nixos/mptcpd: generate config declaratively (#395893)

nixos/modules/services/networking/mptcpd.nix

@@ -5,11 +5,9 @@
   ...
 }:
 let
-
   cfg = config.services.mptcpd;
-
+  settingsFormat = pkgs.formats.ini { };
 in
-
 {
 
   options = {
@@ -20,15 +18,57 @@ in
 
       package = lib.mkPackageOption pkgs "mptcpd" { };
 
+      settings = lib.mkOption {
+        type = settingsFormat.type;
+        default = { };
+        example = lib.literalExpression ''
+          {
+            core = {
+              "addr-flags" = "subflow";
+              "notify-flags" = "existing,skip_link_local,skip_loopback,check_route";
+              "load-plugins" = "addr_adv,sspi";
+            };
+          }
+        '';
+        description = ''
+          mptcpd configuration written to {file}`/etc/mptcpd/mptcpd.conf`.
+
+          See {manpage}`mptcpd(8)` for details about available options and syntax.
+        '';
+      };
+
     };
 
   };
 
   config = lib.mkIf cfg.enable {
 
+    # Disable NetworkManager from configuring the MPTCP endpoints.
+    # See https://github.com/multipath-tcp/mptcpd/blob/48942b2110805af236ca41164fde67830efd7507/README.md?plain=1#L19-L38
+    networking.networkmanager.connectionConfig = {
+      "connection.mptcp-flags" = 1;
+    };
+
+    environment.etc."mptcpd/mptcpd.conf".source = settingsFormat.generate "mptcpd.conf" cfg.settings;
+
     environment.systemPackages = [ cfg.package ];
 
+    services.mptcpd.settings = {
+      core = {
+        log = lib.mkDefault "journal";
+        "plugin-dir" = "${cfg.package}/lib/mptcpd";
+        "path-manager" = lib.mkDefault "addr_adv";
+      };
+    };
+
     systemd.packages = [ cfg.package ];
+    systemd.services.mptcp = {
+      wantedBy = [ "multi-user.target" ];
+      serviceConfig.ExecStart = [
+        ""
+        "${cfg.package}/libexec/mptcpd"
+      ];
+    };
 
   };
 

pkgs/by-name/mp/mptcpd/package.nix

@@ -47,9 +47,15 @@ stdenv.mkDerivation (finalAttrs: {
   postConfigure = "doxygen -u";
 
   configureFlags = [
+    "--sysconfdir=/etc"
     "--with-systemdsystemunitdir=${placeholder "out"}/lib/systemd/system"
   ];
 
+  installFlags = [
+    # The NixOS module generates /etc/mptcpd/mptcpd.conf declaratively.
+    "pkgsysconfdir=$out/etc/mptcpd"
+  ];
+
   # fix: 'Fontconfig error: Cannot load default config file: No such file: (null)'
   env.FONTCONFIG_FILE = "${fontconfig.out}/etc/fonts/fonts.conf";