refactor(tailscale): rename exit-node to subnet-router; drop --advertise-exit-node

The host was always intended as a LAN subnet router (--advertise-routes),
not a full exit node (--advertise-exit-node). Rename every trace of
"exit-node" to "subnet-router" and remove the --advertise-exit-node flag
from extraSetFlags; the operator supplies --advertise-routes at first
tailscale up and Tailscale persists it in state across reboots.

Routing sysctls (useRoutingFeatures = "server"), openFirewall, and
trustedInterfaces = ["tailscale0"] are still required for subnet routing
to work, so the module is kept — just correctly named.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TF2dsuKZAiyZWQ1D7CuHJm

flake.nix

@@ -96,9 +96,9 @@
         proxmox-pxe-boot = mkTarget { platform = "proxmox"; buildType = "pxe-boot"; hostPath = ./hosts/pxe-boot/host.nix; };
         lxc-pxe-boot = mkTarget { platform = "lxc"; buildType = "pxe-boot"; hostPath = ./hosts/pxe-boot/host.nix; };
 
-        linode-tailscale-exit-node = mkTarget { platform = "linode"; buildType = "tailscale-exit-node"; hostPath = ./hosts/tailscale-exit-node/host.nix; };
-        proxmox-tailscale-exit-node = mkTarget { platform = "proxmox"; buildType = "tailscale-exit-node"; hostPath = ./hosts/tailscale-exit-node/host.nix; };
-        lxc-tailscale-exit-node = mkTarget { platform = "lxc"; buildType = "tailscale-exit-node"; hostPath = ./hosts/tailscale-exit-node/host.nix; };
+        linode-tailscale-subnet-router = mkTarget { platform = "linode"; buildType = "tailscale-subnet-router"; hostPath = ./hosts/tailscale-subnet-router/host.nix; };
+        proxmox-tailscale-subnet-router = mkTarget { platform = "proxmox"; buildType = "tailscale-subnet-router"; hostPath = ./hosts/tailscale-subnet-router/host.nix; };
+        lxc-tailscale-subnet-router = mkTarget { platform = "lxc"; buildType = "tailscale-subnet-router"; hostPath = ./hosts/tailscale-subnet-router/host.nix; };
 
         lxc-tor-relay = mkTarget { platform = "lxc"; buildType = "tor-relay"; hostPath = ./hosts/tor-relay/host.nix; };
       };

hosts/tailscale-exit-node/host.nix

@@ -1,12 +0,0 @@
-_:
-
-{
-  networking.hostName = "exit-node";
-
-  # No networking.hostId: only ZFS-touching hosts (server, docker) need one
-  # for pool-import safety, and this host does neither.
-
-  # A genuinely new host (not a pre-refactor carry-over), so it tracks the
-  # flake's current nixpkgs release rather than being pinned to an older one.
-  system.stateVersion = "26.05";
-}

hosts/tailscale-subnet-router/host.nix

@@ -0,0 +1,10 @@
+_:
+
+{
+  networking.hostName = "tailscale-router";
+
+  # No networking.hostId: only ZFS-touching hosts (server, docker) need one
+  # for pool-import safety, and this host does neither.
+
+  system.stateVersion = "26.05";
+}

modules/build-types/tailscale-exit-node.nix

@@ -1,21 +0,0 @@
-{ ... }:
-
-{
-  imports = [
-    ../tailscale/exit-node.nix
-  ];
-
-  # "server", not "both": this build type only ever advertises itself as an
-  # exit node (see ../tailscale/exit-node.nix) -- it doesn't advertise LAN
-  # subnet routes, so it doesn't need the "client"-side loose reverse-path
-  # filtering that "both" would also turn on. Deliberately left unbundled
-  # from LAN-subnet-route advertisement so this build type stays valid on
-  # every platform, including linode (a remote VPS with no network path to
-  # the home LAN at all).
-  services.tailscale.useRoutingFeatures = "server";
-
-  # Forwarded exit-node traffic arrives on tailscale0 already
-  # tailscale-authenticated -- the firewall's normal per-port allow-list
-  # would otherwise drop it. Standard NixOS/Tailscale exit-node guidance.
-  networking.firewall.trustedInterfaces = [ "tailscale0" ];
-}

modules/build-types/tailscale-subnet-router.nix

@@ -0,0 +1,19 @@
+{ ... }:
+
+{
+  imports = [
+    ../tailscale/subnet-router.nix
+  ];
+
+  # "server", not "both": this build type advertises LAN subnet routes but
+  # doesn't use another tailscale exit node itself, so it doesn't need the
+  # "client"-side loose reverse-path filtering that "both" would also enable.
+  # Deliberately kept explicit here (not just relying on subnet-router.nix's
+  # own setting) so the intent is clear at the build-type level.
+  services.tailscale.useRoutingFeatures = "server";
+
+  # 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" ];
+}

modules/tailscale/exit-node.nix

@@ -1,27 +0,0 @@
-_:
-
-{
-  imports = [ ./enable-service.nix ];
-
-  services.tailscale = {
-    # Enables the sysctl forwarding settings exit nodes/subnet routers need;
-    # without this, --advertise-exit-node has no effect.
-    useRoutingFeatures = "server";
-
-    # Lets peers reach this node directly over the tailscale UDP port
-    # instead of relaying through DERP.
-    openFirewall = true;
-
-    # extraSetFlags (tailscale set, via the always-on tailscaled-set
-    # service), not extraUpFlags -- extraUpFlags is only ever applied by
-    # tailscaled-autoconnect, which itself only runs when
-    # services.tailscale.authKeyFile is set (nothing in this repo sets one,
-    # so tailscale up is a manual, one-time operator step on every host that
-    # uses this service). extraSetFlags has no such gate, so
-    # --advertise-exit-node self-reapplies on every boot once the operator
-    # has authenticated the node once.
-    extraSetFlags = [
-      "--advertise-exit-node"
-    ];
-  };
-}

modules/tailscale/subnet-router.nix

@@ -0,0 +1,15 @@
+_:
+
+{
+  imports = [ ./enable-service.nix ];
+
+  services.tailscale = {
+    # Enables the sysctl forwarding settings subnet routers need;
+    # without this, --advertise-routes has no effect.
+    useRoutingFeatures = "server";
+
+    # Lets peers reach this node directly over the tailscale UDP port
+    # instead of relaying through DERP.
+    openFirewall = true;
+  };
+}