nixos/kresd: add listenDoH option

nixos/modules/services/networking/kresd.nix

@@ -5,12 +5,15 @@ with lib;
 let
 
   cfg = config.services.kresd;
-  package = pkgs.knot-resolver;
+  configFile = pkgs.writeText "kresd.conf" ''
+    ${optionalString (cfg.listenDoH != []) "modules.load('http')"}
+    ${cfg.extraConfig};
+  '';
 
-  configFile = pkgs.writeText "kresd.conf" cfg.extraConfig;
-in
-
-{
+  package = pkgs.knot-resolver.override {
+    extraFeatures = cfg.listenDoH != [];
+  };
+in {
   meta.maintainers = [ maintainers.vcunat /* upstream developer */ ];
 
   imports = [
@@ -67,6 +70,15 @@ in
         For detailed syntax see ListenStream in man systemd.socket.
       '';
     };
+    listenDoH = mkOption {
+      type = with types; listOf str;
+      default = [];
+      example = [ "198.51.100.1:443" "[2001:db8::1]:443" "443" ];
+      description = ''
+        Addresses and ports on which kresd should provide DNS over HTTPS (see RFC 7858).
+        For detailed syntax see ListenStream in man systemd.socket.
+      '';
+    };
     # TODO: perhaps options for more common stuff like cache size or forwarding
   };
 
@@ -104,6 +116,18 @@ in
       };
     };
 
+    systemd.sockets.kresd-doh = mkIf (cfg.listenDoH != []) rec {
+      wantedBy = [ "sockets.target" ];
+      before = wantedBy;
+      partOf = [ "kresd.socket" ];
+      listenStreams = cfg.listenDoH;
+      socketConfig = {
+        FileDescriptorName = "doh";
+        FreeBind = true;
+        Service = "kresd.service";
+      };
+    };
+
     systemd.sockets.kresd-control = rec {
       wantedBy = [ "sockets.target" ];
       before = wantedBy;