Over on the Fediverse, I sort of had a question :

I wonder if there's any way in standard WireGuard to have a zero-trust network relay, so that two WG peers that are isolated from each other (eg both behind NAT) can talk directly. The standard pure-WG approach has a public WG endpoint that everyone talks to and which acts as a router for the internal WG IPs of everyone, but this involves decrypting and re-encrypting the WG traffic.

By 'talk directly' I mean that each of the peers has the WireGuard keys of the other and the traffic between the two of them stays encrypted with those keys all the way through its travels. The traditional approach to the problem of two NAT'd machines that want to talk to each other with WireGuard is to have a WireGuard router that both of them talk to over WireGuard, but this means that the router sees the unencrypted traffic between them. This is less than ideal if you don't want to trust your router machine, for example because you want to make it a low-trust virtual machine rented from some cloud provider.

Since we love indirection in computer science, you can in theory solve this with another layer of traffic encapsulation (with a lot of caveats). The idea is that all of the 'public' endpoint IPs of WireGuard peers are actually on a private network, and you route the private network through your public router. Getting the private network packets to and from the router requires another level of encapsulation and unless you get very clever, all your traffic will go through the router even if two WireGuard peers could talk directly. Since WireGuard automatically keeps track of the current public IPs of peers, it would be ideal to do this with WireGuard, but I'm not sure that WG-in-WG can have the routing maintained the way we want.

This untrusted relay situation is of course one of the things that 'automatic mesh network on top of WireGuard' systems give you, but it would be nice to be able to do this with native features (and perhaps without an explicit control plane server that machines talk to, although that seems unlikely). As far as I know such systems implement this with their own brand of encapsulation, which I believe requires running their WireGuard stack.

(On Linux you might be able to do something clever with redirecting outgoing WireGuard packets to a 'tun' device connected to a user level program, which then wrapped them up, sent them off, received packets back, and injected the received packets into the system.)