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I've been meaning to look into this with wireguard, but I'm having trouble searching for/finding how to do this. Is "bastion host" what I'd want? Also is there a way to ensure the VPS cannot access the network as well, and just tunnels it essentially?First, yes a search phrase like that should get you the right terms, though there isn't anything inherently special about it. If multiple systems are connected to one system with wireguard giving them all access to a given subnet is straight forward. As far as the VPS, it can indeed access that subnet too, since it's acting as part of the subnet, but you can use normal firewall rules on the far side internally to control what can talk to what and how. And in this kind of specific instance the WG is more about controller public facing surface area, the Bitwarden/Vaultwarden traffic in flight is itself encrypted.
Second though, having said all that I think if you worried about the VPS bit (or even if not) you should take a look at the Nebula SDN [0, 1] instead. It's built on the Noise encryption framework as well. There, the fixed IP node (the "Lighthouse") primarily acts to let other nodes know their mutual addresses, and they then attempt to form a direct link with no bouncing through a bastion, it's a real mesh. This generally works even if both are NAT'd, and if not it's transparent fallback and still encrypted between them. Depending on distance between nodes this can be a lot lower latency as well. With Nebula you establish an internal CA (super easy built-in tool for it) and that doesn't (and absolutely shouldn't) live on the lighthouse.
I'm fortunate enough to have fixed IPs available to me at home and office and have tended to use WG a lot just because it's had more advanced support and performance in constrained environments for me (kernel support in Linux and now BSDs). Nebula has been super slick though and I've been using it more and more. It makes all this really easy.
Anyway, hope this helps a bit. It's really exciting to me how much open source networking power is now available to everyone. It's a bit of a counter decentralization force IMO to the last few decades push towards central service providers.
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0: https://github.com/slackhq/nebula
1: https://arstechnica.com/gadgets/2019/12/how-to-set-up-your-o... (note 3 years old, there are now Android/iOS clients as well and things are further refined)