Route-based VPN using VTI

VPN tunnels are normally set up based on an IPsec policy. This is called a policy-based VPN. In libreswan, these policies are specified with leftsubnet= and rightsubnet= and optionally also with leftprotoport= and rightprotport=. Some VPN devices - notably from Cisco - allow you to setup a route-based VPN. This is basically a policy-based VPN with leftsubnet=0.0.0.0/0 and rightsubnet=0.0.0.0/0. Then a special Virtual Tunnel Interface ("VTI") device is created that is attached to the IPsec policy. Now, whenever a packet is routed into this VTI device, it will be encrypted. If the packet's route misses the interface, the packet leaves in the clear. While this type of setup is much less secure, it is easier to manage because you just need to update the routing table instead of adding or modifying IPsec policies. While libreswan supported this with KLIPS using the ipsec0 interface, when using XFRM/NETKEY this was not supported. As of libreswan-3.18, this is now supported using the Linux VTI interface and network MARKing.

An additional advantage of using libreswan with VTI is that you have a real interface (unlike the nflogXX interface) that supports firewalling with iptables, running tcpdump, etc. It even fixes tcpdump on the non-VTI device, so it really functions as the old KLIPS ipsec0 interface.

Creating a virtual ethernet connection

An example configuration is shown below. It is a regular VPN connection with the additional options to turn it into a route-based VPN.

conn routed-vpn
    left=192.1.2.23
    right=192.1.2.45
    authby=rsasigkey
    leftsubnet=0.0.0.0/0
    rightsubnet=0.0.0.0/0
    auto=start
    # route-based VPN requires marking and an interface
    mark=5/0xffffffff
    vti-interface=vti01
    # do not setup routing because we don't want to send 0.0.0.0/0 over the tunnel
    vti-routing=no

This will create the vti01 interface. If you want to encrypt all traffic to 10.0.0.0/8 using this VPN, simply run:

ip route add 10.0.0.0/8 dev vti01

You can also use more complicated routing using "ip rule", shorewall or anything else.

If you want to see the pre-encrypt and post-decrypt traffic, run:

tcpdump -i vti01 -n

And you can add firewall rules if you want:

# do not allow IRC traffic on port 6666
iptables -I INPUT -j DROP -p tcp --dport 6666 -i vti01

To see the post-encrypt and pre-decrypt (assuming your external interface is en0), run:

tcpdump -i en0 -n esp or udp port 4500

You can see information about vti tunnels using:

ip tunnel show
ip -s tunnel show
ifconfig vti01

Create a single VTI device for all VPN clients

If you run a VPN server, it is difficult to monitor all VPN connections using tcpdump because it mixes up encrypted and unencrypted traffic, and doesn't show all packets due to the way XFRM/NETKEY steals the packet for encryption. However, if you use a VTI device, all pre-encrypt and post-decrypt traffic appears on the VTI interface. All post-encrypt and pre-decrypt traffic appears on the regular physical interface.

conn roadwarriors
    # Regular certificate based VPN server
    left=1.2.3.4
    leftsubnet=0.0.0.0/0
    right=%any
    rightaddresspool=10.0.1.0/24
    authby=rsasig
    leftcert=mycert
    leftid=%fromcert
    auto=add
    rekey=no
    # Create route-based VPN using VTI
    mark=12/0xffffff
    vti-interface=vti02
    vti-routing=yes

Now you can monitor all connected VPN clients traffic by running:

tcpdump -i vti02 -n

One minor issue is that the vti02 device will not be created until the first client connects. And so you cannot preload it with firewall rules. You can manually create the VTI device to ensure it exists before you start your firewall and libreswan by running:

# the local IP and key mark must match the above configuration
ip tunnel add vti02 local 1.2.3.4 remote 0.0.0.0 key 12
ip link set ipsec0 up