feat: add TCP DNS plugin with SOCKS5 proxy support

Extract shared DNS wire-format helpers into src/derp/dns.py so both
the UDP plugin (dns.py) and the new TCP plugin (tdns.py) share the
same encode/decode/build/parse logic.

The !tdns command routes queries through the SOCKS5 proxy via
derp.http.open_connection, using TCP framing (2-byte length prefix).
Default server: 1.1.1.1.

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

tests/test_tdns.py

@@ -0,0 +1,128 @@
+"""Tests for the TCP DNS plugin."""
+
+import asyncio
+import importlib.util
+import struct
+import sys
+from pathlib import Path
+from unittest.mock import AsyncMock, MagicMock, patch
+
+import pytest
+
+from derp.dns import encode_name
+
+# plugins/ is not a Python package -- load the module from file path
+_spec = importlib.util.spec_from_file_location(
+    "plugins.tdns", Path(__file__).resolve().parent.parent / "plugins" / "tdns.py",
+)
+_mod = importlib.util.module_from_spec(_spec)
+sys.modules[_spec.name] = _mod
+_spec.loader.exec_module(_mod)
+
+from plugins.tdns import _query_tcp, cmd_tdns  # noqa: E402
+
+
+def _make_a_response(ip_bytes: bytes = b"\x01\x02\x03\x04") -> bytes:
+    """Build a minimal A-record DNS response."""
+    tid = b"\x00\x01"
+    flags = struct.pack("!H", 0x8180)
+    counts = struct.pack("!HHHH", 1, 1, 0, 0)
+    qname = encode_name("example.com")
+    question = qname + struct.pack("!HH", 1, 1)
+    answer = qname + struct.pack("!HHIH", 1, 1, 300, len(ip_bytes)) + ip_bytes
+    return tid + flags + counts + question + answer
+
+
+class TestQueryTcp:
+    def test_sends_length_prefixed_query(self):
+        response = _make_a_response()
+        framed = struct.pack("!H", len(response)) + response
+
+        reader = AsyncMock()
+        reader.readexactly = AsyncMock(side_effect=[
+            framed[:2],   # length prefix
+            framed[2:],   # payload
+        ])
+        writer = MagicMock()
+        writer.drain = AsyncMock()
+        writer.wait_closed = AsyncMock()
+
+        mock_open = AsyncMock(return_value=(reader, writer))
+        with patch.object(_mod, "_open_connection", mock_open):
+            rcode, results = asyncio.run(_query_tcp("example.com", 1, "1.1.1.1"))
+
+        assert rcode == 0
+        assert results == ["1.2.3.4"]
+
+        # Verify the written data has a 2-byte length prefix
+        written = writer.write.call_args[0][0]
+        pkt_len = struct.unpack("!H", written[:2])[0]
+        assert pkt_len == len(written) - 2
+
+    def test_closes_writer_on_error(self):
+        reader = AsyncMock()
+        reader.readexactly = AsyncMock(side_effect=asyncio.IncompleteReadError(b"", 2))
+        writer = MagicMock()
+        writer.drain = AsyncMock()
+        writer.wait_closed = AsyncMock()
+
+        mock_open = AsyncMock(return_value=(reader, writer))
+        with patch.object(_mod, "_open_connection", mock_open):
+            with pytest.raises(asyncio.IncompleteReadError):
+                asyncio.run(_query_tcp("example.com", 1, "1.1.1.1"))
+
+        writer.close.assert_called_once()
+
+
+class TestCmdTdns:
+    def test_no_args(self):
+        bot = AsyncMock()
+        msg = MagicMock()
+        msg.text = "!tdns"
+        asyncio.run(cmd_tdns(bot, msg))
+        bot.reply.assert_called_once()
+        assert "Usage" in bot.reply.call_args[0][1]
+
+    def test_ip_auto_ptr(self):
+        bot = AsyncMock()
+        msg = MagicMock()
+        msg.text = "!tdns 1.2.3.4"
+
+        mock_tcp = AsyncMock(return_value=(0, ["host.example.com"]))
+        with patch.object(_mod, "_query_tcp", mock_tcp):
+            asyncio.run(cmd_tdns(bot, msg))
+
+        reply = bot.reply.call_args[0][1]
+        assert "PTR" in reply
+        assert "host.example.com" in reply
+
+    def test_explicit_type(self):
+        bot = AsyncMock()
+        msg = MagicMock()
+        msg.text = "!tdns example.com MX"
+
+        mock_tcp = AsyncMock(return_value=(0, ["10 mail.example.com"]))
+        with patch.object(_mod, "_query_tcp", mock_tcp):
+            asyncio.run(cmd_tdns(bot, msg))
+
+        reply = bot.reply.call_args[0][1]
+        assert "MX" in reply
+
+    def test_custom_server(self):
+        bot = AsyncMock()
+        msg = MagicMock()
+        msg.text = "!tdns example.com A @8.8.8.8"
+
+        mock_tcp = AsyncMock(return_value=(0, ["93.184.216.34"]))
+        with patch.object(_mod, "_query_tcp", mock_tcp):
+            asyncio.run(cmd_tdns(bot, msg))
+
+        assert mock_tcp.call_args[0][2] == "8.8.8.8"
+
+    def test_unknown_type(self):
+        bot = AsyncMock()
+        msg = MagicMock()
+        msg.text = "!tdns example.com BOGUS"
+        asyncio.run(cmd_tdns(bot, msg))
+        reply = bot.reply.call_args[0][1]
+        assert "Unknown type" in reply