nixos/tailscale: Add useRoutingFeatures option

nixos/doc/manual/from_md/release-notes/rl-2305.section.xml

@@ -282,6 +282,20 @@
           to match upstream.
         </para>
       </listitem>
+      <listitem>
+        <para>
+          The new option
+          <literal>services.tailscale.useRoutingFeatures</literal>
+          controls various settings for using Tailscale features like
+          exit nodes and subnet routers. If you wish to use your machine
+          as an exit node, you can set this setting to
+          <literal>server</literal>, otherwise if you wish to use an
+          exit node you can set this setting to
+          <literal>client</literal>. The strict RPF warning has been
+          removed as the RPF will be loosened automatically based on the
+          value of this setting.
+        </para>
+      </listitem>
     </itemizedlist>
   </section>
 </section>

nixos/doc/manual/release-notes/rl-2305.section.md

@@ -81,3 +81,5 @@ In addition to numerous new and upgraded packages, this release has the followin
 - The `services.fwupd` module now allows arbitrary daemon settings to be configured in a structured manner ([`services.fwupd.daemonSettings`](#opt-services.fwupd.daemonSettings)).
 
 - The `unifi-poller` package and corresponding NixOS module have been renamed to `unpoller` to match upstream.
+
+- The new option `services.tailscale.useRoutingFeatures` controls various settings for using Tailscale features like exit nodes and subnet routers. If you wish to use your machine as an exit node, you can set this setting to `server`, otherwise if you wish to use an exit node you can set this setting to `client`. The strict RPF warning has been removed as the RPF will be loosened automatically based on the value of this setting.

nixos/modules/services/networking/tailscale.nix

@@ -4,10 +4,7 @@ with lib;
 
 let
   cfg = config.services.tailscale;
-  firewallOn = config.networking.firewall.enable;
-  rpfMode = config.networking.firewall.checkReversePath;
   isNetworkd = config.networking.useNetworkd;
-  rpfIsStrict = rpfMode == true || rpfMode == "strict";
 in {
   meta.maintainers = with maintainers; [ danderson mbaillie twitchyliquid64 ];
 
@@ -38,14 +35,23 @@ in {
       defaultText = literalExpression "pkgs.tailscale";
       description = lib.mdDoc "The package to use for tailscale";
     };
+
+    useRoutingFeatures = mkOption {
+      type = types.enum [ "none" "client" "server" "both" ];
+      default = "none";
+      example = "server";
+      description = lib.mdDoc ''
+        Enables settings required for Tailscale's routing features like subnet routers and exit nodes.
+
+        To use these these features, you will still need to call `sudo tailscale up` with the relevant flags like `--advertise-exit-node` and `--exit-node`.
+
+        When set to `client` or `both`, reverse path filtering will be set to loose instead of strict.
+        When set to `server` or `both`, IP forwarding will be enabled.
+      '';
+    };
   };
 
   config = mkIf cfg.enable {
-    warnings = optional (firewallOn && rpfIsStrict) ''
-      Strict reverse path filtering breaks Tailscale exit node use and some subnet routing setups. Consider setting:
-
-        networking.firewall.checkReversePath = "loose";
-    '';
     environment.systemPackages = [ cfg.package ]; # for the CLI
     systemd.packages = [ cfg.package ];
     systemd.services.tailscaled = {
@@ -75,6 +81,13 @@ in {
       stopIfChanged = false;
     };
 
+    boot.kernel.sysctl = mkIf (cfg.useRoutingFeatures == "server" || cfg.useRoutingFeatures == "both") {
+      "net.ipv4.conf.all.forwarding" = mkDefault true;
+      "net.ipv6.conf.all.forwarding" = mkDefault true;
+    };
+
+    networking.firewall.checkReversePath = mkIf (cfg.useRoutingFeatures == "client" || cfg.useRoutingFeatures == "both") "loose";
+
     networking.dhcpcd.denyInterfaces = [ cfg.interfaceName ];
 
     systemd.network.networks."50-tailscale" = mkIf isNetworkd {