Exiting ssh when MaxSessions=0

Damien Miller djm at mindrot.org
Sat Aug 30 05:30:11 EST 2008

On Fri, 29 Aug 2008, Iain Morgan wrote:

> Hi,
> I've been experimenting with MaxSessions=0 in the sshd_config and have
> encountered one unfortunate problem. Once the client authenticates to
> the server, it ceases to respond to keyboard input.
> At first glance, it looks like the client is in a hung state and does
> not time out. If port forwarding was requested on the command-line and
> the server accepts the request, that continues to work. But the tilde
> escapes (and ^C) do not work. Apparently, you have to kill the session
> from another terminal.
> Once the session is killed, any buffered input is handled by the shell.
> In cases where you know the server will have MaxSessions=0, this is not
> a huge issue; you just have to remember to use the -f option. It is a
> bit unfortunate if you forget to use -f.
> It would be nice if ~. worked in this situation. I suppose ~C would also
> be nice in order to add port forwardings after the fact. I'm not sure if
> it would be problematic to add such support.

Yes, this is a bug. I think this patch fixes it, but I need to think
though the consequences more:

Index: channels.c
RCS file: /var/cvs/openssh/channels.c,v
retrieving revision 1.273
diff -u -p -r1.273 channels.c
--- channels.c	16 Jul 2008 12:42:06 -0000	1.273
+++ channels.c	29 Aug 2008 19:25:04 -0000
@@ -2311,8 +2311,8 @@ channel_input_open_failure(int type, u_i
-	/* Free the channel.  This will also close the socket. */
-	channel_free(c);
+	/* Schedule the channel for cleanup/deletion. */
+	chan_mark_dead(c);

The difference if you are curious, is that chan_mark_dead() will schedule
the channel for asynchronous cleanup, via channel_garbage_collect().
That path runs the channel->detach_user callback which is what we rely
on to determine that our main session channel has exited. 

channel_free() doesn't run any callbacks, so we never noticed that the
session channel went away.


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