feat(modules/server): srv module automatically calculates a default gateway

1 files changed, 37 insertions, 2 deletions

diff --git a/modules/server/srv.nix b/modules/server/srv.nix
index 7701ee5..0ec63bd 100644
--- a/modules/server/srv.nix
+++ b/modules/server/srv.nix
@@ -41,9 +41,44 @@
       };
 
   in {
-    networking.interfaces = mkIf
+    networking = mkIf
       (config.srv.static != null)
-      (netinterfaces config);
+      {
+        interfaces = netinterfaces config;
+
+        # Calculate the gateway using the given static ip and prefix
+        # We're assuming the gateway is the first (min) host on the network
+        defaultGateway.address =
+          let
+            # 192.168.0.100 -> [ "192" "168" "0" "100" ]
+            ipBytes = builtins.map
+              (x: lib.toInt x)
+              (lib.splitString "." config.srv.static.ip);
+
+            # 2 ^^ x
+            pow2 = x: if x == 0 then 1 else 2 * (pow2 (x - 1));
+            # 24 -> 0b11111111_11111111_11111111_00000000
+            mask = 4294967295 - ((pow2 (32 - config.srv.static.prefixLength)) - 1);
+
+            # 0b11111111_11111111_11111111_00000000 -> [ 255 255 255 0 ]
+            maskBytes = [
+              (builtins.bitAnd (mask / 16777216) 255)
+              (builtins.bitAnd (mask / 65536)    255)
+              (builtins.bitAnd (mask / 256)      255)
+              (builtins.bitAnd mask              255)
+            ];
+
+            # ipBytes & maskBytes
+            networkIP = lib.zipListsWith lib.bitAnd ipBytes maskBytes;
+            # min possible host
+            gatewayIP = (lib.take 3 networkIP) ++ [ ((lib.last networkIP) + 1) ];
+          in
+            lib.mkDefault
+              (lib.concatMapStringsSep
+                "."
+                builtins.toString
+                gatewayIP);
+      };
 
     virtualisation.vmVariant.networking.interfaces = mkIf
       (config.srv.static != null)