppp debug = yes
pppoptfile = /etc/ppp/options.xl2tpd
length bit = yes

6./etc/ppp/options.xl2tpd

name l2tpd
require-mschap-v2
ms-dns 8.8.8.8
ms-dns 8.8.4.4
ipcp-accept-local
ipcp-accept-remote
#ms-dns 8.8.8.8
noccp
auth
crtscts
idle 1800
mtu 1410
mru 1410
nodefaultroute
debug
lock
proxyarp
connect-delay 5000

7./etc/ppp/chap-secrets   #设置拨号用户名密码

# Secrets for authentication using CHAP
# client server secret IP addresses
username * userpass *

service xl2tpd start

——————————————————————————————————————–

常见问题

1.拨入后无法上网。

增加iptables nat规则。如下

[root@server ~]# iptables -t nat -A POSTROUTING -o eth1 -j MASQUERADE
[root@server ~]# iptables -A FORWARD -s 10.254.251.0/24 -p tcp -m tcp --tcp-flags FIN,SYN,RST,ACK SYN -j TCPMSS --set-mss 1356

2.ipsec验证时出现以下错误（内核转发原因）

Please disable /proc/sys/net/ipv4/conf/*/send_redirects
or NETKEY will cause the sending of bogus ICMP redirects! [FAILED]

You need to disable send and accept:

# Disable send redirects
echo 0 > /proc/sys/net/ipv4/conf/all/send_redirects
echo 0 > /proc/sys/net/ipv4/conf/default/send_redirects
echo 0 > /proc/sys/net/ipv4/conf/eth0/send_redirects
echo 0 > /proc/sys/net/ipv4/conf/lo/send_redirects

# Disable accept redirects
echo 0 > /proc/sys/net/ipv4/conf/all/accept_redirects
echo 0 > /proc/sys/net/ipv4/conf/default/accept_redirects
echo 0 > /proc/sys/net/ipv4/conf/eth0/accept_redirects
echo 0 > /proc/sys/net/ipv4/conf/lo/accept_redirects