Use Windows user for upstream proxy

Program.cs

@@ -14,12 +14,8 @@ class Program
 {
     static string USER = "user";
     static string PASS = "pass";
-    static readonly string UPSTREAM_USER = Environment.GetEnvironmentVariable("UPSTREAM_USER");
-    static readonly string UPSTREAM_PASS = Environment.GetEnvironmentVariable("UPSTREAM_PASS");
-    static readonly bool UPSTREAM_ALLOW_EMPTY_PASSWORD = string.Equals(
-        Environment.GetEnvironmentVariable("UPSTREAM_ALLOW_EMPTY_PASSWORD"),
-        "1",
-        StringComparison.OrdinalIgnoreCase);
+    static readonly string UPSTREAM_USER = ResolveUpstreamUser();
+    static readonly string UPSTREAM_PASS = Environment.GetEnvironmentVariable("UPSTREAM_PASS") ?? string.Empty;
     static readonly string UPSTREAM_PROXY_AUTH_HEADER = BuildUpstreamProxyAuthHeader();
     static long _connectionSeq;
 
@@ -32,6 +28,8 @@ class Program
         Log.Info($"http://127.0.0.1:{port}");
         Log.Info($"login: {USER}");
         Log.Info($"pass : {PASS}");
+        Log.Info($"upstream auth user: {UPSTREAM_USER}");
+        Log.Info("upstream auth pass: <empty>");
 
         var listener = new TcpListener(IPAddress.Any, port);
         listener.Start();
@@ -365,11 +363,8 @@ class Program
     static string BuildUpstreamProxyAuthHeader()
     {
         if (string.IsNullOrWhiteSpace(UPSTREAM_USER))
-            return string.Empty;
-
-        if (string.IsNullOrEmpty(UPSTREAM_PASS) && !UPSTREAM_ALLOW_EMPTY_PASSWORD)
         {
-            Log.Info("UPSTREAM_PASS is empty; upstream Basic auth header is disabled");
+            Log.Warn("UPSTREAM_USER is empty; upstream Proxy-Authorization is disabled");
             return string.Empty;
         }
 
@@ -377,6 +372,19 @@ class Program
         string encoded = Convert.ToBase64String(Encoding.ASCII.GetBytes($"{UPSTREAM_USER}:{pass}"));
         return $"Proxy-Authorization: Basic {encoded}\r\n";
     }
+
+    static string ResolveUpstreamUser()
+    {
+        var fromEnv = Environment.GetEnvironmentVariable("UPSTREAM_USER");
+        if (!string.IsNullOrWhiteSpace(fromEnv))
+            return fromEnv;
+
+        var domain = Environment.UserDomainName;
+        var user = Environment.UserName;
+        if (!string.IsNullOrWhiteSpace(domain) && !string.IsNullOrWhiteSpace(user))
+            return $"{domain}\\{user}";
+        return user ?? string.Empty;
+    }
 }
 
 // ================= WINHTTP =================