Comment by antoniomika

We’re actually using Unix sockets as the underlying transport layer for this. We’re also not using sshd, we custom wrote our own daemon that’s entire job is tunneling. If you’re curious about this, you can find the project here: https://github.com/antoniomika/sish

sish was actually my first foray into SSH apps. It was a lot of fun to write and pretty much implements tunnels with a routing system on top. It manages connectivity, routing, and reverse proxying all within user space. No namespaces required!

tuns can actually even tunnel UDP traffic over SSH, also entirely in user space. Docs for that can be found here: https://pico.sh/tuns#udp-tunneling