IPSec LAN to LAN VPN

Environment: CentOS 5.5 to CentOS 5.5
Example:
Site A
   WAN: 100.100.100.100/24
   LAN: 192.168.0.1/24

Site B
   WAN: 200.200.200.200/24
   LAN: 192.168.1.1/24

Install Openswan (formerly FreeSWAN)
yum -y install openswan

Edit Site A /etc/ipsec.conf
conn tunnel
   left=100.100.100.100 # left for local
   leftsubnet=192.168.0.0/24
   right=200.200.200.200 # right for remote
   rightsubnet=192.168.1.0/24
   pfs=yes # pfs for Perfect Forward Secrecy
   type=tunnel
   authby=secret
   auto=start

Edit Site A /etc/ipsec.secrets # left right order is not important, PSK must be the same as Site B
100.100.100.100 200.200.200.200: PSK "yourpsk"

Edit Site B /etc/ipsec.conf
conn tunnel
   left=200.200.200.200
   leftsubnet=192.168.1.0/24
   right=100.100.100.100
   rightsubnet=192.168.0.0/24
   pfs=yes
   type=tunnel
   authby=secret
   auto=start

Edit Site A /etc/ipsec.secrets
100.100.100.100 200.200.200.200: PSK "yourpsk"

Site A and Site B
service ipsec start

Show status
# /etc/init.d/ipsec status
/usr/libexec/ipsec/addconn Non-fips mode set in /proc/sys/crypto/fips_enabled
IPsec running - pluto pid: 30061
pluto pid 30061
1 tunnels up
some eroutes exist

it work!

Check from Site A
ping -I 192.168.0.1 192.168.1.1
Check from Site B
ping -I 192.168.1.1 192.168.0.1

If you ping failed, check your iptables rules, no MASQUERADE between Site A and Site B