Generalise common networking options

- allow disabling IPv4 or v6 (both enabled by default)
- allow specifying interface (defaults to eth0)

infra/common/nixos/networking.nix

@@ -1,7 +1,7 @@
 { config, lib, ... }:
 
 let
-  inherit (lib) mkDefault;
+  inherit (lib) mkDefault mkIf mkMerge;
 
 in
 {
@@ -11,41 +11,14 @@ in
       settings.PasswordAuthentication = false;
     };
 
-    networking = {
+    networking = mkMerge [
+      {
         hostName = config.fediversityVm.name;
         domain = config.fediversityVm.domain;
 
         ## REVIEW: Do we actually need that, considering that we have static IPs?
         useDHCP = mkDefault true;
 
-      interfaces = {
-        eth0 = {
-          ipv4 = {
-            addresses = [
-              {
-                inherit (config.fediversityVm.ipv4) address prefixLength;
-              }
-            ];
-          };
-          ipv6 = {
-            addresses = [
-              {
-                inherit (config.fediversityVm.ipv6) address prefixLength;
-              }
-            ];
-          };
-        };
-      };
-
-      defaultGateway = {
-        address = config.fediversityVm.ipv4.gateway;
-        interface = "eth0";
-      };
-      defaultGateway6 = {
-        address = config.fediversityVm.ipv6.gateway;
-        interface = "eth0";
-      };
-
         nameservers = [
           "95.215.185.6"
           "95.215.185.7"
@@ -58,6 +31,29 @@ in
           enable = true;
           rulesetFile = ./nftables-ruleset.nft;
         };
+      }
+
+      ## IPv4
+      (mkIf config.fediversityVm.ipv4.enable {
+        interfaces.${config.fediversityVm.ipv4.interface}.ipv4.addresses = [
+          { inherit (config.fediversityVm.ipv4) address prefixLength; }
+        ];
+        defaultGateway = {
+          address = config.fediversityVm.ipv4.gateway;
+          interface = config.fediversityVm.ipv4.interface;
         };
+      })
+
+      ## IPv6
+      (mkIf config.fediversityVm.ipv6.enable {
+        interfaces.${config.fediversityVm.ipv6.interface}.ipv6.addresses = [
+          { inherit (config.fediversityVm.ipv6) address prefixLength; }
+        ];
+        defaultGateway6 = {
+          address = config.fediversityVm.ipv6.gateway;
+          interface = config.fediversityVm.ipv6.interface;
+        };
+      })
+    ];
   };
 }

infra/common/options.nix

@@ -89,6 +89,17 @@ in
     };
 
     ipv4 = {
+      enable = mkOption {
+        default = true;
+      };
+
+      interface = mkOption {
+        description = ''
+          The interface that carries the machine's IPv4 network.
+        '';
+        default = "eth0";
+      };
+
       address = mkOption {
         description = ''
           The IP address of the machine, version 4. It will be injected as a
@@ -114,6 +125,17 @@ in
     };
 
     ipv6 = {
+      enable = mkOption {
+        default = true;
+      };
+
+      interface = mkOption {
+        description = ''
+          The interface that carries the machine's IPv6 network.
+        '';
+        default = "eth0";
+      };
+
       address = mkOption {
         description = ''
           The IP address of the machine, version 6. It will be injected as a