Mesh VPN

Gluon integrates several layer 2 tunneling protocols to allow connections between local meshes through the internet.

Protocol handlers

There are currently three protocol handlers which can be selected as a feature:

mesh-vpn-fastd

fastd is a lightweight userspace tunneling daemon that implements cipher suites that are specifically designed to work well on embedded devices. It offers encryption and authentication. The primary drawback of fastd’s encrypted connection modes is the necessary context switches when forwarding packets. A kernel-supported L2TPv3 offloading option is available to work around the context-switching bottleneck, but it comes at the cost of losing the ability to protect tunnel connections against eavesdropping or manipulation.

mesh-vpn-tunneldigger

Tunneldigger always uses L2TPv3, generally achieving the same performance as fastd with the null@l2tp method, but offering no security. Tunneldigger’s primary drawback is the lack of IPv6 support. It also provides less configurability than fastd.

mesh-vpn-wireguard

WireGuard is an encrypted in-kernel tunneling protocol that provides encrypted transmission and at the same time offers high throughput.

fastd

Methods

fastd offers various different connection “methods” with different security properties that can be configured in the site configuration.

The following methods are currently recommended:

salsa2012+umac: Encrypted + authenticated
null+salsa2012+umac: Unencrypted, authenticated
null@l2tp: Unencrypted, unauthenticated

Multiple methods can be listed in site.conf. The first listed method supported by both the node and its peer will be used.

The use of the null@l2tp method with offloading enabled can provide a considerable performance gain, especially on weaker embedded hardware. For L2TP offloading, the mesh-vpn-fastd-l2tp feature needs to be enabled in site.mk.

Gateway / Supernode Configuration

When only using the null or null@l2tp methods without offloading, simply add these methods to the front of the method list. null@l2tp should always appear before null in the configuration when both are enabled. fastd v22 or newer is needed for the null@l2tp method.

It is often not necessary to enable L2TP offloading on supernodes for performance reasons. Nodes using offloading can communicate with supernodes that don’t use offloading as long as both use the null@l2tp method.

Offloading on Gateways / Supernodes

To enable L2TP offloading on the supernodes, it is recommended to study the fastd documentation section pertaining to the offload configuration option.

However, the important changes to the fastd config on your Supernode are:

Set mode multitap;

Every peer gets their own interface.

Replace interface "foo": with interface "peer-%k";

%k is substituted for a portion of the peers public key.

Set offload l2tp yes;

This tells fastd to use the l2tp kernel module.

Set persist interface no;

This tells fastd to only keep interfaces around while the connection is active.

Note that in multitap mode, which is required when using L2TP offloading, fastd will create one interface per peer on the supernode’s. This allows offloading the L2TP forwarding into the kernel space. But this also means added complexity with regards to handling those interfaces.

There are two main options on how you can handle this:

create on up and on down hooks

to handle interface setup and destruction

preferably using the async keyword, so hooks are not blocking fastd

use a daemon like systemd-networkd

Examples for both options can be found in the Wiki.

Configurable Method

From the site configuration, fastd can be allowed to offer toggleable encryption in the config mode with the intent to increase throughput.

There is also an older unprotected method null. Use of the newer null@l2tp method is generally recommended over null, as the performance gains provided by the latter (compared to the encrypted and authenticated methods) are very small.

Site configuration

Add the feature web-mesh-vpn-fastd in site.mk
Set mesh_vpn.fastd.configurable = true in site.conf
Optionally, add null@l2tp to the mesh_vpn.fastd.methods table if you want “Performance mode” as default (not recommended)

Config Mode

The resulting firmware will allow users to choose between secure (encrypted) and fast (unencrypted) transport.

To confirm whether the correct cipher is being used, the log output of fastd can be checked using logread.

WireGuard

In order to support WireGuard in Gluon, a few technologies are glued together.

VXLAN: As Gluon typically relies on batman-adv, the Mesh VPN has to provide OSI Layer 2 transport. But WireGuard is an OSI Layer 3 tunneling protocol, so additional technology is necessary here. For this, we use VXLAN. In short, VXLAN is a well-known technology to encapsulate ethernet packages into IP packages. You can think of it as kind of similar to VLAN, but on a different layer. Here, we use VXLAN to transport batman-adv traffic over WireGuard.

wgpeerselector: To connect all gluon nodes to each other, it is common to create a topology where each gluon node is connected to one of the available gateways via Mesh VPN respectively. To achieve this, the gluon node should be able to select a random gateway to connect to. But such “random selection of a peer” is not implemented in WireGuard by default. WireGuard only knows static peers. Therefore the wgpeerselector has been developed. It randomly selects a gateway, tries to establish a connection, and if it fails, tries to connect to the next gateway. This approach has several advantages, such as load balancing VPN connection attempts and avoiding problems with offline gateways. More information about the wgpeerselector and its algorithm can be found here.

On the gluon node both VXLAN and the wgpeerselector are well integrated and no explicit configuration of those tools is necessary, once the general WireGuard support has been configured.

Attention must by paid to time synchronization. As WireGuard performs checks on timestamps in order to avoid replay attacks, time must be synchronized before the Mesh VPN connection is established. This means that the NTP servers specified in your site.conf must be publicly available (and not only through the mesh). Be aware that if you fail this, you may not directly see negative effects. Only when a previously connected node reboots the effect comes into play, as the gateway still knows about the old timestamp of the gluon node.

gluon-mesh-vpn-key-translate

Many communities already possess a collection of active fastd-keys when they plan migrating their community to WireGuard. These public keys known on the server-side can be derived into their WireGuard equivalent using gluon-mesh-vpn-key-translate. The routers do the necessary reencoding of the private key seamlessly when updating firmware from fastd to the WireGuard variant.

Gateway / Supernode Configuration

On the gateway side, a software called wireguard-vxlan-glue is necessary. It is a small daemon that dynamically adds and removes forwarding rules for VXLAN interfaces, so traffic is sent correctly into the WireGuard interface. Thereby the forwarding rules are only installed if a client is connected, so unnecessary traffic in the kernel is avoided. The source can be found here.