[Bug 2017] New: Multiple dynamically allocated remote ports all connect to the same local port

bugzilla-daemon at bugzilla.mindrot.org bugzilla-daemon at bugzilla.mindrot.org
Sat Jun 9 01:10:10 EST 2012


             Bug #: 2017
           Summary: Multiple dynamically allocated remote ports all
                    connect to the same local port
    Classification: Unclassified
           Product: Portable OpenSSH
           Version: 6.0p1
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P2
         Component: ssh
        AssignedTo: unassigned-bugs at mindrot.org
        ReportedBy: jdunn14 at gmail.com

>From the documentation:
  If the port argument is ‘0’, the listen port will be dynamically
allocated on the server and reported to the client at run time.  When
used together with -O forward the allocated port will be printed to the
standard output.

When using this functionality I found that if multiple remote ports
were dynamically allocated then connections to any of those ports will
forward back to the first forward requested.  Though not a severe
problem this behavior can be extremely misleading.

On the local host (margarita):
jdunn at margarita~$ ssh www.tdt.com -R 0:localhost:22 -R 0:localhost:80
Allocated port 33029 for remote forward to localhost:22
Allocated port 33719 for remote forward to localhost:80

On the remote host (www):
jdunn at www:~$ nc localhost 33029
jdunn at www:~$ nc localhost 33719

Apparently both remote ports are being mapped to the same local port. 
Looking through the code I found that 0 is being passed as the listen
port to channel_connect_by_listen_address.  The for loop in that
function then finds the first permitted_opens[i] with a listen port of

I was hoping to provide a simple fix, but it appears to require
changing the client-server protocol since the dynamically allocated
port is not supplied in the server packet unpacked in
client_request_forwarded_tcpip.  I suspect that the correct solution is
just to replace the listen_port used in the communication with the
dynamically allocated port.  The port number would also have to be
updated in the permitted_opens list when the server allocated the
remote port.

I was able to replicate this first in Fedora17, Debian Squeeze, and
then checked the nightly snapshots of Portable OpenSSH where I found
the same problem.

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