2.7.6. Subnet Extrusion Using Libreswan
IPsec is often deployed in a hub-and-spoke architecture. Each leaf node has an IP range that is part of a larger range. Leaves communicate with each other via the hub. This is called subnet extrusion. In the example below, we configure the head office with 10.0.0.0/8 and two branches that use a smaller /24 subnet.
At the head office:
conn branch1
left=1.2.3.4
leftid=@headoffice
leftsubnet=0.0.0.0/0
leftrsasigkey=0sA[...]
#
right=5.6.7.8
rightid=@branch1
rightsubnet=10.0.1.0/24
rightrsasigkey=0sAXXXX[...]
#
auto=start
authby=rsasig
conn branch2
left=1.2.3.4
leftid=@headoffice
leftsubnet=0.0.0.0/0
leftrsasigkey=0sA[...]
#
right=10.11.12.13
rightid=@branch2
rightsubnet=10.0.2.0/24
rightrsasigkey=0sAYYYY[...]
#
auto=start
authby=rsasig
At the “branch1” office, we use the same connection. Additionally, we use a pass-through connection to exclude our local LAN traffic from being sent through the tunnel:
conn branch1
left=1.2.3.4
leftid=@headoffice
leftsubnet=0.0.0.0/0
leftrsasigkey=0sA[...]
#
right=10.11.12.13
rightid=@branch2
rightsubnet=10.0.1.0/24
rightrsasigkey=0sAYYYY[...]
#
auto=start
authby=rsasig
conn passthrough
left=1.2.3.4
right=0.0.0.0
leftsubnet=10.0.1.0/24
rightsubnet=10.0.1.0/24
authby=never
type=passthrough
auto=route