Babeld is a loop-avoiding distance-vector routing protocol. It could auto update link cost according to network delay, which is different from other distance-vector routing protocols like RIP. This unique feature makes it useful for personal overlay network. Wireguard is a simple and fast layer 3 VPN software.

Wireguard doesn’t support IPv6 multicast, which is used by Babeld to do neighbor discovery. Wireguard forwards packets by matching allowed-ips option for its node. For a central node which other nodes connect to, you could decide which peer to forward a packet to by including the packet’s destination IPv6 address in the peer’s allowed-ips. These allowed-ips works like some kind of routing table. However, allowed-ips for different peers cannot overlap, which means you can not assign one address to multiple peers’ allowed-ips. As a result, you could only assign the multicast IPv6 address to one peer, and other peers would never receive Babeld Hello message from this central node. So you could only use Wireguard as a point-to-point layer 3 tunnel to run Babeld over it, or Babeld would fail to discover all links. As a result, for a n nodes full mesh network, each nodes need a Wireguard tunnel to all other n - 1 nodes.

By default, Wireguard would not assign IPv6 link local address to interface. In fact, Wireguard’s interface doesn’t have MAC address since it is an layer 3 VPN software. However, Babeld requires a IPv6 link local address to work. I use the following code to generate random IPv6 link local address for such interface to make Babeld happy:

import random

def random_mac():
    digits = [0x00, 0x16, 0x3e, random.randint(0x00, 0x7f), random.randint(0x00, 0xff), random.randint(0x00, 0xff)]
    return ":".join(map(lambda x: "%02x" % x, digits))

def mac_to_ipv6(mac):
    parts = mac.split(":")
    parts.insert(3, "ff")
    parts.insert(4, "fe")
    parts[0] = "%x" % (int(parts[0], 16) ^ 2)
    ipv6_parts = []
    for i in range(0, len(parts), 2):
        ipv6_parts.append("".join(parts[i:i + 2]))
    return "fe80::%s/64" % (":".join(ipv6_parts))

def random_ipv6():
    return mac_to_ipv6(random_mac())
if __name__ == "__main__":

Then just add these random generated IPv6 link local address to all interface(as well as normal IPv4 addresses) and set both ends of a Wireguard tunnel’s allowed-ips to,::/0, which simply allow everything to go through the tunnel. Finally, start Babeld on all the nodes and all the interface with your configuration, it shall work.