git

modules/applications/openssh.nix

@@ -16,6 +16,7 @@ in
         PermitRootLogin = "no";
         PasswordAuthentication = false;
         KbdInteractiveAuthentication = false;
+        AllowAgentForwarding = true;
         PubkeyAuthentication = "yes";
       };
     };

modules/applications/ssh/default.nix

@@ -3,6 +3,7 @@
   config,
   ...
 }:
+
 let
   cfg = config.my.applications.ssh;
 in
@@ -13,33 +14,71 @@ in
   ];
 
   options.my.applications.ssh = {
-    enable = lib.mkEnableOption "OpenSSH client";
+    enable = lib.mkEnableOption "OpenSSH client and agent configuration";
 
-    defaultIdentityFile = lib.mkOption {
+    defaultIdentityFiles = lib.mkOption {
+      type = lib.types.listOf lib.types.str;
+      default = [
+        "~/.ssh/id_ed25519"
+      ];
+      description = ''
+        Default local SSH identity files.
+
+        These are used as the normal fallback identities when no agent key
+        is accepted, or when no forwarded agent is available.
+      '';
+    };
+
+    fidoIdentityFile = lib.mkOption {
       type = lib.types.str;
-      default = "~/.ssh/id_ed25519";
-      description = "Default SSH identity file";
+      default = "~/.ssh/id_ed25519_sk_rk";
+      description = ''
+        Local FIDO2 resident-key SSH identity handle.
+
+        This file is only added to SSH identity candidates when a FIDO2
+        device is actually visible. Do not use file existence to decide
+        whether this key is usable.
+      '';
+    };
+
+    githubIdentityFiles = lib.mkOption {
+      type = lib.types.listOf lib.types.str;
+      default = [ ];
+      description = ''
+        Extra GitHub-specific SSH identity files.
+
+        Leave this empty if GitHub should use the normal agent, FIDO key,
+        and default identity fallback order.
+      '';
     };
 
     addKeysToAgent = lib.mkOption {
       type = lib.types.str;
       default = "no";
-      description = "Add keys to SSH agent";
+      example = "1h";
+      description = ''
+        Value for OpenSSH AddKeysToAgent.
+
+        Recommended default is "no" for this setup, because FIDO resident-key
+        handle files should not be added to the agent accidentally.
+      '';
     };
 
     matchBlocks = lib.mkOption {
-      type = lib.types.attrs;
+      type = lib.types.attrsOf lib.types.anything;
       default = { };
-      description = "SSH match blocks";
-    };
+      description = ''
+        Additional Home Manager OpenSSH settings blocks.
 
-    githubIdentityFiles = lib.mkOption {
-      type = lib.types.listOf lib.types.str;
-      default = [
-        "~/.ssh/id_ed25519_sk_rk"
-        "~/.ssh/id_ed25519"
-      ];
-      description = "SSH identity files for GitHub (tried in order)";
+        Use this for host-specific options such as ForwardAgent = true.
+      '';
+      example = lib.literalExpression ''
+        {
+          "proxmox-* *.home.arpa *.internal" = {
+            ForwardAgent = true;
+          };
+        }
+      '';
     };
   };
 

modules/applications/ssh/home.nix

@@ -4,9 +4,40 @@
   config,
   ...
 }:
+
 let
   cfg = config.my.applications.ssh;
   hmCfg = config.my.applications.ssh.homeManager;
+
+  hasFidoDevice = pkgs.writeShellScript "ssh-has-fido-device" ''
+    ${pkgs.libfido2}/bin/fido2-token -L 2>/dev/null \
+      | ${pkgs.gnugrep}/bin/grep -q .
+  '';
+
+  userMatchBlockNames = lib.attrNames (cfg.matchBlocks or { });
+
+  userMatchBlocks = lib.mapAttrs (
+    _name: value: lib.hm.dag.entryBefore [ "my-github" "my-default" ] value
+  ) (cfg.matchBlocks or { });
+
+  githubBlock = {
+    header = "Host github.com";
+
+    HostName = "github.com";
+    User = "git";
+
+    IdentityAgent = "SSH_AUTH_SOCK";
+
+    IdentitiesOnly = false;
+
+    ForwardAgent = false;
+
+    AddKeysToAgent = cfg.addKeysToAgent;
+  }
+  // lib.optionalAttrs (cfg.githubIdentityFiles != [ ]) {
+    IdentityFile = cfg.githubIdentityFiles;
+  };
+
 in
 {
   options.my.applications.ssh.homeManager = {
@@ -14,40 +45,61 @@ in
   };
 
   config.home-manager.sharedModules = [
-    {
-      config = lib.mkIf hmCfg.enable {
-        home.packages = [
-          pkgs.openssh
-        ];
+    (
+      { lib, ... }:
+      {
+        config = lib.mkIf hmCfg.enable {
 
-        systemd.user.sockets.gcr-ssh-agent.Install.WantedBy = lib.mkForce [ ];
+          programs.ssh = {
+            enable = true;
+            enableDefaultConfig = false;
 
-        services.ssh-agent.enable = true;
+            settings = userMatchBlocks // {
+              "my-local-fido-sk-rk" =
+                lib.hm.dag.entryBefore
+                  (
+                    [
+                      "my-github"
+                      "my-default"
+                    ]
+                    ++ userMatchBlockNames
+                  )
+                  {
+                    header = ''Match exec "${hasFidoDevice}"'';
+                    IdentityFile = cfg.fidoIdentityFile;
+                  };
 
-        home.sessionVariables = {
-          SSH_AUTH_SOCK = "\${XDG_RUNTIME_DIR}/ssh-agent";
-        };
+              "my-github" = lib.hm.dag.entryBefore [ "my-default" ] githubBlock;
 
-        programs.ssh = {
-          enable = true;
-          enableDefaultConfig = false;
+              "my-default" =
+                lib.hm.dag.entryAfter
+                  (
+                    [
+                      "my-local-fido-sk-rk"
+                      "my-github"
+                    ]
+                    ++ userMatchBlockNames
+                  )
+                  {
+                    header = "Host *";
 
-          settings = cfg.matchBlocks // {
-            "github.com" = {
-              IdentityFile = cfg.githubIdentityFiles;
-              AddKeysToAgent = cfg.addKeysToAgent;
-            };
+                    IdentityAgent = "SSH_AUTH_SOCK";
+                    IdentitiesOnly = false;
 
-            "*" = {
-              IdentityFile = cfg.defaultIdentityFile;
-              AddKeysToAgent = cfg.addKeysToAgent;
-              SetEnv = {
-                TERM = "xterm";
-              };
+                    ForwardAgent = false;
+
+                    IdentityFile = cfg.defaultIdentityFiles;
+
+                    AddKeysToAgent = cfg.addKeysToAgent;
+
+                    SetEnv = {
+                      TERM = "xterm";
+                    };
+                  };
             };
           };
         };
-      };
-    }
+      }
+    )
   ];
 }

modules/applications/ssh/system.nix

@@ -15,9 +15,14 @@ in
   config = lib.mkIf cfg.enable {
     environment.systemPackages = with pkgs; [
       openssh # OpenSSH client and server
+      libfido2 # FIDO2 support for SSH
     ];
 
-    programs.ssh.startAgent = false;
-    services.gnome.gcr-ssh-agent.enable = false;
+    programs.ssh = {
+      startAgent = true;
+      agentTimeout = "24h";
+    };
+    programs.gnupg.agent.enableSSHSupport = lib.mkForce false;
+    services.gnome.gcr-ssh-agent.enable = lib.mkForce false;
   };
 }