本文基于centos6.5安装openswan-2.6.32-37.el6.x86_64
首先安装预环境：
yum -y install gmp gmp-devel gawk flex bison
安装软件
yum install -y openswan lsof traceroute
for vpn in /proc/sys/net/ipv4/conf/*;do echo 0 > $vpn/accept_redirects;echo 0 > $vpn/send_redirects;done
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
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
virtual_private=
oe=off
# Enable this if you see "Failed to find any available worker"
# nhelpers=0
#You may put your configuration (.conf) file in the "/etc/ipsec.d/" and uncomment this.
include /etc/ipsec.d/*.conf
修改lan-to-lan的配置文件vi /etc/ipsec.d/lan1-to-lan2.conf
conn lan1-to-lan2 #唯一标识一个连接的名字
type=tunnel
authby=secret
left=%defaultroute
leftid=本端公网地址1
leftnexthop=%defaultroute
leftsubnet=192.168.90.0/24
right=对端公网地址
rightsubnet=10.248.200.0/21 #可以配置vhost:%no,%priv 匹配对端子网，但必须保证一边有明确的子网配置。
pfs=yes
auto=start
vi /etc/ipsec.d/idcsubnet1-to-awsvpc1.secrets #配置方式 源ID 目的ID：PSK “密码” 密码两端要配置相同
本端公网地址 对端公网地址: PSK "123456"
配置完成后，建立VPN的两端都需要重启ipsec服务加载配置文件，笔者就是在建VPN时，没有重启中心端的ipsec服务，导致VPN一直建立不起来，相关命令 #service ipsec start/stop/restart
设置开机启动 ，命令#chkconfig ipsec on
命令#ipsec verify,没有fail项即可
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
