Tunnel-only user for SSH on NixOS

I really enjoy reading Solène's website and I stumbled upon one of her posts again while reading more about SSH tunneling. In this post, she talks about setting up dedicated user for ssh tunneling only on OpenBSD¹. Of course, as a Nix user, I have to show you how easy it is to do the same on NixOS.

Generating ssh keys

Well, this process is the same for NixOS. You use ssh-keygen just as usual, but in this case I will generate a ed25519 key:

$ ssh-keygen -t ed25519

Adding a user

This is the fun part, NixOS provides a set of modules to configure users to configure users in a declarative manner. Let's leverage it to create a user that has no shell access nor a home:

users.groups.tunnel = { };
users.users.tunnel = {
  group = "tunnel";
  isSystemUser = true;
  shell = "${pkgs.shadow}/bin/nologin";
  createHome = false;
  openssh.authorizedKeys.keys = [ "<your-ssh-key>" ];
};

Connect using SSH

Again, this is the same for a NixOS and a Non-NixOS system. The key part here is the required -N flag:

$ ssh -N -L 10000:localhost:993 tunnel@host