initial commit: standalone SoftEther VPN client in Go

Built-in DHCP (raw Ethernet frames through tunnel), automatic reconnection,
host route management, classless static routes (option 121/249), DNS config.
Single static binary, Linux only.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

flake.nix

@@ -0,0 +1,38 @@
+{
+  description = "softether-go - Standalone SoftEther VPN client in Go";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, flake-utils }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = import nixpkgs { inherit system; };
+      in
+      {
+        packages.default = pkgs.buildGoModule {
+          pname = "softether-go";
+          version = "0.1.0";
+          src = ./.;
+          vendorHash = null;
+          subPackages = [ "cmd/softether-go" ];
+
+          meta = with pkgs.lib; {
+            description = "Standalone SoftEther VPN client";
+            license = licenses.mit;
+            platforms = platforms.linux;
+          };
+        };
+
+        devShells.default = pkgs.mkShell {
+          buildInputs = with pkgs; [
+            go
+            gopls
+            gotools
+          ];
+        };
+      }
+    );
+}