Merge pull request 'fix(tailscale-router): masquerade LAN traffic into Tailscale' (#69) from worktree-lovely-spinning-bubble into main

Reviewed-on: #69

modules/build-types/tailscale-router.nix

@@ -1,4 +1,4 @@
-{ ... }:
+{ vars, ... }:
 
 {
   imports = [
@@ -12,8 +12,21 @@
   # own setting) so the intent is clear at the build-type level.
   services.tailscale.useRoutingFeatures = "server";
 
+  # Advertise the LAN subnet so Tailscale peers can route back to LAN machines.
+  # Must also be approved in the Tailscale admin console (Machines → Edit route settings).
+  services.tailscale.extraUpFlags = [ "--advertise-routes=${vars.lanCidr}" ];
+
   # Forwarded subnet-router traffic arrives on tailscale0 already
   # tailscale-authenticated -- the firewall's normal per-port allow-list
   # would otherwise drop it. Standard NixOS/Tailscale subnet-router guidance.
   networking.firewall.trustedInterfaces = [ "tailscale0" ];
+
+  # SNAT traffic from LAN machines going out through Tailscale so the remote
+  # peer sees it sourced from this router's Tailscale IP (100.x.x.x) rather
+  # than a raw LAN IP.  Without this, Tailscale drops the forwarded packets
+  # because the source is not a recognised Tailscale address.
+  networking.nat = {
+    enable = true;
+    externalInterface = "tailscale0";
+  };
 }

modules/tailscale/subnet-router.nix

@@ -1,4 +1,4 @@
-_:
+{ pkgs, ... }:
 
 {
   imports = [ ./enable-service.nix ];
@@ -12,4 +12,24 @@ _:
     # instead of relaying through DERP.
     openFirewall = true;
   };
+
+  # Tailscale recommends these ethtool flags on the uplink interface to get
+  # full UDP GRO throughput on subnet routers (https://tailscale.com/s/ethtool-config-udp-gro).
+  # The interface is derived from the default route so it works regardless of
+  # what the NIC is named on a given host.
+  systemd.services.tailscale-udp-gro = {
+    description = "Enable UDP GRO forwarding on uplink for Tailscale subnet router";
+    after = [ "network-online.target" ];
+    wants = [ "network-online.target" ];
+    wantedBy = [ "multi-user.target" ];
+    path = [ pkgs.ethtool pkgs.iproute2 ];
+    serviceConfig = {
+      Type = "oneshot";
+      RemainAfterExit = true;
+      ExecStart = pkgs.writeShellScript "tailscale-udp-gro" ''
+        NETDEV=$(ip -o route get 8.8.8.8 | cut -f 5 -d " ")
+        ethtool -K "$NETDEV" rx-udp-gro-forwarding on rx-gro-list off
+      '';
+    };
+  };
 }