Route-based VPN using VTI

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Route-based VPNs using libreswan

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=. Libreswan 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.

New ipsec.conf keywords for VTI support

The following new keywords have been added to support VTI:

option description
mark= The mark number to use for this connection's IPsec SA policy. It will be used for all instances as well.
markin= / markout= Same as the above but setting different marks for inbound and outbound.
vti-interface= The interface name of the VTI device to use, eg vti01
vti-routing= Whether routes should automatically be created into the VTI device (default yes)
vti-shared= Whether this vti device is shared with other connections (default no). if not shared, and not a template connection with %any, the VTI device will be deleted when tunnel goes down
leftvti=address/mask Configure the address/mask on the vti-interface when connection is established.

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
    # If you run a subnet with BGP (quagga) daemons over IPsec, you can configure the VTI interface
    leftvti=10.0.1.1/24

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


Setting up a route-based VPN with Amazon

[ to be filled in ]

VTI related issues

This functionality is all very new, so not all issues or features have been resolved yet. This is a list of known interesting things.

updown script

The standard updown script (_updown.netkey) tries to autodetect what scenario is being deployed and will reconfigure the VTI interface and network options accordingly. As this feature is still new, we expect it to not be perfect yet. If your scenario is not covered, please contact the developers and explain your use case.

interface options

Each interface has a standard set of options associated with it. These can be found in /proc/sys/net/ipv4/conf/vtiXXX/*. The default updown script sets some of these (rp_filter, forwarding, policy_disable) but specific use cases might require different settings. Setting disable_xfrm will cause the interface to completely fail, so do not do that.

MTU

We noticed different kernels create different MTU sizes for new VTI devices. Currently the MTU is not set by libreswan.

combing different IPsec gateways into one VTI interface [PARTIALLY SOLVED]

You cannot yet have a vti tunnel that uses local 0.0.0.0 and remote 0.0.0.0. Kernel patches for these are pending. This means that you cannot yet combine two different gateways on the same VTI device, that have a different local IP address. However, you can combine different connections that use the same local address but use a different remote address by using the vti-shared=yes option. Obviously, the marks and interface name must be the same for all shared connections.

conn west
     left=1.2.3.4
     right=6.7.8.9
     [...]
     mark=10/0xffffff
     vti-interface=vti01
     vti-routing=yes
     vti-shared=yes

conn east
     left=10.11.12.13
     right=9.8.7.6
     [...]
     mark=10/0xffffff
     vti-interface=vti01
     vti-routing=yes
     vti-shared=yes