Subnet to subnet VPN

Building a tunnel between two endpoints for multiple subnets is pretty simialar to a host to host VPN tunnel. Except you will see we are adding leftsubnets/rightsubnets statements. We used the also= keyword to avoid adding the same information into each connection.

# /etc/ipsec.conf
# The version 2 is only required for compatibility with openswan
version 2

config setup
    protostack=netkey

conn mysubnet
     also=mytunnel
     leftsubnet=192.0.1.0/24
     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
    leftid=@west
    left=192.1.2.23
    leftrsasigkey=0sAQOrlo+hOafUZDlCQmXFrje/oZm [...] W2n417C/4urYHQkCvuIQ==
    rightid=@east
    right=192.1.2.45
    rightrsasigkey=0sAQO3fwC6nSSGgt64DWiYZzuHbc4 [...] D/v8t5YTQ==
    authby=rsasig
    # use auto=start when done testing the tunnel

To test the tunnel on "west":

# ping -n -c 4 -I 192.0.1.254 192.0.2.254
PING 192.0.2.254 (192.0.2.254) from 192.0.1.254 : 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 192.1.2.23, 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=192.0.1.0/24
     leftsourceip=192.0.1.254
     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.