diff --git a/test/dns_proxy_test.dart b/test/dns_proxy_test.dart
index 4017c48..a8ea5be 100644
--- a/test/dns_proxy_test.dart
+++ b/test/dns_proxy_test.dart
@@ -55,8 +55,16 @@ void main() {
       await t.start();
       addTearDown(t.close);
       final r = DnsResolver(config: () => custom('127.0.0.1'), port: t.port);
-      expect((await r.lookup('big.test')).single.address, '10.9.9.9');
-      expect(t.tcpQueries, greaterThan(0));
+      // 전체 실행에서만 실패한 적이 있다 - 실패하면 실제 오류와 가짜 DNS 의 기록을 남긴다
+      try {
+        expect((await r.lookup('big.test')).single.address, '10.9.9.9');
+        expect(t.tcpQueries, greaterThan(0));
+      } catch (e, st) {
+        // ignore: avoid_print
+        print('TC 시험 실패: $e (${e.runtimeType}${e is SocketException ? ', errno ${e.osError?.errorCode}' : ''})\n'
+            '가짜 DNS: port ${t.port}, ${t.events}, 질문 ${t.queries}\n$st');
+        rethrow;
+      }
     });
 
     test('서버에 닿지 않으면 unreachable 오류 (기기 DNS 로 바꾸지 않음)', () async {
diff --git a/test/support/fake_dns.dart b/test/support/fake_dns.dart
index c0f1032..83ef0ee 100644
--- a/test/support/fake_dns.dart
+++ b/test/support/fake_dns.dart
@@ -13,6 +13,9 @@ class FakeDns {
   final queries = <String>[];
   int tcpQueries = 0;
 
+  /// 시작 · 응답 중에 생긴 일 (시험이 실패하면 남겨 원인을 본다)
+  final events = <String>[];
+
   FakeDns(this.records, {this.truncateUdp = false, this.ttl = 300});
 
   int get port => _udp.port;
@@ -29,8 +32,10 @@ class FakeDns {
         try {
           _udp = await RawDatagramSocket.bind(InternetAddress.loopbackIPv4, tcp.port);
           _tcp = tcp;
+          events.add('bound tcp+udp ${tcp.port} (try $i)');
           break;
-        } on SocketException {
+        } on SocketException catch (e) {
+          events.add('udp bind ${tcp.port} failed: ${e.osError?.errorCode} ${e.message}');
           await tcp.close();
           if (i > 50) rethrow;
         }
@@ -43,6 +48,7 @@ class FakeDns {
       _udp.send(_answer(d.data, tc: truncateUdp), d.address, d.port);
     });
     _tcp?.listen((s) async {
+      events.add('tcp accepted ${s.remotePort}');
       final buf = BytesBuilder();
       await for (final c in s) {
         buf.add(c);