Index: twisted/conch/test/test_transport.py
===================================================================
--- twisted/conch/test/test_transport.py	(revision 28913)
+++ twisted/conch/test/test_transport.py	(working copy)
@@ -61,8 +61,17 @@
         self.unimplementeds = []
         self.debugs = []
         self.ignoreds = []
+        self.gotUnsupportedVersion = None
 
+    def _unsupportedVersionReceived(self, remoteVersion):
+        """
+        Intercept unsupported version call.
 
+        @type remoteVersion: C{str}
+        """
+        self.gotUnsupportedVersion = remoteVersion
+        return transport.SSHTransportBase._unsupportedVersionReceived(self, remoteVersion)
+
     def receiveError(self, reasonCode, description):
         """
         Store any errors received.
@@ -800,7 +809,26 @@
         self.assertTrue(proto.gotVersion)
         self.assertEquals(proto.otherVersionString, "SSH-1.99-OpenSSH")
 
+    def test_supportedVersionsAreAllowed(self):
+        """
+        Test that versions configured as supported work.
+        """
+        proto = MockTransportBase()
+        proto.supportedVersions = ("9.99", )
+        proto.makeConnection(proto_helpers.StringTransport())
+        proto.dataReceived("SSH-9.99-OpenSSH\n")
+        self.assert_(not proto.gotUnsupportedVersion)
 
+    def test_unsupportedVersionsCallUnsupportedVersionReceived(self):
+        """
+        Test that versions not configured as supported don't work.
+        """
+        proto = MockTransportBase()
+        proto.supportedVersions = ("2.0", )
+        proto.makeConnection(proto_helpers.StringTransport())
+        proto.dataReceived("SSH-9.99-OpenSSH\n")
+        self.assertEquals("9.99", proto.gotUnsupportedVersion)
+
     def test_badPackets(self):
         """
         Test that the transport disconnects with an error when it receives
Index: twisted/conch/topfiles/4428.feature
===================================================================
--- twisted/conch/topfiles/4428.feature	(revision 0)
+++ twisted/conch/topfiles/4428.feature	(revision 0)
@@ -0,0 +1 @@
+twisted.conch.ssh.transport.SSHTransportBase now allows supported ssh protocol versions to be overriden.
Index: twisted/conch/ssh/transport.py
===================================================================
--- twisted/conch/ssh/transport.py	(revision 28913)
+++ twisted/conch/ssh/transport.py	(working copy)
@@ -67,6 +67,9 @@
     @ivar supportedLanguages: A list of strings representing languages
         supported, from most-preferred to least.
 
+    @ivar supportedVersions: A container of strings representing supported ssh
+        protcol version numbers.
+
     @ivar isClient: A boolean indicating whether this is a client or server.
 
     @ivar gotVersion: A boolean indicating whether we have receieved the
@@ -149,6 +152,7 @@
     supportedPublicKeys = ['ssh-rsa', 'ssh-dss']
     supportedCompressions = ['none', 'zlib']
     supportedLanguages = ()
+    supportedVersions = ('1.99', '2.0')
     isClient = False
     gotVersion = False
     buf = ''
@@ -282,7 +286,18 @@
         self.incomingPacketSequence += 1
         return payload
 
+    def _unsupportedVersionReceived(self, remoteVersion):
+        """
+        Called when an unsupported version of the ssh protocol is received from
+            the remote endpoint.
 
+        @param remoteVersion: remote ssh protocol version which is unsupported
+            by us.
+        @type remoteVersion: C{str}
+        """
+        self.sendDisconnect(DISCONNECT_PROTOCOL_VERSION_NOT_SUPPORTED,
+            'bad version ' + remoteVersion)
+
     def dataReceived(self, data):
         """
         First, check for the version string (SSH-2.0-*).  After that has been
@@ -300,10 +315,9 @@
                 if p.startswith('SSH-'):
                     self.gotVersion = True
                     self.otherVersionString = p.strip()
-                    if p.split('-')[1] not in ('1.99', '2.0'): # bad version
-                        self.sendDisconnect(
-                            DISCONNECT_PROTOCOL_VERSION_NOT_SUPPORTED,
-                            'bad version ' + p.split('-')[1])
+                    remoteVersion = p.split('-')[1]
+                    if remoteVersion not in self.supportedVersions:
+                        self._unsupportedVersionReceived(remoteVersion)
                         return
                     i = lines.index(p)
                     self.buf = '\n'.join(lines[i + 1:])