feat(hosts/ubuntu/electra): wire up ssh server

Mirror the NixOS openssh config on system-manager: no password auth, no
root login, ed25519 host key only, and gabriel's authorized keys. Its
module runs Ubuntu's /usr/sbin/sshd, so PAM keeps working.

Also enable programs.ssh for system-wide known hosts, and teach the NixOS
side about system-manager hosts so they know electra.

home/gabriel/features/cli/ssh.nix

@@ -6,8 +6,9 @@
   ...
 }: let
   nixosConfigs = builtins.attrNames outputs.nixosConfigurations;
+  systemConfigs = builtins.attrNames outputs.systemConfigs;
   homeConfigs = map (n: lib.last (lib.splitString "@" n)) (builtins.attrNames outputs.homeConfigurations);
-  hostnames = lib.unique (homeConfigs ++ nixosConfigs);
+  hostnames = lib.unique (homeConfigs ++ nixosConfigs ++ systemConfigs);
 in {
   programs.ssh = {
     enable = true;

hosts/nixos/common/global/openssh.nix

@@ -4,7 +4,12 @@
   config,
   ...
 }: let
-  hosts = lib.attrNames outputs.nixosConfigurations;
+  # Every host we know a key for, mapped to the file holding it.
+  hostKeyFiles =
+    lib.genAttrs (lib.attrNames outputs.nixosConfigurations)
+    (hostname: ../../${hostname}/ssh_host_ed25519_key.pub)
+    // lib.genAttrs (lib.attrNames outputs.systemConfigs)
+    (hostname: ../../../ubuntu/${hostname}/ssh_host_ed25519_key.pub);
 
   # Sops needs acess to the keys before the persist dirs are even mounted; so
   # just persisting the keys won't work, we must point at /persist
@@ -36,21 +41,23 @@ in {
 
   programs.ssh = {
     # Each hosts public key
-    knownHosts = lib.genAttrs hosts (hostname: {
-      publicKeyFile = ../../${hostname}/ssh_host_ed25519_key.pub;
-      extraHostNames =
-        [
-          "${hostname}.m7.rs"
-        ]
-        ++
-        # Alias for localhost if it's the same host
-        (lib.optional (hostname == config.networking.hostName) "localhost")
-        # Alias to m7.rs and git.m7.rs if it's alcyone
-        ++ (lib.optionals (hostname == "alcyone") [
-          "m7.rs"
-          "git.m7.rs"
-        ]);
-    });
+    knownHosts =
+      lib.mapAttrs (hostname: publicKeyFile: {
+        inherit publicKeyFile;
+        extraHostNames =
+          [
+            "${hostname}.m7.rs"
+          ]
+          ++
+          # Alias for localhost if it's the same host
+          (lib.optional (hostname == config.networking.hostName) "localhost")
+          # Alias to m7.rs and git.m7.rs if it's alcyone
+          ++ (lib.optionals (hostname == "alcyone") [
+            "m7.rs"
+            "git.m7.rs"
+          ]);
+      })
+      hostKeyFiles;
   };
 
   # Passwordless sudo when SSH'ing with keys

hosts/ubuntu/common/global/default.nix

@@ -18,6 +18,7 @@ in {
       ./greetd.nix
       ./network.nix
       ./nix.nix
+      ./openssh.nix
       ./pam.nix
       ./sops.nix
     ]

hosts/ubuntu/common/global/openssh.nix

@@ -0,0 +1,68 @@
+{
+  outputs,
+  lib,
+  systemManagerHostName,
+  ...
+}: let
+  # Every host we know a key for, mapped to the file holding it.
+  hostKeyFiles =
+    lib.genAttrs (lib.attrNames outputs.nixosConfigurations)
+    (hostname: ../../../nixos/${hostname}/ssh_host_ed25519_key.pub)
+    // lib.genAttrs (lib.attrNames outputs.systemConfigs)
+    (hostname: ../../${hostname}/ssh_host_ed25519_key.pub);
+in {
+  services.openssh = {
+    enable = true;
+    settings = {
+      # Harden
+      PasswordAuthentication = false;
+      PermitRootLogin = "no";
+
+      # Automatically remove stale sockets
+      StreamLocalBindUnlink = "yes";
+      # Allow forwarding ports to everywhere
+      GatewayPorts = "clientspecified";
+      # Let WAYLAND_DISPLAY be forwarded
+      AcceptEnv = ["WAYLAND_DISPLAY"];
+      X11Forwarding = true;
+    };
+
+    # Unlike NixOS, nothing generates these: System Manager's openssh module
+    # runs Ubuntu's /usr/sbin/sshd (so it keeps working with Ubuntu's PAM), and
+    # openssh-server already created the key at install time. sops reads the
+    # same file, see ./sops.nix.
+    hostKeys = [
+      {
+        path = "/etc/ssh/ssh_host_ed25519_key";
+        type = "ed25519";
+      }
+    ];
+  };
+
+  programs.ssh = {
+    # Takes over /etc/ssh/ssh_config (Ubuntu's is backed up), which is what
+    # makes the known hosts below system-wide.
+    enable = true;
+    # No default on non-NixOS, and X11Forwarding above asserts on it.
+    setXAuthLocation = true;
+
+    # Each hosts public key
+    knownHosts =
+      lib.mapAttrs (hostname: publicKeyFile: {
+        inherit publicKeyFile;
+        extraHostNames =
+          [
+            "${hostname}.m7.rs"
+          ]
+          ++
+          # Alias for localhost if it's the same host
+          (lib.optional (hostname == systemManagerHostName) "localhost")
+          # Alias to m7.rs and git.m7.rs if it's alcyone
+          ++ (lib.optionals (hostname == "alcyone") [
+            "m7.rs"
+            "git.m7.rs"
+          ]);
+      })
+      hostKeyFiles;
+  };
+}

hosts/ubuntu/common/users/gabriel/default.nix

@@ -1,5 +1,6 @@
 {
   config,
+  lib,
   pkgs,
   systemManagerHostName,
   ...
@@ -28,6 +29,7 @@ in {
       createHome = false;
       shell = pkgs.fish;
       ignoreShellProgramCheck = true;
+      openssh.authorizedKeys.keys = lib.splitString "\n" (builtins.readFile ../../../../../home/gabriel/ssh.pub);
       extraGroups = [
         "audio"
         "netdev"

hosts/ubuntu/electra/ssh_host_ed25519_key.pub

@@ -0,0 +1 @@
+ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIFOj95/ELxnTsYKnbaZwHLc9DN4lxZ6TLuOFkdrfHrpn root@electra