[Bug 1503] New: Possible Intermittent False-negative on Port Forwarding.
bugzilla-daemon at bugzilla.mindrot.org
bugzilla-daemon at bugzilla.mindrot.org
Tue Aug 12 03:22:00 EST 2008
https://bugzilla.mindrot.org/show_bug.cgi?id=1503
Summary: Possible Intermittent False-negative on Port
Forwarding.
Classification: Unclassified
Product: Portable OpenSSH
Version: 5.0p1
Platform: Other
OS/Version: Other
Status: NEW
Severity: minor
Priority: P2
Component: ssh
AssignedTo: unassigned-bugs at mindrot.org
ReportedBy: THanson at CardinalPeak.com
While debugging other issues I have seen occasional reports of "Remote
port forwarding failed..." At least some of them appear to be
incorrect because forwarding was actually working.
Investigation revealed a probable cause: SSH2_MSG_GLOBAL_REQUEST
packets are being used for 2 purposes. They are used to request/cancel
port forwarding AND they are used as "keep alive" messages. Problem
is, the server ( server_input_global_request() ) sets type to FAILURE
for any packet other than "tcpip-forward" or "cancel-tcpip-forward".
Thus, "keep alive" requests are always set to FAILURE. When received
by the Client, it can not differentiate a "keep alive" response from a
forwarding request response. If timing is such that a keep alive was
sent just prior to a forwarding request, the keep-alive response may be
read by the forwarding code as a failure.
2 notes:
1) I have not been able to reproduce this and document the packet
traffic.
2) It appears that the server is expecting one of
[SSH2_MSG_CHANNEL_FAILURE, SSH2_MSG_REQUEST_SUCCESS,
SSH2_MSG_REQUEST_FAILURE ] as a keep alive message BUT the client
_appears_ to be sending SSH2_MSG_GLOBAL_REQUEST in order to trigger a
[SSH2_MSG_REQUEST_FAILURE |SSH2_MSG_REQUEST_SUCCESS] response which the
client wants as a keep-alive. Combining this pattern with
packet_read_poll_seqnr() resetting "keep_alive_timeouts" for every
packet, regardless of type, suggests that the keep-alive hand-shaking
scheme may be broken.
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