nichts/nyx/docs/notes/2023-07-14-openssh-custom-port.md

Notes for 14th of July, 2023

My VPS, which hosts some of my infrastructure, has been running NixOS for a while now. Although weak, I use it for distributed builds alongside the rest of my NixOS machines on a Tailscale network.

This server, due to it hosting my infrastructure that communicates with the rest of the internet (i.e my mailserver), is somewhat responsive to queries from the public - which includes very agressive portscans (thanks, skiddies!)

To mitigate that, I have decided to change the ssh port from the default 22 to something different. While this is not exactly a pancea, it helps alleviate the insane log spam I get from failed ssh requests.

The OpenSSH Configuration

First thing we've done is to configure openssh to listen on the new port on your server configuration

services.openssh = {
  ports = [2222];
}

With this set, openssh on the server will now be listening on the port 2222 instead of the default 22. For the changes to take effect after a rebuild, you might need to run systemctl restart sshd.socket.

Then we want to configure our client to use the correct port for our server instead of the default 22.

programs.ssh.extraConfig = ''
    Host nix-builder
      HostName nix-builder-hostname # if you are using Tailscale, this can just be the hostname of a device on your Tailscale network
	  Port 2222
'';

And done, that is all for the ssh side of things. Next up, we need to configure out builder to use the correct host.

Nix Builder Configuration

Assuming you already have a remote builder configured, you will only need to patch the hostName with the one on your openssh.extraConfig.

nix.buildMachines = [{
    hostName = "nix-builder-hostname";
    sshUser = "nix-builder";
    sshKey = "/path/to/key";
    systems = ["x86_64-linux"];
    maxJobs = 2;
    speedFactor = 2;
    supportedFeatures = ["kvm"];
}];

If you have added the correct hostName and sshUser, the builder will be picked up automatically on the next rebuild.

Home-Manager

If you are using Home-Manager, you might also want to configure your declarative ~/.config/ssh/config to use the new port. That can be achieved through programs.ssh.matchBlocks option under Home-Manager

programs.ssh.matchBlocks = {
  "builder" = {
    hostname = "nix-builder-hostname";
    user = "nix-builder";
    identityFile = "~/.ssh/builder-key";
    port = 2222;
  };
}

And that will be all. You are ready to use your new non-default port, mostly safe from port scanners.