🔒 use exclusive ssh key for chungus

components/network/sshd/default.nix

@@ -8,7 +8,6 @@
 with lib;
 with types;
 let
-  defaultRootKeyFiles = [ "${assets}/mrvandalo_rsa.pub" ];
   cfg = config.components.network.sshd;
 
   # maybe ascii-image-converter is also nice here
@@ -30,11 +29,6 @@ in
       type = bool;
       default = true;
     };
-    rootKeyFiles = mkOption {
-      type = with types; listOf path;
-      default = [ ];
-      description = "keys to root login";
-    };
     onlyTincAccess = mkOption {
       type = bool;
       default = false;
@@ -64,14 +58,13 @@ in
         # settings.LoginGraceTime = 0;
       };
 
-      users.users.root.openssh.authorizedKeys.keyFiles = cfg.rootKeyFiles ++ defaultRootKeyFiles;
-
       # todo enable again when I can it's possible to set the `-q` ssh option in clan
       #services.openssh.banner = builtins.readFile sshBanner;
 
     })
 
     (mkIf (cfg.onlyTincAccess && cfg.enable) {
+      # fixme: this is not working
       networking.firewall.extraCommands = ''
         iptables --table nat --append PREROUTING ! --in-interface tinc.+ --protocol tcp --match tcp --dport 22 --jump REDIRECT --to-ports 0
       '';

flake.nix

@@ -224,18 +224,11 @@
           ];
         };
 
-      defaultModules = [
-        # make flake inputs accessiable in NixOS
-        {
-          _module.args.self = self;
-          _module.args.inputs = self.inputs;
-        }
-        # ssh keys
-        (
+      defaultAuthorizedKeys =
         { config, pkgs, ... }:
         {
           users.users.root.openssh.authorizedKeys.keyFiles = [
-              # master key
+            # yubikey key
             ./assets/mrvandalo_rsa.pub
             # backup key
             "${config.clan.core.clanDir}/machines/chungus/facts/ssh.syncoid.id_ed25519.pub"
@@ -243,8 +236,14 @@
             "${config.clan.core.clanDir}/machines/chungus/facts/ssh.paperless-ngx.id_ed25519.pub"
           ];
           environment.systemPackages = [ pkgs.borgbackup ];
+        };
+
+      defaultModules = [
+        # make flake inputs accessiable in NixOS
+        {
+          _module.args.self = self;
+          _module.args.inputs = self.inputs;
         }
-        )
         {
           # disable emergency mode everywhere, although it might be needed on laptops
           boot.initrd.systemd.emergencyAccess = false;
@@ -409,6 +408,7 @@
               name = "cream";
               host = "cream.bear";
               modules = [
+                defaultAuthorizedKeys
                 zerotierModules
                 nixos-hardware.nixosModules.framework-12th-gen-intel
                 retiolum.nixosModules.retiolum
@@ -445,6 +445,12 @@
                 {
                   clan.core.machineDescription = "Laptop";
                 }
+                {
+                  users.users.root.openssh.authorizedKeys.keyFiles = [
+                    # yubikey key
+                    ./assets/mrvandalo_rsa.pub
+                  ];
+                }
               ];
             };
 
@@ -466,6 +472,15 @@
                 {
                   clan.core.machineDescription = "Home Server";
                 }
+                (
+                  { config, ... }:
+                  {
+                    # keys only to access chungus
+                    users.users.root.openssh.authorizedKeys.keyFiles = [
+                      "${config.clan.core.clanDir}/machines/cherry/facts/ssh.root.chungus.id_ed25519.pub"
+                    ];
+                  }
+                )
               ];
             };
 
@@ -474,6 +489,7 @@
               host = "orbi.bear";
               #host = "95.216.66.212";
               modules = [
+                defaultAuthorizedKeys
                 healthchecks.nixosModules.default
                 homeManagerModules
                 stylixModules
@@ -496,6 +512,7 @@
               #host = "167.235.205.150";
               host = "95.217.18.54";
               modules = [
+                defaultAuthorizedKeys
                 homeManagerModules
                 stylixModules
                 srvos.nixosModules.hardware-hetzner-cloud
@@ -517,6 +534,7 @@
               #host = "usbstick.bear";
               host = "10.100.0.100";
               modules = [
+                defaultAuthorizedKeys
                 homeManagerModules
                 stylixModules
                 zerotierModules

machines/cherry/configuration.nix

@@ -20,6 +20,8 @@
     ./37c3.nix
     ./topology.nix
 
+    ./ssh.nix
+
   ];
 
   #time.timeZone = lib.mkForce "Asia/Bangkok";

machines/cherry/ssh.nix

@@ -0,0 +1,39 @@
+{ config, factsGenerator, ... }:
+{
+
+  # Defines the root SSH key to be used exclusively for accessing the backup server.
+  # The need for this arises because deployments using the 'clan' command-line tool (e.g. 'clan machines update')
+  # make use of the 'ssh -A' option, which forwards the SSH agent from the client to the target machine.
+  # If the target machine becomes compromised by an attacker,
+  # they could potentially leverage the forwarded SSH agent to access the backup server.
+  # This file prevents that scenario by restricting access strictly to the defined SSH key,
+  # which is only used to access the backup server, so no other ssh-agent will contain this ssh key
+
+  clan.core.facts.services."mainUser.ssh.chungus" = factsGenerator.ssh { name = "root.chungus"; };
+
+  systemd.tmpfiles.settings.mainUser = {
+    "/run/facts/ssh.mainUser.chungus.id_ed25519"."C+" = {
+      user = config.users.users.mainUser.name;
+      group = config.users.users.mainUser.group;
+      mode = "400";
+      argument =
+        config.clan.core.facts.services."mainUser.ssh.chungus".secret."ssh.root.chungus.id_ed25519".path;
+    };
+  };
+
+  home-manager.users.mainUser.programs.ssh.matchBlocks = {
+    "chungus.bear" = {
+      identityFile = "/run/facts/ssh.mainUser.chungus.id_ed25519";
+      identitiesOnly = true;
+    };
+    "chungus.private" = {
+      identityFile = "/run/facts/ssh.mainUser.chungus.id_ed25519";
+      identitiesOnly = true;
+    };
+    "chungus.wg0" = {
+      identityFile = "/run/facts/ssh.mainUser.chungus.id_ed25519";
+      identitiesOnly = true;
+    };
+  };
+
+}