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Re: Odd ctrl-s problem.

Peter Saunders pajs@fodder.org.uk
Wed, 27 Feb 2008 06:31:25 -0800 (PST)

A follow up..

I did some testing after this problem occured again.

In 1 window:
	while true; do pkill conserver; conserver start; sleep 7; done
(some times for testing, make it a pkill -SEGV - to simulate when
conserver crashes)

In other window
	while true; do console <hostname>; done

On the host itself
	while true; do date > /dev/console; sleep 1; done

After about 20-30 conserver restarts - the end host required a ctrl q to
send output again.

So, my next test. I created a file which contained ^Eco to force a
console restart.

	while true; do
		cat /tmp/eco | console <hostname>
		sleep 5

Again, after about 20-30 restarts, flow contol has occured again.

For my last test, I took conserver out of the equation, and wrote an
expect script that connected to the console directly:

#!/opt/sfw/bin/expect -f
spawn /usr/bin/ssh -o StrictHostKeyChecking=no -o ForwardX11=no -c 3des user:port@terminalserver
set timeout 60
expect -nocase -re "password:"
send -- "PASSWORD\n"

expect -nocase -re "\n"
expect -nocase -re "\n"
expect -nocase -re "\n"


And ran this in a while true loop. (So doing exactly what conserver
would do, spawn an ssh connection and reading it). If it was a terminal
server bug, i would expect this to behave the same as conserver, e.g. on
the 20-30 time, need a ctrl q. However, this ran every time, never
having an issue. It just kept working forever.

So, it does look like it is something in conserver causing this to
happen, unfortunately intermittently.

I tried setting the default "options" to:

	options "!ixon,!ixoff,autoreinit,reinitoncc";

and even

	options "ixon,ixoff,autoreinit,reinitoncc";

However, this made no difference.

Getting a ^Q sent at startup (in the initcmd) does stop this happening,
as does sending it in the idlestring. So I have a workaround to my
specific issue, but it would be intresting to know why this happens at
all. Any thoughts?


On Thu, Feb 07, 2008 at 10:42:44AM +0000, Peter Saunders wrote:
> Many thanks for your replies,
> I still haven't got to the bottom of what actually caused it, but, it
> seems odd if was the terminal server. Its a reasonably recent event that
> conserver went live on these machines - and in the past, the terminal
> servers didn't have a connection to them from the network, so, all the
> serial traffic was silently thrown away. This is what I was expecting to
> happen during the conserver restart window. (People used to ssh to the
> terminal server only when they needed the console)
> However, I think i'll change the the idlestring to contain ^q - so in
> the event of a restart causing it again, at least after 5 minutes of
> inactivity, conserver would sent a ctrl-q to it again. (Assuming I can
> get it to do this?)
> Cheers
> Pete
> On Wed, Feb 06, 2008 at 12:42:25PM -0500, Greg A. Woods wrote:
> > 
> > On 6-Feb-08, at 11:31 AM, Brodie, Kent wrote:
> > 
> > > Well, it's been a long while since I've had to mess with older  
> > > terminal
> > > servers, but I seem to recall that some flavors of decservers for
> > > example allowed you to control the pass-through of various control
> > > characters to the port.   Specifically, you could control whether  
> > > things
> > > like BREAK, control-x, control-s, etc got passed through or trapped.
> > 
> > The problem here is definitely not with pass-through of control  
> > characters.
> > 
> > It's more to do with the terminal server generating control servers  
> > in its own attempt to control the data flowing from the attached device.
> > 
> > Disabling flow control on the port could probably prevent the problem  
> > from happening, however then flow control during normal use would  
> > then be impossible (pass-through of flow control characters would not  
> > have the desired effect -- they don't get passed through SSH and  
> > TELNET connections in the way you would want them to be transmitted  
> > since buffering in the various network layers would defeat any  
> > attempt to use a raw connection).
> > 
> > Proper flow control for interactive use requires that the terminal  
> > server perform flow control directly itself (and that the various  
> > network layers use whatever mechanisms they have to do flow control  
> > properly, right down to the connection to the attached device).
> > 
> > Eg. you want output to stop almost immediately when you hit ^S but  
> > you don't want anything to be lost.  That means the final output  
> > device in front of the user (eg. xterm) interpret the ^S from the  
> > user and immediately stop generating output, while at the same time  
> > pushing the flow control request back through the various layers  
> > (CONSERVER -> SSH -> TELNET -> RS232 or whatever) so that eventually  
> > a flow control request reaches the device generating the data in the  
> > appropriate form and that all buffered data is preserved in all the  
> > various layers in anticipation of the user hitting ^Q to see some  
> > more (or that it all be flushed if the user hits ^C or whatever).   
> > Note that this may sometimes involve translating the flow control  
> > request into a hardware signal change on the RS323 line, such as de- 
> > asserting CTS.
> > 
> > Note that flow control may have to work properly though all the  
> > layers for more than just interactive uses too.  If you don't want  
> > data from your attached devices to be lost by conserver in its logs,  
> > for example, then you need fully working flow control back through  
> > all the layers to the attached devices.  If you don't have fully  
> > working flow control through all layers then something like a minor  
> > network glitch may cause a buffer to fill and all data between that  
> > time and the draining of the buffer to be lost forever.
> > 
> > -- 
> > 						Greg A. Woods
> > 						<woods@weird.com>
> > 
> > 
> > 
> > _______________________________________________
> > users mailing list
> > users@conserver.com
> > https://www.conserver.com/mailman/listinfo/users
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