networking: expose Ollama and mDNS on Wi-Fi LAN

Serve Ollama on kusanagi's Wi-Fi LAN while removing the blanket tailscale0 firewall trust so tailnet traffic follows explicit host firewall rules instead of inheriting every listening service.

Add shared LAN networking for both hosts: allow ping, publish and resolve .local names with Avahi, and scope mDNS to each machine's declared Wi-Fi interface.

Assisted-by: OpenAI:gpt-5

3 files changed, 39 insertions, 2 deletions

diff --git a/common/core.nix b/common/core.nix
index 5d36646..9e9ca7a 100644
--- a/common/core.nix
+++ b/common/core.nix
@@ -16,8 +16,8 @@
   # Enable networking
   networking.networkmanager.enable = true;
 
-  # for tailscale
-  networking.firewall.trustedInterfaces = [ "tailscale0" ];
+  # for tailscale; open only its discovery/connection UDP port and let the
+  # normal host firewall rules apply to tailnet traffic too.
   networking.firewall.allowedUDPPorts = [ 41641 ];
   services.tailscale.enable = true;
 
diff --git a/common/default.nix b/common/default.nix
index 946b3a7..e5bffc0 100644
--- a/common/default.nix
+++ b/common/default.nix
@@ -5,5 +5,6 @@
 {
   imports = [
     ./core.nix
+    ./net.nix
   ];
 }
diff --git a/common/net.nix b/common/net.nix
new file mode 100644
index 0000000..9341245
--- /dev/null
+++ b/common/net.nix
@@ -0,0 +1,36 @@
+# LAN reachability and mDNS discovery shared by all machines.
+{
+  config,
+  lib,
+  ...
+}:
+
+let
+  cfg = config.lanDiscovery;
+in
+{
+  options.lanDiscovery.interface = lib.mkOption {
+    type = lib.types.str;
+    description = "Wi-Fi LAN interface used for mDNS discovery.";
+  };
+
+  config = {
+    # Allow IPv4 ICMP echo requests so hosts can be found with ping.
+    networking.firewall.allowPing = true;
+
+    # Avahi/mDNS: publish <hostname>.local and resolve other *.local LAN names.
+    # Keep the multicast DNS firewall opening scoped to the host's Wi-Fi LAN interface.
+    networking.firewall.interfaces.${cfg.interface}.allowedUDPPorts = [ 5353 ];
+    services.avahi = {
+      enable = true;
+      nssmdns4 = true;
+      openFirewall = false;
+      allowInterfaces = [ cfg.interface ];
+      publish = {
+        enable = true;
+        addresses = true;
+        workstation = true;
+      };
+    };
+  };
+}