refactor(nixos): simplify SSH configuration

configs/nixos/hosts/neogaia/ssh_user_ed25519_key.pub

@@ -0,0 +1 @@
+ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIGxQ4kWsBo2OGYIPOkFe0vNEcB3yoJwAu0y9wrdQzALE alexion@neogaia

configs/nixos/hosts/pikachu/ssh_user_ed25519_key.pub

@@ -0,0 +1 @@
+ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAINNqJIC6VRXyvrNf3n9su9KdPCikC3CjK/QrCK2reHdB alexion@pikachu

configs/nixos/hosts/zeus/ssh_user_ed25519_key.pub

@@ -0,0 +1 @@
+ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIIRBcHHuubptXCdvED/p2dR3MpjGN/AimqLasLY5984G alexion@zeus

configs/nixos/modules/network/ssh.nix

@@ -4,344 +4,208 @@
   registry,
   ...
 }:
-# SSH on this machine, in both directions.
+# Configures inbound and outbound SSH access.
 let
-  cfg = config.modules.network.ssh; # NOLINT(let_order)
-  targetType = types.submodule (
-    { name, ... }:
-    {
-      options = {
-        hostName = mkOption {
-          type = types.str;
-          default = name;
-          description = ''
-            The network address OpenSSH connects to for this target.
-          '';
-        };
+  cfg = config.modules.network.ssh;
+  user = config.user.name; # NOLINT(let_order)
 
-        user = mkOption {
-          type = types.str;
-          default = config.user.name;
-          description = ''
-            The remote login name OpenSSH uses for this target.
-          '';
-        };
+  # NOLINT(let_order)
+  workstationNames = [
+    "neogaia"
+    "zeus"
+  ];
+  serverNames = [ "pikachu" ];
 
-        port = mkOption {
-          type = types.port;
-          default = 22;
-          description = ''
-            The TCP port OpenSSH uses for this target.
-          '';
-        };
+  isWorkstation = lib.elem config.networking.hostName workstationNames; # NOLINT(let_order)
+  isServer = lib.elem config.networking.hostName serverNames; # NOLINT(let_order)
 
-        aliases = mkOption {
-          type = types.listOf types.str;
-          default = [ name ];
-          description = ''
-            Host patterns written into the generated OpenSSH client block.
-          '';
-        };
+  # NOLINT(let_order)
+  clientKeyFor = host: builtins.readFile (../../hosts + "/${host}/ssh_user_ed25519_key.pub");
+  hostKeysFor =
+    host:
+    map builtins.readFile [
+      (../../hosts + "/${host}/ssh_host_ed25519_key.pub")
+      (../../hosts + "/${host}/ssh_host_rsa_key.pub")
+    ];
 
-        clientKey = mkOption {
-          type = types.nullOr types.str;
-          default = null;
-          description = ''
-            The public key this target offers when it connects outward.
-            Other machines admit this key according to the host groups below.
-          '';
-        };
+  # NOLINT(let_order)
+  managedTargets = lib.genAttrs (workstationNames ++ serverNames) (name: {
+    hostName = registry.address.${name}.self;
+    clientKey = clientKeyFor name;
+    hostKeys = hostKeysFor name;
+  });
 
-        hostKeys = mkOption {
-          type = types.listOf types.str;
-          default = [ ];
-          description = ''
-            The public keys this target presents when it accepts inbound SSH.
-            These keys generate system-wide known-host entries.
-          '';
-        };
-      };
-    }
-  );
+  extraTargets = {
+    # TODO: Pin Gitea's host key after migrating it from Proxmox.
+    gitea = {
+      hostName = "git.alexion.dev";
+      port = 2022;
+      user = "gitea";
+      aliases = [
+        "gitea"
+        "git.alexion.dev"
+      ];
+    };
+  };
 
+  # NOLINT(let_order)
+  mkTarget = name: target: {
+    hostName = target.hostName or name;
+    user = target.user or user;
+    port = target.port or 22;
+    aliases = target.aliases or [ name ];
+    clientKey = target.clientKey or null;
+    hostKeys = target.hostKeys or [ ];
+  };
+  targets = lib.mapAttrs mkTarget (managedTargets // extraTargets); # NOLINT(let_order)
+
+  clientKeysFor = names: lib.filter (key: key != null) (map (name: targets.${name}.clientKey) names); # NOLINT(let_order)
+  # NOLINT(let_order)
   defaultAuthorizedKeys = lib.flatten (
-    clientKeysFor workstationNames ++ optional isServer (clientKeysFor serverNames)
+    clientKeysFor workstationNames ++ lib.optional isServer (clientKeysFor serverNames)
   );
 
-  clientKeysFor = names: filter (key: key != null) (map (name: cfg.targets.${name}.clientKey) names);
+  outboundTargetNames = # NOLINT(let_order)
+    let
+      groupTargets =
+        if isServer then
+          builtins.attrNames extraTargets ++ serverNames
+        else if isWorkstation then
+          builtins.attrNames extraTargets ++ serverNames ++ workstationNames
+        else
+          builtins.attrNames extraTargets;
+    in
+    lib.filter (name: name != config.networking.hostName) groupTargets;
 
-  workstationNames = targetNamesIn cfg.hosts.workstations;
-  targetNamesIn = names: filter (name: hasAttr name cfg.targets) names;
-
-  isServer = elem currentHost cfg.hosts.servers;
-  currentHost = config.networking.hostName;
-  serverNames = targetNamesIn cfg.hosts.servers;
-
-  knownHosts = listToAttrs (
-    concatLists (
-      mapAttrsToList (
+  knownHosts = lib.listToAttrs (
+    lib.concatLists (
+      lib.mapAttrsToList (
         targetName: target:
-        imap0 (
+        lib.imap0 (
           i: key:
-          nameValuePair "${targetName}-${toString i}" {
+          lib.nameValuePair "${targetName}-${toString i}" {
             hostNames = knownHostNamesFor target;
             publicKey = keyWithoutComment key;
           }
         ) target.hostKeys
-      ) cfg.targets
+      ) targets
     )
   );
   knownHostNamesFor =
     target:
     let
       withPort = name: if target.port == 22 then name else "[${name}]:${toString target.port}";
-      names = unique (target.aliases ++ [ target.hostName ]);
+      names = lib.unique (target.aliases ++ [ target.hostName ]);
     in
     map withPort names;
 
-  keyWithoutComment = key: concatStringsSep " " (lib.take 2 (lib.splitString " " key));
+  keyWithoutComment = key: lib.concatStringsSep " " (lib.take 2 (lib.splitString " " key));
 
   sshSettingsFor =
     name:
     let
-      target = cfg.targets.${name};
+      target = targets.${name};
     in
     {
-      header = "Host ${concatStringsSep " " target.aliases}";
+      header = "Host ${lib.concatStringsSep " " target.aliases}";
       HostName = target.hostName;
       User = target.user;
     }
-    // optionalAttrs (target.port != 22) { Port = target.port; };
-
-  outboundTargetNames =
-    let
-      groupTargets =
-        if isServer then
-          [ "gitea" ] ++ cfg.hosts.servers
-        else if isWorkstation then
-          [ "gitea" ] ++ cfg.hosts.servers ++ cfg.hosts.workstations
-        else
-          [ "gitea" ];
-    in
-    filter (name: name != currentHost) (targetNamesIn groupTargets);
-
-  isWorkstation = elem currentHost cfg.hosts.workstations;
+    // lib.optionalAttrs (target.port != 22) { Port = target.port; };
 
   hostKeySecret = type: "ssh-host-${type}-key";
-  user = config.user.name;
-
-  inherit (lib)
-    concatLists
-    concatStringsSep
-    elem
-    filter
-    genAttrs
-    hasAttr
-    imap0
-    listToAttrs
-    mapAttrs
-    mapAttrsToList
-    mkIf
-    mkMerge
-    mkOption
-    nameValuePair
-    optional
-    optionalAttrs
-    types
-    unique
-    ;
-
+  hostKeyTypes = [
+    "ed25519"
+    "rsa"
+  ];
   userKeySecret = "ssh-user-ed25519-key";
 
 in
 {
   options.modules.network.ssh = {
-    enable = lib.mkEnableOption "the OpenSSH daemon, with host keys restored from secrets";
+    enable = lib.mkEnableOption "the OpenSSH daemon";
 
-    hosts = {
-      servers = mkOption {
-        type = types.listOf types.str;
-        default = [ "pikachu" ];
-        description = ''
-          Hosts that serve durable services.
-          They admit workstation keys and server keys, and they receive aliases for other servers and the forge.
-        '';
-      };
-
-      workstations = mkOption {
-        type = types.listOf types.str;
-        default = [
-          "neogaia"
-          "zeus"
-        ];
-        description = ''
-          Hosts the operator works from.
-          Their keys are admitted by every host, and they receive aliases for the whole fleet and the forge.
-        '';
-      };
-    };
-
-    targets = mkOption {
-      type = types.attrsOf targetType;
-      default = {
-        gitea = {
-          hostName = "git.alexion.dev";
-          port = 2022;
-          user = "gitea";
-          aliases = [
-            "gitea"
-            "git.alexion.dev"
-          ];
-        };
-
-        neogaia = {
-          hostName = registry.address.neogaia.self;
-          clientKey = "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIGxQ4kWsBo2OGYIPOkFe0vNEcB3yoJwAu0y9wrdQzALE alexion@neogaia";
-          hostKeys = [
-            "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIJS+wp7K123+4BT6G4f954R6WyrbWveY7VlpoBUf6I5p neogaia"
-            "ssh-rsa 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 neogaia"
-          ];
-        };
-
-        zeus = {
-          hostName = registry.address.zeus.self;
-          clientKey = "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIIRBcHHuubptXCdvED/p2dR3MpjGN/AimqLasLY5984G alexion@zeus";
-          hostKeys = [
-            "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIJ4GBPt966LR4LpmXgiMMkepIqNIlXTUBh4zdNeek/lN root@zeus"
-            "ssh-rsa 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 root@zeus"
-          ];
-        };
-
-        pikachu = {
-          hostName = registry.address.pikachu.self;
-          clientKey = "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAINNqJIC6VRXyvrNf3n9su9KdPCikC3CjK/QrCK2reHdB alexion@pikachu";
-          hostKeys = [
-            "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIKljRf4pJO+pqEqjpPz08gOYq3g1PpxvE66xVw7uMEnA root@pikachu"
-            "ssh-rsa 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 root@pikachu"
-          ];
-        };
-      };
-      description = ''
-        SSH targets known to the fleet.
-        The inventory holds connection details plus public keys used for authorization and host verification.
-      '';
-    };
-
-    extraAuthorizedKeys = mkOption {
-      type = types.listOf types.str;
-      default = [ ];
-      description = ''
-        Additional client public keys admitted by this host.
-      '';
-    };
-
-    authorizedKeys = mkOption {
-      type = types.nullOr (types.listOf types.str);
-      default = null;
-      description = ''
-        Complete override for client public keys admitted by this host.
-        Leave null to derive access from `modules.network.ssh.hosts` and `modules.network.ssh.targets`.
-      '';
-    };
-
-    extraSettings = mkOption {
-      type = types.attrsOf types.anything;
-      default = { };
-      description = ''
-        Additional OpenSSH client settings merged into the generated Home Manager configuration.
-      '';
-    };
-
-    hostKeys.restore = mkOption {
-      type = types.bool;
+    hostKeys.restore = lib.mkOption {
+      type = lib.types.bool;
       default = true;
       description = ''
-        Restore the host keys from secrets rather than letting the daemon generate its own.
-        A machine with its own identity keeps its fingerprint across a reimage by restoring committed keys.
-        A guest carries no host identity, so it turns this off and presents a self-generated key instead.
+        Restores host keys from secrets.
+        Preserves the fingerprint across reimages.
+        Guests disable this option and generate their own host keys.
       '';
     };
 
-    hostKeys.sopsFile = mkOption {
-      type = types.nullOr types.path;
+    hostKeys.sopsFile = lib.mkOption {
+      type = lib.types.nullOr lib.types.path;
       default = null;
       description = ''
-        Encrypted file holding this host's SSH host private keys, one entry per key type, named `ssh-host-<type>-key`.
-        Required when `restore` is on.
+        Sets encrypted file that contains host private keys.
+        Names each secret `ssh-host-<type>-key`.
+        Requires a value when `restore` is enabled.
       '';
     };
 
-    hostKeys.types = mkOption {
-      type = types.listOf types.str;
-      default = [
-        "ed25519"
-        "rsa"
-      ];
-      description = ''
-        Key types to restore, naming both the entries read from the encrypted file and the algorithms the daemon offers.
-      '';
-    };
-
-    userKey.sopsFile = mkOption {
-      type = types.nullOr types.path;
+    userKey.sopsFile = lib.mkOption {
+      type = lib.types.nullOr lib.types.path;
       default = null;
       description = ''
-        Encrypted file holding this machine's SSH client private key, under the entry `ssh-user-ed25519-key`.
-        Left unset on a machine that authenticates to no remote server, such as a guest.
+        Sets encrypted file that contains the SSH client private key.
+        Names the secret `ssh-user-ed25519-key`.
+        Guests leave this unset because they do not authenticate to remote servers.
       '';
     };
   };
 
-  config = mkIf cfg.enable (mkMerge [
-    {
-      services.openssh.enable = true;
+  config = lib.mkIf cfg.enable (
+    lib.mkMerge [
+      {
+        services.openssh.enable = true;
 
-      users.users.${user}.openssh.authorizedKeys.keys =
-        if cfg.authorizedKeys != null then
-          cfg.authorizedKeys
-        else
-          defaultAuthorizedKeys ++ cfg.extraAuthorizedKeys;
+        users.users.${user}.openssh.authorizedKeys.keys = defaultAuthorizedKeys;
 
-      programs.ssh.knownHosts = knownHosts;
+        programs.ssh.knownHosts = knownHosts;
 
-      home-manager.users.${user}.programs.ssh = {
-        enable = true;
-        enableDefaultConfig = false;
-        settings =
-          mapAttrs (name: _: sshSettingsFor name) (genAttrs outboundTargetNames (name: name))
-          // cfg.extraSettings;
-      };
-    }
+        home-manager.users.${user}.programs.ssh = {
+          enable = true;
+          enableDefaultConfig = false;
+          settings = lib.mapAttrs (name: _: sshSettingsFor name) (
+            lib.genAttrs outboundTargetNames (name: name)
+          );
+        };
+      }
 
-    (mkIf cfg.hostKeys.restore {
-      assertions = [
-        {
-          assertion = cfg.hostKeys.sopsFile != null;
-          message = "modules.network.ssh.hostKeys.restore requires modules.network.ssh.hostKeys.sopsFile to name the encrypted host keys.";
-        }
-      ];
+      (lib.mkIf cfg.hostKeys.restore {
+        assertions = [
+          {
+            assertion = cfg.hostKeys.sopsFile != null;
+            message = "Set `modules.network.ssh.hostKeys.sopsFile` when host-key restoration is enabled.";
+          }
+        ];
 
-      sops.secrets = genAttrs (map hostKeySecret cfg.hostKeys.types) (_: {
-        inherit (cfg.hostKeys) sopsFile;
-        mode = "0400";
-        restartUnits = [ "sshd.service" ];
-      });
+        sops.secrets = lib.genAttrs (map hostKeySecret hostKeyTypes) (_: {
+          inherit (cfg.hostKeys) sopsFile;
+          mode = "0400";
+          restartUnits = [ "sshd.service" ];
+        });
 
-      services.openssh.hostKeys = [ ];
-      services.openssh.extraConfig = concatStringsSep "" (
-        map (type: "HostKey ${config.sops.secrets.${hostKeySecret type}.path}\n") cfg.hostKeys.types
-      );
-    })
+        services.openssh.hostKeys = [ ];
+        services.openssh.extraConfig = lib.concatStringsSep "" (
+          map (type: "HostKey ${config.sops.secrets.${hostKeySecret type}.path}\n") hostKeyTypes
+        );
+      })
 
-    (mkIf (cfg.userKey.sopsFile != null) {
-      sops.secrets.${userKeySecret} = {
-        inherit (cfg.userKey) sopsFile;
-        mode = "0400";
-        owner = user;
-      };
+      (lib.mkIf (cfg.userKey.sopsFile != null) {
+        sops.secrets.${userKeySecret} = {
+          inherit (cfg.userKey) sopsFile;
+          mode = "0400";
+          owner = user;
+        };
 
-      home-manager.users.${user}.programs.ssh.settings."*".IdentityFile =
-        config.sops.secrets.${userKeySecret}.path;
-    })
-  ]);
+        home-manager.users.${user}.programs.ssh.settings."*" = {
+          IdentityFile = config.sops.secrets.${userKeySecret}.path;
+          IdentitiesOnly = true;
+        };
+      })
+    ]
+  );
 }