populate ip address based on hostname and populate the hostfile with other vm ips

nixos/machines/ghatanothoa/configuration.nix

@@ -4,7 +4,6 @@
     ../../modules/jitsi.nix
     ../../roles
     ../../roles/vm.nix
-    ../../modules/vmNetwork.nix
   ];
 
   services.mathebau-jitsi = {
@@ -14,6 +13,5 @@
 
   # System configuration here
   networking.hostName = "ghatanothoa";
-  vmNetwork.ipv4 = "192.168.0.25";
   system.stateVersion = "23.11";
 }

nixos/machines/lobon/configuration.nix

@@ -4,7 +4,6 @@
     ../../modules/mailman.nix
     ../../roles
     ../../roles/vm.nix
-    ../../modules/vmNetwork.nix
   ];
 
   # System configuration here
@@ -16,7 +15,6 @@
   };
 
   networking.hostName = "lobon";
-  vmNetwork.ipv4 = "192.168.0.22";
   system.stateVersion = "23.11";
 
   sops.secrets = {

nixos/machines/nodens/configuration.nix

@@ -3,7 +3,6 @@
     ./hardware-configuration.nix
     ../../roles
     ../../roles/vm.nix
-    ../../modules/vmNetwork.nix
   ];
 
   # System configuration here
@@ -11,6 +10,5 @@
   environment.systemPackages = [pkgs.git];
 
   networking.hostName = "nodens";
-  vmNetwork.ipv4 = "192.168.0.18";
   system.stateVersion = "24.11";
 }

nixos/machines/nyarlathotep/configuration.nix

@@ -4,7 +4,6 @@
     ../../modules/mail.nix
     ../../roles
     ../../roles/vm.nix
-    ../../modules/vmNetwork.nix
   ];
 
   # System configuration here
@@ -40,7 +39,6 @@
   };
 
   networking.hostName = "nyarlathotep";
-  vmNetwork.ipv4 = "192.168.0.17";
   system.stateVersion = "24.05";
 
   sops.secrets = let

nixos/modules/vmNetwork.nix

@@ -1,48 +0,0 @@
-{
-  lib,
-  config,
-  ...
-}: let
-  inherit
-    (lib)
-    mkOption
-    types
-    last
-    init
-    ;
-  inherit
-    (lib.strings)
-    splitString
-    concatStringsSep
-    toInt
-    ;
-  cfg = config.vmNetwork;
-in {
-  imports = [];
-
-  options.vmNetwork = {
-    ipv4 = mkOption {
-      type = types.str;
-      description = "the ipv4 adress of this machine";
-    };
-  };
-
-  config = {
-    networking = {
-      interfaces.enX0.ipv4.addresses = [
-        {
-          address = cfg.ipv4;
-          prefixLength = 16;
-        }
-      ];
-      defaultGateway = let
-        addr = splitString "." cfg.ipv4;
-        addrInit = init addr;
-        addrLastInt = builtins.toString (toInt (last addr) + 127);
-      in
-        concatStringsSep "." (addrInit ++ [addrLastInt]);
-      # https://www.hrz.tu-darmstadt.de/services/it_services/nameserver_dns/index.de.jsp
-      nameservers = ["130.83.22.63" "130.83.22.60" "130.83.56.60"];
-    };
-  };
-}

nixos/roles/hostmap.nix

@@ -0,0 +1,54 @@
+# This data is taken from /etc/hosts from azatoth
+{
+  bragi = {
+    ipv4 = "192.168.1.11";
+  };
+  tsathoggua = {
+    ipv4 = "192.168.0.13";
+  };
+  nyogtha = {
+    ipv4 = "192.168.0.14";
+  };
+  hastur = {
+    ipv4 = "192.168.0.15";
+  };
+  cthulhu = {
+    ipv4 = "192.168.0.16";
+  };
+  nyarlathotep = {
+    ipv4 = "192.168.0.17";
+  };
+  nodens = {
+    ipv4 = "192.168.0.18";
+  };
+  uvhash = {
+    ipv4 = "192.168.0.19";
+  };
+  aphoom-zhah = {
+    ipv4 = "192.168.0.20";
+  };
+  dagon = {
+    ipv4 = "192.168.0.21";
+  };
+  lobon = {
+    ipv4 = "192.168.0.22";
+  };
+  yibb-tstll = {
+    ipv4 = "192.168.0.23";
+  };
+  eihort = {
+    ipv4 = "192.168.0.24";
+  };
+  ghatanothoa = {
+    ipv4 = "192.168.0.25";
+  };
+  toth = {
+    ipv4 = "192.168.0.26";
+  };
+  ithaqua = {
+    ipv4 = "192.168.0.27";
+  };
+  cthugha = {
+    ipv4 = "192.168.0.30";
+  };
+}

nixos/roles/vm.nix

@@ -1,5 +1,6 @@
 {modulesPath, ...}: {
   imports = [
     (modulesPath + "/virtualisation/xen-domU.nix")
+    ./vmNetwork.nix
   ];
 }

nixos/roles/vmNetwork.nix

@@ -0,0 +1,46 @@
+{
+  lib,
+  config,
+  ...
+}: let
+  inherit (lib) mapAttrsToList;
+  inherit (lib.attrsets) foldAttrs concatMapAttrs;
+  inherit (lib.asserts) assertMsg;
+  inherit (lib.lists) filter last init;
+  inherit (lib.strings) splitString toInt concatStringsSep;
+  inherit (builtins) elem toString;
+  hostmap = import ./hostmap.nix;
+  myhostName = config.networking.hostName;
+  # To turn the hostmap around suitable for networking.hosts the following simple code almost works
+  # concatMapAttrs (hostname: ipData: { ${ipData.ipv4} = [hostname]; }) hostmap
+  # but breaks as soon as we want to map two different names to the same ip.
+  # So the code looks uglier than one would expect.
+  globalhosts = foldAttrs (a: b: a ++ b) [] (mapAttrsToList (hostname: ipData: {${ipData.ipv4} = [hostname];}) hostmap);
+  # We replace our own ip with 127.0.0.1 in /etc/hosts
+  myhosts = concatMapAttrs (ip: hosts:
+    if (elem myhostName hosts)
+    # nixos maps the hostname to the loopback 127.0.0.2 by default, so we exclude it here.
+    # there is also a default localhost to 127.0.0.1 in place
+    then {"127.0.0.1" = filter (x: x != myhostName) hosts;}
+    else {${ip} = hosts;})
+  globalhosts;
+  myIp = assert (assertMsg (hostmap ? ${myhostName}.ipv4) "${myhostName} has no ip configured in nixos/roles/hostmap.nix"); hostmap.${myhostName}.ipv4;
+in {
+  networking = {
+    hosts = myhosts;
+    interfaces.enX0.ipv4.addresses = [
+      {
+        address = myIp;
+        prefixLength = 16;
+      }
+    ];
+    defaultGateway = let
+      addr = splitString "." myIp;
+      addrInit = init addr;
+      addrLastInt = toString (toInt (last addr) + 127);
+    in
+      concatStringsSep "." (addrInit ++ [addrLastInt]);
+    # https://www.hrz.tu-darmstadt.de/services/it_services/nameserver_dns/index.de.jsp
+    nameservers = ["130.83.22.63" "130.83.22.60" "130.83.56.60"];
+  };
+}