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Re: ACK char issue

From: david kerns
Subject: Re: ACK char issue
Date: Thu, 20 Aug 2020 01:03:45 -0700

if you do this:
screen -L
printf "\x6X\x6\n"

then look at the screenlog.0 file, does it contain your ACK ?

On Tue, Aug 18, 2020 at 11:47 PM Dro Ghazarian <> wrote:
  Hi David,
  Thank you for your quick reply.  I believe I did not explain the problem correctly.  I do not want to see the actual ACK character on the screen but rather being able to get the packet information that contains the ACK character. To demonstrate the issue, it can be done as the following.

1. from my windows machine I use putty to ssh to a linux machine.
2. Before connecting to the linux machine, I configure the putty to log all the SSH packets.
3. Once connected, I issue the following command printf "test\6\n"   , I am using this command to just demonstrate the issue, we do have our command processor that uses ACK as an acknowledgement of a received data
4. When I look at the incoming packet log I can see that I received the ACK character.  which is the following: 28 0d 0a 74 65 73 74 06 0d 0a  .......(..test..
5. From the same putty terminal I create a screen session by doing :  screen -dmS test
6. Then I connect to the session by:  screen -Rr test
7. issue the printf command again:  printf "test\6\n"
8. When looking at the incoming packets, the ACK character is missing:  27 0d 0a 74 65 73 74 0d 0a  .......'..test..
9. It appears that the screen is removing the ACK character and any other non printable character as a matter of fact.  standard characters such as bell (\7) and etc are not removed, so if I issue printf "test\7\n" I could hear the bell sound and the incoming packet will contain 07 chracter.
10. I have looked at the screen settings to configure the screenrc and also changing stty settings but I am not having any luck figuring out how to stop the screen from eating out the ACk character.

I would really appreciate your help

Thank you
Dro Ghazarian

On Fri, Aug 14, 2020 at 1:28 PM david kerns <> wrote:
ACK is a non-printable char ... if you run your app in a non screen window, I'd be surprised if you saw them.
Try running your app like this: app | cat -v
should print "^F" for every ACK ... independent of screen

On Fri, Aug 14, 2020 at 12:59 PM Dro Ghazarian <> wrote:
  I have been trying to use screen to run a process in detached mode.  Our process prints ACK characters to the screen, but unfortunately the screen is somehow removing non printable characters.  Other known characters such as bell would come through but not ACK. Is there a setting that I can change to let the screen handle ACK characters?

Thank you
Dro Ghazarian 

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