feat(tailscale-router): serve ts.net DNS forward zone for LAN hosts

FreeIPA (the new authoritative DNS) cannot reach 100.100.100.100
(Tailscale's internal MagicDNS resolver) directly because the DC is not
a Tailscale node. The tailscale-router IS a Tailscale node and can
reach 100.100.100.100 via tailscale0, so it now runs a dnsmasq
instance on its LAN interface that forwards all ts.net queries to
Tailscale's resolver.

After deploying this host, configure FreeIPA with:
  kinit admin
  ipa dnsforwardzone-add ts.net \
    --forwarder=192.168.2.222 \
    --forward-policy=only

This replaces Pi-hole's conditional forwarder for ts.net and restores
resolution of Tailscale MagicDNS names (e.g. raspberrypi.tail13f623.ts.net)
for all LAN hosts using FreeIPA as their DNS server.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ULXzafSDwGhmFGnn3LtDSQ

modules/build-types/tailscale-router.nix

@@ -3,6 +3,7 @@
 {
   imports = [
     ../tailscale/subnet-router.nix
+    ../tailscale/ts-dns-forwarder.nix
     ../beszel/enable-agent.nix
   ];
 

modules/tailscale/ts-dns-forwarder.nix

@@ -0,0 +1,45 @@
+{ vars, ... }:
+
+{
+  # Run dnsmasq on the LAN interface as a forwarding-only resolver for
+  # *.ts.net (Tailscale MagicDNS names). FreeIPA's bind-dyndb-ldap
+  # cannot reach 100.100.100.100 (Tailscale's internal resolver) directly
+  # because the DC is not a Tailscale node. This host IS a Tailscale node
+  # and can reach 100.100.100.100 via its tailscale0 interface, so it
+  # acts as an intermediary: FreeIPA has a conditional forward zone for
+  # ts.net pointing here (vars.tailscaleRouterIp), and this dnsmasq
+  # instance forwards those queries onward to Tailscale's resolver.
+  #
+  # Configure FreeIPA once after deploying this host:
+  #   kinit admin
+  #   ipa dnsforwardzone-add ts.net \
+  #     --forwarder=${vars.tailscaleRouterIp} \
+  #     --forward-policy=only
+  services.dnsmasq = {
+    enable = true;
+    settings = {
+      # Listen only on the LAN interface — not tailscale0 or loopback.
+      # bind-interfaces prevents dnsmasq from binding to 0.0.0.0 and
+      # then filtering by interface later; combined with `interface` this
+      # ensures it genuinely listens only on eth0.
+      bind-interfaces = true;
+      interface = [ vars.lxcLanInterface ];
+
+      # Forward-only: no local /etc/hosts or /etc/resolv.conf reading,
+      # no negative caching of NXDOMAIN for names this instance doesn't
+      # serve. All ts.net queries come from FreeIPA's conditional forwarder
+      # and must be answered by Tailscale's resolver.
+      no-hosts = true;
+      no-resolv = true;
+
+      # Tailscale's internal "Quad100" resolver — reachable from any
+      # Tailscale node via the tailscale0 interface. All *.ts.net queries
+      # (MagicDNS hostnames like raspberrypi.tail13f623.ts.net) are
+      # forwarded here exclusively.
+      server = [ "/ts.net/100.100.100.100" ];
+    };
+  };
+
+  networking.firewall.allowedUDPPorts = [ 53 ];
+  networking.firewall.allowedTCPPorts = [ 53 ];
+}