Fix a pseudo-bug re: responding to MSG_UNIMPLEMENTED w/ itself

4 files changed, 49 insertions, 5 deletions

diff --git a/dev-requirements.txt b/dev-requirements.txt
index 1da876b8..c192f144 100644
--- a/dev-requirements.txt
+++ b/dev-requirements.txt
@@ -4,6 +4,7 @@ invocations>=1.2.0,<2.0
 # NOTE: pytest-relaxed currently only works with pytest >=3, <3.3
 pytest>=3.2,<3.3
 pytest-relaxed==1.1.2
+mock==2.0.0
 # Linting!
 flake8==2.4.0
 # Coverage!
diff --git a/paramiko/transport.py b/paramiko/transport.py
index 1317f372..d8cb3434 100644
--- a/paramiko/transport.py
+++ b/paramiko/transport.py
@@ -81,6 +81,8 @@ from paramiko.common import (
     DEFAULT_WINDOW_SIZE,
     DEFAULT_MAX_PACKET_SIZE,
     HIGHEST_USERAUTH_MESSAGE_ID,
+    MSG_UNIMPLEMENTED,
+    MSG_NAMES,
 )
 from paramiko.compress import ZlibCompressor, ZlibDecompressor
 from paramiko.dsskey import DSSKey
@@ -1958,11 +1960,22 @@ class Transport(threading.Thread, ClosingContextManager):
                         if len(self._expected_packet) > 0:
                             continue
                     else:
-                        self._log(WARNING, "Oops, unhandled type %d" % ptype)
-                        msg = Message()
-                        msg.add_byte(cMSG_UNIMPLEMENTED)
-                        msg.add_int(m.seqno)
-                        self._send_message(msg)
+                        # Respond with "I don't implement this particular
+                        # message type" message (unless the message type was
+                        # itself literally MSG_UNIMPLEMENTED, in which case, we
+                        # just shut up to avoid causing a useless loop).
+                        name = MSG_NAMES[ptype]
+                        self._log(
+                            WARNING,
+                            "Oops, unhandled type {} ({!r})".format(
+                                ptype, name
+                            ),
+                        )
+                        if ptype != MSG_UNIMPLEMENTED:
+                            msg = Message()
+                            msg.add_byte(cMSG_UNIMPLEMENTED)
+                            msg.add_int(m.seqno)
+                            self._send_message(msg)
                     self.packetizer.complete_handshake()
             except SSHException as e:
                 self._log(ERROR, "Exception: " + str(e))
diff --git a/sites/www/changelog.rst b/sites/www/changelog.rst
index b00f03f0..207bfe59 100644
--- a/sites/www/changelog.rst
+++ b/sites/www/changelog.rst
@@ -2,6 +2,11 @@
 Changelog
 =========
 
+- :bug:`-` Modify protocol message handling such that ``Transport`` does not
+  respond to ``MSG_UNIMPLEMENTED`` with its own ``MSG_UNIMPLEMENTED`` message.
+  This behavior probably didn't cause any outright errors, but it doesn't seem
+  to conform to the RFCs and could cause (non-infinite) feedback loops in some
+  scenarios (usually those involving Paramiko on both ends).
 - :support:`1292 backported` Backport changes from :issue:`979` (added in
   Paramiko 2.3) to Paramiko 2.0-2.2, using duck-typing to preserve backwards
   compatibility. This allows these older versions to use newer Cryptography
diff --git a/tests/test_transport.py b/tests/test_transport.py
index 13fb302e..3ea2cb26 100644
--- a/tests/test_transport.py
+++ b/tests/test_transport.py
@@ -30,6 +30,7 @@ import threading
 import random
 from hashlib import sha1
 import unittest
+from mock import Mock
 
 from paramiko import (
     Transport,
@@ -47,6 +48,7 @@ from paramiko import OPEN_SUCCEEDED, OPEN_FAILED_ADMINISTRATIVELY_PROHIBITED
 from paramiko.common import (
     MSG_KEXINIT,
     cMSG_CHANNEL_WINDOW_ADJUST,
+    cMSG_UNIMPLEMENTED,
     MIN_PACKET_SIZE,
     MIN_WINDOW_SIZE,
     MAX_WINDOW_SIZE,
@@ -1027,3 +1029,26 @@ class TransportTest(unittest.TestCase):
             assert "forwarding request denied" in str(e)
         else:
             assert False, "Did not raise SSHException!"
+
+    def _send_unimplemented(self, server_is_sender):
+        self.setup_test_server()
+        sender, recipient = self.tc, self.ts
+        if server_is_sender:
+            sender, recipient = self.ts, self.tc
+        recipient._send_message = Mock()
+        msg = Message()
+        msg.add_byte(cMSG_UNIMPLEMENTED)
+        sender._send_message(msg)
+        # TODO: I hate this but I literally don't see a good way to know when
+        # the recipient has received the sender's message (there are no
+        # existing threading events in play that work for this), esp in this
+        # case where we don't WANT a response (as otherwise we could
+        # potentially try blocking on the sender's receipt of a reply...maybe).
+        time.sleep(0.1)
+        assert not recipient._send_message.called
+
+    def test_server_does_not_respond_to_MSG_UNIMPLEMENTED(self):
+        self._send_unimplemented(server_is_sender=False)
+
+    def test_client_does_not_respond_to_MSG_UNIMPLEMENTED(self):
+        self._send_unimplemented(server_is_sender=True)