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Route-based VPNs

Generally IPsec processing is based on policies. After regular route lookups are done the OS kernel consults its SPD for a matching policy and if one is found that is associated with an IPsec SA the packet is processed (e.g. encrypted and sent as ESP packet). Refer to IPsecDocumentation for details.

Depending on the operating system it is also possible to configure route-based VPNs. Here IPsec processing does not (only) depend on negotiated policies but may e.g. be controlled by routing packets to a specific interface.

Most of these approaches also allow easy capture of plaintext traffic, which, depending on the operating system, might not be that straight-forward with policy-based VPNs (see CorrectTrafficDump). Another advantage this approach could have is that the MTU can be specified for the tunneling devices allowing to fragment packets before tunneling them in case PMTUD does not work properly.

VTI Devices on Linux

Disclaimer: VTI devices are supported since the Linux 3.6 kernel, but some important changes were added later (3.15+). The information below might not be accurate for older kernel versions.

VTI devices act like a wrapper around existing IPsec policies. This means you can't just route arbitrary packets to a VTI device to get them tunneled, the established IPsec policies have to match too. However, you can negotiate 0.0.0.0/0 traffic selectors on both ends to allow tunneling anything that's routed via VTI device.

To make this work, that is, to prevent packets not routed via VTI device from matching the policies (if 0.0.0.0/0 is used every packet would match) marks are used. Only packets that are marked accordingly will match the policies and get tunneled, for other packets the policies are ignored. Whenever a packet is routed to a VTI device it automatically gets the configured mark applied so it will match the policy and get tunneled.

It's important to note that VTI tunnel devices are a local feature, no additional encapsulation (like with GRE, see below) is added, so the other end does not have to be aware that VTI devices are used in addition to regular IPsec policies.

A VTI device may be created with the following command:

ip tunnel add <name> local <local IP> remote <remote IP> mode vti key <number equaling the mark>

<name> can be any valid device name (e.g. ipsec0, vti0 etc.). But note that the ip command treats names starting with vti special in some instances (e.g. when retrieving device statistics). The IPs are the endpoints of the IPsec tunnel. The number at the end has to match the mark configured for the connection. It is also possible to configure different marks for in- and outbound traffic using ikey/okey <mark>, but that is usually not required.

After creating the device it has to be enabled (ip link set <name> up) and then routes may be installed (routing protocols may also be used). To avoid duplicate policy lookups it is also recommended to set sysctl -w net.ipv4.conf.<name>.disable_policy=1. All of this also works for IPv6.

Statistics on VTI devices may be displayed with ip -s tunnel show [<name>]. Note that specifying a name will not show any statistics if the device name starts with vti.

A VTI device may be removed again with ip tunnel del <name>.

Configuration

To avoid that routes installed by the IKE daemon cause conflicts disable route installation with charon.install_routes=0 in strongswan.conf.

Then configure a regular site-to-site connection either with the traffic selectors set to 0.0.0.0/0 on both ends (local|remote_ts=0.0.0.0.0/0 in swanctl.conf or left|rightsubnet=0.0.0.0/0 in ipsec.conf) or set to specific subnets. As mentioned above, only traffic that matches these traffic selectors will then actually be forwarded, other packets will be rejected with an ICMP error message (destination unreachable/destination host unreachable).

The most important configuration option is the mark (mark_in|out in swanctl.conf, mark in ipsec.conf). After applying the optional mask (default is 0xffffffff) to the mark set on the VTI device, and applied by it to the routed packets, the value has to match the configured mark.
So referring to the example above, to match the mark on vti0 configure mark_in = mark_out = 42 and to match the mark on ipsec0 set the value to 0x01000201 (but something like 0x00000001/0x0000000f would also work).

Sharing VTI Devices

VTI devices may be shared by multiple IPsec SAs (e.g. in roadwarrior scenarios, to capture traffic or lower the MTU) by setting the remote endpoint of the VTI device to 0.0.0.0. For instance:

ip tunnel add ipsec0 local 192.168.0.1 remote 0.0.0.0 mode vti key 42

Then assuming virtual IPs for roadwarriors are assigned from the 10.0.1.0/24 subnet a matching route may be installed with ip route add 10.0.1.0/24 dev ipsec0.

Connection-specific VTI Devices

With a custom updown script it is also possible to setup connection-specific VTI devices.

For instance, to create a VTI device on a roadwarrrior client that receives a dynamic virtual IP (courtesy of Endre Szab├│):

This does not work when two roadwarriors are connected from the same IP. The kernel rejects the creation of a VTI where
the remote and local addresses of the VTI are already in use by another VTI.

If there is more than one subnet in the remote traffic selector this might cause conflicts as the updown script will be called for each combination of local and remote subnet.

Marks on Linux

One of the core features of VTI devices, dynamically specifying which traffic to tunnel, can actually be replicated directly with marks and firewall rules. By configuring connections with marks and then selectively marking packets directly with Netfilter rules via MARK target in the PREROUTING or FORWARD only specific traffic will get tunneled.

This may also be used to create multiple identical tunnels for which firewall rules dynamically decide which traffic is tunneled though which IPsec SA (e.g. for QoS/DiffServ).

GRE

An alternative to VTI devices is using GRE, which is a generic point-to-point tunneling protocol that adds an additional encapsulation layer (at least 4 bytes). But it provides a portable way of creating route-based VPNs (running a routing protocol on-top is also easy).

While VTI devices depend on site-to-site IPsec connections in tunnel mode, GRE uses a host-to-host connection that can also be run in transport mode (avoiding additional overhead). But while VTI devices may be used by only one of the hosts, GRE must be used by both of them.

Creating a GRE tunnel on Linux can be done as follows:

ip tunnel add <name> local <local IP> remote <remote IP> mode gre

<name> can be any valid interface name (e.g. ipsec0, gre0 etc.). But note that the ip command treats names starting with gre special in some instances (e.g. when retrieving device statistics). The IPs are the endpoints of the IPsec tunnel.

After creating the device it has to be enabled (ip link set <name> up) and then routes may be installed.

Statistics on GRE devices may be displayed with ip -s tunnel show [<name>]. Note that specifying a name will not show any statistics if the device name starts with gre.

A GRE device may be removed again with ip tunnel del <name>.

Configuration

As mentioned above, a host-to-host IPsec connection in transport mode can be used. The traffic selectors may even be limited to just the GRE protocol (local|remote_ts=dynamic[gre] in swanctl.conf or left|rightsubnet=%dynamic[gre] in ipsec.conf).

libipsec And TUN Devices

Based on our own userland IPsec implementation and the kernel-libipsec plugin it is possible to create route-based VPNs with TUN devices. Similar to VTI devices the negotiated IPsec policies have to match the traffic routed via TUN device.
In particular because packets have to be copied between kernel and userland it is not as efficient as the solutions above (also read the notes on kernel-libipsec).

Problems

Make sure to disable the connmark plugin when running the VTI. Otherwise, it will insert iptables rules into the *mangle table that prevent the VTI from working.