VPN（virtual private network，虚拟专用网）就是在两个网络实体之间建立的一种受保护的连接，这两个实体可以通过点到点的链路直接相连，但通常情况下他们会相隔较远的距离。vpn方式有三种：
Site-to-Site（站点到站点或者网关到网关）：如3个机构分布在互联网的3个不同的地方，各使用一个ipsec vpn网关相互建立VPN隧道，企业内网（若干PC）之间的数据通过这些网关建立的IPSec隧道实现安全互联。
End-to-End（端到端或者PC到PC）： 两个PC之间的通信由两个PC之间的IPSec会话保护，而不是网关。
End-to-Site（端到站点或者PC到网关）：两个PC之间的通信由网关和异地PC之间的IPSec进行保护。
本次讲述第一种方式Site-to-Site（站点到站点或者网关到网关）
1、需求分析
PC1部署Ipsec VPN，配置私网地址，在无线路由器做NAT；
华为USG做总部的VPN服务器；
因考虑PC1访问互联网需要通过无线路由器做nat，所ipsec vpn要考虑到nat穿越，后面openswan会有相关配置；
2、安装openswan
pc1部署centos7操作系统，在linux上部署openswan
yum install -y openswan lsof
我们改动内核参数，允许IP转发、永久性禁止重定向。
sed -i 's#net.ipv4.ip_forward = 0#net.ipv4.ip_forward = 1#g' /etc/sysctl.conf
sed -i 's#net.ipv4.conf.default.rp_filter = 1#net.ipv4.conf.default.rp_filter = 0#g' /etc/sysctl.conf
查看内核参数，确保禁止重定向sysctl -a | egrep "ipv4.*(accept|send)_redirects"
net.ipv4.conf.all.accept_redirects = 0
net.ipv4.conf.all.send_redirects = 0
net.ipv4.conf.default.accept_redirects = 0
net.ipv4.conf.default.send_redirects = 0
net.ipv4.conf.lo.accept_redirects = 0
net.ipv4.conf.lo.send_redirects = 0
net.ipv4.conf.eth0.accept_redirects = 0
net.ipv4.conf.eth0.send_redirects = 0
重新装入/etc/sysctl.conf
sysctl -p
3、修改ipsec配置文件 vi /etc/ipsec.conf
config setup
plutodebug=all
plutostderrlog=/var/log/ipsec.log
# Debug-logging controls: "none" for (almost) none, "all" for lots.
# klipsdebug=none
# plutodebug="control parsing"
# For Red Hat Enterprise Linux and Fedora, leave protostack=netkey
protostack=netkey
nat_traversal=yes
#实现nat穿越
virtual_private=
oe=off
# Enable this if you see "failed to find any available worker"
# nhelpers=0
4、修改lan-to-lan的配置文件 [root@localhost ~]# cat /etc/ipsec.d/site1-tosite2.conf
conn site1-tosite2
type=tunnel
#隧道模式
authby=secret
#共享秘钥方式
auto=start
ike=3des-sha1;modp1024
keyexchange=ike
phase2=esp
phase2alg=3des-sha1
pfs=no
left=%defaultroute
leftid=alice
leftnexthop=%defaultroute
leftsubnet=192.168.1.0/24
right=172.17.0.1
rightsubnet=192.168.2.0/24
5、配置ipsec vpn共享秘钥
vi /etc/ipsec.d/idcsubnet1-to-awsvpc1.secrets
本端公网地址 对端公网地址: PSK "123456"
6、检查命令 设置开机启动 ，命令#chkconfig ipsec on 命令 #ipsec verify ,没有fail项即可
[root@localhost ~]# ipsec verify
Verifying installed system and configuration files
Version check and ipsec on-path [OK]
Libreswan 3.25 (netkey) on 3.10.0-1062.9.1.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]
Two or more interfaces found, checking IP forwarding [OK]
Checking rp_filter [ENABLED]
/proc/sys/net/ipv4/conf/all/rp_filter [ENABLED]
/proc/sys/net/ipv4/conf/ens33/rp_filter [ENABLED]
rp_filter is not fully aware of IPsec and should be disabled
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 FIPS [OK]
Checking for obsolete ipsec.conf options [OBSOLETE KEYWORD]
Warning: ignored obsolete keyword 'nat_traversal'
ipsec verify: encountered 5 errors - see 'man ipsec_verify' for help
命令 #service ipsec status ，检查进程状态，tunnel up数量 命令 #ip xfrm policy ，检查tunnel的详细信息，源/目的subnet、下一跳等
