Topology: 

host 1 ------------------- vpn-gw 1 <----------------------------> vpn-gw 2 ------------------ host 2
                                               |                                       |
                                |                                     |
<--192.168.50.0/24-->    10.15.109.36                  10.15.109.5      <--192.168.1.0/24-->

Setup:

  1. Install the openswan package. Now we got 2 main files: /etc/ipsec.secrets and /etc/ipsec.conf
  2. Edit ipsec.secrets for vpn-gw 1: 
     10.15.109.36 10.15.109.5 : PSK "password
  3. Edit ipsec.secrets for vpn-gw 1: 
     10.15.109.5 10.15.109.36 : PSK "password"

  4. Edit ipsec.conf for vpn-gw 1 & vpn-gw 2 (same exact stuff): 

     conn vpn
 authby=secret
 left=10.15.109.36
 leftsubnet=192.168.50.0/24
 leftnexthop=10.15.109.5
 right=10.15.109.5
 rightsubnet=192.168.1.0/24
 rightnexthop=10.15.109.36
  5. Restart ipsec: 
     /etc/init.d/ipsec restart
  6. Now you can ping from host 1 to host 2!