nixbld: factor out network interface names

nixbld-etc-nixos/configuration.nix

@@ -5,6 +5,9 @@
 { config, pkgs, ... }:
 
 let
+  netifWan = "enp0s31f6";
+  netifLan = "enp3s0";
+  netifWifi = "wlp4s0";
   hydraWwwOutputs = "/var/www/hydra-outputs";
 in
 {
@@ -31,25 +34,25 @@ in
       allowedTCPPorts = [ 80 443 631 5901 ];
       allowedUDPPorts = [ 53 67 631 ];
     };
-    networkmanager.unmanaged = [ "interface-name:wlp4s0" "interface-name:enp3s0" ];
-    interfaces."enp3s0".ipv4.addresses = [{
+    networkmanager.unmanaged = [ "interface-name:${netifLan}" "interface-name:${netifWifi}" ];
+    interfaces."${netifLan}".ipv4.addresses = [{
       address = "192.168.1.1";
       prefixLength = 24;
     }];
-    interfaces."wlp4s0".ipv4.addresses = [{
+    interfaces."${netifWifi}".ipv4.addresses = [{
       address = "192.168.12.1";
       prefixLength = 24;
     }];
     nat = {
       enable = true;
-      externalInterface = "enp0s31f6";
-      internalInterfaces = ["enp3s0" "wlp4s0"];
+      externalInterface = netifWan;
+      internalInterfaces = [ netifLan netifWifi ];
     };
   };
 
   services.hostapd = {
     enable        = true;
-    interface     = "wlp4s0";
+    interface     = netifWifi;
     hwMode        = "g";
     ssid          = "M-Labs";
     wpaPassphrase = (import /etc/nixos/secret/wifi_password.nix);
@@ -57,11 +60,11 @@ in
   services.dnsmasq = {
     enable = true;
     extraConfig = ''
-      interface=enp3s0
-      interface=wlp4s0
+      interface=${netifLan}
+      interface=${netifWifi}
       bind-interfaces
-      dhcp-range=interface:enp3s0,192.168.1.10,192.168.1.254,24h
-      dhcp-range=interface:wlp4s0,192.168.12.10,192.168.12.254,24h
+      dhcp-range=interface:${netifLan},192.168.1.10,192.168.1.254,24h
+      dhcp-range=interface:${netifWifi},192.168.12.10,192.168.12.254,24h
     '';
   };
 
@@ -106,7 +109,7 @@ in
 
   # Enable CUPS to print documents.
   services.avahi.enable = true;
-  services.avahi.interfaces = ["enp3s0"];
+  services.avahi.interfaces = [ netifLan ];
   services.avahi.publish.enable = true;
   services.avahi.publish.userServices = true;
   services.printing.enable = true;