Patch for a port forwarding problem in serverloop.c

[jam at McQuil.Com]

Hello all,

I encountered a problem when trying to setup a remote portforward
using OpenSSH 3.5p1.

If the port that I was trying to listen to on the remote side
was already in use, a message 'cannot bind to port: 5000' was
written to the syslog, but the connection still persisted.

I had no way on the client side to know that the listening
wasn't taking place.  that is, the portforwarding of port 5000
failed, but the ssh connection on port 22 was still there.

I found the spot in serverloop.c, where it seemed to be
ignoring the return status from the 'channel_setup_remote_fwd_listener'
call.

The fix for me was a simple testing of the success, and calling
fatal() to indicate that the port forwarding failed.

Here is the patch:


--- serverloop.c.orig	Sat Feb 22 11:35:39 2003
+++ serverloop.c	Sat Feb 22 11:19:29 2003
@@ -989,6 +989,9 @@
 			/* Start listening on the port */
 			success = channel_setup_remote_fwd_listener(
 			    listen_address, listen_port, options.gateway_ports);
+                        if( !success ){
+				fatal("Cannot bind to the remote port");
+			}
 		}
 		xfree(listen_address);
 	}


And, here is the commandline that I used to initiate the connection:


while :; do

   ssh -N -R 5000:localhost:5000  user at remotesystem

   sleep 10

done  


My patch is not terribly robust.  I suppose that if you didn't use
the '-N' option, you may not want the connection to disappear, but
in my case, if the port forwarding fails, I really need the
connection to drop, so I can go into a loop and retry it.

The reason I need this, is that i'm setting up a forwarded port
when I boot the client.  If I reboot the client, the old connection
lingers for a couple of minutes on the remote machine, before
it realizes that the client has gone away.  When the client comes
back up, and tries to re-establish the forwarded port, it can't.

So, now with my patch, the client will realize that the port forward
request failed, and it will sit in a loop, and try every 10 seconds,
until the connection succeeds.

Anyway, thanks, and I hope you find my patch useful.

Jim McQuillan
jam at Ltsp.org
-------------- next part --------------
--- serverloop.c.orig	Sat Feb 22 11:35:39 2003
+++ serverloop.c	Sat Feb 22 11:19:29 2003
@@ -989,6 +989,9 @@
 			/* Start listening on the port */
 			success = channel_setup_remote_fwd_listener(
 			    listen_address, listen_port, options.gateway_ports);
+                        if( !success ){
+				fatal("Cannot bind to the remote port");
+			}
 		}
 		xfree(listen_address);
 	}