Subnet to subnet VPN with PSK
Building a tunnel between two endpoints for multiple subnets with PSK authentication is pretty simialar to a host to host VPN with PSK tunnel. We used the also= keyword to avoid adding the same information into each connection.
# /etc/ipsec.conf config setup protostack=netkey conn mysubnet also=mytunnel leftsubnet=188.8.131.52/24 rightsubnet=192.0.2.0/24 auto=start conn mysubnet6 also=mytunnel connaddrfamily=ipv6 leftsubnet=2001:db8:0:1::/64 rightsubnet=2001:db8:0:2::/64 auto=start conn mytunnel left=184.108.40.206 right=220.127.116.11 authby=secret
To test the tunnel on "west":
# ping -n -c 4 -I 18.104.22.168 192.0.2.254 PING 192.0.2.254 (192.0.2.254) from 22.214.171.124 : 56(84) bytes of data. 64 bytes from 192.0.2.254: icmp_seq=1 ttl=64 time=0.XXX ms 64 bytes from 192.0.2.254: icmp_seq=2 ttl=64 time=0.XXX ms 64 bytes from 192.0.2.254: icmp_seq=3 ttl=64 time=0.XXX ms 64 bytes from 192.0.2.254: icmp_seq=4 ttl=64 time=0.XXX ms --- 192.0.2.254 ping statistics --- 4 packets transmitted, 4 received, 0% packet loss, time XXXX rtt min/avg/max/mdev = 0.XXX/0.XXX/0.XXX/0.XXX ms
The reason why you need to specify the source address for the ping command is that Linux will always pick the "nearest IP" automatically. Since the nearest IP would be 126.96.36.199, and that IP is not part of the 192.0.2.0/24 subnet, the ping would go out unencrypted. If you want all communication between the gateways themselves to be encrypted, and it is okay that they will talk to each other on their internal IP addresses, you can use the leftsourceip= and rightsourceip= options:
conn mysubnet also=mytunnel leftsubnet=188.8.131.52/24 leftsourceip=184.108.40.206 rightsubnet=192.0.2.0/24 rightsourceip-192.0.2.254 auto=start
libreswan will than add a route to the system for the remote subnet using the "src <ipaddress>" parameter to accomplish this.
Alternatively, you could add IPsec tunnels for the host-host connection, but you would also need to add tunnels for the host-subnet and subnet-host connections. This is a little cumbersome, so usually people just use the sourceip= options.