How To Setup IPSEC Site-To-Site VPN on Linux CentOS
Linux Gateway Server with 2 Lan Cards on Each office Location. Lets Call them A and B.
A is Head Office Linux Server and B is Branch Office Linux Server.
A has CentOS 6.4 , Extenal IP Address 220.127.116.11 , Local Lan Subnet 192.168.100.0/24
B has CentOS 5.9, External IP Address 18.104.22.168, Local Lan Subnet 10.1.1.0/24
To Accomplish the task we will use OpenSwan.